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OCR for page 79
6
Vadose Zone Technical Element
The Integration Project's Vadose Zone Technical Element
supports studies to obtain a better understanding of contaminant behavior
in the unsaturated zone at the Hanford Site and to develop conceptual
models, numerical models, and parameter databases for the System
Assessment Capability (SAC; see Chapter 4~. The vadose zone is
arguably the most important region of the Hanford Site from both a
scientific and an environmental restoration perspective: it contains most of
the chemical and radionuclide contaminants that have been discharged or
leaked into the environment and is host to the site's waste storage and
disposal facilities, including the high-level waste tanks, burial pits and
trenches, disposal ponds and cribs, and injection (or"reverse") wells
(Chapter 2~. The present-day distributions and chemical forms of
contaminants in the vadose zone are poorly known, as are the fate and
transport processes that will govern the future migration of these
contaminants to the groundwater and the Columbia River.
This chapter provides a brief review and assessment of the work
supported under this technical element. The main sources of information
used in this assessment are the Integration Project Roadmap (DOE
2000a), other DOE documents (DOE 1 999e, 20009), and briefings
received during the committee's information-gathering meetings. It was
apparent to the committee from these briefings that the Vadose Zone
Technical Element is still in the early stages of development and that the
schedule for S&T work is in flux owing mainly to budget reductions
(Chapter 10~.
THE VADOSE ZONE: WHAT IS IT, AND WHY IS IT POORLY
UNDERSTOOD?
The vadose zone, also called the unsaturated zone,' is that
portion of the earth's crust between the land surface and the water table.
It includes the capillary fringe (a region above the water table that
The adjective vadose, from the Greek word "shallow," was introduced by
Posepny in 1894 to designate water in the unsaturated zone, although
subsequent usage included shallow groundwater as well (Meinzer and Wenzel,
1942~. In recent years, however, the term vadose zone has been used more or
less synonymously with unsaturated zone, and the committee uses these two
terms interchangeably in this report.
79
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80
Science and Technology for Environmental Cleanup
contains water held by capillary action), perched water bodies, and other
features that may be temporarily or permanently filled with water.
The unsaturated zone contains solids, liquids, and gases. The
solid phase consists of rock and mineral particles interspersed with
organic solids, as well as plant and animal life. Solid particles vary in size
from fractions of a micron in clays to millimeters in sands and gravels.
The largest particles can be meters across and have substantial internal
porosity.
The liquid phase is composed primarily of aqueous solutions that
exhibit variable concentrations throughout the vadose zone. The
distribution of solutes varies within individual pores due to electrical and
chemical gradients at liquid-solid and liquid-gas interfaces. At sites such
as Hanford, the liquid phase contains a variety of inorganic and organic
contaminants, including nonaqueous liquids. Some, such as alcohol, mix
completely with water. Others, such as carbon tetrachloride, form distinct
phases known as nonaqueous phase liquids (NAPLs).
The gas phase is generally similar in composition to that of the
above-ground atmosphere, except for elevated concentrations of water
vapor and carbon dioxide. At contaminated sites like Hanford, the vadose
zone gas phase contains semivolatile and volatile organic compounds as
well. Gas transport is driven by compositional, pressure, and thermal
gradients. Near sources of contamination, this transport is difficult to
model.2
The vadose zone typically contains from 20 to 50 percent porosity
by volume. Pores have irregular shapes and complex interconnections
that elude precise description. Pores in sediment arise from depositional
features that are modified by postdepositional processes. Modification of
pores occurs during soil formation, weathering, and biological processes.
Consequently, pore geometries tend to be spatially heterogeneous and
anisotropic. In addition to the interconnected pore space between grains,
passageways for fluids include burrows, root channels, fractures, and
human artifacts including well bores and corroded pipes.
Small particles (e.g., clay minerals) may contain large amounts of
porosity and surface area, up to hundreds of square meters per gram.
Surfaces of wetted clays are electrically charged and interact with
charged species in the liquid phase. At Hanford, electrochemical
interactions were assumed to bind certain contaminants, particularly
cesium, strongly to the solid phase, retarding their migration (see Chapter
1~.
Unsaturated zones are chemical and mechanical systems in
disequilibrium in which fluids and solutes move in response to gradients in
2Neither diffusion theory nor advection theory alone accurately predicts gas
transport near sources of contamination.
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Vadose Zone Technical Element
81
free energy. Transport rates are approximately proportional to gradients in
free energy, but the proportionalities are nonlinear functions of saturation
(Figure 6.1~. Because pores contain varying amounts of gas and liquid,
transport parameters are represented by saturation-dependent functions
rather than by constant values as in the saturated zone. Additionally,
some transport parameters exhibit hysteresis as a function of saturation-
that is, they have different values depending on whether the system is
being wetted or dried.
~ 1 E-04
o
c'
IJJ
~ 1 E-05
LLl
oh
1 E-06
LIJ
1 E-07
>
~ 1 E-08
At
o
c' 1 E-09
J
1E-10
1E-11 . ,
-
Zen
,~
E]
Her
-
0 0.1 0.2 0.3 0.4
VOLUME OF WATER PER BULK VOLUME
Figure 6.1. Dependence of unsaturated hydraulic conductivity on
saturation, expressed by the volume of water per bulk volume, in
sand. Different methods of determination (squares, diamonds,
and triangles) are necessary to span the range of interest
accurately. SOURCE: Stonestrom, 1996.
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Science and Technology for Environmental Cleanup
These nonlinearities make it difficult to obtain representative
measurements and tend to amplify modeling errors. A 5 to 10 percent
uncertainty in saturation, for example, can lead to an order-of-magnitude
uncertainty in predicted transport rates. Standard methods for hydraulic
conductivity determination are limited to one or two orders of magnitude in
range and become impractical at low water content. Hydraulic
conductivities are often inferred but rarely measured at saturations less
than 50 percent.
Quantifying water and solute movement through the vadose zone
is particularly difficult in arid regions. Most precipitation reaching the
ground returns to the atmosphere through evapotranspiration;
groundwater recharge is thus the difference between two nearly equal
quantities. The amount of water crossing the land surface as liquid or
vapor depends on dynamic meteorological and plant conditions that
change by the hour. Evapotranspiration is therefore difficult to measure
and model. Because of this, water-balance estimates of recharge are
subject to large errors. These uncertainties are amplified by climate
change, which can alter flora and fauna and produce major shifts in
recharge locations and amounts.
In summary, the vadose zone is a complex system of interacting
physical, chemical, and biological processes. Mathematical models of
transport incorporate parametric functions that exhibit nonlinearity and
hysteresis, complicating hydrogeological characterization.
Heterogeneities exist at scales from individual mineral grains to geologic
formations, further complicating characterization. For all of these reasons,
modeling the fate and transport of contaminants through the vadose zone
presents a difficult technical challenge.
SCOPE OF VADOSE ZONE TECHNICAL ELEMENT
il
The Vadose Zone Technical Element comprises five broad
science and technology (S&T) activities and, within these, 27 individual
"projects" (Table 6.1~:
1. Field investigations of representative sites: This activity
ncludes six projects to develop an improved understanding of
contaminant distributions beneath selected tank farms and at 200 Area
soil waste sites.3
3V\laste sites (e.g., tanks, ponds, cribs, trenches, landfills) in the 200 Area
have been grouped based on waste inventories (DOE, 1997c), and efforts are
under way to characterize representative sites from each of these groups. These
sites are referred to by the Department of Energy as 200 Area soil waste sites.
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Vedose Zone Technical Element
83
2. Transport modeling: This activity includes eight projects to
obtain an improved understanding of fate and transport processes
beneath selected tank farms and at 200 Area soil waste sites.
3. Waste and sediment experiments and models: This activity
includes six projects to obtain kinetic and thermodynamic data on key
contaminants to determine first-order hydrochemical reactions controlling
contaminant behavior in sediments beneath tank farms and at 200 Area
soil waste sites.
4. Vadose zone transport field studies: This activity includes four
projects to develop an improved understanding of water and solute
movement, reactive transport, and migration pathways in vadose zone
it
sediments.
5. Advanced vadose zone characterization: This activity
ncludes three projects on advanced characterization technologies to
support the vadose zone transport field studies in the 200 East Area and
200 West Area and to evaluate tools for monitoring contaminant plumes
in the vadose zone beneath tank farms.
As shown in Table 6.1, work on projects under the Vadose Zone
Technical Element is planned to run from fiscal year 1999 through fiscal
year 2004, and some of the early work was being completed as the
committee finished its information gathering for this report. The total
planned funding for this technical element is about $42.6 million, of which
$17.8 million is being provided to Environmental Management Science
Program (EMSP) projects from the fiscal year 1999 competition.4 The
actual budgets for the Vadose Zone Technical Element have been lower
than indicated in Table 6.1 owing to funding cutbacks (see Chapter 10~.
EVALUATION OF WORK PLANNED UNDER THE VADOSE ZONE
TECHNICAL ELEMENT
As of early 2001, most of the technical work to be done within the
Vadose Zone Technical Element either had not been started or was not
yet completed. Consequently, there is little scientific or technical output in
the form of peer-reviewed reports or papers available for the committee's
evaluation. The committee has therefore focused its efforts on reviewing
the written plans for this work and providing responses to the following
five questions that were developed to address the statement of task for
this study (Chapter 1~:
4The science program projects are under way and are scheduled to be
completed in fiscal year 2003.
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Science and Technology for Environmental Cleanup
TABLE 6.1 Summary of S&T Activities and Planned S&T Projects Under the Vadose
Zone Technical Element
S&T Activity
S&T Project Objectives
Projects
Planned
Project
Duration
(fiscal
years)
2001-2004 7,830
Hanford
Funding
(thousand
dollars)
EMSP
Funding
(thousand
dollars)
1,600
Field
investigations
of
representative
sites
6
Transport 8
modeling
Waste and 6
sediment
experiments
and models
Vadose zone 4
transport field
studies
Advanced
vadose zone
charactenz-
ation
Develop an improved
understanding of
contaminant
distributions beneath
selected tank farms
and at 200 Area soil
waste sites
Obtain an improved
understanding of fate
and transport
processes beneath
selected tank farms
and at 200 Area soil
waste sites
Obtain kinetic and
thermodynamic data
on key contaminants to
determine first-order
hydrochemical
reactions controlling
contaminant behavior
in sediments beneath
tank farms and at
representative 200
Area soil waste sites
Develop an improved
understanding of water
and solute movement,
reactive transport, and
migration pathways in
vadose zone
sediments in the 200
East Area and 200
West Area
Use advanced
characterization
technologies to
support the vadose
zone transport field
studies in Me 200 East
Area and 200 West
Area, and evaluate
tools for monitoring
contaminant plumes in
the vadose zone
beneath tank farms.
2001-2004 3,840 600
2001-2004 3,500 8,000
1999-2004 8,120 3,900
2000-2003 1,500 3,700
NOTE: EMSP = Environmental Management Science Program
SOURCE: DOE, 2000a, Figure 4-1, Table ~1.
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Vadose Zone Technical Element
1. Can the objectives of the planned work be achieved?
2. Does the planned work represent new science?
3. Can the planned work have an impact on cleanup decisions
at the Hanford Site?
4. Does the planned work address the important issues?
5. Are there other concerns, comments, or suggestions that
should be considered by the Integration Project in executing the planned
work?
85
The five S&T activities are described and evaluated in the
following sections. More written documentation is available for some
projects in this technical element than In the Inventory Technical Element
(Chapter 5~. Consequently, the committee is able to provide a more
detailed review.
Field Investigations of Representative Sites
Six separate projects are planned under this activity to improve
understanding of contaminant distributions in the vadose zone in the 200
Area. These projects are designed around field investigations at what the
Integration Project calls "representative sites," that is, sites designed by
the Integration Project to be broadly representative of the population of
waste sites that exist in the 200 Area based on characteristics such as
waste type and vadose zone geology.
The scale of evaluation for most of the projects under this activity
is the individual mineral, although studies of intact cores and
homogenized core material will be undertaken to examine questions
related to contaminant migration. Three specific processes and/or
attributes of waste-soil interactions will be examined for (1 ) the potential
for immobilization of technetium and cesium; (2) the influence of
temperature; and (3) aluminum activity on subsurface mobility of waste
constituents. Other task objectives are more open-ended.
Two projects (VZ-15 and VZ-3) are focused on understanding
chemical and hydrochemical processes beneath leaking single-shell tanks
in the S-SX Tank Farm, which contain highly concentrated waste from the
PUREX (Plutonium-Uranium Extraction) process, and the B-BX-BY Tank
Farm, which contains dilute high-level waste from other chemical
processing operations. Some of this work is being conducted in
cooperation with the Office of River Protection, which is drilling wells in
the tank farms to obtain contaminated core samples from beneath tanks
that are suspected to have leaked.
5The projects under each of the six activities are given these identification
numbers in DOE (2000a, Table 4-1~.
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86
Science and Technology for Environmental Cleanup
One project (VZ-2) is focused on understanding chemical and
hydrochemical processes beneath other 200 Area soil waste sites,
especially sites that received significant inventories of technetium,
actinides, and dense non aqueous phase liquid (DNAPLs). Some of this
work also is being conducted in cooperation with Hanford core projects
(Chapter 3~.
Three projects (VZ4, VZ-5, and VZ-6) are focused on developing
conceptual models of the important processes controlling contaminant
distributions beneath leaking single-shell tanks and soil waste sites in the
200 Area. This information will serve as input to future revisions of the
System Assessment Capability (see Chapter 4~.
Can the objectives of the planned work be achievecl?
A significant portion of the proposed tasks involves
characterization of contaminant-sediment associations. Presumably, once
such associations are elucidated, the development of hypotheses
regarding the mechanism of interaction will follow. An objective of the
conceptual model development is to obtain a comprehensive
understanding of the important processes controlling contaminant
distribution beneath waste tanks. Criteria for the successful completion of
the tasks are unclear, and the level of understanding required to meet
data needs is not defined.
Does the planned work represent new science?
The scientific merit of the proposed characterization work
appears to be good, particularly with the application of state-of-the-art
analytical techniques such as x-ray absorption spectroscopy. Experience
gained from working on Hanford Site materials should be applicable to
contaminant-sediment interaction questions at other Department of
Energy (DOE) sites.
Can the planned work have an impact on cleanup decisions at the
Hanford Site?
The proposed work will focus on materials of specific concern to
the Hanford Site. However, an important question remains to be
answered: Is the scale of analysis appropriate for the scale at which site
decisions must be made? A goal of this work is the incorporation of
conceptual models into the SAC Rev. 3 (see Chapter 4), but the S&T
program has not demonstrated how mineral-scale studies will fit into a
site-wide simulation model such as SAC. Translating the information
derived from mineral-grain studies up to the spatial scales represented by
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Vadose Zone Technical Element
site-wide models like the SAC is not a trivial task (see Sidebar 6.1~. This
is especially true for subsurface structure, where the challenge is to
understand the dominant components of heterogeneity at large scales.
Does the planned work address the important issues?
The Integration Project has not provided an explicit link between
87
the planned work and the issues to be addressed, especially within the
context of future decisions to be made at the site. Nor have the data
quality objectives or criteria for success been determined.
Developing an understanding of mechanisms of contaminant-
sediment interaction is important for providing confidence that reactive
transport models are conceptually correct. However, the scope of the
problem and site heterogeneity will confound efforts to achieve closure on
the physicochemical controls on contaminant migration. It is not clear how
the importance of a process will be determined. In short, how is it possible
to make conclusions regarding the relevance of processes to site needs
from 500-gram sediment samples (DOE, 20009, p. 5.4~? What will be the
criteria for selecting samples for detailed analyses?
Are there other concerns, comments, or suggestions that should be
considered by the Integration Project in executing the planned work?
The committee is concerned about how the S&T program intends
to set the data quality objectives for supporting sound management
decisions. Data quality objectives include the type and distribution of data
(e.g., What are the cesium concentrations in the vadose zone at an
appropriate spatial distribution and sampling clensity?) and uncertainty
requirements (i.e., How well does a particular parameter value need to be
knowing.
The data quality needs can be considered only in the context of a
specific management tool (e.g., SAC), because not all data will be critical
to uncertainty reduction. For example, it may be necessary to know the
value of the sorption parameter only to within an order of magnitude in a
particular system to estimate a particular risk component. However, the
level of certainty (precision) that has to be achieved cannot be defined in
the absence of identifying the specific need.
Transport Modeling
According to the Integration Project Roadmap (DOE 2000a), eight
separate projects are planned to improve understanding of fate and
transport processes in the vadose zone in the 200 Area. Three projects
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OCR for page 89
Vadose Zone Technical Element
s-orptiOnof=ntamin.a.ntsby.g - ~ at - q if. ~ 1-d ~~s~.a-pld~
-c-ompared.to:the-.~e Negro. : lo I; :
contaminant maybe ~ : : it i ;
b Her wise, Wnp' lo kinetic~moddis~m~ust be~used~to Credit system l
~--~ II! ic :nnt:=lw~ ~rI~ 1 ~ i tb ~ i .-
89
~ ~ ~ ~ ~ ~ ~ ~ . ~. ~ ~ ~ ~ ~ . ~ ~ ~ .. ~. ~ ~. ~ ~ ~. ~ ~ ~ :. ~ . ~ . ~ ~ . ... ... ~ ~ ~ . . . . .
~ ~ :the- ~rsutlook~l~s ~bv no -means 31eak. 1- yl rol4Oists~ have, - or examp e'~ ~ ~
:~achieved.s.'u'c2c.es,sinu,.n,de '' ' all': :''h' :'i fl '''' ''f'' ' 1'
. t~nspod para'''met'er.2s-0pe,,n,dix, :.C).I.n'. soffi ' ' ' ' ' ''i' ':-ma' -b' . ' ' ' 'ibl '
~-~to understand-co~a.mi~nant-behavior o~n~.long~.tim.e. i ;
obsew.at~~n-of n~~~ analogues—~--exampl-e- ;~mtand-~ng - ~-- ~ ~ :
~co.ntmison.~~' m t~nspod th~ugh~ an ex m~nat'~n the
processes:.that. lead .t.o. .the~.~-at'po ~ran~um depos'is.- - ~r, ~- ::
. . , . . . ~ ~ . . .
many.~.- t. ~e co-n. tam~na.n.ts -a an. or -- ~a ~ nq- o ? ~gus - na u~ -:
a~nalogu~es..~ The ~challeng~e ~-. th~en. ~ its to- . pred.ict system ~e~,olution..~at
.~- ~ ~-~ . ~..~, ~ ~ .~ ~.~. ~ .. ~.~ .- - . . .~ - - - ~ ~ - ~ . ~ .~ .~ ~ ~ ~ ,.~,~ -~', ~ ~ ~ ~.~ . -. . .~ ~ - .~- ~ . ~. ~ ~-~. ~ . ~
~sp~atia.l.-.a~n~d~:~tem~po.ral-scales-..lGor wh~:h-..th.e.re is--.n-o~-.env.ir.onmen.ta-l..-.
~n~Ina''~ ~r annndoni~ ~-f Ilv f
(VZ-07, VZ-09, VZ-10) target selected waste management areas
containing single-shell tanks (S-SX Tank Farm, B-BX-BY Tank Farm, T-
TX-TY Tank Farm), and one project (VZ-08) targets "high-priority' but
unspecified 200 Area soil waste sites. All four projects have as their
scope "preliminary evaluation of key transport processes affecting
contaminant transport' (DOE, 2000a, Table 4-1~.
One project (VZ-1 1 ) will provide the SAC Rev. 2 with "evaluations
of key contaminant transport processes beneath SSTs [single-shell
tanks]." Another project (VZ-12) will provide the SAC Rev. 3 with
evaluations of "coupled fluid flow and multicomponent reactive transport"
(DOE, 2000a, Table 4-1).
Two projects (VZ-13, VZ-14) will provide modeling support for the
experimental design of field-scale infiltration and reactive tracer
experiments. The first will be carried out at an uncontaminated site in the
200 East Area (the site of the current vadose zone field transport
experiment). The second will be carried out at an uncontaminated site in
the 200 West Area that is yet to be selected.
Each of these eight projects is to produce "a documented suite of
process models and simulation results" for the targeted area (DOE,
2000a, Table 4-1~. In addition, one EMSP project is linked to this activity.
That project is entitled "Quantifying Vadose Zone Flow and Transport
Uncertainties Using a Unified, Hierarchical Approach." Its purpose is to
develop "a general approach for modeling flow and transport in a
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9o
Science and Technology for Environmental Cleanup
heterogeneous vadose zone using geostatistical analysis, media scaling,
and conditional simulation to estimate soil hydraulic parameters at
unsampled locations from field-measured water content data and a set of
scale-mean hydraulic parameters" (DOE, 2000a, Table 2-1~.
Can the objectives of the planned work be achieved ?
It appears likely that objectives will be achieved only to a limited
extent. An important limiting factor is the lack of data for calibration and
testing purposes. There is little information on the three-dimensional
distribution of contaminants under the tanks, and almost no data exist on
conditions in the deep (~30 meters) vadose zone. Data to assess lateral
movement of contaminants from tank areas are largely unavailable.
1 00,000
1 0,000
-
y 1,000
a)
Cal
U'
._
Cal
Oh
1
0.1
l
it
_ Appro~dmate rime covered I
by e~dsffr~ Hanfad ecological I
and water qual ty data set ~
l
1
l
100
) _
Debris flows; landsildes;
channel shim; river meander
cutoff; channelizaffon; diversion
overdammir~ by humans
Minor glac anon fluc~affon, very large
landsildes; alluvial or colluvial valley
filling; river meander development
Technic uplift, subs dance;
sea level dianges; gladaffon
Hanford Site scale
Sediment accumulation/
washout small bank failures;
flood flow scour/deposldon
Annual flow; seasonal
meaophyle grow~/scour
0.1 1 10 100 1,000
K. . . S - And Ye
. . . of current Hanford Sly kna..,I - ~~e ars
10,000 100,000 1,000,000
Figure 6.2 Spatial and temporal scales of geologic and hydrologic
processes relative to the Hanford Site process data set.
SOURCE: Adapted from Frissell et al., 1986.
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Vaclose Zone Technical Element
Does the planned work represent new science?
91
Although modeling reactive transport through heterogeneous
sediments is not new, several features of the targeted systems are unique
to the Hanford Site. Unique features include substantial contaminant
chemistries (high pH, high ionic strength, and unusual compositions) and
thermal effects. Hanford-specific features also include high levels of
sediment heterogeneity in the glacial-lake outburst flood deposits that
underlie the site.
Can the planned work have an impact on cleanup decisions at the
Hanford Site?
Well-constructed and calibrated transport codes could directly
inform remediation and stewardship decisions at the site, especially if
their use has been formally linked to specific site decisions. Defensible
transport codes should help form the scientific and technical bases for
cleanup. Even if transport codes are not sufficiently accurate or well
constrained for management decisions, an important role of modeling
activities can be to identify important gaps in understanding.
Does the planned work address the important issues?
The Integration Project has not provided an explicit link between
the planned work and the issues to be addressecl, but the committee sees
several such links. The planned work could help explain the cesium
"anomaly' discovered in the deep vadose zone beneath one of the tank
farms (Chapter 1~. In fact, the transport-modeling activity emphasizes the
tank farms, where pressing decisions on tank waste retrieval and tank
closure loom (Chapter 2~. The activity also emphasizes modeling to
support future revisions of SAC. Given the inability of models that have
been used in the past to predict the observed migration of contaminants,
the transport-modeling projects address some of the most important
issues at Hanford. It is also likely, however, that the cribs, ponds, tile
drains, and plutonium production canyons are not receiving the S&T
attention they require. Also, there are potentially significant S&T gaps
related to modeling microbiological and sediment transport processes.
Are there other concerns, comments, or suggestions that should be
considered by file Integration Project in executing the planned work?
This review was based on descriptions of S&T activities in the
Integration Project Roadmap (DOE 2000a) and other DOE documents
(DOE 1 999e, 2000g). The committee was unable to offer detailed
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Science and Technology for Environmental Cleanup
critiques of these S&T activities because documentation of objectives and
work plans were either missing or quite limited. Nevertheless, one of the
main concerns that emerges from this review is that with the exception of
the targeted tank farms, most vadose zone waste sites (the 200 Area soil
waste sites) are not being studied, which leaves a large knowledge gap.
Given the large number of waste management areas at Hanford, the S&T
effort seems to be spread thin. It is worth noting that the schedule in DOE
(2000, Figure 4-1 ) has slipped and that fiscal year 2000 funding for the
transport-modeling activity is one-third of the planned amount (DOE,
2000g, p. 5-1~. One aspect of the S&T program that is yielding very
positive results is the solicitation and active involvement of modeling
talent from other DOE national laboratories (Los Alamos National
Laboratory, Lawrence Berkeley National Laboratory, and Lawrence
Livermore National Laboratory).
It is unlikely that any modeling effort will provide usable results
unless there are appropriate data for calibration and verification. There
will also have to be formal procedures for comparing the field
observations with the modeled predictions in view of the substantial
uncertainties expected in both. Acquisition of such data must be an
integral part of the S&T plan. The Integration Project is directing some of
the needed data collection, but the committee believes that such efforts
must be increased.
Waste and Sediment Experiments and Models
The stated goals of this activity are (1 ) to develop an improved
understanding of key geochemical phenomena in target waste sites by
conducting kinetic and thermodynamic studies of contaminants of concern
using uncontaminated and contaminated sediments to determine proximal
chemical and hydrochemical reactions and (2) to use the data from the
first goal in the development of numerical models for describing
contaminant transport through unsaturated columns. Six projects are
planned under this activity to meet these objectives. Four of the projects
(VZ-15, VZ-16, VZ-17, VZ-18) will involve kinetic and thermodynamic
studies to understand hydrochemical reactions beneath the S-SX Tank
Farm, B-BX-BY Tank Farm, T-TX-TY Tank Farm, and as yet unspecified
200 Area soil waste sites. The remaining two projects (VZ-19, VZ-20) will
focus on the development of numerical models that describe these
reactions.
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Vadose Zone Technical Element
Can the objectives of the planned work be achieved?
The goals of these projects are rather open-ended- for example,
"an improved understanding" and "to develop data." The data quality
objectives are not specified.
Does the planned work represent new science?
Because much of the work will focus on Hanford Site materials,
the work will be new. It is unclear, however, whether the laboratory
experiments will be new in approach and whether the questions
addressed, and the modeling techniques employed, will be new in a
generic sense (i.e., an advance in the science).
Can the planned work have an impact on cleanup decisions at the
Hanford Site?
Because there is not a clear link between the technical element
activities and specific management decisions, the applicability of these
tasks to cleanup decisions is not evident. No specific hypotheses are
listed for testing, and the tasks give one the impression that they are
meant to characterize system attributes rather than address testable
hypotheses.
Does the planned work address the important issues?
The task descriptions provided to the committee are not
sufficiently defined for it to ascertain the central issues to be resolved.
Are there other concerns, comments, or suggestions that should be
considered by the Integration Project in executing the planned work?
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As with the field investigation tasks, the problem remains of how
to set the data quality objectives for supporting management decisions. A
key aspect is the accurate characterization and modeling of chemical
speciation and transformations in time and space.
Vadose Zone Transport Field Studies
The stated primary objective of these field studies, to be
conducted at uncontaminated sites, is to collect data sets to verify
conceptual and numerical models that describe transport through the
vadose zone. A secondary objective is to test advanced characterization
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techniques at the Hanford Site under controlled conditions. Some key
science issues driving these studies are plume identification and
delineation, upscaling techniques (Sidebar 6.1), effects of elevated salt
concentration, and preferred pathway analysis. The additional science
issues of thermal and accelerated recharge effects on contaminant
migration from tank leaks and colloidal transport in coarse heterogeneous
sediments are identified in the fiscal year 2000 work plan (DOE, 1 999e)
but not in the fiscal year 2001 plan (DOE, 2000g).
In fiscal year 2001, a new field experiment with high-salt-
concentration tracers is to be conducted at the existing test facility in the
200 East Area, the so-called "Sisson and Lu site," which consists of a
concentric array of wells around a central injection well. Plans for a new
field testing facility, tentatively in the 200 West Area, are to be developed
starting in fiscal year 2002. The crucial issue of upscaling methodologies
is deferred to a workshop in fiscal year 2002 as part of the development
of a test plan for the deep (>20 feet)6 vadose zone transport studies in the
200 West Area.
Reactive transport field experiments (VZ-22 and VZ-24) are
identified prominently in the project descriptions given in Table 4-1 in the
Integration Project Roadmap (DOE 2000a), but aside from the high salt
concentration reactive transport experiment, they do not seem to be a
major consideration in the detailed work plans (DOE, 1999e, 20009~. The
issue of field-scale reactive transport is largely unresolved and would
seem to be central to many problems at Hanford. Overall, the field
investigations are intended to integrate with the field investigations of
representative field sites and the transport-modeling activities, and are to
provide results that will be used by the SAC and the Office of River
Protection project for model verification tests.
Can the objectives of the planned work be achieved ?
Clearly, many objectives of the proposed field experiment are
achievable, but it is not clear that the resulting data collected will be
adequate to definitively resolve the scientific issues identified. The
approach to be taken to the difficult issues of upscaling and preferential
pathways is not clear from the available documentation, and the efforts
directed to these issues are deferred until late in the project. Field
experimentation of this kind is very important from both basic and applied
perspectives, but it is generally difficult to anticipate the outcome of such
efforts.
6The description of what constitutes the adeep" vadose zone is different in
various documents reviewed by the committee.
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Does the planned work represent new science?
Field experimentation of this kind is unique, particularly for the
deep, dry, unusually heterogeneous vadose zone at Hanford. If these
experiments can be used to establish an effective approach to
characterize and simulate such large-scale heterogeneous nonlinear
systems, this would be a major scientific contribution.
Can the planned work have an impact on cleanup decisions at the
Hanford Site?
Certainly field-tested and validated techniques for predicting
large-scale, long-term fate and transport of contaminants in the vadose
zone at Hanford would be useful in cleanup decisions, both for individual
contaminated sites and for a site-wide effort such as the SAC.
Does the planned work address the important issues?
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Field experiments of this kind, if adequately designed and
executed, are central to efforts to reliably assess the fate and transport of
contaminants currently in the vadose zone at Hanford and to predict the
behavior of wastes that may be deposited in the vaclose zone in the
future. Improved characterization techniques for both contamination and
media properties are also very important.
Are there other concerns, comments, or suggestions that should be
considered by the Integration Project in executing the planned work?
A major concern is the lack of emphasis on upscaling techniques
early in the effort. If experiments of this kind are designed around specific
upscaling techniques from the very beginning, it is Much more likely that
the necessary and sufficient data will be collected and definitive
conclusions will evolve. Another concern is the unrealistic time frame for
the completion of these experiments. The processes involved are very
slow, particularly for the deep vadose zone experiments with reactive
transport, whose initiation of which is deferred until late in the project. It is
unrealistic to suggest, as implied by Figure 4-1 of DOE (2000a), that
meaningful field experiments of this kind can be completed by early fiscal
year 2003.
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Advanced Vadose Zone Characterization
Three projects (VZ-25, VZ-26, VZ-27) are planned under this
activity. Two involve field tests of characterization technologies for
delineating moisture and contaminant plumes at the vadose zone field
transport study sites in the 200 East Area and 200 West Area (VZ-25 and
VZ-26~. The third project (VZ-27) will evaluate characterization tools to
support single-shell tank retrieval and closure decisions (VZ-27~.
Characterization tools being evaluated include tracers, tensiometers,
neutron-logging devices, pore-water monitoring devices, cone
penetrometers, and geophysical imaging techniques.
Outcomes of the first two projects will be documented tests that
describe the performance of the characterization techniques in the field-
scale transport studies. The outcome of the third project will be an
evaluation of tools for delineating plumes of non-gamma emitting
contaminants such as technetium-99.
In addition, five EMSP projects are linked to this activity. Two deal
with developing sensors for technetium and organochloricles. The other
three deal with geophysical techniques for characterizing flow and
transport in the vadose zone.
Can the objectives of the planned work be achieved?
This is an area is which investments in S&T could yield high
returns. Techniques for characterizing the shallow vadose zone (i.e., from
0 to about 15 meters in depth) have already been evaluated in field tests
that started in May 2000. Characterization of the deeper vadose zone still
appears problematic. Surface-based geophysical techniques lose
resolution with depth. Subsurface techniques are limited by access
limitations and concerns about creating pathways for preferential flow. It is
unclear, however, whether characterization objectives for the deep
vadose zone can be achieved.
Does the planned work represent new science?
The limited scope of the projects under this activity supports
relatively little development of new or Hanford-specific techniques.
Rather, advanced techniques developed at other sites are being tested
and evaluated in Hanford sediments. Several of the techniques being
evaluated represent emerging scientific advances.
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Can the planned work have an impact on cleanup decisions at the
Hanford Site?
There is wide agreement that lack of vadose zone
characterization hampers remediation decisions. To a large degree, lack
of vadose zone characterization reflects limitations of available
techniques. Advances in characterization technology will significantly
support cleanup decisions. This point is discussed further in Chapter 5.
Does the planned work address the important issues?
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All of the parameters currently being targeted are important. The
issue of deep characterization and monitoring needs more attention.
Tensiometers (used for measuring pore-water pressure) are limited to
relatively moist conditions; generalized measurement of pore-liquid
potential has to be addressed. The lack of techniques for measuring pore-
liquid chemistry appears to be a significant gap. Techniques for thermal
and microbiological characterization also appear to have gaps.
Are there other concerns, comments, or suggestions fhaf should be
considered by the Integration Project in executing the planned work?
This review is based on brief descriptions of S&T activities in the
Integration Project Roadmap (DOE 2000a) and other DOE documents
(DOE 1999e, 2000g) and is limited in breadth and depth because activity
descriptions are lacking in detail. The advanced vadose zone
characterization technical element appears to have been folded into the
vadose zone transport field transport studies. Although the field study
provides a valuable opportunity to test advanced characterization
techniques, the magnitude of the S&T need would seem to warrant
dedicated laboratory, theoretical, and field-based efforts beyond the
immediate scope of the vadose zone transport field transport studies.
DISCUSSION AND RECOMMENDATIONS
In general, the research activities planned under the Vadose
Zone Technical Element address important unresolved scientific issues
relevant to subsurface remediation problems at Hanford. However, the
technical merits of the individual projects are difficult to assess because
appropriate details on the approaches to be used are frequently lacking.
The different activities are well integrated, largely through a focus on the
vadose zone field studies, but the direct importance of the individual
studies to remediation decisions is unclear.
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The planned vadose zone field studies are an important element
of the research program because they integrate geochemical
investigations, transport modeling, and advanced characterization
techniques and provide data that can be used to evaluate upscaling
methodologies. However, much of the new information that would be
obtained through the S&T work reflects laboratory or small-scale field
observations and consequently is not directly applicable to the large field
scales pertinent to remediation. Moreover, the long period of time
required to carry out vadose zone field experiments in dry environments
such as Hanford is not considered adequately in the planning.
One of the main "owners" of S&T results from the Vadose Zone
Technical Element will be the SAC, which can use these results to
develop more realistic models for contaminant transport in the vadose
zone. The hydraulic and transport parameters to be used in the vadose
zone models in SAC will be derived in part from laboratory measurements
on centimeter-scale core samples and will then be extrapolated to the
hundred-meter scales relevant to field transport. The scientific basis of an
upscaling algorithm to calculate "effective" parameters for a large block of
heterogeneous sediments from highly variable measurements on small
samples has not yet been developed and demonstrated. A basic problem
is that small core samples cannot capture large-scale geometric features
that often dominate contaminant transport in highly heterogeneous
hydrogeologic settings.
Consequently, a sound upscaling framework is essential to
provide the link between readily measured laboratory properties and field-
scale behavior pertinent to remediaton problems, thereby establishing a
basis for assessing the importance of new information in remediation
decisions. However, the development of an upscaling approach that could
bridge this scale gap is deferred until late in the project. The lack of early
emphasis on an unsealing framework is a serious weakness of current
plans because this framework should play a central role in the design of
field experiments and also can be used to assess more directly the impact
of new information in remediation decisions, thereby providing a basis for
setting research priorities. To address this weakness, the committee
recommends that the upscaling work planned as part of the vadose
zone transport field studies be initiated as soon as possible.
The vadose zone transport field studies could provide critical data
for scaling hydrologic parameters and elucidating three-dimensional flow
in the subsurface at time scales relevant to site remediation. These
studies are scientifically complex and costly, and their outcome could
have important impacts on other Integration Project work, particularly the
SAC, and on several core projects (Tank Farm Vadose Zone Project, 200
Area Remedial Action Project, Immobilized Low-Activity Waste Project).
Consequently, it is essential that these studies be done well the first time.
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The committee therefore recommends that peer reviews be
established specifically to provide continuing oversight of these
field studies. This peer review should occur during all stages of the
studies that is, from initial planning and design of the experiments
through analysis and interpretation of results.
7See Chapter 10 for a definition of peer review.
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Representative terms from entire chapter:
technical element
