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Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

5
Technology Overview

The Navy requested an update of previous reviews of innovative technologies for cleanup of groundwater, soils, and sediment (NRC 1994, 1997a, 1999a, 2000). This chapter discusses a variety of innovative technologies the Navy might consider during adaptive site management (ASM), for example, for initial remedy selection, as replacements for existing remedies that have proved to be unsuccessful, or as additions to current remedies to better achieve cleanup goals or reduce cleanup time. Because the Navy defined sediment contamination and solvents and metals in soil and groundwater as its most pressing current problems, the focus is on these types of contamination and on applicable remedial technologies, including the concept of treatment trains designed to meet multiple goals for multiple contaminants. The emphasis is on those technologies showing the greatest promise, particularly those technologies being developed and evaluated by the Department of Defense (DoD) and by the U.S. Environmental Protection Agency (EPA) and its Technology Innovation Office in association with the Federal Remediation Technologies Roundtable (FRTR). Although petroleum hydrocarbon sites remain a significant problem because of their sheer number (as discussed in Chapter 1), they are not the focus of this chapter at the request of the Navy and because their cleanup is generally considered to be well understood.

Both in situ and ex situ technologies can be identified according to applicable contaminant groups. Using the FRTR grouping of contaminants (see Table 1-1), eight contaminants groups—halogenated and non-halogenated volatile organic compounds (VOCs), halogenated and non-halogenated semivolatile organic compounds (SVOCs), fuels, inorganics, radionuclides, and explosives—can be defined and linked to the treatment technologies listed in Table 5-1 in terms of both in situ and ex situ procedures. Contaminants and technologies germane to soils, sediments,

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

TABLE 5-1 Primary Treatment Technologies

In Situ Soil and Sediment

Ex Situ Soil and Sediment

In Situ Groundwater

Ex Situ Groundwater

Biosparging

Bioventing

Bioremediation

Capping

Chemical Reduction/Oxidation

Dual-Phase Extraction

Dynamic Underground Stripping

Electrokinetics

Hot Air Injection

Heating

Phytoremediation

Soil Flushing (in situ)

Soil Vapor Extraction

Solidification/ Stabilization

Steam Extraction

Thermally Enhanced Recovery (e.g., EM, in situ RF, ISTD)

Vitrification

Bioremediation— Composting

Bioremediation— Land Treatment

Bioremediation— Slurry Phase

Chemical Reduction/Oxidation

Contained Recovery of Oily Waste

Critical Fluid Extraction

Cyanide Oxidation

Dehalogenation

Hydraulic Dredging

Incineration (offsite)

Incineration (onsite)

Landfill Disposal

Mechanical Dredging

Physical Separation

Plasma High-temperature

Metals Recovery

Pyrolysis

Solar Detoxification

Soil Washing

Solidification/ Stabilization

Solvent Extraction

Thermal Desorption

Vitrification

Aeration

Air Sparging

Bioremediation

Bioslurping

Chemical Reduction/Oxidation

Circulating Wells

Cosolvent Flushing

Dual-Phase Extraction

Dynamic Underground Stripping

Electrokinetics

Hot Water/Steam Flushing/Stripping

Monitored Natural Attenuation

Permeable Reactive Barrier

Phytoremediation

Surfactants/Surfactant Flushing

Vertical Barrier Wall

Free Product

Recovery

Pump and Treat with:

Air Stripping

Bioreactors

Carbon Adsorption

Chemical Reduction/Oxidation

Chemical Treatment

Distillation

Electrochemical Treatment

Filtration

Precipitation

Reverse Osmosis

Solar Detoxification

Solvent Extraction

Supercritical Water Oxidation

UV/Oxidation

 

SOURCE: Adapted from EPA (1997a).

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

and groundwater can be further categorized according to the purpose of the technology and its relative maturity. Accordingly, as indicated in Table 5-2, screening of potential technologies can be facilitated to assist remedial project managers (RPMs) in selecting a remedial alternative. Each technology is defined in a glossary at the end of this chapter.

Key reference information useful for identifying and selecting technologies and combinations of technologies responsive to cleanup needs has been consolidated into a matrix published elsewhere (EPA, 1997a; http://www.frtr.gov). Other sources of information include technology-specific fact sheets produced by a joint effort between the Department of Energy (DOE) and the Air Force Base Conversion Agency (as well as those from other federal agencies). The objective of these fact sheets is to provide RPMs with information on optimizing cleanup technologies, on presenting multiple lines of evidence about remedy performance, on preparing five-year reviews, on operating remedy demonstrations, and on communicating progress to the public. The FRTR website maintains a database of many remediation technologies, their applications, conditions of use, performance data, and cost (although it is not comprehensive). This database would be even more useful if universities, states, and the private sector were encouraged to submit additional information where appropriate. The lack of a central, comprehensive database is likely to hamper the data analysis exercises (see Chapter 3) that characterize full-scale ASM. In addition, federal facility database systems are aligned to measure progress of the cleanup process (see Figures 1-1 and 1-2) versus measuring cleanup performance—an approach to data collection and analysis that will need to shift in order for ASM to be successfully implemented.

Although site conditions and contaminant sources limit the selection of applicable treatment technologies, most sites can be remediated by three primary strategies—destruction or alteration of contaminants, extraction or separation of contaminants from environmental media, and immobilization of contaminants. Currently, destruction technologies include both in situ and ex situ thermal, biological, and chemical methods. Extraction and separation technologies include thermal desorption, soil washing, solvent and vapor extraction for soils and sediments, and phase separation, adsorption, stripping, and/or ion exchange for groundwater. Immobilization technologies include stabilization, solidification, and containment. Generally, no single technology can remediate an entire site, and the use of treatment trains, sometimes combining in situ and ex situ techniques, is common, as discussed subsequently.

The main advantage of in situ treatment is that it allows remediation

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

to occur without costly removal of the contaminant source. However, in situ treatment generally requires more time, and there is less certainty about attaining cleanup goals in terms of the extent and uniformity of treatment because of the usual heterogeneity of the source location and problems with treatment verification. In contrast to in situ treatment, the main advantage of ex situ treatment is that it generally requires shorter time periods to complete, and there is more certainty about the extent and uniformity of treatment. However, ex situ treatment incurs costly source excavation/removal and possible permitting and exposure implications. The control and proper disposition of emissions and residuals from ex situ treatment are important considerations that require compliance with permit conditions and the application of best management practices associated with each technology or combination of technologies. It should be noted that disposal actions may also be necessary for such in situ technologies as permeable reactive barriers and phytoremediation. Further discussion of this issue for individual technologies can be found in the references provided in Table 5-2.

Beyond considering the potential advantages and disadvantages of in situ and ex situ technologies, an important consideration in the evaluation of a remedy is the physical/chemical properties and the behavior of the contaminant and its source. For instance, subsurface contamination by nonhalogenated or halogenated VOCs potentially exists in four phases: (1) as vapors in the unsaturated zone, (2) as compounds sorbed on soil particles in both saturated and unsaturated zones, (3) as contaminants dissolved into pore water according to their solubility in both saturated and unsaturated zones, and (4) as a nonaqueous phase liquid (NAPL). The preferred remediation may involve a treatment train approach (e.g., air sparging/soil vapor extraction, liquid-phase carbon adsorption, and catalytic oxidation for nonhalogenated VOCs, or groundwater pumping, activated carbon adsorption with adsorbate reinjection, and offsite disposal of spent activated carbon for halogenated VOCs). In the case of soils or sediments, vapor extraction, thermal desorption, and incineration exemplify a corresponding treatment train.

A similar scenario could be developed for nonhalogenated or halogenated SVOCs. They can occur in the subsurface as vapors in the saturated zone, as contaminants sorbed or partitioned onto the soil or aquifer material in both the saturated and unsaturated zones or on sediments, as contaminants dissolved into pore water in both saturated and unsaturated zones, and as NAPLs. Common ex situ treatment technologies for SVOCs in groundwater include carbon adsorption and UV oxidation. In

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

TABLE 5-2 Candidate Technologies for Soil, Sediment, and Groundwater Remediation

Technologya

Purposeb

Target Contaminantsc

a

b

c

d

e

f

a

b

c

d

e

f

In situSoil and Sediment Remediation

Bioventing

X

 

X

 

X

 

Capping

 

X

 

X

 

X

X

X

X

Chemical oxidation/reduction

X

 

X

 

X

X

 

X

In situ heating

X

 

X

 

X

 

X

X

X

 

Phytoremediation

X

 

X

 

X

X

X

X

 

X

Soil flushing

X

 

X

 

X

X

X

 

X

Soil vapor extraction

X

 

X

 

X

X

X

 

X

 

Vitrification

 

X

 

X

 

X

X

X

X

 

X

Ex SituSoil and Sediment Remediation

Composting

X

 

 

 

 

 

X

X

 

X

 

Confined aquatic disposal

 

X

X

 

X

X

X

X

Hydraulic dredging

X

 

 

X

X

X

X

Incineration

X

 

 

X

X

X

X

X

 

Landfills

X

X

X

X

X

X

X

 

X

Land treatment

X

X

 

X

X

 

X

 

Mechanical dredging

X

 

 

X

X

X

X

Slurry-phase bioremediation

X

X

X

X

X

X

 

X

 

Soil washing

X

 

X

X

 

X

X

X

X

Solidification/stabilization

 

X

 

X

 

X

Thermal desorption

X

 

X

 

X

X

X

X

X

X

 

Groundwater Remediation

Air sparging

X

X

 

X

 

X

X

X

 

X

 

Bioremediation

X

X

X

X

X

 

X

 

X

Bioslurping

X

 

X

X

 

X

X

X

Circulating wells

 

X

X

X

X

X

 

X

Cosolvents and surfactants

X

 

X

X

 

X

X

X

Dual-phase extraction

X

X

X

X

X

X

 

X

Dynamic underground stripping

X

X

X

X

 

X

X

X

Chemical oxidation/reduction

 

X

X

 

X

X

X

X

X

X

Natural attenuation

 

X

X

 

X

 

X

 

X

 

Permeable reactive barriers

 

X

X

X

X

X

X

X

 

X

Phytoremediation

X

X

X

 

 

X

X

X

X

 

X

Pump-and-treat

X

X

 

 

X

X

X

X

X

 

Steam flushing

X

 

X

X

X

X

X

X

 

X

Vertical barrier walls

 

X

 

X

 

X

X

X

X

X

X

aSee Glossary at end of this chapter

b(a) Source conversion/removal, (b) plume remediation, (c) containment, (d) remediation enhancement, (e) isolation, (f) pretreatment

c(a) Nonhalogenated VOCs, (b) halogenated VOCs, (c) nonhalogenated SVOCs, (d) halogenated SVOCs, (e) fuels, (f) inorganics, (g) radionuclides, (h) explosives

d(a) Emerging, (b) innovative, (c) established/conventional

SOURCES: Adapted from FRTR (1997, 1998).

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

 

Maturityd

Relevant References

g

h

a

b

c

 

X

AAEE, 1995, 1997; FRTR, 1998

X

 

X

EPA, 1994; Evanko and Dzombak, 1997; NRC, 1997b, 1999a; EPRI, 1999; McLellan and Hopman, 2000

 

X

 

NRC, 1997a; EPA, 1998a

 

X

 

Fountain, 1998; FRTR, 1998

 

X

 

X

 

Schnoor, 1998; Fiorenze et al., 2000

 

X

 

NRC, 1999a

 

X

AAEE, 1997; FRTR, 1998; NRC, 1999a

X

 

X

AAEE, 1997; Evanko and Dzombak, 1997; NRC, 1999a

 

X

 

X

AAEE, 1995, 1997

X

 

X

 

EPA, 1994; NRC, 1997b; EPRI, 1999; McLellan and Hopman, 2000

X

 

X

EPA, 1994; NRC, 1997b; EPRI, 1999; McLellan and Hopman, 2000

 

X

 

X

AAEE, 1994, 1997; FRTR, 1998

X

 

X

AAEE, 1994, 1997; FRTR, 1998

 

X

AAEE, 1995, 1997; FRTR, 1998

X

 

X

EPA, 1994; NRC, 1997b; EPRI, 1999; McLellan and Hopman, 2000

 

X

 

X

AAEE, 1995, 1997

 

X

 

X

AAEE, 1993, 1997; FRTR, 1998; NRC, 1999a

X

 

X

AAEE, 1994, 1997; Evanko and Dzombak, 1997

 

X

 

X

AAEE, 1993, 1997; FRTR, 1998

 

 

X

Miller, 1996a; NRC, 1999a

X

 

AAEE, 1995, 1997; FRTR, 1998; NRC, 2000

 

X

Miller, 1996b

X

 

Miller and Roote, 1997

X

 

Jafvert, 1996

X

 

AAEE, 1997

X

 

Fountain, 1998; Balshaw-Biddle et al., 2000; NRC, 1999a

X

 

X

 

EPA, 1998a; NRC, 1999a

 

 

X

 

NRC, 2000

X

 

X

Vidic and Pohland, 1996; EPA, 1998b

X

 

X

 

EPA, 1999e; Schnoor, 1998; Schnoor, 2002

 

 

X

FRTR, 1998; NRC, 1999a

X

Fountain, 1998; NRC 1999a

X

NRC, 1999a

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

soil and sediment, biodegradation, incineration, and excavation with off-site disposal are typical. Associated treatment trains may involve thermally enhanced soil vapor extraction followed by in situ bioremediation for nonhalogenated SVOCs, and excavation, ex situ dehalogenation, soil washing/dewatering and land application for halogenated SVOCs.

Inorganic contaminants such as metals may be found in the elemental form, but more often exist as salts mixed in soil or sediment. The fate of metals depends on their physical and chemical properties, the associated waste matrix, and the environmental phase within which they reside. The most common reservoirs for metals are soil and sediment, and the most common treatment technologies include solidification/stabilization, excavation and offsite disposal, and extraction. Depending upon solubility and mobilization potential, metals may also exist in groundwater, and are most frequently treated by ex situ precipitation, filtration, and ion exchange, although in situ treatment by oxidation/reduction and vitrification has occurred. A representative treatment train may be the combination of electrokinetics and phytoremediation.

OPTIMIZATION OF REMEDIES

Before discussing innovative technologies, it is worthwhile to consider the optimization of existing remedies to make them more efficient and effective. This process can utilize data and information from both routine monitoring conducted during remedy implementation as well as from evaluation and experimentation efforts to better define the site conditions. Periodically reevaluating the entire remedial design to determine whether it should be adjusted is critical because the remedial system is dynamic and will lead to changes in in situ conditions as the remedy is being implemented. As one would expect, optimization is more developed for technologies that have been in use for longer periods, like pump-and-treat.

Experiential Optimization

As discussed in Chapter 2, the term “optimization” is used here to mean any adjustment in a single remedy to make it more efficient or cost-effective to implement. To distinguish it from mathematical optimization, the report further defines “experiential optimization” to mean remedy adjustments such as eliminating redundancy, replacing over-

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

designed components with appropriately sized ones, or relocating or adding some components. In this approach, the technical staff responsible for operation of the remedial system evaluates all components of system design and determines, using engineering judgment, whether any components are redundant, overdesigned, or poorly located and whether additional components are needed. Table 5-3 summarizes examples of experiential optimization for a variety of remedial systems, including soil vapor extraction, air sparging, bioventing, bioslurping, in situ chemical oxidation, reactive permeable barriers, light nonaqueous phase liquid (LNAPL) free product recovery, dense nonaqueous phase liquid (DNAPL) removal and containment, groundwater extraction for hydraulic containment, groundwater extraction for mass removal, and groundwater monitoring. The table entries specifically address optimizing existing remedies and do not include changes to alternate remedies. Additional detail can be found in NAVFAC (2001). These examples demonstrate that a good deal of engineering judgment and expertise are required to implement the suggested schemes. Seventeen case studies mentioning the use of optimization in revising cleanup strategies can be found at the FRTR website (http://www.frtr.gov), although information is not provided on how the optimization was carried out.

Mathematical Optimization

In the peer-reviewed, archival literature, optimization of a remedial scheme is defined more restrictively to mean mathematical simulation of subsurface fluid flow and/or transport coupled with a linear, nonlinear, or dynamic programming algorithm to predict an optimal configuration or management of remedial system components. Formal mathematical optimization of any remedial system is theoretically possible but in practice has principally been applied to pump-and-treat systems.

EPA has recently begun to promote the use of formal mathematical optimization coupled with groundwater modeling for pump-and-treat applications as a potential means to save funds and energy (EPA, 1999a). EPA (1999a) presents a screening model that allows a user to make a rapid determination of whether additional expenditure on a mathematical optimization is worthwhile. In cases where many wells are pumping at a significant rate, where an optimal strategy is not obvious, or where the cost of additional wells is insignificant in comparison to the total amount currently being expended on pumping/energy costs, the screening model will usually indicate that an optimization exercise is worth pursuing. In a

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

second volume (EPA, 1999b), EPA provides details of how mathematical optimization of a groundwater pump-and-treat situation can be accomplished. The user of the available software should have access to or should be able to construct a groundwater model of the site, and in addition be able to understand and implement the optimization algorithms suggested by EPA. The level of technical competence of the user is presumed to be relatively high.

Typically, pump-and-treat systems are designed based on experience and are adapted to site-specific conditions by carrying out field-scale pilot tests. To assist in the design process, users can use 2- or 3-dimensional numerical groundwater models (e.g., MODFLOW; McDonald and Harbaugh, 1996) to predict groundwater flow paths and hydraulic head distributions at a field site in response to imposed injection or withdrawal stresses, given that site lithology is adequately characterized in terms of spatially varying soil and rock permeabilities. This allows the user to answer questions regarding the number of wells to install and the effects of well placement and pumping rates on the movement of water through the saturated zone. It is possible to find an efficient design by simulating a number of combinations of well numbers, well placement, and injection or withdrawal rates to achieve either desired hydraulic containment or water removal.

However, the best design may not be found by such an iterative procedure. There are many possible combinations of design parameters, and identification of a best set of choices for test simulations may not be readily apparent for heterogeneous soils and complicated site boundary conditions. A more advanced level of design technology that builds on the numerical simulation approach is formal optimization of the design variables, where the best combination is found by mathematical techniques used in the field of operations research (e.g., Bradley et al., 1977; Gill et al., 1981). To optimize pump-and-treat design, mathematical programming algorithms can be coupled with a 2- or 3-dimensional groundwater flow model defining the physical system to determine the optimal set of design parameters for achieving pumping or injection objectives. This approach is the topic of EPA’s recent set of reports (EPA, 1999a,b) and is also the subject of textbooks written within the last decade (e.g., Gorelick et al., 1993; Ahlfeld and Mulligan, 2000).

Optimization as a formal mathematical methodology that can be used to improve system performance has been in use for some time. Indeed, a literature review reveals that the concept of coupling simulation and optimization models dates back to 1958 (Lee and Aronofsky, 1958) and has been applied in the areas of petroleum and gas production, water supply,

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

TABLE 5-3 Summary of Experiential System Optimization of Certain Remedies

Technology

Component Evaluated for Optimization

Recommended Action

Justification

Soil Vapor Extraction

Characterization of subsurface heterogeneity

Check for level of detail of characterization

Improved detail will aid in better placement of extraction well screens

3D distribution of vapor monitoring probes

Check for adequate number of vapor monitoring points

Improved placement/numbers will aid in determining adequacy of (1) volume of influence of vacuum system and (2) air flow velocities

Flow rates at extraction wells

Determine mass removal from each well; decrease flow from unproductive wells and increase flow to more contaminated areas

Improve distribution of total energy used for vacuum application

Continued high contaminant concentration in vapor

Check for unidentified or uncontrolled source areas

Presence of continuing source area will extend cleanup times

Economics of aboveground vapor treatment system

Check treatment efficiency

Lower vapor concentrations may cause change in existing treatment efficiency; switching of treatment technology as vapor concentrations get lower could generate cost savings

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

Technology

Component Evaluated for Optimization

Recommended Action

Justification

Soil Vapor Extraction (con’t.)

Location/activity of extraction wells

Conduct equilibrium tests by shutting off all wells for 3– 6 weeks

Rebounding will occur in hot spots; focus additional contaminant removal on these locations

 

Vertical location of extraction intervals

Vertical profile testing to determine air flow rates and contaminant concentration with depth

Determine location of unproductive screened intervals that can be packed off; also want to avoid extracting water from wells that are too close to water table

Air Sparging

Zone of influence

Check for design zone of influence. If not being achieved, increase air flow to injection wells or install additional wells outside current zone of influence; evaluate system for short-circuiting

Design zone of influence needs to be achieved to attain cleanup goals

 

Increasingly high injection pressures required to maintain flow

Check wells for plugging; redevelop or replace affected wells

Cleanup will not be achieved or will be delayed if injection wells are plugged.

 

Control of sparging vapors

May need to install SVE system

Need to keep sparging vapors from migrating to undesirable areas

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

 

Slope of contaminant concentration as a function of time

Check for target slopes; if slopes are too shallow, increase airflow to injection wells; install additional wells; evaluate system for short-circuiting; identify uncontrolled source area; evaluate alternative technologies

Desire to reduce cleanup times

 

Asymptotic contaminant concentrations due to desorption or diffusion limitations

Pulse injection wells, install additional wells in contaminated areas, or evaluate alternative technologies

Desire to reduce cleanup times

Bioventing

Percent oxygen in soil gas

Adjust air flow and blower pressure to achieve at least 5% oxygen in soil gas

Permeability of soil dictates combination of pressure and air flow required to force air into the pore space

 

Excessive air flow

Reduce air flow until oxygen is between 5% and 15%

Can achieve energy efficiencies by replacing oversized blowers with properly sized blowers

 

When to stop clean up

In-situ respiration testing

Measured rate of biodegradation is indicator of low hydrocarbon supply

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

Technology

Component Evaluated for Optimization

Recommended Action

Justification

Bioventing (con’t. )

Radius of influence

Check to see whether radius is as designed. Increase air flow, install additional wells, evaluate for short-circuiting

Desire to achieve design radius of influence to effect desired cleanup

 

High contaminant concentrations

Excavate hot spots or evaluate alternative technology

Concentrations may be too high for microbial activity to be effective

LNAPL Free Product Recovery

Recovery options

Conduct pilot baildown tests, limited pump down tests, and vacuum-enhanced recovery tests

Free product recovery is usually on the order of not more than 10%

 

Declining recovery rate

Check to see if well screens are clogged

Lower recovery will extend cleanup times

 

Ratio of fuel to water pumped

Check placement of pumps in wells; check to see if pumping rate is greater than necessary

If ratio is too low, recovery time will be extended

 

Radius of influence or containment of free product

Increase pumping rates or install downgradient interceptor trenches

Incomplete containment of free product will increase cleanup times

DNAPL Containment

Detail of site characterization

Tightly spaced soil borings; partitioning tracer test

Guidance for locating DNAPL

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

Bioslurping

Declining recovery rate over time

Check for biological fouling or mineral buildup at well screen

Lower recovery rate will extend recovery time and increase costs

 

Design recovery rate never achieved

Check well development and well screen locations

Inability to meet design recovery rate will extend recovery time and increase costs

 

Location of suction tubes

Check to make sure suction tubes intersect free product

Tubes located above the water table will cause groundwater mounding; tubes too far below the free product will expend energy pumping excessive groundwater

 

Vacuum rate

If vacuum rate is below design rate, check for short-circuiting and proper sizing of vacuum pump

Operation below design rate will reduce the radius of influence and extend cleanup times

 

Migration of free product

Check on adequate location and number of recovery wells

Desire to prevent free product migration

Permeable Reactive Barriers

Location of monitoring wells

Need wells upgradient, downgradient, laterally, and within reactive barrier

Desire for accurate evaluation of system performance

 

Breach of reactive barrier

Upgrade or reinstall barrier; consider alternative technology; grout any leaks between barrier and funnel walls

Desire to contain/treat contamination

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

Technology

Component Evaluated for Optimization

Recommended Action

Justification

In situ Chemical Oxidation

Radius of influence

Check against design radius of influence. If radius of influence is below design radius, refine permeability characterization; reassess injection volume of reagent

Permeability may be too low for reagent to effectively reach contaminant’ inadequate injection volume of reagent will result in incomplete oxidation

 

Chemical concentrations remaining after treatment

If chemical removal is incomplete or rebounds, check on well locations, volume of chemical reagent, refinement of site characterization, chemistry of aquifer material

Desire to attain complete oxidation reaction by having all reactant reach contamination and by having minimal interference by reactions with aquifer material

Groundwater Extraction for Hydraulic Containment

Mapping of dissolved phase

Check for level of detail of characterization; utilize direct push probes and discrete sampling for additional detail

Improved level of detail will aid in better placement of extraction well screens

 

Source controls

Possible addition of source-control well, in situ chemical destruction, or in situ barriers or treatment walls

Without removal of source, rates of mass removal will become asymptomatic; with source control, volume of water pumped in downgradient areas may be able to be reduced

 

 

Evaluate potential for natural attenuation

Other source control or mass removal may not be necessary

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

 

Location of extraction wells, total pumping rates

Mathematical optimization

Identify better combinations of pumping locations, rates, and schedules. May be able to achieve objective of plume containment with lower than maximum pumping rates

 

Well design

Evaluation of well design, construction techniques, well materials

Possible improvement of system efficiency; identify potential of well clogging if rates have decreased over time

 

Monitoring wells

Check for adequate number of monitoring wells

Evaluate whether entire plume is being contained

Groundwater Extraction for Mass Removal

Extraction rates

Evaluate mass removal for each location

Decrease extraction rates at unproductive wells, increase extraction rates in more contaminated areas

 

Pumping rates

Check on whether contaminant removal is limited by chemical solubility or diffusion; possibly lower or cycle pumping rates

Pumping rates in solubility-limited and diffusion- limited systems may be too high and ineffective; cost savings can be realized by reducing pumping rates

 

Pumping rates

Check on whether design rates have been achieved

Failure to attain design rates may prevent plume containment

 

Location/activity of extraction wells

Complete equilibrium tests by shutting off wells for three months

Define hot spots where remediation efforts should be focused

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

Technology

Component Evaluated for Optimization

Recommended Action

Justification

Groundwater Extraction for Mass Removal (Con’t.)

Vertical location of extraction intervals

Complete vertical profile testing

Identify intervals containing greatest masses of recoverable contaminants; allow determination of unproductive intervals to pack off

 

Map of dissolved phase

Check to make sure that plume is being contained while being removed; increase pumping rates as needed

Contaminant plume migration increases plume volume and possible receptor exposure.

 

Location of extraction wells, total pumping rates

Mathematical optimization

Find better combinations of pumping location/rates/schedule to increase mass removal and/or decrease cleanup costs

 

Above-ground treatment system

Evaluate for economic efficiency

As contaminant concentrations change, an alternate treatment system may be more cost-effective

 

Above-ground treatment system

Evaluate for design treatment efficiency

Unit may not be operating properly and could be repaired

 

Above-ground treatment system

Evaluate monitoring versus maintenance costs

Dollars spent monitoring maybe better suited to maintenance

 

Above-ground treatment system

Evaluate pumps and blowers for overdesign

Potential cost savings as concentrations begin to decrease

 

Above-ground treatment system

Evaluate cost of remote monitoring vs. onsite labor

Possible cost savings via remote monitoring

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

 

Number of wells

Identify redundant wells for elimination (with regulator)

Potential cost savings

Groundwater Monitoring

Frequency of sampling

Evaluate appropriateness of sampling less frequently based on remediation progress

Potential cost savings

 

Sampling and analytical protocols

Ensure that correct protocols are being applied to monitoring well samples

Potential cost savings if all monitoring wells are not required to undergo same protocols as point-of-compliance wells

 

SOURCES: Adapted from Air Force (2001) and NAVFAC (2001).p

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

wastewater injection, excavation dewatering, and hydraulic containment of groundwater contaminant plumes. The objective functions specified in problem formulations vary widely and have included, for example, maximizing profit, maximizing production, maximizing sum of hydraulic heads, maximizing total injection/withdrawal flow rates, minimizing costs, minimizing difference in desired versus actual production, and minimizing total injection/withdrawal flow rates. For the specified objective functions, decision variables have included flow rates at wells, head or pressure at wells, and well installation (binary or yes/no decision variables). Optimization algorithms that have either been proposed or actually used to solve these problems include linear, quadratic, nonlinear, and mixed linear-integer programs; some algorithms for solving certain optimization problems are widely available as commercial software packages (e.g., Murtaugh and Saunders, 1983, available from http://www.sbsi-sol-optimize.com/Minos.htm; Schrage, 1997, available from http://www.lindo.com).

EPA presents several case studies demonstrating that application of optimization to existing pump-and-treat well fields can save on the order of hundreds of thousands of dollars per site, depending on the objective. If, for example, the objective is plume containment, often it will be found that adequate hydraulic gradients toward the center of the plume can be maintained by reducing the pumping rates of the wells at the site, thereby reducing annual energy costs. In other cases, it can be shown that additional well placement and reevaluation of pumping and injection rates can also save additional dollars beyond the present scenario. A case study of mathematical optimization is presented in Box 5-1.

EPA notes that hydraulic modeling does not address mass removal or desired contaminant concentrations. To model such contaminant concentrations or masses, contaminant transport modeling must be coupled with optimization algorithms. This approach appeared in the literature over 15 years ago (Gorelick et al., 1984) and is now being pursued by EPA. Transport modeling is more complicated in that there are more parameters that need to be specified (dispersivities, sorption coefficients, biodegradation rates) and the process is nonlinear in contaminant concentration.

The principles discussed above can be applied to mathematical optimization of remediation of the vadose zone. An optimization handbook for soil vapor extraction is under development by EPA. A recent discussion of the mathematical approach to optimization of soil vapor extraction system design is provided by Sun et al. (1998).

One deficiency in the use of mathematical optimization not widely

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

recognized is that rarely is the uncertainty in the predicted optimal scheme quantified. Aquifers are naturally heterogeneous such that the three-dimensional spatial variability of the rock and soil structure can never be known precisely. The uncertainty in the distribution of soil properties affects predictions of flow and transport. To address the issue of uncertainty in groundwater flow and transport modeling, statistical methods are used to generate a synthetic geologic structure between points of observed head/solute concentration, and often Monte Carlo methods are employed to evaluate equally likely realizations of geologic structure that obey the assumed underlying statistical pattern. In this way, the effect of the uncertainty of the inputs (soil/rock hydraulic conductivity distribution) on the outputs (hydraulic head and solute concentrations) is quantified. The practice of quantifying uncertainty in subsurface flow and transport modeling is virtually ignored in the literature on coupling flow and transport models with optimization algorithms for improving well placement/pumping rates. Inclusion of the consideration of uncertainty would provide a range of possible optimal scenarios instead of just one scenario.

There is no documentation indicating that the Navy has been using the mathematical optimization approach championed by EPA as a method of saving remediation costs for pump-and-treat scenarios. The Navy may wish to consider implementing mathematical optimization for improving the efficiency of pump-and-treat systems and ultimately saving hundreds of thousands of dollars in pumping costs. However, a high level of technical expertise is needed to (1) calibrate a groundwater model to existing site hydrogeology and (2) couple site-specific groundwater modeling results with the mathematical optimization tools available from EPA. This of course requires an investment in personnel resources. The Navy could consider utilizing EPA’s screening methodology (EPA, 1999a) to decide whether a full-blown optimization effort would be economical to undertake. According to EPA, implementation of the screening model costs about $15,000.

***

At the current time, mathematical optimization is readily available only for pump-and-treat remediation schemes, such that experiential optimization will be needed for other remedies. Although few quantitative criteria are available for implementing experiential optimization, checklists provided by, for example, NAVFAC (2001) and the FRTR should be useful until a more complete database of experience is developed.

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

BOX 5-1 Mathematical Optimization of a Groundwater Pump-and-Treat System SOURCE: EPA (1999b).

Figure 5-1 illustrates contamination contours of a 1,2-dichloroethane (EDC) plume in a sand, clay, and gravel aquifer, beneath a site adjacent to a river in Kentucky. The saturated thickness of the aquifer varies from 100 feet at the southern border to 30–50 feet at the river. At the time of the study, a pump-and-treat system had been operating at the site since 1992. Twenty-three wells (18 from an original design plus five added subsequent to the original design, all labeled BW on Figure 5-1) had been installed principally for preventing migration of groundwater contaminants to the river, eight (“SW”) wells were installed near the plume centers for the purpose of accelerating mass removal, and eight “OW” wells were installed to prevent plume migration to adjacent properties. The typical pumping rates for the three kinds of wells were 420–580 gallons per minute (gpm), 80–160 gpm, and 25–100 gpm, respectively. A range of pumping rates for each type of well reflects adjustments in the system to respond to variations in the water table elevation caused by variations in the river level.

EPA chose this site as a case study for illustrating the application of mathematical optimization because of the large number of existing wells in operation as well as the high annual expense of operation. Contaminants removed from the aquifer were being treated by steam stripping, and the treated water was discharged to the river. The cost of pumping and treatment by steam stripping was on the order of $1.8 million per year in 1999. A screening analysis by EPA (1999a) determined that it would be economically justifiable to expend funds ($40,000) to conduct groundwater modeling and optimization analysis of the current system to see if cleanup objectives could be attained at a lower cost by installing new wells and/or utilizing different pumping rates at existing wells. The screening analysis suggested that a change in pumping rates and/or in the number of wells pumped could save millions of dollars over the planning horizon (20 years), even if new wells costing $20,000 each were added to the system.

The goals of the hydraulic optimization were to evaluate the following: (1) the potential for reducing pumping rates at the BW wells with continued prevention of plume migration to the river, (2) the tradeoff between the total number of BW wells operating and the total pumping required for containment, (3) the total pumping required for containment with BW wells pumping only, (4) the pumping required for containment as a function of variation in the hydraulic head constraint required, and (5) the tradeoff between adding SW wells and reducing pumping rates at BW wells.

The code used to conduct the optimization was “MODMAN,” consisting of the U.S. Geological Survey groundwater flow code MODFLOW (McDonald and Harbaugh, 1996) coupled with a linear programming algorithm LINDO (Schrage, 1997), to find the optimal set of pumping rates given the physical constraints of the system (EPA, 1999b). The mathematical objective function specified was minimization of the total sum of the pumping rates at the site, which is a surrogate for minimizing costs, since electricity usage is proportional to pumping rate. The annual steam stripping costs were equivalent to about $2000/gpm of water

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

pumped. The physical characteristics of the hydrogeology are captured by first calibrating the groundwater flow code to the site, and subsequently determining site-specific aquifer responses to unit pumping rates at various locations, which are then built into the coefficients of the specified objective function. (This method of including the physical system characteristics as coefficients in the objective function is termed the matrix-response method, see Gorelick et al., 1993). Physical constraints that were mathematically defined included (1) hydraulic head at locations where hydraulic containment was desired, and (2) maximum desirable pumping rates at each well. In the case of the BW wells protecting the river, a hydraulic head constraint along a line between the river and the BW wells was specified, as shown in Figure 5-2 by the cross marks. The numerical value specified was 0.01 ft lower than the head of the river, in order to guarantee a solution that would contain a hydraulic gradient pointing toward the plume and away from the river at the desired locations.

FIGURE 5-1 1,2-dichloroethane concentrations in September 1996 at a facility in Kentucky. SOURCE: EPA (1999b).

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

The results of the first two goals of the optimization runs are discussed here. A first set of runs examined whether the pumping rates of the original 18 BW wells could be reduced, holding the pumping rates of the SW and OW wells constant at their original design rates, to achieve the specified hydraulic constraint of 0.01 ft at the noted locations. The optimization algorithm found that only 17 of 18 BW wells were needed, and that the total pumping rate required at these wells to achieve hydraulic containment was 273 gpm instead of the original design total pumping rate of 549 gpm. This scenario resulted in a savings of $552,000 per year in operating costs. Further runs limiting the total number of wells allowed to operate (runs each with a maximum of 10–16 wells specified) indicated that as few as 14 wells could be pumped (275 gpm or a cost savings of $548,000 per year), with a more modest incremental savings as the number of wells was further limited to be as few as 10 (see Figure 5-3). Only when the number of wells was limited to nine was the solution found to be infeasible, that is, the constraints could not be met. If the optimization algorithm had been used in the design

FIGURE 5-2 Hydraulic head constraint locations and potential additional well locations specified in the hydraulic optimization modeling. SOURCE: EPA (1999b).

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

mode before the wells had been installed, the designers would have found it optimal to install 14 instead of 18 wells to achieve the containment objective, thereby also saving an additional $80,000 in well installation costs for the original design ($180,000 total including the later modification that added five BW wells). This indicates the power of using optimization algorithms to infer information about the physical system that may not otherwise be obvious. Based on these illustrative cost savings, in the summer of 2000 EPA issued two directives requiring that all Superfund sites at which pump-and-treat remediation was being conducted be evaluated using optimization to assess potential cost savings (EPA, 2000a,b), although the emphasis of the guidance is on experiential optimization rather than modeling.

FIGURE 5-3 Total pumping rate versus maximum number of wells allowed to pump for the containment problem in the Kentucky case study. SOURCE: EPA (1999b).

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

PROMISING TECHNOLOGIES FOR THE NAVY’S PRESSING CONTAMINATION PROBLEMS

The following sections describe specific innovative technologies applicable to the types of contamination problems encountered at Navy and other federal facility sites. The discussion includes several technologies because there are more than just two or three that would suffice to cover all of the Navy’s critical problem sites. The innovative technologies for treating solvents in soil and groundwater were chosen because they have recently garnered intense interest from potentially responsible parties (PRPs), including the Navy, and they have proven promising based upon previous applications. Thus, pump-and-treat and other conventional technologies are not included. On the other hand, a broad overview is given of technologies for treating metals sites and contaminated sediment sites that reflects the committee’s professional experience regarding their potential use and efficacy.

Cost issues are not discussed in subsequent sections, primarily because complementary cost data for remediation technologies are not readily available for every type of application. However, a recent cost compendium has been prepared to include current information about the costs of bioremediation, thermal desorption, soil vapor extraction (SVE), onsite incineration, groundwater pump-and-treat, and permeable reactive barriers (PRBs) based upon about 150 projects (EPA, 2001a). The overall findings regarding remediation costs indicated that:

  • correlations between unit costs and quantity of material treated or mass removed were evident for bioventing, thermal desorption, SVE, and pump-and-treat systems,

  • economies of scale were observed for bioventing, thermal desorption, and SVE in that unit costs decreased as larger quantities of soil were treated,

  • costs of technology applications are site-specific and thus are affected by many factors (e.g., properties, distribution, and concentrations of the contaminant; character of treated matrix and hydrogeological setting; market forces; maturity of technology; regulatory requirements; etc.), and

  • some technologies (e.g., PRBs) could not be quantified with respect to cost due to lack of information concerning the longevity of the project, the contaminant quantity treated, and the mass of contaminant removed.

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

It will be important to the eventual success of ASM (particularly at management decision period [MDP] 3) to have greater understanding of the labor, utility, chemical, and disposal costs of different technologies. Presently, most financial data systems do not break down cleanup costs in this way, such that new budgeting requirements and formats will be needed to produce data that can support ASM.

Technologies for Remediation of Organic Contaminants in Soil and Groundwater

Recalcitrant organic contaminants are found at over 54 percent of all Navy facilities (NRC, 1999b), and are common contaminants at federal facilities in general. As discussed in Chapter 1, they pose significant challenges to site remediation, particular when found in karst and fractured rock environments. Three of the innovative technologies discussed below (in situ chemical oxidation, thermal treatment, and enhanced bioremediation) are broadly classified as source removal technologies because their goal is to reduce substantially the source term (be it solid-bound, free-phase or dissolved contamination). In situ oxidation and thermal treatment in particular are noteworthy for reducing contaminant mass over a short timeframe. Barrier walls, in contrast, are effective primarily for contaminant plume treatment and control. To date, they have been developed for a limited number of organic compounds and metals.

In Situ Chemical Oxidation/Reduction

In situ chemical oxidation/reduction (ISO) is a groundwater remediation technology for toxic organic chemicals that has largely been used for source removal and control. The oxidants most commonly employed include peroxide, ozone, and permanganate. Hydrogen peroxide is capable of directly oxidizing organic contaminants—by free radical formation when ferrous iron is used as a catalyst (Fenton’s Reagent). Fenton’s Reagent oxidation is most effective under very acidic conditions, such that the need for pH adjustment is a disadvantage during the application of the technology. The advantages of peroxide include relatively low regulatory resistance, more field experience than for either ozone or permanganate, and a sparsity of byproducts of oxidation.

Ozone gas also can oxidize contaminants directly or through free

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

radical formation, and it is the strongest viable chemical oxidant available. Because ozone is a gas, it is most suitable for treating the vadose zone, or possibly contaminant accumulations (e.g., LNAPL) in the capillary fringe. Like peroxide, ozone reactions are most effective in systems with acidic pH, where they proceed with extremely fast, pseudo first-order kinetics. Because of ozone’s high reactivity and instability, it is produced onsite and requires closely spaced delivery points. In situ decomposition of the ozone can lead to beneficial oxygenation and biostimulation, and it is less costly than either peroxide or permanganate. However, because ozone must be applied as a gas, vapor recovery and possible treatment can add to the cost of the technology.

Permanganate is typically provided as a liquid or solid potassium or sodium salt that dissolves directly in the groundwater, and its reaction stoichiometry in natural systems is complex because of its multiple valence states and mineral forms. Depending on pH, the reaction can include direct oxidation or free radical enhanced oxidation. The reactions proceed at a somewhat slower rate than for peroxide or ozone according to second-order kinetics. Permanganate has a lower cost than peroxide and is effective over a broader pH range, and it is more stable than ozone. However, oxidation via permanganate also produces manganese oxide, which can precipitate and potentially cause reduced porosity. Increased dissolved manganese levels are also a regulatory concern, as is the purple color of groundwater containing unreacted permanganate.

The rate and extent of oxidation of a target contaminant are determined by the properties of the chemical itself and its susceptibility to oxidation as well as by the reaction matrix and its conditions (e.g., pH, temperature, oxidant concentration, other reduced compounds, minerals, and free radical scavengers). Generally, the technology is used on chlorinated volatile organic compounds (CVOCs) such as trichloroethylene (TCE) and on petroleum hydrocarbons. The method of oxidant delivery throughout the reaction matrix is of paramount importance; vertical and horizontal injection wells and sparge points with forced advection to rapidly move the oxidant, particularly for peroxide and ozone, into the subsurface are often deployed. Moreover, all three oxidation reactions (Box 5-2) can lead to (1) a decrease in pH if the system is not effectively buffered, (2) genesis of colloids with reduced permeability, (3) mobilization of redox-sensitive and exchangeable sorbed metals, (4) possible formation of toxic byproducts, (5) evolution of heat and gas, and (6) biological perturbations.

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

BOX 5-2 Simplified Stoichiometry for Oxidation of TCE by Peroxide, Ozone, and Permanganate

Peroxide:

3H2O2 + C2HCl3 2CO2 + 2H2O + 3HCl

Ozone:

O3 + C2HCl3 2CO2 + 3HCl

Permanganate:

2KMnO4 + C2HCl3 2CO2 + 2MnO2 + 2KCl + HCl

The stoichiometric relationships, like those shown in Box 5-2, can be used to estimate the amount of oxidant that would theoretically be needed to destroy the target contaminant. However, for site-specific oxidant demand estimates, bench-scale treatability tests based on soil slurry systems are often conducted to evaluate the feasibility of in situ oxidation and to calculate potential oxidant loading requirements. Results from slurry systems do not take into account preferential flows that are likely to occur in the subsurface, such that in reality, an excess of oxidant is often applied. Example bench-scale testing results are provided in Gates and Siegrist (1995).

Single, multiple, and continuous injections using recirculation of amended fluid have been used to apply the technology. For single or multiple injections, permanent or temporary injection points are used to deliver an aqueous solution containing the oxidant and any needed catalyst under pressure. The oxidant (and catalyst) concentration, the target pH, the injection well spacing (i.e., radius of influence), the number of injections, and the injection pressure are all important design parameters that can affect cost and performance. The use of recirculation, with injection and extraction wells, is intended to increase subsurface mixing and reaction opportunity, but the costs are likely to be higher. In addition, thin screen intervals at different depths more fully saturate the target zone and reduce the need for vertical migration of the oxidant. High injection pressures may be used to create fractures in tighter subsurface materials and thereby encourage migration and mixing of the reactants. Finally, in some cases, vapor extraction is used in conjunction with oxidation in the vadose zone to relieve off-gas pressure, to encourage oxidant migration, and/or to capture any volatile emissions (ESTCP, 1999). Despite these measures, it should be noted that in situ oxidation reagents, particularly Fenton’s Reagent and ozone, are relatively short-lived compared to the rate of groundwater flow in most aquifers, such that oxidant contact with and treatment of contaminants is not significantly mediated

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

by groundwater advection and oxidant dispersion. Table 5-4 provides example calculations of the minimum volume of injectant that must be delivered in an active form to achieve cleanup of different target treatment volumes. In summary, significant volumes of liquid oxidants may be required to treat relatively small areas.

Measuring Performance. Performance measurement should be based on multiple lines of evidence. Contaminant concentration changes over time and space are the most common and useful measurement. However, because contaminant concentration reductions can be caused by oxidation, simple displacement, and/or dilution effects, the measurement of geochemical indicators, tracers, and contaminant-destruction byproducts (e.g., chloride), as well as the use of control wells, should be considered. Geochemical indicators such as dissolved oxygen, redox potential, and conductivity (background chloride, etc.) provide an initial geochemical fingerprint that will change if the oxidant is delivered to a specific monitoring location. Tracer compounds that should be considered for the evaluation of oxidant distribution include both visual tracers and a semi-conservative dissolved tracer (i.e., Mn2+, K+, Na+, etc.); bromide and iodide should be considered when applying liquid oxidants like peroxide. The release of halogenated ions, such as chloride or bromide, from target contaminants is a useful line of evidence if original contaminant concentrations are high enough to result in a significant increase in halogen ion concentrations as a result of contaminant destruction. All injection trials should include one or more control wells where water and tracer are injected into a contaminated zone in order to differentiate dissolved contaminant displacement or dilution from destruction.

TABLE 5-4 Volumes of Liquid Oxidant required to affect Target Radius of Influence

Assumed radius of influence (ft)

Target or injection well screened interval (ft)

Volume of aquifer affected (gal)

Required volume of injectant to achieve assumed radius of influence (gal)a

Approximate Number of injection wells/acreb

Approximate Total volume injectant/ acre (gal)c

10

10

23,500

5,900

140

826,000

10

20

47,000

11,800

140

1,652,000

20

20

188,000

47,000

35

1,645,000

50

20

1,170,000

294,000

6

1,764,000

aEntries equal Column 3 multiplied by an assumed porosity of 0.25.

bNumber of wells per acre is approximated by dividing the surface area of an acre by the surface area coverage of a single well.

cEntries equal Column 4 multiplied by Column 5.

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

Although the theoretical stoichiometry of ISO is known, nontarget materials in the subsurface (e.g., natural organic acids, reduced iron and manganese, and sulfides) can all consume oxidant. Moreover, these sources can affect the potential for heat and off-gas generation or foaming and for rebound of contaminant levels caused by diffusion from untreated sources. Thus, data on the concentrations, masses, and fluxes of these materials in the treatment zone are essential to both rational design and measuring performance.

Because most subsurface environments are highly heterogeneous, the effectiveness of injection and/or reinjection needs to be evaluated both at the initial location and at possible new locations. The presence of target contaminants in lower-permeability layers that are adjacent to more permeable, preferential flow paths should be of special concern since oxidant delivery may be incomplete in lower-permeability zones. Thus, performance monitoring should be conducted in unique lower-permeability and/or high organic carbon layers. Likewise, monitoring for contaminant concentration rebound can guide the design of any subsequent remediation strategies by defining the remaining contaminant reservoir that was not treated by ISO. However, such information should be coupled with measurements of soil pore water chloride concentrations during injection to provide supporting evidence of dechlorination reactions and concomitant loss of the contaminant. For such a soil confirmation program to be useful, it needs to appropriately consider potential spatial and temporal variability of contaminant distribution, and recognize the associated mechanisms (e.g., chemical oxidation, volatilization/air stripping/gas phase partitioning, and dilution) of contaminant reduction.

Technology Evaluation. A recent status review of in situ oxidation (ESTCP, 1999) at 42 government (DoD and DOE) and private sites is summarized in Table 5-5. The review was conducted in two phases; phase I consisted of a survey of sites to identify where ISO had been used. The survey involved contacting ISO vendors and reviewing government (DoD, DOE, and EPA) databases and websites to determine current status of the project, scale, contaminants and media, responsible parties and regulators involved, extent of any available site data, and initial response indicating relative success or failure to satisfy facility-specific performance objectives. Accordingly, 19 sites were deemed successful, six failed, and 17 were uncertain. Of the 42 sites, 19 were partially or primarily contaminated with CVOCs, with TCE being the most prevalent

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

TABLE 5-5 Characteristics of In Situ Oxidation (Phase I) Field Sites

 

Number of Sites

Characteristics

DoDa

DOE

Private

Total

Contaminants

CVOC

6

3

12

21

BTEX/TPH

6

-

13

19

Both

1

-

-

1

Unknown

1

-

-

1

Media Treated

Soil only

0

0

0

0

Groundwater

2

0

17

19

Both

10

3

7

20

Unknown

2

-

1

3

Oxidant

Hydrogen peroxide

12

1

24

37

Potassium permanganate

1

1

2

0

1

0

4

1

Ozone

 

Vendor

Geo-Cleanse

8

1

4

13

Clean-Ox

3

0

13

16

ISTEC

1

0

7

8

Other

2

2

1

5

Scale

Pilot/Demo Only

9

3

15

27

Full Only

1

0

4

5

Both

4

0

6

10

Outcomeb

Success

5

3

11

19

Failure

6

0

0

6

Uncertain

3

0

14

17

Totals

14

3

25

42

SOURCE: ESTCP (1999).

aDoD Breakdown: Navy (NFESC) = 5; Corps of Engineers/Air Force = 7; Army (Base Contract) = 2

bOutcome determinations are relative terms based on available Phase I information provided by facility representative (e.g., direct comments or pilot-scale tests that led to full-scale operations). These terms denote the ISO technology’s ability or lack thereof to satisfy facility-specific program performance objectives.

contaminant of concern. Hydrogen peroxide was used at 37 sites, potassium permanganate at four sites, and ozone at only one site. Five of the 42 were Navy sites.

The results of the Phase I survey were then used to select several

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

sites for more detailed Phase II evaluation, consisting of a review of available site characteristics, design, and performance data to more fully investigate and understand the site conditions and characteristics, the reasons why ISO was selected, the design parameters and rationale, the cost and performance of ISO under real-world conditions, the reasons for success or failure of ISO to meet the project objectives, and the specified technological concern. The Naval Submarine Base at Kings Bay, Georgia (Box 5-3), and the Naval Air Station at Pensacola, Florida, received such detailed site profiling and evaluation of results. At both of these sites, natural attenuation appeared promising after ISO treatment. The location, area and contaminant of concern, regulatory driver, oxidant, scale, remediation objectives, ability to meet objectives, and follow-up actions for these and the other Phase II sites are summarized in Table C-1 in Appendix C.

Collectively, the experiences with ISO indicate varying degrees of success, largely based on the sufficiency of site characterization and technology deployment. Various key factors have been identified and relate to site characterization needs and design and operational issues. The success of ISO is dependent upon effective contact and mixing with target contaminants, compatible subsurface geochemistry, and the maintenance of sufficient oxidation capacity to overcome oxidant losses from nonspecific oxidation reactions (e.g., reactions with the aquifer matrix and spontaneous oxidant decomposition). Major unanswered issues regarding the technology include:

  • the absence of a well-defined screening procedure to evaluate site-specific geochemical parameters for compatibility with ISO techniques,

  • the lack of properly designed pilot-testing procedures,

  • differentiation between dissolved contaminant displacement and dilution versus treatment,

  • oxidant loss due to consumptive reactions with soils and naturally occurring organic and inorganic materials, and estimations of the amount of oxidant necessary to overcome these losses so as to achieve the desired contaminant destruction,

  • effectiveness of ISO for dissolved versus sorbed contaminants,

  • credible analyses of contaminant rebound effects, and

  • compatibility with anaerobic biodegradation processes.

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

BOX 5-3 In Situ Oxidation for Remediation of Chlorinated Solvents in Soil and Groundwater at Kings Bay Naval Submarine Base, Georgia

The site under consideration is a leaking former sanitary landfill, under which a perchloroethylene (PCE) plume has developed that is 120 feet long by 40 feet wide, with a 30- to 40-foot horizon below ground surface (bgs). The plume is moving toward a residential area through sandy soils that have a hydraulic conductivity in the impacted zone of 30 ft/day. The PCE concentrations detected in the landfill source area were as high as 8,500 µg/L, with breakdown products TCE, dichloroethylene (DCE), and vinyl chloride detected at concentrations of more than 9,000 µg/L in groundwater.

The remediation strategy chosen was to conduct full-scale in situ chemical oxidation with Fenton’s Reagent (Geo-Cleanse) at 50 percent peroxide and an equivalent volume of ferrous iron catalyst delivered by injection to the subsurface. A total of 44 injection wells (23 in Phase I; 21 in Phase II) were installed at both deep (40–42 ft bgs) and shallow (32–35 ft bgs) depths. Phase I included two injections of oxidants totaling 12,045 gallons (8,257 gallons November 2–21, 1998; 3,788 gallons February 8–14, 1999). Phase II included two additional injections totaling 11,247 gallons (8,283 gallons June 3–11, 1999; 2,964 gallons July 12–15, 1999). The estimated volume of groundwater treated during Phase I was 78,989 gallons (based on treatment volume of 11,778 cubic yards and a porosity of 22 percent). During both phases, the design injection rate of oxidant was 0.2–1 gpm, while air was injected at 3 cfm to disperse the catalysts.

Following the in situ oxidation treatment, total VOCs in the primary treatment area were reduced from 9,074 µg/L to 90 µg/L, a 99 percent reduction. Subsequent results have shown that concentrations have remained below 100 µg/L. The natural attenuation capacity of the aquifer is expected to polish residuals outside the source area that are present in concentrations of less than 100 µg/L. Modeling exercises are predicting plume collapse in five years, barring the existence of other source areas outside the primary treatment zone. Based on the apparent success of in situ oxidation, the existing pump-and-treat system was discontinued.

Accordingly, uncertainties that have emerged during the demonstration and applications have indicated a need to provide comprehensive information on several factors (ESTCP, 1999). First, there must be better delineation of the contaminant’s location and extent and of its sorption potential, particularly for DNAPL accumulations. The degree of soil layering versus the distribution of contaminants is an important parameter to understand because the distribution of oxidants will be limited to more permeable soil horizons unless injection/distribution approaches are tightly controlled. Mass and volume estimates of total CVOCs be-

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

fore and after treatment are needed to determine the efficacy of natural attenuation, and to be able to estimate the injected fluid volumes of oxidant. Another requirement is vapor monitoring, including detection of potentially explosive vapors in the subsurface, to safeguard against possible health and safety hazards during treatment. The prior consideration of these factors in formulating and optimizing a remedial action plan will enhance the potential success of ISO applications at contaminated sites (NAVFAC, 2001).

Design considerations include determining the radius of influence of injection wells to ensure adequate contact and the enhancement of mixing to promote contact between oxidant and contaminant. It may be necessary to consider multiple injections into the same or preferably new locations to accommodate matrix heterogeneities and circumvent problems with the development of preferential flow paths and short-circuiting caused by plugging of flow paths. Comparisons of the estimated in situ half-life of the oxidant to the groundwater flow velocity will help determine whether natural or induced groundwater flow can significantly distribute the oxidant. Finally, it will be important to incorporate ISO into an overall site management strategy, particularly at DNAPL sites, where source removal or reduction can be complemented by more cost-effective residuals treatment (e.g., natural attenuation or sparging).

Thermal Treatment

There are three general methods that can be used to inject or apply heat to the subsurface to enhance remediation: injection of hot gases such as steam or air, hot water injection, and electrical resistance heating (Davis, 1997, 1998). Steam, hot air, and hot water injection rely on contact between the injected fluid and the contaminant. Steam injection will displace mobile contaminants in front of the steam as well as vaporize volatile residual contaminants, and therefore can recover volatile and semivolatile contaminants in both the liquid and vapor phases. Hot air injection has been used to recover contaminants only in the vapor phase, and it is applicable to water-soluble volatile and semivolatile organics. Because steam has a heat capacity approximately four times that of air and a heat of evaporation of more than 2,000 kJ/kg, steam is often preferred to enhance the recovery of volatile contaminants and oils in soils and aquifers. However, for contaminants that have a high solubility in water, residual contamination remains after steam injection, unlike with hot air injection. Hot water injection is applicable for contaminants in

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

the nonaqueous liquid phase, which tend to have low volatility and very low solubility in water, and this is most effective when the nonaqueous phase is present in quantities greater than the residual saturation.

Electrical energy has been applied to the soil in the low frequency range used for electrical power, that is, electromagnetic (EM), alternating current (AC), or resistivity heating, as well as in the radio frequency (RF) range. In each case, electrical energy heats the soil, increases the volatility of contaminants, and may induce the groundwater to boil and form steam (Fountain, 1998). The contaminants are driven out of the source zone by a combination of volatilization and thermally induced vapor-phase transport. Hence, electrical heating is usually coupled with soil vapor extraction (SVE) or steam injection to recover the volatilized contaminants. DNAPLs will be volatilized if the soil is heated to near the contaminant boiling point; the contaminant may also be mobilized by a reduction in viscosity. For semivolatile organic contaminants, higher temperatures (300°–400°C) obtained using RF energy are required to achieve greater removal and transformation efficiencies.

Electrical heating has proved to be effective in sandy media, and it also has a greater potential than steam or hot water injection in less permeable media such as clays. The higher water content generally found in clay will aid in directing the electromagnetic energy to the clay and promotes both a faster heating rate and higher temperatures. RF heating, however, is limited to the unsaturated zone, and for contaminants trapped below the water table, dewatering would have to be conducted prior to electrical heating.

Each of these thermal treatment methods is applicable only to certain types of sites and contaminants. The permeability of the media, the amount and type of heterogeneity, the amount of sorption, and the solubility of the contaminant must all be considered. For example, electrical heating may be favored in low-permeability media and when there is significant heterogeneity. Hot air or RF heating may be more applicable for highly soluble contaminants where drying of the soil may be necessary, and higher temperatures and/or longer remediation times may be necessary when adsorption is significant. Figure 5-4 can be used to determine which of the techniques is most applicable in a given situation; in some cases, more than one technique may be applicable, such that the selected technology is often the least severe in terms of temperature and pressure requirements (Davis, 1997).

A second important point is that each of these thermal treatment

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

FIGURE 5-4 Guide for selection of thermal techniques applicable to a particular site. SOURCE: Davis (1997).

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

methods is completely dependent on the capture effectiveness of the newly mobilized contaminant. With the possible exception of hot water flooding, all thermal remediation technologies require a highly effective soil vapor extraction system as the ultimate contaminant removal mechanism. The soil vapor capture system must be capable of overcoming condensate formation in situ and in above-ground equipment; it must be capable of fully capturing the “flash” volatilization of heated nonaqueous phase liquid; and, where applicable, it must be designed to effectively capture contaminants mobilized in the saturated zone. Groundwater extraction systems are often used in concert with soil vapor extraction systems at sites where contaminants are present in or adjacent to the saturated zone.

Measuring Performance. Primary remedial performance evaluation parameters for thermal treatment are media-specific contaminant concentrations before and after heating as well as mass removal versus time. The reliance on contaminant concentrations before and after treatment raises important questions regarding the defensibility of using standard groundwater and soil sampling and analysis techniques on “hot” samples. Techniques to cool or otherwise address uncontrolled contaminant loss from volatilization are being developed, but they have not been validated or widely applied. Rebound testing data are limited, but should only be considered valid if a sufficient period of time has elapsed since the return of the subsurface to ambient conditions. The elapsed time necessary for rebound effects to be exhibited is greater with (1) increased degree of soil layering, (2) greater degree of differences in hydraulic conductivities of distinct soil layers, (3) lower groundwater flow velocities, and (4) lower contaminant solubility or volatility.

Given that the success of thermal remediation technologies is directly dependent on capture system effectiveness, performance evaluation of the extraction system is critical. This evaluation should be completed using the standardized techniques for radius of influence or capture zone analysis (pressure profile for SVE—USACE, 1995; potentiometric surface for hot water). However, the evaluation of capture effectiveness of tracer compounds would provide a far more rigorous performance measure. Thus, the injection of an inert gas tracer (e.g., helium) into various areas of the extraction/heating array would provide useful data as to whether volatilized contaminants would be captured by the extraction system. Water-soluble conservative tracers (e.g., bromide and iodide) would verify that the flow path of injected water and its respective capture efficiency were acceptable. These techniques have seen

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

limited application, and technical guidance regarding their appropriate use is sparse, if not nonexistent.

Temperature and pressure monitoring should be maximized since these monitoring networks are relatively cost-effective to install and are particularly informative and straightforward regarding the collection and analysis of these data.

Technology Evaluation. Of the thermal treatment options, steam injection, electrical resistance heating, 3- or 6-phase heating, and microwave and RF heating have been applied for in situ remediation of subsurface contamination. Selected steam injection projects are presented in Table C-2 in terms of target contaminants, treatment designs, and outcomes, and Table C-3 provides a compilation of full-scale and demonstration in situ thermal treatment projects. (Both tables are in Appendix C.) The in situ thermal desorption (ISTD) project at the Naval Facility, Ferndale, CA, is further described in Box 5-4. Given the newness of the

BOX 5-4 In Situ Thermal Desorption of Polychlorinated Biphenyls (PCBs) in Soil at Naval Facility, Ferndale, CA Source: Davis (1997)

The site of interest is on a 30-acre military base used for oceanographic research and undersea surveillance. There are approximately 1,000 cubic yards of PCB-contaminated silty and clayey colluvial soils under and adjacent to a former transformer/diesel generator building. Contamination underneath the building was 2–15 feet below ground surface (bgs), while PCBs adjacent to the building occurred 5–15 feet bgs. In this location, the depth to groundwater is greater than 60 feet. Concentrations of PCB-Aroclor 1254 were found to be 0.15–860 ppm, and PCDD/Fs was detected at levels up to 3.2 ppb 2,3,7,8-TCDD Toxicity Equivalents (TEQ).

The remediation strategy chosen at this site was based on TTEMI thermal well technology. This consists of heater-only and heater-vacuum wells installed at a depth of 15 feet in a hexagonal pattern with 6.0-foot spacing over an area 40 x 30 feet. The cleanup goal for PCB concentration was 1 ppm or lower; for dioxins and furans, the total concentration goal of 2,3,7,8-TCDD TEQ was less than 1.0 ppb. Remedial operation began on November 5, 1998, and ceased on January 15, 1999. Interim sampling was subsequently conducted, followed by a shutdown of soil heating on February 26, 1999.

Confirmation sampling to detect residual levels of contaminant was conducted in April 1999. This revealed that the target treatment area achieved remedial objectives for all samples. Additional contamination was identified outside of the thermal treatment zone because of the presence of unknown utility structures; this contamination was removed by excavation after limited thermal treatment.

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

technology and limited data on performance, it is too early to pronounce judgment of its overall and general efficacy.

Nonetheless, some general guidelines are suggested to improve the chances that thermal treatment is successful. First, it is important to adequately characterize the site with respect to the horizontal and vertical distribution of the contaminant, the heterogeneities of the medium, and the preferred flow paths (e.g., of the injected steam). This information is important for the design of the delivery and extraction systems and also for anticipating the monitoring and analysis requirements. The location and physical/chemical properties of the contaminant will determine the degree of solubilization, volatilization, and desorption and, hence, the removal opportunity. Unless adequately accounted for, subsurface heterogeneities may result in nonuniform heating and incomplete remediation.

With rare exception, all thermal technologies involve the production or transport of steam through the subsurface with the potential to volatilize contaminants, resulting in significant vapor production. The flow path of the steam and mobilized contaminant is determined by the relative permeability of soils. However, the relative permeability of soils and other properties (e.g., moisture and electrical resistance) can change dramatically over time as a direct result of thermal technologies. Condensate will be produced in situ at any location where steam contacts soil at a temperature that is lower that the boiling point of water or of the contaminant. The air permeability of soils is highly sensitive to degrees of water saturation. Thus, the soil vapor extraction network must be designed such that paths of vapor movement are made available even if condensate is formed in situ and air permeability is reduced in certain zones.

Unlike soil vapor capture efficiency, which can be negatively impacted by high water content, the performance of electrical resistance heating technologies can be negatively impacted by low water content. As soils dry out from heating and evaporation, the resistivity of the soils increases. An increase in resistance requires an increase in power input to maintain the original or desired heat input (Balshaw-Biddle et al., 2000). Thus, the application of electrical resistance heating in the vadose zone will likely require electrode irrigation systems and specialized electrode design. However, field experience to date has proved that electrode irrigation alone may not overcome soil power delivery problems because the power is consumed in boiling the irrigation water as opposed to heating of the subsurface at a distance from the electrode.

The compatibility of thermal remediation technologies with subsur-

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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face utility networks or underground wastes must be assessed in detail. Major considerations include subsurface utilities acting as conduits for mobilized contaminants, melting of polyvinyl chloride (PVC) and similar materials, vapor excursion to the surface, and spatial changes in soil conductivity (e.g., metal materials).

Barrier Walls

Barrier walls remove, transform, or otherwise prevent groundwater contaminants from migrating offsite. There are two general types of walls: nonreactive and reactive. Nonreactive barrier walls include slurry walls, sheet-pile walls, and grout walls whose primary function is to prevent offsite movement of contaminated groundwater. Although improvements in nonreactive barrier walls are being made, they are generally not considered innovative technology. Reactive barrier walls have been called passive reactive barriers or, more recently, permeable reactive barriers (PRB). PRBs are used (1) to control migration of and to treat contaminated plumes, (2) to control migration of contaminants from source areas (followed downgradient, perhaps, by pump-and-treat or monitored natural attenuation), or (3) as a polishing step following other in situ technologies (e.g., flushing).

The general concept of a PRB is shown in Figure 5-5. In the most commonly applied approach, a trench is dug and backfilled with permeable, reactive material. Contaminated groundwater then naturally flows (termed continuous wall) or is made to flow using pumping and/or impermeable barriers that direct flow (termed funnel and gate) through the barrier where reactions occur. More recently, techniques have been developed that allow injection of these materials into the subsurface to create reactive barriers at depth.

Contaminant removal can take place by chemical reaction, sorption, precipitation, or biotransformation. This technology is based on reduction, in that highly oxidized contaminants are transformed to nontoxic or immobile products. The most common reactive material is zero-valent iron (Fe(0)), which will be the focus of this section. As the Fe(0) corrodes, electrons are released that can be used to reduce highly oxidized contaminants (Box 5-5).

The range of organic and inorganic pollutants treated and reactive materials used in PRB are summarized in several recent reviews (Sacre, 1997; Gavaskar et al., 1998, 2000, 2001; EPA, 1998b; Scherer et al., 2000). These reviews also discuss advantages and limitations of the

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

FIGURE 5-5 Permeable reactive barrier. SOURCE: EPA (1998b).

technology, design and implementation aspects, barrier emplacement methods, and principles of barrier media selection. Data from Scherer et al. (2000) summarize the general status of reactive barrier technology in terms of the contaminants treated and the materials used (see Table 5-6). Of 124 projects reviewed, 72 were laboratory studies, 26 were field demonstrations, 20 were commercial installations, and six were pilot studies (Sacre, 1997). Cr(VI) and halogenated aliphatics—primarily TCE—are the most common pollutants treated. There is documentation that reactive materials other than Fe(0) are being used in field installations. For example, sodium dithionite is being used to treat a Cr(VI) plume at Hanford, WA. Activated carbon is being used to remediate groundwater contaminated with a mixture of pesticides, xylene, and ethylbenzene. A mixture of municipal compost, leaf compost, and wood chips is being used to remove nickel, iron, and sulfate from mine-tailings contamination. These PRBs have been in operation for only a few years, and although they show promise, their long-term efficacy can not yet be ascertained. It has recently been suggested that bioaugmentation of Fe(0)-PRB may be advantageous for some contaminants (Weathers et al., 1997; Till et al., 1998; Scherer et al., 2000).

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

BOX 5-5 Reduction of Highly Oxidized Contaminants by Fe(0)

Removal of contaminants by Fe(0)-PRB is based on reduction. As Fe(0) corrodes, electrons are released that can be used to reduce highly oxidized contaminants such as trichloroethylene (CHCl=CCl2) and hexavalent chromium (e.g., CrO42-):

Trivalent Cr (Cr3+) produced becomes immobilized as the solid Cr(OH)3 within the barrier.

The electrons can also be used to reduce water-derived protons:

Thus, treatment with Fe(0)-PRB will result in an increase in groundwater pH.

Measuring Performance. Performance of PRBs is typically assessed by measuring contaminant concentration (and potential products) upgradient and downgradient of the barrier. Other geochemical parameters of interest include pH (which increases dramatically across Fe(0)-PRB), dissolved oxygen, total dissolved solids in general and dissolved iron in particular. Measurements of hydraulic conductivity are also important because of the potential for clogging the barrier with mineral precipitates (e.g., CaCO3, iron oxides and carbonates, etc.), hydrogen gas (produced from water during the corrosion of Fe(0)), and microbial growth. Ongoing monitoring should allow for determination of the adequacy of plume capture and of desired residence times (Gavaskar et al.,

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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TABLE 5-6 Organic and Inorganic Contaminants Treated with Permeable Reactive Barrier Technology, and Materials Used for PRBs (percentages are based on a total of 124 projects)

Organic Compounds Treated

%

Inorganics Treated

%

Materials Used for PRBs

%

TCE

DCE

PCE

CCl4

VC

TCA

Other Halogenated Organics

CHCl3

Benzene

Halogenated Methanes

DCA

Toluene

Nitroaromatics

PCBs

Naphthalene

Nonhalogenated VOCs

1,2,3-TCP

CFC-113

26

13

12

9

6

5

4

4

3

2

2

2

2

2

2

2

2

2

Chromium

Lead

Molybdenum

Arsenic

Cadmium

Nitrate

Selenium

Nickel

Copper

Vanadium

Other

31

11

9

9

9

7

4

3

3

3

9

Zero-Valent Iron

Peat

Zeolites

Lime

Geochemical Fixation

Ferric Oxyhydroxide

Surfactant Modified Silicates

Zero-Valent Iron and Sulfur-Containing Materials

Sawdust

Microbes

Chitosan Beads

Hydrogen Sulfide

Other

45

6

6

5

5

4

2

2

2

2

2

2

17

 

SOURCES: Scherer et al. (2000) and adapted from Sacre (1997).

1998, 2000, 2001). Water-level measurements, in-situ flow sensors, and other flow measurements should be used. PRBs are typically designed for a 20- to 30-year life.

As with most remediation technologies, a best-case scenario for process monitoring would be to complete mass and water balances. With PRBs, this is difficult at best. If the contaminant were, for example, TCE, the suite of daughter products of reduction (cis-DCE, vinyl chloride [VC], and ethane) could be measured. If the products are unknown (e.g., from carbon tetrachloride reduction) or are immobilized in the wall, mass balances are not possible. With Fe(0)-PRB, transformation is based on reduction. An electron balance is thus possible, in theory, by measuring Fe(II) entering and exiting the barrier. However, Fe(II) can be oxidized if O2 is present, such that both Fe(II) and Fe(III) minerals are typically precipitated within the barrier (Phillips et al., 2000). Efforts are underway to better understand such phenomena (Liang et al., 2000).

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

Modeling the geochemical and hydraulic behavior of PRBs will be helpful in monitoring and predicting performance (Gavaskar et al., 2001; Morrison et al., 2001).

Technology Evaluation. Because field-scale application of the technology is only seven years old, design procedures and protocols are not yet well developed, although some guidance documents are available (e.g., Gavaskar et al., 1998, 2000; U.S. Air Force, 1997; NAVFAC, 2001). The current technology is limited in applicability to contaminated groundwater (i.e., it is not for soils and sediments). The most successful application is the use of Fe(0)-PRB for remediating groundwater contaminated with chlorinated ethenes. Some success has been reported for Cr(VI) reduction to Cr(III).

Several websites contain summaries of the current status of PRB installations (e.g., http://www.rtdf.org, http://clu-in.org, http://www.frtr.gov, http://www.gwrtac.org/, http://erb.nfesc.navy.mil/). EPA (2001b) indicates that PRBs are part of the remedial action at eight Superfund sites. There was also a recent updated review of some 38 full-scale Fe(0)-PRBs (Vidic, 2001) and a review of DoD installations. The Remediation Technologies Development Forum (RTDF) website and EPA (1999c) summarize a number of field-scale installations of PRBs. Chlorinated solvents, primarily the chlorinated ethenes PCE, TCE, cis-DCE, and VC, and 1,1,1-TCA, are being treated using Fe(0)-PRB at 12 of these installations. The oldest of these installations (Intersil Semiconductor Site, Sunnyvale, CA) has been in operation since 1995 and is briefly described in Box 5-6. Summaries of these full-scale installations indicate that most are working much as designed and are meeting treatment goals, at least for Fe(0)-PRB treating chlorinated solvents. Some success has been reported in the immobilization of Cr(VI) and U(VI) via reduction and precipitation.

From these different reviews, it is clear there are issues not yet resolved regarding long-term performance. These include but are not limited to, potential clogging due to chemical precipitation (some of which is caused by increased pH) or biological growth; competency of the confining layer beneath the PRB in preventing escape of contaminants under the PRB; deterioration of water quality downgradient of the PRB, including the release of incomplete reduction products (e.g., VC from TCE), high pH, and potentially high soluble Fe(II) levels; remobilization of chromium and uranium; the role of microbes in enhancing or reducing treatment effectiveness; longevity of the Fe(0) (i.e., when and how often it will have to be replaced); and hydraulic capture. Some of these issues

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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BOX 5-6 PRB Case Study: Intersil Semiconductor Site, Sunnyvale, CA

Intersil manufactured semiconductors at the site from the early 1970s until 1983 (http://www.rtdf.org/public/permbarr/; Gallinati et al., 1995; Warner et al., 1998). The primary contaminants resulting from this activity are TCE (50–200 µg/L), cis-DCE (450–1,000 µg/L), VC (100–500 µg/L), and Freon 113® (20–60 µg/L). Air stripping was used for remediation at the site until an Fe(0)-PRB was installed in 1996. The contaminated area, a semiconfined aquifer, is 2–4 feet thick. The lower aquitard is clay and silty clay. Low-permeability walls were installed to direct the flow to the PRB (Figure 5-6).

The PRB is 4 feet wide, 36 feet long, and 20 feet deep and is filled with 220 tons of granular iron to a depth of 11 feet. Installation cost $1 million. The cleanup goals are 5 µg/L for TCE, 6 µg/L for cis-DCE, 0.5 µg/L for VC, and 1,200 µg/L for Freon 113®. Since installation of the PRB, concentrations of these VOCs have been below the cleanup goals within the barrier. Some hydraulic mounding has occurred above the PRB, but it has not yet adversely affected performance. An unexpected benefit was observed as a result of placing a pea gravel zone upgradient of the PRB to aid in flow distribution. Some limited mixing of Fe(0) into this zone resulted in conditions favorable for some chemical precipitation of minerals and pretreatment of chlorinated solvents. It is possible that this will extend the life of the barrier itself.

FIGURE 5-6 Fe(0)-PRB at the Intersil site. SOURCE: EPA (1998b).

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

were addressed in a recent report summarizing PRB performance at DoD sites, particularly Moffett Field (Box 5-7).

In summary, PRBs offer three potentially significant advantages: (1) conservation of water and energy, (2) low operation and maintenance costs, and (3) in situ application. However, capital costs may be high, only a few types of redox-sensitive pollutants are amenable to PRB

BOX 5-7 PRB Case Study: Moffett Field Naval Air Station, Mountain View, CA

A pilot-scale PRB facility has been operated at the Moffett Field Naval Air Station since 1996. A funnel and gate system treats a groundwater plume containing PCE, TCE, and cis-DCE as major contaminants. Steel sheet piles with interlocking joints make up the funnel and sides of the gate. Iron filings are the reactive media with pea gravel upgradient and downgradient from the filings. A recent report evaluated various aspects of longevity and hydraulic performance at Moffett Field, as well as at other DoD PRB installations (Gavaskar et al., 2001).

The report indicated that flow through the PRB was progressing as designed. After five years of operation, concentrations of PCE, TCE and cis-DCE were below their respective maximum contaminant levels. However, there has not yet been a front of clean water downgradient from the PRB, although there were signs that this would happen in the future. Several reasons were proposed, one being that the PRB was not tied into an impermeable layer and contaminated groundwater is leaking under and around the PRB. Analyses indicated that Fe(0) reactivity deteriorates with time, although it is still not possible to predict when the Fe(0) will need to be replaced. Analysis of hydraulic performance indicated an average residence time of nine days. The presence of an upgradient pea gravel zone helped to create a more uniform flow entering the PRB. Analysis for mineral precipitates indicated the presence of calcite, geothite, and some calcium-aluminum precipitates, although no discernable effect on flow velocities was reported. A cursory assessment of microbial activity indicated considerably less diversity downgradient of the PRB.

The major lesson learned from the assessment was that geochemical characterization of site groundwater is important. Because of the loss of Fe(0) reactivity, a thicker PRB is needed for groundwater with higher total dissolved solids (TDS>500–1,000 mg/L). Hydrogeologic modeling and monitoring (e.g., water level measurements) before and after installation should help assess hydraulic capture. The report indicates that when PRBs are located within plume boundaries, it is likely that some time will pass before downgradient contaminant concentrations will decrease (i.e., a “clean” front of groundwater appears). Finally, the report recommends additional research to help assess the rate at which Fe(0) loses reactivity and why this happens. Such information is required to estimate how much Fe(0) will be needed and how long it will be before Fe(0) needs to be replaced.

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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treatment (at least in its present form), immobilized contaminants may not be immobilized “forever,” and there is a lack of long-term performance data for these systems. The technology currently is applicable only for contaminated groundwater remediation under appropriate geochemical and hydrologic conditions. The technology is most appropriately applied when plumes are less than about 1,000 feet wide and less than about 30 feet below ground surface (Gavaskar et al., 1998, 2000, 2001). The depth is limited because it is quite expensive with current technology to dig deeper than about 30 feet. However, developments in construction techniques (e.g., hydrofracturing, etc.) may overcome this limitation (Vidic, 2001). Proper design must ensure hydraulic capture of the contaminated plume (i.e., ensure that the plume does not pass over, under, or around the PRB). Thus, PRBs are most effective when they are keyed into an impermeable formation at depth (e.g., bedrock).

Enhanced Bioremediation

Several terms are currently used to describe the use of biological processes to remediate contaminated sites in situ. Enhanced bioremediation is taken to mean that enhancements are made to stimulate the growth of indigenous, subsurface microbes to increase the rate of contaminant removal or immobilization (e.g., Cr(VI)). The Navy currently uses enhanced bioremediation to describe the addition of oxygen and other nutrients and/or cometabolic substrates (e.g., carbon and energy source) to stimulate the growth of indigenous microbes and increase the rate of aerobic biodegradation (http://erb.nfesc.navy.mil). Enhanced bioremediation is to be distinguished from intrinsic biodegradation, where no additions are made to the contaminated site, but rather indigenous microbes are allowed to degrade and/or immobilize contaminants at the rate dictated by the in situ geochemical environment. As noted in a recent NRC report (NRC, 2000), the term being used today is “natural attenuation,” where all naturally occurring processes that act to decrease the concentration and mass of a contaminant are included. (Natural attenuation will be discussed in a separate section.) Bioaugmentation involves adding specific organisms to the subsurface environment. Other in situ bioremediation processes include bioventing, biosparging, bioslurping, and air sparging.

The focus of this section is enhanced bioremediation, which typically involves adding nutrients to the subsurface environment to increase the rate at which contaminants are biodegraded by indigenous organisms.

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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Examples include carbon and energy sources such as sugars, lactate, volatile acids (e.g., acetic acid), complex materials such as molasses and vegetable oils, proprietary compounds that release molecular hydrogen slowly, and gaseous hydrogen; electron acceptors such as oxygen (applied as air, pure oxygen, hydrogen peroxide, or proprietary slow-oxygen-release compounds), nitrate, and sulfate; nutrients, primarily nitrogen and phosphorus; and perhaps buffers. In some cases, it may be advantageous to create anaerobic zones followed by aerobic zones to ensure more complete removal of contaminants and their daughter products. Several excellent reviews are available to describe the fundamentals of enhanced bioremediation and natural attenuation (NRC, 1993, 1997a, 2000; Rittmann and McCarty, 2001). Figure 5-7 provides a general schematic of the process.

Enhanced bioremediation has been reported to remove contaminants aerobically both as a primary substrate—for example, addition of oxygen and nutrients for degradation of petroleum hydrocarbons (Brown et al.,

FIGURE 5-7 Schematic of enhanced bioremediation system. SOURCE: NAVFAC (2001).

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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1993)—and as a cometabolic substrate—for example, addition of toluene and oxygen for removal of TCE (McCarty et al., 1998). Similarly, enhanced bioremediation has been used to remove contaminants anaerobically as a primary substrate—for example, removal of PCE via dehalorespiration by addition of vegetable oil (Boulicault et al., 2000) or H2 (Newell et al., 2000)—and as a cometabolic substrate—for example, addition of acetate for the removal of carbon tetrachloride in the presence of nitrate (Semprini et al., 1992). Although there are many reports of success with enhanced bioremediation in field demonstrations, few reports present the types of evidence described below as needed for conclusive proof that bioremediation is responsible for most of the contaminant removal. There are exceptions (e.g., Beeman et al., 1994; McCarty et al., 1998).

The Navy considers enhanced aerobic bioremediation of petroleum hydrocarbons to be “conventional” (i.e., established) treatment and enhanced aerobic cometabolism of chlorinated aliphatic hydrocarbons to be an “emerging” technology, which is in agreement with recent NRC reports (NRC, 1993, 2000). There is considerable current interest in adding electron donors to stimulate reductive dechlorination of chlorinated aliphatic hydrocarbons, primarily the chlorinated ethenes. The recent discovery of dehalorespiring bacteria that use chlorinated ethenes (e.g., PCE, TCE) as electron acceptors and H2 as a preferred energy source in support of growth has led to intensive efforts to discover ways to deliver a slow, steady supply of H2 to these organisms. Molecular H2 can be delivered directly to the subsurface (Newell et al., 2000), or it can be produced via fermentation and anaerobic oxidation of a variety of substrates. Many substrates have been tried in the laboratory, and there are several reports of field tests and full-scale operations. Soluble substrates that have been tried include acetate, propionate, butyrate, lactate, benzoate, methanol, and simple sugars. Insoluble substrates, typically termed “slow-release compounds,” include biomass, compost, molasses, edible oils, tetrabutyl orthosilicate, wood chips, and proprietary compounds (typically polymers that hydrolyze and dissolve slowly in water). One reason for using substrates that are fermented or oxidized to release H2 is to minimize competition for H2 among the dehalorespiring bacteria and methanogens and sulfate reducers. Dehalorespiring bacteria have been shown to outcompete other organisms for available H2 when H2 concentrations are low (Fennell and Gossett, 1998; Yang and McCarty, 1998). Soluble substrates such as benzoate, lactate, and propionate are useful for water recirculation systems (EPA, 2000c; Leigh et al., 2000; Yang and McCarty, 2000a). Insoluble (or slightly soluble) substrates are useful for

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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more passive approaches that create “biologically active zones” (biobarriers) in the subsurface. Examples include the use of molasses (EPA, 2000c; Hansen et al., 2000), plant biomass (Haas et al., 2000), soybean oil (Boulicault et al., 2000), and proprietary compounds (Boyle et al., 2000).

It should also be noted that recent evidence indicates that source zones or near-source zones may be treated with enhanced anaerobic bioremediation (Carr et al., 2000; Yang and McCarty, 2000b). Dehalorespiring bacteria can enhance the dissolution of DNAPL.

Measuring Performance. Several recent reports (NRC, 1993, 2000; NAVFAC, 2001; Rittmann and McCarty, 2001) have outlined how performance of enhanced bioremediation should be measured. It is not a trivial undertaking for a number of reasons. For example, success is defined differently by the wide variety of parties involved. These include regulators, buyers of bioremediation, the public, researchers, and developers of bioremediation. No single measure is universally applicable to the wide variety of sites being addressed. Finally, contaminated sites are frequently heterogeneous, making it impossible to fully characterize them. Thus, it is difficult to conclusively prove the success of in situ bioremediation.

The key to evaluating success is to directly link observed loss of contamination with microbial activity. NRC (1993) recommends an approach relying on three types of evidence:

  1. documented loss of contaminants from the site,

  2. laboratory assays showing that microorganisms at the site have the potential to transform the contaminants under the expected site conditions, and

  3. one or more pieces of evidence showing that the biodegradation potential is actually realized in the field.

It is this third type of evidence that is the most crucial and, unfortunately, the most difficult to obtain. Details describing the scientific bases for these measurements are included in NRC (1993) and updated in NRC (2000).

There are techniques that provide principal evidence and those that provide confirmatory evidence (Rittmann and McCarty, 2001). Principal evidence is that which is capable of proving success or failure—for example, stoichiometric consumption of electron acceptors, formation of inorganic-C that originated from organic-C, and/or increased degradation

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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rates over time (quantification is very important). Confirmatory evidence is that which can support the proof of success, but its absence does not prove failure. Examples include an increase in protozoan population, the detection of intermediate metabolites, and an increase in the ratio of nondegradable to degradable components.

Technology Evaluation. Potential advantages of enhanced bioremediation are that it destroys contaminants (e.g., via mineralization or conversion to benign organics such as ethene), it can be applied in situ, and it is less expensive than other technologies. However, it remains a challenge to deliver the enhancements, monitor effectiveness, and demonstrate conclusively that biodegradation is responsible for contaminant removal. As currently employed for contaminated aquifers, the technology is best applied in situations where the hydraulic conductivity is sufficiently high (say greater than 10-3 cm/sec), residual NAPL is absent or has been removed, contaminants are not overly hydrophobic, and contamination is not too deep. At the present time, enhanced bioremediation has been shown to work best with petroleum hydrocarbons—primarily benzene, toluene, ethylbenzene, and xylene (BTEX)—in relatively simple hydrogeologic environments. Increasingly, amendments, primarily electron donors that release H2, are being added to stimulate dehalorespiring bacteria. The Navy is particularly interested in the potential of slow-release compounds for this purpose.

Many slow-release compounds are low-cost; for example, molasses and edible oils may be quite inexpensive compared to the alternatives (Harkness, 2000). However, there are several concerns regarding the use of so-called slow-release compounds. Typically, less than about 20 percent (perhaps less than 5 percent) of the substrate may be used for dehalogenation (Yang and McCarty, 2000a), resulting in an excess of organic carbon (e.g., organic acids such as acetate, propionate, etc.) being available for transport with the groundwater or for degradation by other organisms. During fermentation, and depending on the geochemistry, alkalinity may be consumed, and the pH may decrease significantly. This may trigger other undesirable water quality changes (e.g., metal dissolution). Subsequent degradation of the organic acids will increase downgradient alkalinity and pH. Production of undesirable hydrogen sulfide from sulfate reduction may occur. There is the potential for the production of methane gas, which could decrease hydraulic conductivity, as could excess microbial growth. For some slow-release compounds (e.g., edible oils), some removal from the aqueous phase will result simply from partitioning of the chlorinated organics into the nonaqueous phase

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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rather than from degradation. Although this should not affect long-term performance, it must be considered in the design, operation, and monitoring of such a system. Addition of slow-release compounds may simply displace the polluted groundwater, giving the illusion of success. It is very important to characterize the hydrogeology of the site to determine if the slow-release compounds can be distributed effectively (Hansen et al., 2000). If the site is aerobic, this technology may not be appropriate. Hansen et al. (2000) also point out the importance of increased biosurfactant production as a result of stimulating microbial growth in the subsurface. Such surfactants can solubilize contaminants and temporarily increase aqueous concentrations.

A case study of enhanced bioremediation is given in Box 5-8. It provides “proof-of-concept” evidence for enhanced anaerobic bioremediation. It also indicates that limited data are available on the effectiveness of this technology, and although results are promising, it is too soon to assess long-term cost and performance.

Several issues need to be resolved in order to declare enhanced anaerobic bioremediation a proven technology. First, it is clear that its major application is for bioremediation of PCE and TCE. Success will be determined by the complete conversion of these compounds into ethene; conversion to vinyl chloride is an unacceptable endpoint. Although the addition of slow-release electron donors can stimulate anaerobic cometabolism of chlorinated ethanes and chlorinated methanes, complete anaerobic conversion of these compounds to ethane and methane, respectively, has not been demonstrated. Using techniques described above, it must be demonstrated conclusively that biodegradation, not physical displacement or dilution, is responsible for decreases in contaminant concentration. To date, there has been no effort to assess the effect of the degradation of slow-release compounds on downgradient water quality (possible changes include decreased pH, increased dissolution of metals, and increased biological oxygen demand from volatile acids). Finally, the technology needs to be proven cost-effective.

Technologies for Remediation of Inorganics in Soil and Groundwater

The most frequently occurring metal contaminants at Navy sites are lead, zinc, copper, nickel, barium, cadmium, vanadium, aluminum, and beryllium. These heavy metals and other inorganic contaminants (e.g., arsenic, cyanides, perchlorate, and radionuclides) pose a great challenge

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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BOX 5-8 Enhanced Bioremediation Case Study: Emeryville, CA

Enhanced anaerobic bioremediation is being used to remediate an abandoned manufacturing facility in Emeryville, CA (EPA, 2000c). Metal-plating operations resulted in contamination of the groundwater with degreasing solvents and metals. The primary pollutants of concern are TCE and Cr(VI). Monitoring in 1995 indicated PCE concentrations in the tens of µg/L, TCE concentrations of up to 17,000 µg/L, cis-DCE concentrations as high as 900 µg/L, and VC concentrations generally lower than 20 µg/L. Most of the data focus on the chlorinated ethenes, although Cr(VI) concentrations in excess of 100,000 µg/L were also found at the site. The soil type is interbedded sand and clay units, the depth to groundwater is between 3.5 and 8 feet, and the groundwater velocity was 60 ft/yr. Monitoring data indicated that limited reductive dechlorination was occurring, and the rate was limited by lack of organic carbon (electron donor) and/or the environment was not sufficiently reducing. A pilot study was undertaken to establish a reactive anaerobic zone by adding a mixture of molasses, anaerobic digester supernatant, and tap water. The supernatant was added because preliminary analyses indicated low bacterial counts in the subsurface. The pilot study lasted approximately six months and indicated that an anaerobic zone could be created that would support reductive dechlorination of the organic compounds and the reduction of Cr(VI) to immobile Cr(III).

The full-scale system was installed and has been operating since April 1997. It consists of 91 injection points installed to a depth of 24 feet below ground surface using a GeoprobeTM. During the first injection event, a mixture of 25 gallons of molasses, 1 gallon of supernatant, and 125 gallons of water was made. October 1998 data indicated that concentrations of PCE, TCE, cis-DCE, and VC near the source area fell below 5 µg/L. It was reported that Cr(VI) concentrations were reduced by approximately 99 percent, and that in some areas where historic concentrations were above 100,000 µg/L, concentrations are now below 5 µg/L. Data concerning organic carbon levels (fate of added molasses), pH, and other changes in geochemistry were not available. Thus, although it appears that molasses addition is stimulating conversion of TCE to ethene, long-term monitoring is needed to confirm lasting effectiveness.

for remedial efforts. Unlike many organics, chemical and biological transformations of inorganics can change the form of the contaminant, but cannot destroy it (Evanko and Dzombak, 1997; EPA, 1997b). Furthermore, the chemical form of the inorganic contaminant influences its solubility, mobility, and toxicity in the subsurface (EPA, 1997c). The form, or speciation, of inorganic contaminants depends on the source of the waste and the geochemistry of the subsurface at the site. For example, zinc usually occurs in the +2 oxidation state and tends to form soluble compounds at neutral and acid pH values (Evanko and Dzombak,

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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1997). Under these conditions, zinc can become a mobile metal in groundwater. At higher pHs, zinc can form carbonate and hydroxide complexes, which reduces its solubility. Furthermore, zinc readily precipitates under reducing conditions—for example, ZnS(s). The inability of transformation reactions to destroy inorganic contaminants and the influence of geochemistry on inorganic contaminant mobility present major challenges for remediation of sites containing inorganic contaminants.

Because inorganic contaminants cannot be destroyed, site management strategies focus either on containing the contaminants by decreasing compound mobility and toxicity, separating/extracting the contaminants from the subsurface, or using institutional controls to restrict human exposure to the contamination (NRC, 1997a; Evanko and Dzombak, 1997; EPA, 1997b). This discussion of remedial technologies for inorganics is confined to the former two approaches. Also, because the top nine inorganic contaminants at Navy sites are metals, this discussion will exclude remediation of nonmetal contaminants such as perchlorate, cyanides, and nitrate. Management of radioactive wastes is also excluded.

The listing of candidate technologies for soil, sediment, and groundwater remediation in Table 5-2 identifies 16 technologies for the remediation of sites contaminated with inorganics. Descriptions of these technologies appear on several web sites (e.g., http://clu-in.org, http://www.epareachit.org, http://www.frtr.gov, http://www.gwrtac.org, http://www.rtdf.org, and http://erb.nfesc.navy.mil/). Based on the strategy used to control the contamination, technologies for remediation of inorganics in the subsurface can be grouped into the following five categories:

  1. Excavation. Contaminated materials are removed by digging and are transported to an offsite disposal facility.

  2. Containment. Containment technologies attempt to prevent the transport of contaminants by isolating or solidifying them within a designated area. Examples are capping, subsurface barriers, solidification/stabilization, and vitrification.

  3. Toxicity and/or mobility reduction. Chemical or biological reactions are used to alter the form (speciation) of metal contaminants in order to decrease their toxicity and/or mobility. Examples are chemical treatment, permeable reactive barriers, biological treatment, and phytoremediation.

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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  1. Physical separation. Ex situ processes are used to selectively remove contaminated material from the soil usually for the purpose of reducing the amount of material requiring subsequent treatment. Examples are screening, classification, gravity concentration, froth flotation, and magnetic separation.

  2. Extraction. The metal is removed from the rest of the soil or groundwater by ex situ or in situ techniques such as soil washing, soil flushing, and electrokinetic treatment.

Measuring Performance

As in all remediation technologies, documenting the success of technologies for control of inorganic contaminants requires evidence that the technology reduces risk by decreasing the mass, concentration, mobility, and/or toxicity of the contaminants and requires identification of the operative mechanism(s). The latter is needed to ensure that there is a cause-and-effect relationship between the implementation of the technology and the observed reduction in risk.

A complete mass balance both before and after remediation provides the most confidence in the assessed performance of a technology. Concentration data with monitoring well and soil core samples are typically used to determine contaminant mass. For stabilization and containment technologies, the most important mass balance information is to demonstrate immobilization of the contaminants. Furthermore, the integrity of the stabilized material must be determined for the site-specific groundwater flow and chemical conditions. For technologies that transform the inorganics to less harmful or less mobile species, monitoring must prove that the reaction processes are taking place. Here, the mass balance analysis must confirm the stoichiometry between the reactants and products. Finally, for technologies that rely on in situ or ex situ extraction, a mass balance analysis must be conducted to determine the contaminant extraction efficiency and to confirm that the mass extracted in the outflow stream is correlated with the mass removed from the subsurface.

Technology Evaluation

In the past, the typical remedy for inorganics-contaminated sites has been excavation, transport offsite, and burial at an approved disposal fa-

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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cility (dig-and-haul). Although newer in situ treatment technologies may soon replace excavation, there are nonetheless recent advances that have improved excavation and all ex situ remedies that depend on it—mainly by focusing exclusively on soils (or sediments) that do not meet cleanup standards. Traditional excavation activities for soils rely on work plans that specify the required excavation footprint based on existing characterization data. When excavation is complete, the final dig face is sampled to ensure that remaining soils attain cleanup goals. The characterization data used for excavation design are typically the product of remedial investigation/feasibility study (RI/FS) sampling work, and such data are often inadequate for accurately delineating contamination footprints. Evidence suggests that excavation activities at hazardous waste sites have resulted in significant overexcavation (Durham et al., 1999).

Precision excavation techniques, an alternative to traditional excavation approaches, attempt to limit excavation and subsequent remediation to only those soils that fail to attain cleanup standards. Precision excavation differs from traditional approaches in three key ways: (1) it makes broader use of data collection during the excavation process to provide in situ segregation of soils, (2) its work plans do not specify excavation footprints, but rather identify the decision-making process that will be used to screen soils as excavation work moves forward, and (3) its excavation work is designed in lifts or phases that allow for dig face screening before work proceeds. The feedback and self-adjustment mechanisms built into precision excavation programs are consistent with the adaptive management concepts described in this report.

The viability of a precise excavation approach for soil- or sediment-contaminated sites depends on the availability of rapid field analytical techniques appropriate for the contaminants of concern and their action levels. Chapter 3 discusses advances in field data collection technologies and analytical technologies in more detail and their pertinence to ASM. Within the RCRA program, EPA’s SW-846 contains guidance on acceptable analytical techniques. The latest draft (EPA, 2000d) includes several additions pertinent to precise excavation and Navy contaminants of concern. These include portable x-ray fluorescence spectrometry systems for metals, and portable GC, calorimetric, and immunoassay technologies for addressing explosives, PCB, and PAH contamination in soils and sediments. Differentially corrected global positioning systems are capable of providing relatively accurate locational control for data collection efforts in real time.

Beyond excavation, there are several promising onsite and in situ technologies being developed for addressing contamination by inorgan-

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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ics. Many of these alternatives to dig-and-haul have been successful when tested in beakers at the laboratory scale or in small-scale pilot tests. Limited documentation exists on the performance of treatments for inorganics contamination at the field scale or at Navy sites. For example, only seven Navy sites with contamination by inorganics in soil, sludge, and sediment are included in the compilation by EPA (EPA, 2000e) of over 600 innovative remediation technology demonstration projects in North America. No Navy case studies were listed for groundwater contaminated by inorganics. The technologies employed include phytoremediation, soil washing, ex situ extraction, electrokinetics, solidification and stabilization, and ex situ physical separation/chemical treatment. A review of containment technologies, biological treatment technologies, and physical/chemical treatment technologies approved for use by the Navy (from http://erb.nfesc.navy.mil/restoration) identified case studies employing capping, biotransformation, constructed wetland, phytoremediation, electrokinetic extraction, and solidification/stabilization for control of contamination by inorganics. Several of these case studies involved small-scale experiments. Additional information on four of the most frequently cited technologies for inorganics contamination is presented below and more detail is found in NRC (1999a). Solidification/stabilization is applicable to a wide range of inorganic contaminants and site conditions. The latter three technologies—electrokinetics, phytoremediation, and chemical treatment—are much more restrictive for certain contaminants and conditions.

Solidification/Stabilization. Solidification/stabilization refers to processes that encapsulate a waste to form a solid material or that involve chemical reactions that reduce the leachability of a waste. Examples include chemical additives (e.g., cements and polymers) and thermal fusing/glassification. From FY1982 through FY1998, solidification/stabilization projects were the second most common type of source control treatment technology implemented at Superfund sites, representing 24 percent of all source control projects (EPA, 2000f). Solidification/stabilization projects were mainly implemented for metal contaminants. Fifty-six percent of the applications were used to treat metals only, whereas 90 percent of the applications were used to treat metals alone or in combination with organics or radioactive metals. The major limitations of solidification/stabilization are uncertainty in long-term effectiveness, the need for long-term monitoring because untreated contaminants remain on the site, and questions about future site use with containment technologies in place (EPA, 1999d).

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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Electrokinetics. Electrokinetic remediation describes technologies that separate and extract heavy metals, radionuclides, and organic contaminants from saturated and unsaturated soils, sludges, and sediments (EPA, 1995; NRC, 1999a) as well as groundwater (GWRTAC, 1997). The strategy uses an electrical field imposed by electrodes implanted in the vicinity of the contaminant source. It has been proved successful in the laboratory and in small-scale pilot tests, particularly for removal of metals in low-permeability soils that are difficult to flush. Many issues remain to be resolved prior to full-scale commercialization of electrokinetic remediation. Testing at the Naval Air Weapons Station, Point Mugu, California, indicated that small-scale experiments give a false indication of the applicability for remediation of chromium and cadmium in the field (EPA, 2000c). Additional research is needed to determine the effect of naturally occurring ions on mobilization and removal of the target metals, to identify the site-specific factors that control the performance, to determine the relationship between electrode design and electric field shape, and to determine the optimum configuration of the electrodes in the field.

Phytoremediation. Phytoremediation employs metal-accumulating plants to either remove inorganic contaminants from the shallow subsurface or to withdraw soil moisture through evapotranspiration, which can provide hydraulic containment of contaminants (EPA, 1999e; Lasat, 2002). Phytoremediation is in the early stage of commercialization and is best suited for sites with (1) widely dispersed contamination (large land area), (2) low contaminant concentrations to prevent plant toxicity, and (3) and contaminant depths not exceeding the root zone. The effectiveness of phytoremediation depends upon the interaction among contaminants, soil, plants, and microbes. A variety of factors affect this complex interaction, such as climatic conditions, site hydrogeology, and agronomic practices. The case study described in Box 5-9 demonstrates some reduction in total soil lead with phytoremediation, but several growing cycles/seasons will be required to address all of the hot spots and realize near complete lead removal. Much greater knowledge of plant/soil/contaminant interactions is needed in order to optimize phytoremediation performance at a given site.

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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BOX 5-9 Case Study: Phytoremediation at Simsbury, Connecticut

Phytoremediation of lead in soil was tested in 1998 at the Ensign-Bickford Company site, Simsbury, Connecticut (available at: http://bigisland.ttclients.com/frtr/00000164.html). Near surface soils at the site were contaminated with lead from open burn/open detonation activities. The test area of 2.35 acres initially contained an average total lead concentration of 635 mg/kg; concentrations were higher than 1,000 mg/kg in many areas of the site, with some areas exceeding 4,000 mg/kg. Treatment crops of Indian mustard (Brassica juncea) and sunflower (Helianthus annus) were cultivated over a six-month period. Lime and fertilizers (nitrogen, phosphorus, and potassium) were tilled into the soil to a depth of 15–20 cm. An overhead irrigation system was used to provide moisture. Supplemental foliar fertilizers were added through the irrigation system.

Plant growth for each of the treatment crops was generally good. However, certain soils within the treatment area remained saturated, which caused less than optimal plant growth and required replanting. The six-month test was considered a success because average lead uptake measured in the sunflower and Indian mustard plant materials from all crops was approximately 1,000 mg/kg dry weight. Total lead concentrations in the surface soils decreased from an average of 635 mg/kg to 478 mg/kg. After phytoremediation, no collected soil samples exceeded 4,000 mg/kg. Initially, 7 percent of the treatment area had had soil lead concentrations in excess of 2,000 mg/kg, and after six months of phytoremediation, only 2 percent still exceeded 2,000 mg/kg. Further treatment cycles are planned for the site.

Chemical Treatment. Two case studies are available that demonstrate the use of chemical addition to achieve in situ reduction of Cr(VI) to Cr(III) (EPA, 2000c). At White Sands Missile Range, New Mexico, H2S gas was injected into the subsurface in an attempt to reduce the hexavalent chromium to the less mobile trivalent chromium. Test results indicated that channeling of the H2S occurred through strata having higher relative permeability. Furthermore, observed consumption of H2S was higher than predicted from small-scale laboratory column tests. At the DOE’s Hanford Site, a dithionite solution was injected to react with natural iron in the subsurface and form reduced iron (Fe(II)). The Fe(II) reacted with Cr(VI) and reduced it to Cr(III). Concentrations of chromium in groundwater were decreased to less than 8 µg/L in one month. Two years after treatment was complete, the treatment zone remained anoxic and Cr(VI) remained below detection limits. The anoxic zone is estimated to have a life of 7–13 years without further addition of dithionite. A major uncertainty is the long-term ability of these technologies to

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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prevent remobilization of the chromium.

Summary

The Navy has inorganic contaminants at many of its sites, which poses a great challenge. Because the behavior and speciation of inorganics is strongly coupled to site-specific conditions, it is not possible to make generic statements about the application of a given technology. The Navy’s problems with inorganic contaminants are not unique, but the level of priority the Navy has given to remediation of metal contaminants does not appear to be commensurate with the high frequency of occurrence of these contaminants at their sites. The Navy should devote more effort to developing strategies for managing inorganic contaminants. Implementation of the remediation technology needs to be coupled to a specific performance goal or objective. Because inorganic contaminants cannot be destroyed, efforts need to continue to focus on containment and on performing a complete mass balance.

Technologies for Remediation and Management of Contaminated Sediment

Throughout its history, the Navy has focused significant activity in coastal ports. Contaminated sediments have resulted from handling and disposal of fuels, bilge water, antifoulants, and other compounds, and from handling of wastewater on shoreline facilities. As discussed in Chapter 1, as many as 110 Navy facilities have identified sediment contamination, with most cleanup efforts still in the RI/FS stage.

For a variety of reasons, management of contaminated sediments poses one of the most difficult site remediation issues faced today. The technologies applied to contaminated sediments, for example containment by clean capping layers or removal by dredging and disposal, are often conventional. The safe and effective application of these conventional technologies in a dynamic subaqueous environment, however, requires innovation in design and care in implementation. Contaminated sediments typically reside in spatially variable and dynamic systems subject to seasonal flow variations and episodic storm events. The volume of sediments that must be managed often exceeds a million cubic yards, dwarfing many contaminated soil sites. These sediments are also associated with equally daunting volumes of water, and efforts to remove the

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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contamination typically entrain even more water. All management options leave a residual risk that must be evaluated and managed. In some cases, the sources that led to the contamination may not be completely controlled, leading to the potential for recontamination of the sediments. In addition, long periods are usually required to observe the resources at risk and demonstrate recovery, making the assessment of success difficult.

A range of options is applicable to the management and remediation of contaminated sediments. Among the regulatory and nonregulatory approaches to reducing and managing risks of contaminated sediments are:

  • control of environmental releases contributing to sediment contamination,

  • socioeconomic options including reduction of exposure through fish advisories, institution of catch and release fisheries, relocation of exposed communities, and the introduction of economic or other acceptable offsets1,

  • natural attenuation including intrinsic biodegradation and natural capping by deposition of clean sediment,

  • other in situ management via containment or treatment, including capping, and

  • removal and ex situ management, which requires application of dredging technologies, pretreatment technologies, ex situ treatment and disposal technologies, and technologies for the management of residual contaminants, including contaminated gaseous and liquid effluents or the residual contaminants in the treated dredged material.

Control of the environmental releases leading to sediment contamination is a critical first step in managing contaminated sediments. Unlike most sources of soil contamination, the sources of sediment contamination may not have been fully characterized; even if they have been fully characterized, they may be difficult or impossible to adequately control. Thus, the degree to which these sources continue to contribute to sediment contamination must be assessed and incorporated into a conceptual model of the system. This should include a clear understanding of the source of sediment contamination, the vertical and areal distribution of contaminated sediments, key fate and transport processes, and how these

1  

An example of such an offset would be the industrial development of contaminated sediment areas following an approach similar to that of land-based Brownfields.

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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relate to the risks to the resources of concern. After assessment or control of releases leading to sediment contamination, the identification and selection of management options should be based on a wide range of considerations, including effectiveness, permanence, implementability, risks associated with implementation, cost, and state and community acceptance. All of these factors must be evaluated relative to the identified goals and based upon site-specific conditions (NRC, 2000).

Natural Attenuation

Natural attenuation occurs via any of a number of processes that can contain, destroy or dilute contaminants in the environment. In contaminated sediments, contaminants are often relatively immobile and refractory, and the most important natural attenuation process is often the stable burial of contaminants by sedimentation or deposition of clean sediments. Although the concentration or mass of the contaminant may be unchanged, significant reduction in exposure and risk may occur via this process. Some natural attenuation processes, such as dispersion, dilution, and volatilization, may transfer the risk from one location to another, which may or may not reduce overall risk. Other processes, such as biotransformation, sorption, and containment by burial with clean sediments, may directly reduce risk. Biotransformation processes that may significantly reduce risk include biodegradation of organic compounds, reductive dechlorination of halogenated compounds, and binding of metals into insoluble or non-bioavailable sulfides. Sorption of contaminants into a soil fraction that limits the rate or extent of desorption may also reduce bioavailability and subsequent exposure and risk.

Even in situations where natural attenuation is not the primary management approach, it still serves to manage the residual contamination not addressed by other approaches, which includes marginally contaminated areas outside of the zone being actively remediated or the residual contamination remaining within the remediated zone. It is anticipated that if natural attenuation is coupled with other more active remedial approaches, the duration of the monitoring may be shorter, although monitoring intensity should be unchanged. Because natural attenuation must be relied upon to some extent at all contaminated sites, an evaluation of the change in risk with time posed by natural attenuation processes should be a component of all remediation proposals.

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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Other In Situ Treatment

In situ treatment options are often designed to enhance natural attenuation processes. Biological degradation or transformation processes could be encouraged in situ, but most sediment contaminants degrade only slowly or to a limited extent in sediments, and an adequate delivery system for nutrients and other required reagents has not been identified. An effective delivery system would likely involve mixing of the sediment, which would encourage resuspension and loss of both sediments and contaminants. The lack of an effective delivery and homogenization system has also hindered the application of in situ stabilization systems. A demonstration in Manitowoc Harbor, Wisconsin, revealed difficulties in the management of pore water released by the solidification process (Fitzpatrick, 1994, as referenced in EPA, 1994).

In situ treatment options that do not involve delivery of chemicals to the sediments have also been proposed. In situ vitrification employs electricity to raise sediments to sufficiently high temperatures to produce a glasslike product. The energy costs of heating high-moisture-content sediments to glass formation temperatures are formidable, and the technology has not been used except for small volumes of highly contaminated sediments. Electrochemical geooxidation employs electricity to encourage redox reactions in sediments. This technology is under development and has not been demonstrated on a large scale for sediment remediation. In general, in situ treatment and stabilization technologies that are effective and commercially available have not been demonstrated (PIANC, 2000).

The remainder of this section is devoted to capping, which is the process of placing clean sediment or sand on top of the contamination, much like as occurs with natural deposition. In situ sediment capping is primarily designed to stabilize or contain contaminated sediments, isolate contaminants from benthic organisms, and slow contaminant migration out of the underlying sediments. Guidance exists for the design, placement, and monitoring of a cap as a sediment management option (Palermo et al., 1998). This guidance includes quantitative information on design of armoring layers, design for contaminant containment, and stability analysis during cap placement.

After placement, contaminants will migrate by diffusion in the pore space or by advection due to consolidation or groundwater seepage through the cap. After an initial transient movement, quasi-steady release rates are realized that are typically much lower than release rates prior to capping. The length of the transient period is longer for caps that

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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contain sorbing material or for thick caps. The quasi-steady release rate depends primarily on the thickness of the cap and the extent to which groundwater seepage drives contaminant transport. The amount of contaminant that accumulates in the upper layers of the cap depends upon the sorptive characteristics of the cap. Thus, a weakly sorbing cap such as sand will tend to reach quasi-steady release rates relatively rapidly but not accumulate significant contamination in the upper layers of the cap. A strongly sorbing cap, however, will release essentially no contaminant for a long time but will ultimately accumulate contaminants in the upper layers of the cap, although typically at concentrations much below those originally found in the contaminated sediments.

Thin layer (5 to 15 cm) capping to enhance natural attenuation is the process most closely related to natural deposition processes. By placing a thin layer of clean sediment over the contaminated sediment, the process is potentially less disruptive to the benthic community. Because the sediment-water interface tends to approach an equilibrium state, the small modification provided by a thin layer cap is potentially more stable than thicker caps without additional armoring. A layer of only 5–15 cm will generally isolate the bulk of the contaminants from the benthic community and the overlying water. Isolated penetrations of a thin layer cap can still occur, but they are unlikely to lead to aquatic organism exposure to significant contaminant mass. Primary concerns associated with thin layer capping are the long-term stability of the capping layer without armoring and the ability to accurately place a thin layer of sediment.

Thick layer capping is the conventional approach to containment of contaminated sediments. Cap thickness is normally 20 cm to as much as 1 m. The greater thickness helps ensure that an isolating cap layer remains even if there is significant heterogeneity in placement thickness or small amounts of post-placement erosion. The larger depth of capping material, however, may result in load bearing problems for an underlying soft sediment or require placement in multiple layers to allow the underlying sediment to consolidate and develop sufficient strength to support the cap layer. A number of examples exist of successful cap placement over very soft sediments—that is, undrained shear strength of 0.2 kPa or less (Zeman and Paterson, 1997; Palermo et al., 1998).

Clean sediment caps can be used to contain sediments and contaminants in situ or ex situ. The ex situ application, referred to as confined aquatic disposal (CAD), involves dredging of the contaminated sediment and placement in a submerged pit, followed by placement of additional clean dredged material to serve as a cap. This approach is most useful

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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where the sediments must be dredged for navigation purposes or when capping in the original location is inappropriate. Resuspension losses during removal and during placement must be assessed, but maintaining the contaminated sediments in the aquatic environment may be advantageous, especially for contaminants that are mobilized upon exposure to air. Box 5-10 illustrates the use of a CAD cell for managing contaminated sediments.

Because capping does not normally encourage degradation or transformation of the contaminants, long-term monitoring to ensure cap stability and contaminant containment would normally be required. These requirements may be more extensive for capping than for removal

BOX 5-10 Management of Contaminated Sediments with a CAD cell at Puget Sound Naval Shipyard

The Puget Sound Naval Shipyard, Bremerton, Washington, has been in operation for more than 100 years, and over that time a variety of contaminants have been introduced to the near-shore sediments. The Shipyard was placed on the National Priorities List in 1994. Sediment sampling in 1994 and 1995 identified a number of locations with contaminant concentrations in excess of State of Washington sediment quality standards. Cleanup of these sediments was complicated by the need for navigational dredging to allow large aircraft carriers to dock at the port. Navigational needs may place additional constraints upon sediment remedial approaches and may encourage removal of material beyond that required or desirable based upon environmental concerns. In this case, dredging was required even though the most desirable disposal method was deemed to be subaqueous disposal, resulting in some contaminant release and exposure during removal. Cost for the combined navigational and cleanup dredging and upland disposal was estimated to be $44 million. Ultimately, the chosen remedy was dredging followed by placement in a submerged pit and containment by a clean sediment cap (confined aquatic disposal, CAD) at an estimated cost of $14 million. This return of the contaminated sediments to a waterbody also resulted in measurable exposures.

The combined cleanup and navigational dredging produced >390,000 yd3 of material for placement in the CAD cell. The CAD cell was 36 ft deep and 400 x 415 ft in a region with 30-ft water depth. The dredging and capping project was complicated by daily tidal exchange, a tight schedule, and the need to strictly control dredging to ensure adequate CAD cell capacity. After dredged-material placement, a 1-ft interim cap was placed, followed by a second 3-ft cap. The project was completed in fall 2001, and monitoring plans evaluating effectiveness and long-term containment are being developed. To date, the dredging and sediment and cap placement appear to be successful, and the result is a remedial project that satisfies the needs of both navigation and remediation.

Additional information is available at http://yosemite.epa.gov.region10.

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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options because more of the contamination would be expected to remain in place after capping. It should be noted, however, that because of residual contamination, long-term monitoring is a requirement of any remedial approach.

Removal Technologies—Dredging

Options that involve removal of contaminated sediments from a waterbody are significantly more complicated than in situ approaches because of the train of treatment that is triggered. Removal options generally require a pretreatment step for dewatering of the dredged materials, treatment or disposal of the removed materials, and treatment and disposal of any residuals left in the waterway or produced during treatment or disposal. Often, the feasibility, cost, and potential risks are determined by these “downstream” technologies in the treatment train rather than by the dredging activity itself. The costs associated with dredging are typically less than $20 per cubic yard while treatment and disposal of the dredged material and associated water may cost $100–$1,000 per cubic yard or more.

Dredges for removal of contaminated sediment fall into one of two basic categories: (1) hydraulic dredges that primarily use suction and hydraulic action to remove sediments and (2) mechanical dredges that remove sediments by direct, mechanical action. Hydraulic dredges are generally capable of high production rates, and they minimize sediment resuspension. Mechanical dredges are generally preferred for high solids content, low water production, greater accuracy, and improved performance in the presence of debris and obstructions. Hybrid dredges have also been used that are predominantly mechanical in action but also withdraw water to control migration of a resuspension plume. For small areas or areas where water can be removed or diverted, dry excavation of the contaminated sediments may be an option. Although this simplifies the excavation process, the potentially greater mobility of contaminants after exposure to air by vaporization or oxidation should be assessed before dry excavation is undertaken. The selection of a particular dredging technology and the risks associated with dredging relative to other remedial alternatives are dependent upon site-specific factors, and only limited general guidance can be provided. Some of the site-specific factors include sediment grain size and cohesiveness, the presence of debris, access to the site, and the conditions controlling the relationship between contaminant release and the exposure and risks faced by sensitive organ-

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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isms.

Dredging is likely to be most effective when the contaminants are present in well-defined areas of relatively homogeneous, debris-free sediments. Debris, bedrock, and large areas of diffuse contamination all work to make dredging less effective as a sediment management option. Even when dredging may be the preferred option, resuspension of contaminated sediments, residual contamination, and dredged material pretreatment, treatment, or disposal requirements may limit the feasibility of the approach, as discussed below.

Resuspension. A significant factor in the selection of dredges for removal of contaminated sediments is the resuspension potential. Sediment contaminants are largely associated with the solid phase, and therefore resuspension of particles leads to resuspension of contaminants. Hydraulic dredges that are operated slowly and with care generally give rise to less resuspension than mechanical dredges or dredges operated to maximize production rate (McCellan et al., 1989). Contaminant losses from resuspension have been estimated to be as low as 0.1 percent to 0.3 percent (Kauss and Nettleton, 1999; Hayes et al., 2000). This loss can be significantly higher with dredging of fine-grained sediments, dredging at high rates without maintaining close operational control, and dredging in the presence of debris. Debris may prevent a bucket from closing during dredging by mechanical means or cause shutdown with hydraulic dredging. Both may lead to significant short-term releases of resuspended sediment. The importance of resuspension losses depends upon a variety of site-specific factors including dredging rate, water flow, and the distribution of contaminants in the sediment column.

In general, improvements (i.e., reductions) in sediment resuspension and contaminant release come at the expense of volumetric efficiency and production rates. The low resuspension rates of the enclosed cable arm dredge noted by Kauss and Nettleton (1999) were aided by continuous monitoring and in-water cycle times of 2–6 minutes during normal operation, much slower than would be expected during navigational dredging. As another example, during hydraulic hot-spot dredging in New Bedford Harbor in 1994 and 1995, efforts to control resuspension led to the capture of 160 million gallons of water, which had to be decanted and treated while targeting the dredging of only 10,000 yd3 of sediments (an average solids concentration based upon targeted sediments of little more than 1 percent) (Foster-Wheeler, 1999). Thus, the cost and time of dredging projects can be strongly influenced by the effectiveness and rate of subsequent wastewater treatment.

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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Residuals. Potentially more important than resuspension is the residual sediment contamination left on the surface after dredging. Because of the mixing that occurs with dredging operations, it is difficult to reduce the residual sediment column concentration below the depth-averaged initial concentration without significant overdredging to ensure clean underlying sediment. Under certain conditions, this layer may be very thin and of little consequence, but data are insufficient for predicting when significant residual contamination will occur. When overdredging is limited by “hardpan” or bedrock, the overall reduction in surficial sediment concentrations that can be obtained could be limited. The residual contamination may require further management by either monitored natural attenuation or more active remedial efforts.

Pretreatment and Water Treatment. Dredged material requires subsequent treatment or disposal, the first step of which is pretreatment to remove and treat excess water and reduce volume (except in the case of CAD). Although mechanical dredging does not add as much water to sediment as hydraulic dredging, some dewatering from in situ densities is generally required. The produced water content adds to the cost and complexities of dredged material disposal and may pose significant concerns for water quality upon return to the waterbody. Dredging is also normally subject to significant variations in production rate. Subsequent treatment or disposal steps often cannot maintain effectiveness if the feed rate is widely variable, and so a temporary storage system is normally required to serve as a basin for watering and volume equalization. The variations in sediment conditions and production rate also make it difficult to provide more aggressive pretreatment of dredged material—for example, by the addition of nutrients or dewatering agents in a dredged material pipeline during hydraulic removal operations.

Water separation and treatment is generally accomplished in a primary settling basin. Potential contaminant concerns in such systems are evaporation of contaminants from the exposed sediment and overlying water (Valsaraj et al., 1995) and carryover of dissolved and suspended contaminants with the effluent water (Myers et al., 1996). Secondary treatment can be accomplished via a variety of conventional approaches, but such treatment is complicated by the variations in sediment quality and by the variety of contaminants that may be present. Conventional pollutants such as oxygen-demanding organic matter and ammonia may pose more serious problems in the water than the toxic contaminants that are the primary focus of the sediment remediation—issues that should be taken into consideration and perhaps will require additional monitoring.

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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Dredged Material Disposal and Treatment. In most sediment remediation activities that have been completed or are underway, the dewatered dredged material is either left in a confined disposal facility (CDF) or transported to a landfill. If disposed of in an upland landfill, dredged material is not subjected to further treatment, other than perhaps further dewatering. For a CDF, the ultimate disposal is typically in the same facility in which primary dewatering has taken place and some treatment may occur. Particular treatment technologies that have been considered in a CDF include biodegradation, phytoremediation, and solidification/stabilization. Problems include the heterogeneity of the dredged material and the difficulty of applying biodegradation and phytoremediation to the entire column of dredged material, which may be tens of feet thick. A completed CDF may be capped for control of leachate production and vaporization, and to provide a physical barrier to direct contact by terrestrial animals. Additional development and field testing are required before the approach will receive widespread acceptance.

Although in principle the public is more supportive of treatment technologies that destroy contaminants, the costs of sediment treatment alternatives have generally not been competitive with landfill or disposal facility placement. A recent review of eight technologies (PIANC, 2000) suggested that contracts of ten or more years involving the treatment of a million or more cubic yards of dredged material per year were required for sufficient economies of scale to make the technologies commercially viable. These volumes are available only in large harbors subject to navigational dredging of sediments that cannot be disposed of in open water (e.g., New York/New Jersey Harbor) or in a few large contaminated sediment sites. It may be possible to build centralized facilities capable of processing the contaminated dredged materials from multiple sites, although significant public acceptance and regulatory barriers would need to be overcome. A second consideration is that treatment technologies require development of a market for the effluents from their processes and regulatory standards that define the acceptability of these effluents for certain uses. Finally, treatment technologies, even those that destroy contaminants, may generate residuals that are released to the environment or need to be disposed of in landfills.

Measuring Performance

Regardless of the remedial option chosen, the most direct indicator

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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of remedial performance is monitoring of the resource at risk from sediment contamination, which is often the body burden of contaminants in edible fish. Monitoring concentrations in fish over time can be used to indicate remedial performance and to compare remedial alternatives as shown in Figure 3-4. A relative comparison of the potential risks, including the risk of no remedial efforts, can be effective in identifying absolute risks. A convenient comparison for any active remedial approach is the effectiveness or potential risks of the approach relative to those expected with natural attenuation processes. Prognostic models are required to predict contaminant processes far into the future to effectively compare outcomes from different remedial alternatives. The ability to accurately predict future conditions in sediments, including identification of key sources of uncertainty, is reviewed in Reible et al. (2002).

Exposure and risk in sediments are largely limited to the near-surface sediments. The layer subject to erosion, even during large storm events, rarely exceeds a few centimeters. As indicated previously, bioturbation is typically limited to the upper 10–15 cm of sediment. In addition, sorption-retarded advection and diffusion in sediments are generally so slow that only the upper few centimeters contribute significantly to flux. Thus, exposure and risk from sediment contaminants is largely related to surficial sediment concentrations. As an indicator of average surficial sediment concentration, the surface area weighted average concentration (SWAC) has been used at some sites (e.g., in the Sheboygan ROD, the Fox River RI/FS, and the Shiawassee ROD) as a surrogate measure of exposure and risk. For those sites where a large fraction of the contaminant mass is deeply buried, the use of SWAC metrics for remediation will lead to significantly different remedial designs than total concentrations. These metrics are most useful and accurate where exposure is controlled by widespread contamination over large surface areas and not where erosion or contaminant release from small hot spots control risk.

The risks associated with different sediment remediation approaches may vary with time. Dredging may increase short-term risk through resuspension of the contaminants, incomplete water treatment, or elevated residual surface concentrations. In situ approaches such as capping, however, may give rise to an elevated risk far into the future. In such case, time-integrated measures of performance may prove useful. For example, the time integral of predicted fish body burdens may represent an average indication of exposure. Similarly, the integral of the SWAC may indicate average exposure for risks controlled by average surficial sediment concentrations.

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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TREATMENT TRAINS

Much of the literature and guidance that describe innovative technologies address one particular technique and its application for a specific class of contaminants. For example, the Naval Facilities Engineering Service Center (NFESC) website provides the user a wealth of information on individual technologies that are grouped by biological, physical/chemical, and containment and removal technologies. However, most hazardous waste sites, including Navy sites, are rarely contaminated by a single chemical group. Rather, mixtures of contaminants with varying physical and chemical properties like chlorinated solvents (VOCs), petroleum hydrocarbons (fuels, oils, and polycyclic aromatic hydrocarbons), PCBs, and one or more heavy metals are much more likely to be present. Furthermore, the contaminants typically reside in different media, such as surface soils, sediments, aquifer solids, groundwater, and surface water. A single remedial technology is normally effective at treating only a subset of the contaminants in a waste mixture or treating one type of media. Two or more remedial technologies applied in combination or sequentially are likely to be necessary to attain cleanup goals for a waste mixture and multimedia contamination. From this perspective, each remedial technology should be viewed as a unit operation that can be linked as part of a treatment train. Creating treatment trains should be considered part of optimizing or adding to existing remedies— a key decision-making point in ASM. The same treatment train strategy is used to treat drinking water or wastewater. In drinking water treatment, processes such as coagulation, sedimentation, filtration, and disinfection are usually combined to achieve potable water. In wastewater treatment, particle removal, biodegradation, and disinfection are often coupled to meet effluent standards.

Another factor contributing to the need for treatment trains is that each waste site is unique. The efficacy and adequacy of any remedial option depends on site-specific characteristics, such as the chemical properties, horizontal and vertical extent of contamination, and hydrogeologic setting. Often, multiple technologies need to be employed for managing the risks at waste sites because of the dynamic and complex nature of the subsurface. The behavior and response with one technology often improve our knowledge of the site, which helps guide the implementation of a second technology.

The concept of treatment trains is now commonly implemented for remediation of certain classes of contaminants. The NFESC website identifies four remedial technologies under the category of “Combined

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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Mechanism,” including constructed wetland, Lasagne™ Process, natural attenuation, and vacuum-enhanced recovery (bioslurping). For example, bioslurping uses vacuum extraction together with in situ biodegradation to remove contaminants. The vacuum extraction component of bioslurping aims to remove the bulk of the volatile compounds and is capable of removing separate-phase globules of contamination. The introduction of oxygen can stimulate the biodegradation of residual contamination. The FRTR website provides reference guides for common treatment trains associated with the eight contaminant groups listed in Table 5-2. For example, free product recovery, venting/air stripping, and in situ biodegradation can be coupled to effectively manage gasoline spills (Lee and Raymond, 1991).

The most common treatment train that is being invoked for addressing organics-contaminated soils and plumes in groundwater is a source treatment technology in conjunction with monitored natural attenuation (MNA) (see Box 5-11 for a description of MNA). MNA can be a primary (stand-alone) technology in some cases, usually for petroleum hydrocarbons. However, remediation of petroleum hydrocarbon contamination is not the focus of this report. Consequently, the application of MNA discussed here is its use as part of a treatment train. The Navy’s approach of coupling a variety of source removal/containment technologies with MNA is consistent with national trends and is described in greater detail below.

At sites with metal contaminants, two or more remedial options applied sequentially to contaminated soil often increase the effectiveness while decreasing the cost of remediation (EPA, 1997b,c). Treatment trains for metal contaminants include soil pretreatment, physical separation designed to decrease the amount of soil requiring treatment, and treatment of process residuals or off-gases. A promising treatment train for remediation of metal-contaminated soil is the combination of electrokinetics and phytoremediation. Electrokinetics is used to remove metals from deep soil and groundwater, whereas phytoremediation is effective at removing metals in surface soils.

The above examples of treatment trains pertain to contaminated soils and groundwater, but the concept is equally applicable to contaminated sediments, especially when the remedial options involve removal of contaminated sediments from a waterbody. Removal options involve not only dredging, but other technologies as well to manage the dredged material, the water produced, and any residuals left in the waterway. Combining processes can also be effective for in situ treatment of contaminated sediments, such as capping and natural attenuation. Cap installa-

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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BOX 5-11 Description and Application of Monitored Natural Attenuation for Groundwater Contamination

Natural attenuation refers to a variety of natural processes that result in a decrease in contaminant concentration and mass. These processes can be physical (dispersion, dilution, sorption, volatilization), chemical (oxidation, reduction, immobilization by precipitation), or biological (biodegradation by indigenous microorganisms and perhaps plants). Monitored natural attention (MNA) refers to the use of natural attenuation as a remedial option. NRC (2000) provides an extensive review of MNA and suggests that proper application of MNA should include three basic steps:

  1. Develop a conceptual model that characterizes the site (e.g., where the contaminant is, how the groundwater is moving, etc.) and identifies what processes, if any, could potentially be responsible for decreasing the concentration and mass of contaminants.

  2. Gather sufficient site-specific data to demonstrate that contaminant loss is due to a given attenuation process (e.g., biodegradation, immobilization of heavy metals by precipitation), and determine whether natural attenuation is sustainable and will meet remediation goals.

  3. Implement a long-term monitoring program to ensure attenuation processes continue to occur and remediation goals are being met.

NRC (2000) lists characteristics required for a comprehensive protocol for MNA that cover three broad subject areas: community concerns, scientific and technical issues, and implementation issues. The NRC reviewed 14 of the available natural attenuation protocols, including the Navy guidance document for MNA (Dept. of the Navy, 1998) and found that none met all the characteristics of a comprehensive protocol. The principal findings concerning the current state of practice of MNA can be summarized as follows:

  • MNA is an established remedy for a limited number of contaminants— primarily BTEX and perhaps chlorinated ethenes under some conditions (NRC, 2000).

  • MNA should only be accepted as a remedial option when attenuation processes have proved to be working and sustainable (see items 1–3 above); it should never be considered a default or presumptive remedy or “no-action” alternative.

  • Rigorous protocols need to be developed to ensure that MNA is analyzed properly.

  • MNA cannot be achieved solely by dilution and/or dispersion.

  • The reaction processes must avoid accumulation of harmful daughter products.

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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  • Affected communities must be involved in the process and have access to all relevant information (e.g., proof that the attenuation processes are working and sustainable).

(Involving affected communities should apply to all remediation activities.)

Measuring Performance

Measuring the efficacy of MNA is similar to measuring the efficacy of enhanced bioremediation. Effective long-term monitoring is absolutely required to ensure that attenuation processes continue to result in reduction in contaminant concentration and mass and in the protection of human and environmental health. Such monitoring may be required for many years, even decades.

Technology Evaluation

Based on the knowledge of natural processes that can affect the movement and fate of contaminants in groundwater, NRC (2000) summarized the likelihood of success of MNA for various classes of contaminants. Only for BTEX, low-molecular-weight alcohols, ketones, esters, and methylene chloride is the likelihood of success rated as “high.” In some cases metals can be immobilized. MNA may be appropriate for sites with contaminant classes with a lower likelihood of success, such as most halogenated aliphatic and aromatic compounds, nitroaromatic compounds, and toxic metals, but evidence for success will usually require extensive effort in site characterization, laboratory studies, modeling, and monitoring. Because natural attenuation processes are always site-specific, information contained in NRC (2000) can only be used as a general guide for the potential of MNA to be successful. Each site must be studied individually to determine if MNA is effective for remediation and for controlling risks from contaminated groundwater.

The Science Advisory Board (SAB) of EPA recently released Natural Attenuation in Groundwater (EPA, 2001c), a report that builds on NRC (2000). The SAB states that MNA, when properly used, is a “knowledge-based remedy in which the engineering informs the understanding, monitoring, predicting, and documenting of the natural processes, rather than manipulating them.” It also makes specific recommendations as to how the science base of MNA needs to be enhanced by EPA for better application to chlorinated solvents, underground storage tanks, inorganics, and sediments.

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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tion stabilizes and contains contaminants for immediate reduction in risk, and then slower natural attenuation processes (e.g., biodegradation) reduce contaminant mass over the long term.

Treatment Trains and Source Removal

A feature of contaminated sites that necessitates the use of treatment trains is that conceptually, a waste site consists of two distinct components. The first component is a contaminant source area where the bulk of the contaminant mass is usually located. The second component is the plume of dissolved contaminants that emanates from the source area. The source/plume concept for subsurface contamination has been well documented in previous reviews (NRC, 1994; Cherry et al., 1997). The approaches and prospects for cleaning up the plume of dissolved contaminants are much different than they are for cleaning up the source areas, necessitating a coupling of treatment technologies that usually involves some measure of source control along with some measure to restore the contaminated groundwater.

Contaminant source areas include near-surface sources such as surface spills, leaking drums and storage tanks, and landfills, but they also include deep subsurface pools or ganglia of NAPLs and metals that have precipitated in mineral phases having low solubility. Sorbed contaminants also constitute a long-term source of dissolved-phase contamination. Source areas are difficult to characterize and locate in their entirety because of poor knowledge of site operating history along with the complexity of the subsurface (NRC, 1994, 1999b). The source areas at waste sites persist for a very long time and are capable of contaminating groundwater over time scales of decades to centuries.

One approach to managing such sites is immobilization or containment of contamination by hydraulic and/or physical barriers, followed by restoration of the dissolved plume (NRC, 1994). The success of this two-step treatment train (source containment and aquifer restoration) relies on maintaining the integrity of the containment system; Jackson (2001) points out that ensuring containment is both technically difficult and costly to achieve at many waste sites. Another concern with such approaches is that the source remains in the subsurface, so there is a long-term threat of slow dissolution of contaminants into groundwater should the containment system fail. Consequently, source removal is perceived by many stakeholders to be a more desirable cleanup approach. Technologies appropriate for source removal are listed in Table

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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5-2. As reviewed in the previous sections, popular technologies being used by the Navy for source removal include in situ chemical oxidation, thermal treatment, enhanced bioremediation, and methods to extract inorganic contaminants.

A reduction in contaminant mass from a source zone is expected to provide several benefits including a decrease in cleanup time, a possible reduction in risk, a decrease in the extent of contaminated groundwater, and improvement in the performance of natural attenuation processes (Sale and McWhorter, 2001). However, there is no consensus among the technical community on the benefits derived from partial contaminant mass removal from source zones. The research results presented in Box 5-12 illustrate that under the assumption of aquifer homogeneity and uniform groundwater flow, even substantial amounts of mass removal may have little impact on the time for cleanup, on groundwater concentrations, and on the exposure pathways for a site. In contrast, other investigators (also Box 5-12) present modeling results for heterogeneous flow fields that demonstrate significant reductions in contaminant fluxes to groundwater (and corresponding significant reductions in risk) for modest degrees of source removal. Additional studies are certainly needed to resolve the disagreement that currently exists regarding the relationship between partial source removal and its impact on contaminant fluxes to groundwater and site risk.

Source removal technologies at contaminated sites are normally implemented without an understanding of how much mass removal is needed to be effective (e.g., in meeting water quality goals, in restoring the plume, and in reducing risk). It is recommended that the Navy perform site-specific analyses of the effectiveness of source zone mass removal to better guide and justify the selection of source removal technologies being implemented at Navy sites. This analysis will also help the Navy determine if enough of the source mass can be removed to warrant the expense of implementing the technology.

Compatibility of Technologies in Treatment Trains

In the selection of technologies that are combined or sequenced into a treatment train, the impact of one process on the performance of other processes must be considered. In some instances, the combination of technologies does not cause any compatibility issues. For example, MNA or enhanced bioremediation could follow downgradient from a PRB as long as the products from the PRB (e.g., Fe(II) species, high pH,

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
×

BOX 5-12 Impact of DNAPL Source Zone Treatment upon Contaminant Concentration and Flux

There is disagreement among researchers on the relationship between degree of contaminant mass removal from source zones and contaminant fluxes to downgradient groundwater. Sale and McWhorter (2001) demonstrate the impact of DNAPL source zone treatment upon downgradient contaminant concentrations. They conceptualize the source zone as containing multiple subzones of DNAPL, as shown in Figure 5-8. A key assumption in this analysis is that the aquifer is homogeneous and thus groundwater flow is uniform within the DNAPL source zone.

The model developed by Sale and McWhorter computes the steady-state concentration distribution in the downgradient groundwater resulting from rate-limited dissolution of DNAPL. As clean groundwater contacts a subzone, DNAPL dissolves at a rate that is proportional to the difference between the aqueous solubility and the local solute concentration. Once dissolved, the fate of the DNAPL is controlled by advection and longitudinal and transverse dispersion. Sale and McWhorter use their model to explore how the steady-state DNAPL concentration is affected by various parameters (e.g., velocity, dispersivity) and also by the size and configuration of the source zone.

Because influent groundwater is not contaminated, this model predicts that most dissolution occurs in the upgradient region of the source zone, because that is where the “driving force” for dissolution is greatest. Moreover Sale and McWhorter find that dissolution rates are sufficiently fast so that groundwater solute concentrations increase to near the solubility limit after only a short travel distance, thus effectively shutting down dissolution from downgradient portions of the source zone. Referring to Figure 5-8, this implies that the pollutant concentration exiting the source zone will remain close to its solubility limit even if a

FIGURE 5-8 Schematic of a DNAPL source zone. SOURCE: Reprinted, with permission, from Sale and McWhorter (2001). © (2001) American Geophysical Union.

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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large portion of the DNAPL mass in the source zone is eliminated. The modeling results indicate that removal of the vast majority of the DNAPL will likely be necessary to achieve significant near-term reductions in groundwater concentrations and reductions in source longevity (Sale, 1998).

Other investigators have argued that such conclusions regarding the efficacy of source-zone mass removal are overly pessimistic because of the assumption of a uniform flow field. In particular, Rao et al. (2002) present modeling results for heterogeneous flow fields that demonstrate significant risk reduction for more modest degrees of source zone treatment. In this model, a random distribution of both hydraulic conductivity and DNAPL saturation was assumed in the source zone; the model also allows for statistical correlation between hydraulic conductivity and saturation. During a simulation, the researchers allow the DNAPL to dissolve away, and at various times they compute the downgradient mass flux and the DNAPL mass remaining. Rao et al. (2002) argues that contaminant mass flux across the downgradient boundary of the source zone is a more meaningful metric for risk than simply the groundwater concentration.

Figure 5-9 shows the results of the Rao et al. model simulations. The fractional flux reduction is plotted on the ordinate, and the fractional mass reduction is plotted on the abscissa. For a given reduction in DNAPL mass, the largest flux reduction is for the negative correlation case where the higher DNAPL saturations are associated with the lower-velocity regions (i.e., less-permeable zones). Achieving the same level of flux reduction for the positive correlation case requires a larger fractional mass reduction since the DNAPL is preferentially located in the high-velocity regions (i.e., more-permeable zones) that are making a large contribution to the flux. Because DNAPL source zone treatment technologies tend to preferentially target the more permeable portions of the aquifer, the conclusion based upon these modeling results is that contaminant mass flux could be significantly reduced even for modest reductions in the DNAPL mass.

FIGURE 5-9 Mass flux reduction versus the fraction of mass removed. SOURCE: Rao et al. (2002).

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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and associated geochemical changes) do not inhibit native microbes. Enhanced bioremediation can be used in conjunction with surfactant flushing, with a variety of oxidation processes, and with low-temperature physical processes such as soil vapor extraction. Because the properties of inorganic contaminants differ markedly from organic contaminants, remediation technologies for inorganics are potentially compatible with a variety of other remediation techniques. Physical separation and extraction processes for inorganics could reduce the amount of material requiring subsequent treatment for organics. If containment in the source zone is effective at immobilizing both inorganics and organics, then the spread of contamination will be eliminated and the remediation of the groundwater plume will be facilitated.

Many of the approaches to managing contaminated sediments are compatible with each other when they are used on different portions of the site. For example, in situ capping in a portion of a site is not likely to interfere with dredging or natural attenuation in other portions of the site. Certain management approaches may also be compatible for application on the same portion of a site. For example, capping may be a useful adjunct to dredging to eliminate any negative consequences of the residual contamination. It may also be appropriate to employ capping as an interim risk reduction measure until additional remedial decisions can be made. However, if dredging is subsequently implemented, the volume, cost, and complexity of the dredging process would likely increase. At many contaminated sediment sites, natural attenuation is necessary as a complementary remedial approach to ultimately achieve risk-based goals. Indeed, it is expected that at the Lower Fox River site risk-based concentration goals will not be achieved until after decades of natural attenuation subsequent to the implementation of the initial remedy (WDNR, 2001).

In other instances, caution must be exercised in combining technologies. This is especially critical when using MNA subsequent to a source removal technology, as source treatment efforts may directly and adversely impact the microbial activity and hence the performance of MNA. One possible detrimental impact is the alteration of the electron acceptor available for microbial metabolism. For example, active source removal technologies that introduce oxygen to the subsurface are likely to shut down the anaerobic biodegradation processes necessary for natural attenuation of chlorinated solvents and that are operative for certain petroleum hydrocarbons and inorganic contaminants (see Box 5-13). Technologies that could introduce oxygen include in situ chemical oxidation, in-well stripping, air sparging, soil vapor extraction, cosolvent or

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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BOX 5-13 Impact of Source Removal on Natural Attenuation of Perchloroethylene

A glacial outwash aquifer in Minnesota was contaminated with PCE from a former dry cleaner supply company (Ferrey et al., 2001). The groundwater chemistry at the site is conducive for reductive dechlorination of PCE [e.g., low dissolved oxygen (DO) concentrations, anaerobic electron acceptors, and reducing conditions]. Consequently, the initial remedial strategy for the site was to install a vacuum vaporizer well in the source area to remove high levels of PCE and to rely on MNA for plume treatment. A vacuum vaporizer well uses in-well sparging with air to strip chlorinated solvents from recirculating groundwater. Operation of the vacuum vaporizer well reduced source area groundwater PCE concentrations from 9,900 to 25 µg/L, but elevated the DO levels to between 2.9 and 3.3 mg/L. DO concentrations prior to PCE source removal were less than 0.7 mg/L. The aquifer aeration by the vacuum vaporizer well caused elevated levels of PCE transformation products in downgradient monitoring wells. At a monitoring well 360 feet downgradient of the vacuum vaporizer well, TCE concentrations increased from <10 to 35 µg/L, cis-DCE concentrations increased from 70 to 370 µg/L, and vinyl chloride concentrations increased from below the detection limit to 83 µg/L. The latter finding is especially problematic as vinyl chloride is more harmful than PCE, the parent compound. After operating for three years, the vacuum vaporizer well was shut down, and concentrations of TCE, cis-DCE, and vinyl chloride returned to pre-sparging levels. The conclusion from this study is that the benefit of a remediation system that alters the groundwater chemistry (i.e., introduction of air to create oxidative conditions) should be balanced against the potentially negative effect that the remedial technology may have on the natural attenuation mechanisms already existing.

surfactant flushing, and thermal treatment. A second possible negative impact is that source removal could remove a contaminant that is used to enhance the biodegradation of another contaminant. An example is the inadvertent removal of petroleum hydrocarbons, phenols, alcohols, or ketones that are serving as primary substrates for microbes involved in the intended biodegradation of chlorinated solvents in the downgradient groundwater plume, which could slow down or completely stop natural attenuation of the chlorinated solvents. Another negative impact that can arise in coupling source control with MNA is alteration of the flow field or mobility of the contaminant, which can enhance contaminant spreading, reduce time available for attenuation reactions, and sterilize the site for an indeterminate period. Potential effects of other remediation activities on MNA are tabulated in a recent NRC report (NRC, 2000, Table 3-2).

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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The first step in establishing the compatibility between a source removal technology and MNA is to create a conceptual model for the site. Development of a conceptual model is an iterative process that involves characterizing the groundwater flow system, delineating the contaminant source and plume, and identifying the reactions contributing to natural attenuation. Data generated by site monitoring are then coupled to the conceptual model to establish if adequate loss of the contaminants is possible and to identify the processes responsible for this loss. Techniques for this data analysis include graphical and statistical analysis of trends in concentrations of contaminants and substances, mass balances to verify reaction stoichiometry, simple modeling of solute transport, and comprehensive flow and solute transport models (NRC, 2000). A mass balance analysis can be used to determine if a given source removal technology will unfavorably alter the chemical environment for natural microbial reactions (e.g., modify electron acceptor conditions). Furthermore, data analysis is needed to establish whether MNA can achieve the desired remediation goal at the appropriate downgradient receptor(s). For example, solute transport modeling can be used to determine if the concentrations emanating from the source zone after treatment are low enough for MNA to be sufficiently protective of human health and the environment.

***

In summary, caution needs to be exercised when combining treatment technologies in order to ensure compatible performance. Incompatibility issues are especially important when source treatment is coupled with MNA as the primary site management approach. In theory, if one can completely delineate the source area and succeed in removing or destroying most of the contaminant mass, then a significant benefit can be achieved when negative effects on natural attenuation are not expected. However, source treatment can interfere with the present or future performance of natural attenuation, principally through disruption of environmental conditions required for the biodegradation reactions (e.g., availability of electron donors and acceptors) and destruction of the microbes (e.g., sterilization via chemical oxidation and thermal treatment). In the former situation, source treatment is undesirable. The latter situation requires selection of an alternate technology. If the attenuation reactions are sustainable for a long period of time, then MNA alone may serve as a long-term remedy for the site.

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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MAJOR CONCLUSIONS AND RECOMMENDATIONS

This chapter has reviewed several innovative technologies that are applicable to the most recalcitrant contamination scenarios found at Navy installations (and other federal facilities). The information is most relevant to those stages of ASM that involve the optimization, replacement, and addition of remedies, particularly MDP3 and MDP4. Although all the technologies have their place, there is no clearly superior single remedy that can address even a small fraction of the Navy’s contamination problems. Remedy selection must remain site-specific. In general, for the innovative technologies reviewed here, there is a lack of refined evaluation procedures and peer-reviewed literature on their cost and performance—partly because their development is vendor-driven— making it impossible to fully evaluate their success or efficacy. Thus, as mentioned in Chapter 4, further testing of innovative or new experimental technologies at selected sites is needed, both for site-specific application and if the results are likely to improve cleanup activities at other sites. When evaluating remedial options and technologies, the full life cycle of the technologies and the management and disposition of all residuals that may be generated by the technologies should be considered.

A routine part of ASM is reevaluation of the current remedy design for possible optimization. Optimization can be as simple as ensuring that system components are still appropriate and are operating at design efficiency. Formal mathematical optimization can be used to evaluate well configuration and pumping rates in pump-and-treat or soil vapor extraction systems for potential cost savings. In the course of taking such action, the remedy must remain protective of human health and the environment. More detailed instruction for site managers on how to optimize various remedial systems is required, because existing information in DoD guidance manuals is presented in very general terms and can be used only by persons who are already quite technically knowledgeable in the remediation field. Recommendations below pertain to specific innovative technologies that hold promise for addressing contamination scenarios identified by the Navy as problematic.

In situ oxidation holds promise for removing organic compounds from the subsurface, although greater confidence in this technology awaits the creation of standardized bench-scale and field-testing protocols. Such protocols should specify that early site screening be conducted for compatible geochemical, natural attenuation, and hydrogeologic conditions. Bench-scale tests should evaluate multiple oxidant

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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dosage rates to include ones that can be realistically implemented at the field scale. Oxidant requirements should be calculated based on the scale of the target treatment volume. Field testing should include an experimental control to assist in the evaluation of contaminant dilution, displacement, and rebound, with rebound being utilized as the ultimate determinant of success or failure. A sufficient time period (often greater than one year) must elapse to allow rebound effects to be exhibited, particularly for sites with low relative groundwater seepage velocities (<100 feet/year) and/or with multiple soil layers across the contaminated region. The Navy should compile the lessons learned and the technical data obtained during Navy field applications of in situ oxidation.

Thermal treatment technologies provide aggressive and potentially successful remediation options for subsurface contaminants. Thermal technologies usually involve production or transport of steam through the subsurface, with the potential to volatilize contaminants. The flow paths of the steam and mobilized contaminants are determined by the heterogeneity and permeability of the subsurface matrix, which are also sensitive to degrees of water saturation. Thus, the application of thermal treatment technologies should be approached in a site-specific fashion, with a primary focus on site characterization and the design of effective vapor capture and dewatering strategies, particularly for sites where contaminants could exist in the saturated zone.

Permeable reactive barriers can effectively treat a limited number of groundwater pollutants under well-defined hydrogeologic conditions. These pollutants include PCE, TCE, cis-DCE, and perhaps Cr(VI). The technology has been applied in the field for approximately seven years, so data on long-term performance are limited. Hydraulic capture remains a key issue in determining effectiveness, and the long-term integrity of these systems is not known.

Enhanced anaerobic bioremediation has considerable potential for treating various types of organic contaminants in the subsurface, although it is not yet a proven, field-tested technology. Enhanced bioremediation can destroy contaminants via mineralization or conversion to benign organics, it can be applied in situ, and it is less expensive than other treatment technologies. Initial applications offer promise for in situ bioremediation of PCE and TCE. Significant questions remain concerning electron donor selection and delivery, long-term effectiveness, and cost.

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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Because metal contaminants cannot be destroyed and their behavior and speciation is strongly coupled to site-specific conditions, remediation approaches for metal contaminants remain a challenge. Given that metals are frequently reported contaminants of concern at Navy sites, the Navy should devote resources to accelerate the development of and field-scale testing of cost-effective technologies for mitigating risks from metal contaminants.

Presently, the only options that are routinely available for managing contaminated sediment include natural attenuation, capping eitherin situor after dredged material removal, and dredging with disposal in confined disposal facilities or in upland landfills. Dredged material treatment options are under development and may be commercially available and viable in the future. Because of the large volumes of sediment dredged to maintain navigation projects in many harbors, it is likely that economies of scale will encourage substantial application of these technologies.

Treatment trains for the remediation of many contaminated sites is an important component of adaptive site management. Most sites are contaminated with multiple contaminants that may require different treatment processes. Treatment trains can often increase the effectiveness in achieving remedial goals while decreasing the cost of remediation. A common treatment train is source control in conjunction with monitored natural attenuation. This approach must be implemented with caution as source removal can disrupt microbial metabolism via redox changes, removal of primary substrates, and creation of inhibitory conditions.

Site-specific analyses of the effectiveness of source removal are needed to better guide and justify remedy selection. Additional studies including controlled field demonstrations are needed to evaluate the benefits (e.g., to groundwater quality) derived from partial contaminant mass removal from source zones and the compatibility of such treatment with natural attenuation. This analysis will also help PRPs determine if enough of the source mass can be removed to warrant the expense of implementing the technology.

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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GLOSSARY


Air sparging.

Removal of volatile chemicals from the subsurface by injecting air beneath the water table and extracting vapors with vacuum and sometimes subjecting the vapors to subsequent treatment.


Bioremediation.

Exploitation of the metabolic activities of microorganisms to transform or destroy contaminants. Enhanced bioremediation refers to the addition of carbon and energy sources and/or electron acceptors to stimulate the growth of indigenous microbes and increase the rate of intrinsic biodegradation. Enhanced aerobic bioremediation of petroleum hydrocarbons is an established treatment, while enhanced aerobic cometabolism of chlorinated aliphatic hydrocarbons is an emerging technology. Enhanced anaerobic bioremediation is an innovative technology that involves adding compounds to stimulate reductive dechlorination of chlorinated aliphatic hydrocarbons.

Bioslurping.

Simultaneous application of vacuum-enhanced extraction/recovery, vapor extraction, and bioventing to remove/transform contaminants, particularly LNAPLs.

Bioventing.

Passing air through the soil to stimulate biodegradation of organic material with minimum volatilization.


Capping.

Providing an impermeable barrier to surface water infiltration into contaminated soil to reduce further contaminant release and transport, or controlled placement of a clean, isolating material cover over contaminated sediments without relocating or causing major disruption to the original bed.

Chemical Oxidation/Reduction.

Use of chemical oxidants or reductants to oxidize or reduce organic and inorganic contaminants.

Circulating wells.

Creation of a groundwater circulation cell around a well through which contaminated groundwater is cycled and treated by the action of an air stripping process.

Composting.

Bioremediation of contaminated soils or sediments in the presence or absence of oxygen.

Confined disposal.

Placing dredged materials within diked near-

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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shore, island, or land-based confined disposal facilities and enclosing with a cap to provide isolation.

Cosolvents and surfactants.

Mobilization or solubilization of NAPLs or sorbed contaminants for facilitated removal after injection and flushing of cosolvents or surfactants into the vadose and saturated zones.


Dual-phase extraction.

Use of a screened vertical well with or without a drop tube under applied vacuum to extract contaminated vapor and both aqueous and nonaqueous liquid above and below the water table, possibly augmented with air injection.

Dynamic underground stripping.

Combination of steam injection and electrical heating for vacuum extraction of nonaqueous phase liquid contaminants from the subsurface.


Electrokinetics.

In situ process that separates and extracts inorganic and organic contaminants from saturated and unsaturated soil, sediments, and groundwater under the influence of an imposed electrical field.

Incineration.

Ex situ thermal process primarily for the destruction or removal of organic compounds from contaminated matrices.


Hydraulic dredging.

Employing centrifugal pumps to draw up sediment in a liquid slurry form for transfer through a pipeline to a placement site.


In situ heating.

Raising the temperature of soils by electrical resistance, microwave, and/or radio frequency heating to increase volatility of contaminants and to form steam for vapor-phase transport.


Landfill disposal.

Placing contaminated materials, with or without pretreatment, in or on the land with liners and covers or caps for containment.

Land treatment.

Managed treatment and disposal involving tillage of contaminated materials into the surface soil to allow natural assimilation for conversion and containment.


Mechanical dredging.

Using bucket-like equipment to scoop up sediment by mechanical force to minimize sediment dispersion and other

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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effects on sediment properties prior to transfer to the placement site.


Natural attenuation.

In situ reduction in mass or concentration of contaminants in groundwater, soil, or surface waters from naturally occurring physical, chemical, and biological processes.


Permeable reactive barriers.

Emplacement of reactive materials in a subsurface structure designed to intercept a contaminant plume, provide flow through the reactive media, and transform contaminants.

Phytoremediation.

Use of natural or engineered vegetation for in situ plant uptake and containment of contaminated soils, sediments, and water.

Pump-and-treat.

Use of a series of wells to pump large amounts of contaminated groundwater to the surface for treatment before ultimate surface discharge or reinjection.


Slurry phase bioremediation.

Biological treatment of contaminated solids and groundwater in suspended growth bioreactors.

Soil flushing.

In situ soil treatment of contaminants using chemical amendments and fluid pumping to mobilize and recover contaminants.

Soil vapor extraction.

Use of induced air flow through the unsaturated zone to vacuum-remove volatile compounds from soil in the vapor phase with subsequent treatment and discharge to the atmosphere.

Soil washing.

Ex situ, water-based process employing chemical and physical extraction and separation to remove contaminants from excavated soil.

Solidification/Stabilization.

Reduction of hazard by converting contaminants into less soluble, mobile, or toxic forms using chemical, physical, and/or thermal processes.

Steam flushing.

Injection of steam into the saturated and unsaturated zones to mobilize and volatilize contaminants before recovery through extraction wells and ex situ treatment.


Thermal desorption.

Direct or indirect ex situ use of heat to physi-

Suggested Citation:"5. Technology Overview." National Research Council. 2003. Environmental Cleanup at Navy Facilities: Adaptive Site Management. Washington, DC: The National Academies Press. doi: 10.17226/10599.
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cally separate and transfer contaminants from soils and sediments before subsequent collection and treatment.


Vertical barrier wall.

Isolation of contaminant source from flowing groundwater with confinement trenches, grouts, or sheet piling to reduce risk and enhance opportunities for remediation.

Vitrification.

Application of electrical heating to elevate temperature sufficiently to melt the soil and form a glass upon cooling for extraction/destruction and containment of contaminants.

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The number of hazardous waste sites across the United States has grown to approximately 217,000, with billions of cubic yards of soil, sediment, and groundwater plumes requiring remediation. Sites contaminated with recalcitrant contaminants or with complex hydrogeological features have proved to be a significant challenge to cleanup on every level—technologically, financially, legally, and sociopolitically. Like many federal agencies, the Navy is a responsible party with a large liability in hazardous waste sites.

Environmental Cleanup at Navy Facilitites applies the concepts of adaptive management to complex, high-risk hazardous waste sites that are typical of the military, EPA, and other responsible parties. The report suggests ways to make forward progress at sites with recalcitrant contamination that have stalled prior to meeting cleanup goals. This encompasses more rigorous data collection and analysis, consideration of alternative treatment technologies, and comprehensive long-term stewardship.

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