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Groundwater Technical Element The Groundwater Technical Element supports research on the saturated zone at the Hanford Site, especially at its interfaces with the vadose zone and Columbia River. The results of the work supported under this technical element will be used by the Hanford Site's "core" groundwater project, which is responsible for site-wide groundwater monitoring and remediation (see Chapter 3), as well as the System Assessment Capability (SAC; see Chapter 4~. Groundwater occurs beneath the entire Hanford Site, and at present, it provides the primary pathway for contaminant transport from the site to potential receptors in the river and surrounding environment. Many radionuclides of concern at the Hanford Site are highly mobile in groundwater and are transported with little or no retardation (e.g., tritium, technetium-99; see Figure 2.8a). Transport of other radionuclides by groundwater tends to be slower, either because they are less soluble (e.g., uranium, plutonium) or because they react strongly with minerals in the vadose zone before they reach the groundwater (e.g., cesium-137~. Chemical contaminants such as carbon tetrachloridc a dense, nonaqueous phase liquid (DNAPL - are only slightly soluble in groundwater. They tend to be partitioned between groundwater and a pure phase, and their presence in the subsurface can actually modify hydrologic properties (e.g., DNAPLs can partially fill pores, thereby changing water-f~lled porosity and hydraulic conductivity). As discussed in Chapter 2, DNAPL contamination is a serious problem in the 200 Area at the site (see Figure 2.7~. Rates of groundwater flow beneath the Hanford Site generally range from a few to several hundred meters per year, depending on hydraulic gradients and subsurface properties. At the faster rates, contaminants can be transported across the site in a few decades, which has in fact occurred for tritium (Figure 2.8a). Indeed, the groundwater pathway of particular concern at the Hanford Site stretches from the 200 Area on the Central Plateau, where most of the waste inventory and subsurface contamination exist today, to the Columbia River (see Figure 2.1), some 15-20 kilometers distant. As discussed in Chapter 2, chemical processing operations in the 200 Area resulted in the discharge of billions of gallons of water to ponds, cribs, and wells, which raised water table elevations (see Figure 2.6~. These hydraulic mounds have generally accelerated flow rates and, in some cases, have reversed flow directions from natural conditions. Groundwater tends to follow nearly horizontal flow paths in the sediments underlying the Hanford Site. Because of this, groundwater flow 100
Groundwater Technical Element 101 is often modeled as two dimensional, with no vertical structure. In detail, however, groundwater flow is three dimensional. Vertical components of flow may be substantial where contaminants enter the groundwater from disposal areas in the vadose zone. Vertical gradients influence the distribution and transport of contaminants (Figure 7.1 ) and complicate the task of monitoring contaminant movement in the subsurface. The difficulties arising from the three-dimensional nature of contaminant plumes in groundwater are reflected in the science and technology (S&T) plan reviewed in this chapter. 10 E 20 - Q ~ 30 a) 0 40 a) m rat Q 8 so 60 70 . . 1 0 5000 10000 15000 Tc-99 Concentration (pCi/L) Figure 7.1 Technetium-99 concentration gradient below the water table in the 200 West Area. SOURCE: Data for well 299-W10-24, sampled October 9-16, 1998, from Hartman et al., 2000, Table 2.8-3.
102 Science and Technology for Environmental Cleanup Groundwater represents the saturated end member of the vadose zone, and several issues for understanding and characterizing the unsaturated zone discussed in Chapter 6 also apply here, especially with respect to hydrologic complexity and scaling relationships. However, the "upscaling problem" described in Chapter 6 for the vadose zone is of less a concern when describing physical transport mechanisms (i.e., advection, dispersion) in the saturated zone. In particular, established procedures exist for determining large-scale hydrologic properties for Groundwater transport from field tests (e.g., pump tests). Moreover, hydrologic properties of the porous medium can usually be represented by single values (or tensors). Corresponding properties in the vadose zone must be represented by nonlinear functions of saturation that often exhibit hysteresis, as discussed in Chapter 6. Scaling of geochemical properties remains to be developecl, but in general, Groundwater transport of contaminants is easier both to measure and to model than is vadose zone transport. SCOPE OF THE GROUNDWATER TECHNICAL ELEMENT The main sources of information used in this assessment are the Integration Project Roadmap (DOE, 2000a) and briefings received during the committee's information-gathering meetings. The schedule and budget for S&T work under the Groundwater Technical Element are shown in Table 7.1. The Groundwater Technical Element comprises six broad S&T activities and, within these, 20 individual projects (Table 7.1~. To date, only one of these projects has been funded, as discussed in more detail below. 1. Groundwater-vadose zone infefface study. This activity includes four projects (designated GW-1 through GW-4 in Table 4.1 in the Integration Project Roadmap [DOE, 2000a]) designed to better document the relationships between contaminant transport through the vadose zone and the consequent formation and evolution of three-dimensional contaminant plumes in groundwater. The four projects investigate three- dimensional plume structure beneath soil sites for example, cribs and tile drains (GW-1), dilute waste tanks (GW-2), concentrated waste tanks (GW-3), and other waste sites (GWEN. 2. Biogeochemical reactive transport. This activity includes two projects (GW-5, GW-6) to obtain an improved understanding of the effect of redox and complexation reactions on radionuclide (mainly actinide element) transport in Groundwater connected with two plutonium-bearing waste streams and one project (GW-7) to obtain an improved
Groundwafer Technical Element understanding of multiphase reactive transport of DNAPLs, particularly carbon tetrachloride. 103 3. Hydrogeological characterization study. This activity includes five projects (GW-8 through GW-12) to evaluate the variability and scaling of subsurface hydrological parameters that control contaminant transport. One project (GW-8) will approach the problem using existing groundwater data. Two projects (GW-9 and GW-10) will conduct otherwise unspecified "multiple-scale studies" beneath clean and contaminated sites. The remaining two projects (GW-1 1 and GW-12) will synthesize data and construct three-dimensional visualizations of hydrogeological properties under soil and tank sites. 4. Regional plume geometry. The activity includes one project (GW-13) to develop a three-dimensional image of contaminant plumes along a transect extending from the 200 West Area to the Columbia River. 5. Multiscale three dimensional model development. This activity includes three projects (GW-14 through GW-16) to develop approaches for implementing three-dimensional transport models that can be run at multiple scales and three projects to develop methods for incorporating heterogeneity and uncertainty in these models for groundwater under a boiling waste tank (GW-17), a specific retention basin (GW-18), and a dilute waste tank (GW-19~. 6. Groundwater discharge study. This activity includes one project (GW-20) to quantify the three-dimensional plume dynamics at a site along the Columbia River. As shown in Table 7.1, work on these projects is planned to run from fiscal year 2000 through fiscal year 2004. The total planned funding for this technical element is about $16.3 million. Note, however that these funding levels, which were identified in the roadmap, have been revised by the Department of Energy (DOE) (see Table 3.1~. EVALUATION OF WORK PLANNED UNDER THE GROUNDWATER TECHNICAL ELEMENT As of early 2001, only the "groundwater discharge" study (GW-20; see Table 7.1), which is concerned with groundwater in the 100 Area, had been initiated; it was supported by funding from Hanford's "core" groundwater project (Chapter 3~. Consequently, there was little scientific or technical output in the form of peer-reviewed reports or papers available for the committee's evaluation. Accordingly, the committee offers only general comments about the work planned under this technical element, again focused on the five evaluation questions against which the other S&T elements are compared. The lack of specificity in the
104 Science and Technology for Environmental Cleanup TABLE 7.1 Summary of S&T Activities and Planned S&T Projects Under the Groundwater Technical Element S&T Activity S&T Project Objectives Project Hanford EMSP Projects Duration Fundinga Funding Planned (fiscal (thousand (thousand years) dollars) dollars) Groundwater- 4 Obtain an improved 2001-2003 2,100 0 vadose zone understanding of the interface relationships between study contaminant transport through the vadose zone and plume formation in groundwater Biogeochemical 3 Obtain an improved 2001-2004 7,800 0 reactive understanding of redox transport conditions, the role of complexants in transport, and the location and characteristics of DNAPL contamination Hydrogeological 5 Develop an improved 2001-2003 3,000 0 charactenz- understanding and ation study characterization of subsurface heterogeneity on contaminant transport Regional plume 1 Obtain an improved 2001 2,100 0 geometry understanding of the three-dimensional geometry of contaminant plumes in Groundwater Multiscale three- 6 Obtain an improved 2001-2003 1,300 0 dimensional understanding of model heterogeneity and development uncertainty that can be incorporated into multiscale models Groundwater 1 Obtain an improved In ob 0 discharge understanding of progress study contaminant release locations and fluxes to the Columbia River NOTE: EMSP = Environmental Management Science Program The Integration Project intends to seek funding from national S&T programs (e.g., DOE Headquarters) for some of this work. blithe River Monitoring Project (see Chapter 3) is providing funding and leadership for this work. SOURCE: DOE, 2000a, Figure 4-1, Table ~1.
Groundwater Technical Element 105 information available in the groundwater portion of the Integration Project Roadmap precludes a more detailed assessment. Can the objectives of the planned work be achieved? The committee found it difficult to provide a definitive answer to this question because of the lack of technical detail on the planned projects in the Integration Project Roadmap (DOE, 2000a). The S&T objectives may be achievable if the planned work is funded at adequate levels and tied to site decisions. However, some fundamental issues must be resolved to bring these tasks to completion. In particular, the Integration Project may have unrealistic expectations about the time that will be needed to complete some of these studies. For example, task GW-11 (Synthesis and Visualization of Hydrogeology Soil Site) has a 10-month time line. Task GW-12, which has the same general objective for a high-level waste tank site, has a 24- month time line. The expected outcomes of GW-1 1 and GW-12 include providing estimates of small-scale hydrogeological property variability and spatial correlation in a form amenable for use in numerical models, investigating the scale dependence of hydraulic measurements, and investigating important scales of physical and hydrogeological heterogeneity characterization. Issues of scaling that are raised within the scope of these projects are an active focus of research efforts in many scientific disciplines (see Sidebar 6.1 and an expanded discussion of scaling in Appendix C). It is probably more realistic to anticipate that significant progress on scaling issues will be measured on a time scale of 5 to 10 years, rather than the 1- to 2-year time frames allowed for these projects, even if funded at the requested levels. The Integration Project should consider the implications of slower-than-planned progress on these projects for other work at the site (e.g., the SAC) and should adjust the schedules accordingly, if appropriate. Does the planned work represent new science? Again, the lack of detailed information makes it impossible for the committee to identify specific areas of new science. However, opportunities appear to exist to develop new understanding and better quantification of issues such as the three-dimensional nature of contaminant plumes and hydrogeological characterization, both of which are identified in Table 7.1. The research planned on each of these topics is generalizable beyond Hanford. Underlying questions and anticipated outcomes apply in a broad sense to many contaminated sites where remediation and stewardship are planned or under way. It is critical,
106 Science and Technology for Environmental Cleanup however, that this research be conducted at Hanford, in light of the mix of contaminants that have been released to the environment and with respect to transport to and interactions with the Columbia River. Can the planned work have an impact on cleanup decisions at the Hanford Site? Better characterization of groundwater pathways and contaminant fate and transport in the saturated zone has obvious relevance to issues of site remediation and long-term stewardship, especially in the 200 Area. It is clear, however, that the understanding of groundwater flow and transport is more mature than that for vadose zone flow and transport. Consequently, uncertainties in groundwater models at the Hanford Site are small relative to uncertainties in vadose zone and river models. Therefore, S&T directed at refining the understanding of groundwater transport may not be a good investment relative to S&T efforts that are needed to improve the understanding of vadose zone and river transport (see Chapters 6 and 8~. Does the planned work address the important issues? The broad tasks outlined in the Groundwater Technical Element address the core issues that have to be resolved with respect to contaminant fate and transport in groundwater at the Hanford Site. In the course of these studies, sophisticated computational tools may be developed that can aid in making sound site management decisions. Valuable basic data on the hydrogeology of the saturated zone and contaminant distribution in the groundwater system also may be obtained. These data may be important for achieving progress in a number of other site projects, such as the development of long-term monitoring plans for the groundwater system at Hanford. Are there other concerns, comments, or suggestions that should be considered by the Integration Project in executing the planned work? The committee has two concerns. First, as noted previously, detailed project descriptions do not appear to exist in many cases, and written descriptions of the projects in the Integration Project Roadmap (DOE, 2000a) were too brief to determine how likely it is that the projects will meet their objectives. Second, although the projects may provide valuable contributions to science, it is not clear whether the S&T results are needed for site decision making. Essentially all of the projects are assigned to the priority ranking "Critical to the success of the Accelerated Cleanup: Path to Closure" project in the Integration Project Roadmap
Groundwater Technical Element 107 (DOE, 2000a). The discussion in Appendix B of DOE (2000a) of the consequences of not filling a particular research need, although valid, provides little substantive information to guide a prioritization effort. Therefore, the committee recommends that a more selective system of prioritization be developed for these projects and that each project be referenced to this prioritization system before subsequent funding cycles begin. A more detailed discussion of prioritization is provided in Chapter 10. DISCUSSION The Integration Project has clearly assigned a lower priority to the Groundwater Technical Element than to the Vadose Zone Technical Element, as shown by the planned funding levels and schedules in Tables 3.1, 6.1, and 7.1. According to the Integration Project Roadmap (DOE, 2000a), there were plans to start 14 of the 20 Groundwater activities by February 2001. As of March 2001, only one Groundwater activity (GW-20) was under way. Although the documentation of detailed research plans is sparse, the planned S&T activities in the Groundwater Technical Element appear to identify a set of projects and investigations that can add confidence to the assessment of contaminant migration in Groundwater at Hanford. Because Groundwater modeling has progressed to a greater degree than many other S&T issues discussed in this report, the committee agrees with the Integration Project's decision to assign a lower priority to the Groundwater Technical Element relative to the other technical elements. The committee notes, however, that the basis for this decision does not appear to be documented and was therefore not reviewable. Among the activities included with the Groundwater Technical Element, assignment of the highest priority to the groundwater-river interface study (GW-20) is clearly driven by the intensive restoration efforts under way along the Columbia River (see Chapter 2~. This research activity is likely to have a more immediate return in better managing current cleanup activities along the river corridor than the other projects planned under this technical element.
