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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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82 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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84 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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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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92 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? 93 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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94 Science and Technology for Environmental Cleanup 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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Vadose Zone Technical Element 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? 95 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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96 Science and Technology for Environmental Cleanup 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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Vadose Zone Technical Element 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? 97 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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98 Science and Technology for Environmental Cleanup 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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Vadose Zone Technical Element 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. 99
Representative terms from entire chapter: