Contaminants in the Subsurface Source Zone Assessment and Remediation (2005) / Chapter Skim
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4 Objectives for Source Remediation
4 Objectives for Source Remediation
Pages 125-177
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From page 125... ...
The project manager and other stakeholders must know the full range of site remedial objectives, their relative priorities, and how they are defined operationally as specific metrics, in order to determine whether source remedial actions will contribute to meeting objectives for the site. The primary purpose of this chapter is to describe the many objectives possible at sites for which source remediation is a viable option, many of which have been institutionalized within regulatory, risk assessment, and economic frameworks for site cleanup.
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From page 126... ...
This is illustrative of a situation in which an explicit operational statement of site objectives (e.g., a reduction of human health risk as estimated by the procedures specified in EPA's Risk Assessment Guidance for Superfund, RAGS) , if made prior to the attempt at source remediation at this site, might have led to a decision not to attempt source remediation.
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From page 127... ...
An unambiguous delineation of objectives and metrics will, however, allow the decision on source remediation to be more clearly evaluated. The relevance of source remediation to different stakeholders' objectives can be identified in advance, and progress can be measured.
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From page 128... ...
. The distinction between absolute and functional objectives is important, because trade-offs among different absolute objectives cannot be accomplished at a technical level.
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From page 129... ...
. On the other hand, it may serve the higher-level functional objectives of limiting human health risk to an acceptable level (by reducing exposure potential)
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From page 130... ...
These include mass removal, concentration reduction, mass flux reduction, reduction of source migration potential, plume size reduction, and changes in toxicity or mobility of residuals. The specification of metrics for performance evaluation with respect to these objectives is typically easy since the objectives are related to physical, measurable properties.
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From page 131... ...
The obvious associated metric is contaminant concentration. Like mass removal, concentration reduction can serve as an absolute objective (e.g., meeting MCLs)
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From page 132... ...
Such concentration reductions have ranged over one half to two orders of magnitude. While it is possible that local concentration levels in the treatable zones may be substantially reduced by mass removal, local concentrations in less accessible DNAPL zones will likely remain high (at or near aqueous solubility levels)
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From page 133... ...
A more extensive numerical investigation of this sort suggests that a twoorders-of-magnitude mass flux reduction may be achievable following partial source zone mass removal in Type I media (Lemke et al., 2004)
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From page 134... ...
Alternatively, in Figure 4-1 maximum saturations of PCE do not exceed 31 percent. The potential influence of source zone mass removal on DNAPL distributions is illustrated in Figures 4-1(B)
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From page 135... ...
135 t et 0.10 100 surfactan Lemke 0.08 and after ppm (2003)
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From page 136... ...
136 satu- from 0.10 100 ted 8 PCE Adap 0.08 6 initial ppm (A)
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From page 137... ...
Concep tually, one might view these flux-averaged concentrations as more representative of the risk to downstream receptors, since they incorporate the dilution effect, similar to that which would be measured by a fully screened well directly downstream of the source. Another way to present the results of these types of analyses is to plot the relationship between mass removal and mass flux reduction.
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From page 138... ...
The mass flux reduction "plateau" exhibited by the Figure 4-2 scenario is associated with the persistence of a pool that contains much of the PCE mass. Although limited simulations are available to extrapolate to other aquifer types, release scenarios, and/or remediation technologies, the illustrations given above suggest that source mass removal may offer substantial benefits, if mass flow rate is the metric of choice.
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From page 139... ...
field characterization efforts or due to aging of the contaminant that leads to alterations in the interfacial properties controlling the capillary forces. As discussed in Chapter 2, the capillary force that acts to retain DNAPL in a pore is controlled by the pore size, the wettability characteristics of the solid, and the interfacial tension between the water and DNAPL.
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From page 140... ...
Primarily through pool mass removal, source zone remediation activities can result in a reduction in the local saturations of the DNAPL left in place. As noted above, this may result in a reduction of mobility and a reduction in the risk of further migration.
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From page 141... ...
Under some conditions, where substantial mass removal has been achieved in source zone treatment, monitored natural attenuation may even be a feasible follow-on treatment choice. The metrics associated with this objective are variable, but they commonly include reductions in contaminant concentrations, mass, and mass fluxes.
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From page 142... ...
Knowing which of the physical functional objectives is most appropriate for achieving exposure reduction at a site is not a trivial undertaking, as illustrated in Box 4-2. The complex interconnections between mass removal, concentration
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From page 143... ...
Two physical functional objectives for remediation are being considered at this site: DNAPL mass removal from the source zone and aqueous concentration reduction within the source zone. Selection of the functional objective is a complex task and, in this scenario, is dependent on the chemical and hydrogeologic setting.
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From page 144... ...
Financial Objectives Cost minimization is typically one of the absolute objectives of any cleanup decision. That is, most stakeholders agree that, all else being equal, the lowercost option should be selected, thereby freeing up funds for other beneficial uses.
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From page 145... ...
Even when life cycle cost is defined as the metric to be used, divergent cost estimates may be obtained due to differences in assumptions regarding the scope and boundaries of the analysis and in projections regarding the future of technology, regulations, human and institutional behavior, and other factors, as discussed below. Overview of Life Cycle Cost Analysis Key to life cycle cost analysis is a full cost inventory that includes all direct and indirect costs ranging from project start-up costs (e.g., design, studies to prove a technology or obtain permits)
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From page 146... ...
A life cycle cost analysis would include all labor, equipment, and material costs associated with the cost elements in the table. In practice, analysts typically use a TABLE 4-1 Example Cost Elements in a Life Cycle Cost Analysis WBS Element Name 1.0 Research, Development, Test, and Evaluation 1.01 Design and Engineering 1.02 Prototype 1.03 Project Management 1.04 System Test and Evaluation 1.05 Training 1.06 Data 1.07 Equipment 1.08 Facilities 1.09 Other Research, Development, Test, and Evaluation 2.0 Preparation/Mobilization 2.01 Planning/Engineering 2.02 Site Preparation 2.03 Regulatory Compliance/Permitting 2.04 Mobilization continued
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From page 147... ...
4.11 Transportation 4.12 Waste Management/Disposal 5.0 Site Restoration 5.01 Demobilization 5.02 Capping 5.03 Decommissioning/Closure 5.04 Restoration 6.0 Long-Term Management 6.01 Institutional Controls 6.02 Sampling/Monitoring 6.03 Remedy Failure/Repair/Replacement 6.04 Natural Resource Damage Liability 6.05 Other Long-Term Liability SOURCE: Adapted from EPA (2000) and Department of the Army (2002)
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From page 148... ...
Decisions on discount rates can play an important role in remedial action decision making, particularly in determining what remedy to choose to meet the cost objective -- for example, contaminant mass removal vs. contaminant isolation or long-term stewardship measures.
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From page 149... ...
Life cycle cost estimation over long timeframes is complex not only due to differences of opinion on the choice of discount rate, but also, and perhaps more important, due to the large uncertainty in projections of future costs given the uncertainties surrounding the site conceptual model, future technology, regulatory policies, societal norms, land use, population density, etc. Major sources of uncertainty in cost estimates relate to the site characterization model and the effectiveness of a selected technology at the specific site.
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From page 150... ...
Conversely, budgetary pressures and/or a desire to avoid boombust cycles and minimize disruption to the local economy may lead to preference for a level funding profile. Either case may have a substantial effect on the remedial design, the work plans, and, consequently, the life cycle cost estimates.
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From page 151... ...
Schedule objectives may be absolute objectives or functional objectives. For example, accelerating site cleanup is a functional objective when it is used as a means to achieve the absolute objective of reduction of risk to human health and the environment.
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From page 152... ...
0 to 10 PL Easement settlement year 3 to 30 YL Discount rate (%)
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From page 153... ...
Remediation Easements Two variables related to remediation easements (settlement probability and settlement year) were chosen for inclusion in the Monte Carlo simulation.
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From page 154... ...
The easement settlement year was Y(NRD) FIGURE 4-6 Distribution of NRD settlement year (min = 5 yrs, max = 30 yrs)
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From page 155... ...
Scenario 2 Scenario 2 examines the potential liability reduction that could be achieved with the implementation of an aggressive source remediation strategy designed to reduce plume size. For this example, it is assumed that plume reduction from 50 acres (20 hectares)
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From page 156... ...
associated with implementing the aggressive source remediation under Scenario 2. The cost ranges presented in the table represent 90 percent confidence intervals estimated from the Monte Carlo analysis.
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From page 157... ...
In general, the Army, DoD, and other institutions charged with hazardous waste remediation should be cognizant of selecting remedial actions, in particular source remediation, that take into account the absolute and functional objectives held by stakeholders. Decisions as to how to manage historical releases occur within a broad social context, involving multiple parties that may each have diverse and dynamic sets of drivers.
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From page 158... ...
By noting for each stakeholder whether particular remedial objectives are considered absolute or functional, the RPM will be able to make more informed judgments about the evaluation of source remediation. Thus, if the regulatory authority has a preference for "reducing mass or toxicity," technologies that can be demonstrated to achieve this (and convincing metrics that indicate whether or not they have achieved this)
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From page 159... ...
The major categories of existing frameworks can be broadly classified as regulatory, which defines the legal goals and objectives of the remedial action, and risk assessment, which defines the existing threat to human health and the environment and the level of remediation required to reduce this risk to acceptable levels. An understanding of these existing frameworks for site remediation is essential to developing an unambiguous set of absolute objectives for site remediation and corresponding functional objectives that define whether those absolute objectives have been obtained.
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From page 160... ...
If the site scores above the HRS threshold, then the entire facility is placed on the NPL for possible remedial action. Following that, site managers generally undertake a remedial investigation and feasibility study (RI/FS)
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From page 161... ...
and lifetime health advisory levels for the explosives of concern in this report. These values are commonly set as the objectives of source remediation.
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From page 162... ...
162 given the b MCL (µg/L)
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From page 163... ...
163 to = to not 2B day. = a refer D appears water Acute probable, EPA of = possible, and by 2A = liters Int.
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From page 164... ...
Thus, the relationship between monitoring well concentrations and source remediation is complex, which should be kept in mind when selecting MCLs as metrics. NCP Balancing Criteria The five balancing criteria in the NCP, designed to guide the selection of the most appropriate among several remedies that could meet the threshold criteria,
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From page 165... ...
That EPA has placed them in the least valued of its categories clearly indicates that these objectives are not, from an agency point of view, absolute objectives. Applying the above-described criteria, EPA attempts to select the remedial alternative that utilizes "permanent solutions and treatment" to the "maximum extent practicable" and that is "cost-effective," in the sense that the costs it entails are in proportion to the treatment effectiveness.
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From page 166... ...
The two indicators, "Current Human Exposures under Control" and "Migration of Contaminated Groundwater under Control," measure whether people are currently being exposed to unacceptable levels of environmental contamination, and whether existing groundwater contamination is growing and/or affecting nearby surface waterbodies. The functional objectives that are frequently called for in site-specific agreements between owners and operators of TSDFs and regulatory authorities are typically defined in terms of concentrations of particular contaminants as measured at the boundaries of given units of real property.
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From page 167... ...
, it is likely that at identified Army sites where source remediation is an option, a risk assessment has already been conducted or will be conducted in the future. Distinctions between Human Health and Ecological Risk Assessment The methods typically used in human health risk assessment are highly prescribed by RAGS and similar risk-based methodologies.
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From page 168... ...
Box 4-4 further discusses the role of uncertainty in risk assessment calculations, and how uncertainty can be more quantitatively assessed in lieu of using the default assumptions discussed above. The ecological risk assessment process is far less prescribed in the published risk-based methodologies than the human health risk evaluation process for several reasons.
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From page 169... ...
, differences in exposure frequencies or duration, or, in the case of ecological risk assessments, inter- and intraspecies variability in doseresponse relationships. EPA risk assessment guidance (EPA, 1989)
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From page 170... ...
If a risk estimate is conducted using probabilistic methods, the uncertainty associated with the best estimate of the exposure or risk distribution can be quan titatively estimated using a two-dimensional Monte Carlo analysis. This analysis can provide a quantitative measure of the confidence in the fraction of the popula tion with a risk exceeding a particular level.
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From page 171... ...
These methods do not factor the lifetime of a source of contamination into risk estimates. They do not typically evaluate the size of the population potentially at risk, nor do they consider risk beyond the lifetime of an individual (i.e., they do not consider cumulative risk to the entire population exposed for the lifetime of the source of contamination)
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From page 172... ...
, but these also fail to address timeframes of contamination that may span centuries. Accordingly, existing risk metrics may be unable to demonstrate benefits from source remediation efforts, if the primary effect of those efforts is to reduce the time over which the source contributes to elevated contaminant concentrations in groundwater.
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From page 173... ...
Indeed, within the Army several source remediation technologies have been piloted and then selected for scaling up in the absence of having specific cleanup objectives prior to the pilot projects. As an illustration of this, in situ chemical oxidation might have been attempted for a small portion of the source zone during a pilot study and found to achieve a certain percentage of mass removal.
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From page 174... ...
, such that alternative actions with significant differences in terms of the speed with which a site can be remedied cannot be distinguished. Within life cycle cost analysis, the chosen timeframe and discount rate can significantly affect cost estimations for different remedies.
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From page 175... ...
1997b. Ecological Risk Assessment Guidance for Superfund: Process for Designing and Con ducting Ecological Risk Assessments.
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From page 176... ...
2004. DNAPL source zone remediation: influence of hydraulic property correlation on predicted source zone architecture, DNAPL recovery, and contaminant mass flux.
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From page 177... ...
1996. Monte Carlo modeling of time-dependent exposures using a Microexposure event approach.
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