F.4.0 FLOW AND TRANSPORT MODEL
The vadose zone and groundwater modeling effort used VAM2D to predict contaminant migration through the vadose zone and groundwater. VAM2D has been previously used for flow and transport assessments at the Hanford Site. The model formulation used in the code is a descendant of that used in the SATURN code presented by Huyakorn et al. (Huyakorn et al. 1984, 1985) and was developed by HydroGeoLogic Inc. (Huyakorn et al. 1991).
The approach used for this modeling effort relies as much as possible on extensive previous work completed at the Hanford Site (e.g., hydrogeological investigations and modeling studies). Understanding and being able to predict changes in the hydraulic head of the unconfined aquifer is in an advanced stage at the Hanford Site. However, contaminant transport in the unconfined aquifer and flow and contaminant transport in the vadose zone are still in relatively early stages of understanding and development. The modeling approach was as follows:
- A combined groundwater flow and transport code (VAM2D) was used.
- Hydrogeologic and contaminant transport parameters from previous studies, including Wood et al. (Wood et al. 1995), Kincaid et al. (Kincaid et al. 1993), and Wurstner and Devary (Wurstner-Devary 1993) were used in this modeling effort.
- The VAM2D flow model of the unconfined aquifer at the Hanford Site was developed based on a previously published Sitewide calibrated groundwater flow model developed with the CFEST code (Wurstner-Devary 1993). The VAM2D flow model of the unconfined aquifer was then benchmarked against these results.
Details of the approach used to test the model are provided in the following sections.
F.4.1 FEATURES OF THE VAM2D FLOW AND TRANSPORT CODE
VAM2D is a 2-D, finite element model developed for simulating saturated and unsaturated flow and transport. Using a single model code for both vadose zone and groundwater modeling simplified the combined modeling effort. VAM2D is capable of performing flow and transport simulations in vertical cross-sections as well as horizontal orientations.
The capabilities of VAM2D applicable to the TWRS EIS modeling effort include the following:
- Simulates flow and transport in saturated and unsaturated zones;
- Solves flow and transport simultaneously or sequentially;
- Accommodates a wide range of field conditions;
- Computes hysteretic effects on flow because of wetting and drying cycles; and
- Computes the effects of variable anisotropic hydraulic conductivities on flow in stratified media.
F.4.2 LIMITATIONS OF THE TRANSPORT MODEL
Limitations of VAM2D specified in the user's manual (Huyakorn et al. 1991) include the following:
- Does not simulate three-dimensional flow. However, a 2-D analysis is appropriate for the site in that there is a lower confirming bed in the Ringold Formation, and sufficient data to develop a three-dimensional flow and transport model may not be available;
- In performing variably saturated flow, the code handles only single-phase flow (i.e., water) and ignores the flow of a second phase (i.e., air or other nonaqueous phase). This is not a concern as aqueous phase liquids are not reported in site inventory;
- The code does not address kinetic sorption effects and/or reversible chemical reactions; and
- The groundwater flow portion of the model was executed for steady-state conditions. This did not allow simulation of the decay of the groundwater mounds associated with waste disposal activities.
F.4.3 RELIABILITY TESTING OF CALCULATED RESULTS
Several tests may be performed to demonstrate a model's ability to reasonably predict flow and contaminant transport. These include:
- Verification - Comparing the numerical solutions generated by the model with one or more analytical solution or with other solutions.
- Benchmarking - Testing the model solution against the solution of other models for the same problem.
- Calibration - Establishing that the model can reproduce field-measured conditions.
- Validation - Comparing model results with detailed field data.
- Parameter Sensitivity - Quantifying the uncertainty in the calibrated model caused by uncertainty in the estimates of the parameters used.
The following sections describe reliability testing performed on VAM2D for the Hanford Site.
F.4.3.1 Verification
A number of tests were performed to ensure reliability of the code on the computer platform used for the modeling effort (IBM RS/6000 workstation) and to compare results with known analytical solutions. These included the following:
- Initially verifying the model against sample problem 1 in the VAM2D User's Manual;
- Verifying results for saturated flow against an analytical solution (Dupuit solution) (Fetter 1994); and
- Verifying results for saturated transport against an analytical solution (Domenico solution [Domenico 1985]).
The sample problem results matched the results published in the VAM2D User's Manual for head, saturated value, and x-velocity. Y-velocity values differed slightly, however the differences were less than 2.0E-13. The results published in the user's manual were for VAM2D Version 5.2; Version 5.3 was used for this modeling effort.
For the flow problem to be solved with the Dupuit solution, a simple model was constructed and solved for unconfined flow with fixed head boundaries at each end and fixed across a transect. The analytical solution was calculated for several points and compared to the model results. VAM2D model results very closely matched the Dupuit solution. Results calculated by VAM2D compared to the analytical solution are provided as follows.
| Distance from
Left Boundry |
Head Calculated
by VAM2D |
Head Calculated
by Dupuit Solutions |
| 12.5 m | 5.844 m | 5.846 m |
| 37.5 m | 4.90 m | 4.981 m |
For the transport problem to be solved with the Domenico solution, a simple 2-D model was constructed and solved for transient transport. A contaminant was input at one grid node and a transient model run was performed to predict contaminant concentrations for several node points at a specified point in time (300,000 days). Concentrations based on the Domenico solution were calculated for several points and compared to the model results. VAM2D model results very closely matched the Domenico solution. Results calculated by VAM2D compared to the analytical solution are provided as follows.
| Grid Location | Concentration Calculated | Concentration Calculated | |
| Delta X | Delta Y | by VAM2D | by Domenico Solution |
| 10 m | 0 m | 453.3 mg/L | 455.71mg/L |
| 20 m | 5 m | 85.08 mg/L | 83.15 mg/L |
| 30 m | 0 m | 270.5 mg/L | 273.13.73 mg/L71mg/L |
| 40 m | 10 m | 14.60 mg/L | |
F.4.3.2 Benchmarking
The groundwater flow model effort was developed and benchmarked as follows:
- Unconfined aquifer flow parameters and boundary conditions used to set up the VAM2D model were developed from published groundwater flow modeling work using CFEST (Wurstner-Devary 1993). This effort has undergone verification, calibration, and quasi-validation efforts, which were initiated in the mid-1960's.
- VAM2D predictions of hydraulic head were compared to these published CFEST results (Wurstner-Devary 1993). Basic differences in the model input requirements and the grid used required minor adjustments, primarily to boundary discretization, to obtain a closer match.
Figure F.4.3.1 shows both the published results from prior model development and the hydraulic heads calculated with the VAM2D model. As expected, this figure indicates good agreement between the VAM2D results and the previously published results.
F.4.3.3 Calibration
Calibration of a groundwater model consists of comparing its results to an independent standard. Changing flow conditions at the Hanford Site make an absolute calibration infeasible. However, a qualitative calibration can be performed. This qualitative calibration begins by examining the geometry of a tritium plume that is present on the Hanford Site and estimating tritium travel times from the 200 East Area to the Columbia River. Contaminants originating from the 200 East Area are estimated to take approximately 20 to 25 years to reach the Columbia River. The estimated travel time is based on site operations beginning in the 1940's and detection of contaminants in springs and groundwater in the 1970's.
In this qualitative calibration effort, the VAM2D model was used to simulate a contaminant concentration of 200,000 mg/L source originating from B Pond in the 200 East Area. Discharge fluid fluxes were based on 1979 data, and the transient transport simulation was based on the steady-state field (also based on 1979 data). Figure F.4.3.2 provides estimates of tritium levels observed in groundwater based on 1977 environmental monitoring (Meyers 1978). Figure F.4.3.3 provides the 300 mg/L isoconcentration lines for tritium at 10, 20, and 30 years, assuming this constant discharge rate. Figure F.4.3.3 demonstrates that the travel times calculated by VAM2D correspond well with the assumed 20- to 25-year travel time. Additionally, the plume geometry for the tritium plume originating from the 200 East Area (Figure F.4.3.2) is similar to the predicted plume geometry (Figure F.4.3.3). An exact match between these two plumes should be not expected because discharge amounts varied substantially over time, and the observed tritium plume (Figure F.4.3.2) was created by multiple sources. However, similarities between the two plume geometries indicate that the VAM2D results are reasonable.
F.4.3.4 Validation
Validating a groundwater model consists of comparing model results with detailed field data. However, rigorous validation requires accurate historic data on effluent discharges as a function of time.
Although data are available on flow and transport within the unconfined aquifer, the data set is not sufficient to perform a detailed model validation. Flow conditions have changed dramatically since the early 1940's, primarily as the result of changes in wastewater discharges. Historic records of effluent amounts and water quality have not been maintained since that time in sufficient detail to perform a rigorous validation of flow and transport in the unconfined aquifer.
F.4.3.5 Parameter Sensitivity
Parameter sensitivity was investigated for the following areas:
- The effect of higher glass surface areas for the In Situ Vitrification alternative;
- The effect of changing the performance period of the Hanford Barrier from 1,000 to 500 years;
- The effect of the decay of the potentiometric head resulting from groundwater mounding due to discharge to the Hanford Site ponds;
- The effect of variations in filtration rate; and
- The effect of variations in distribution coefficient (Kd).
The approach and conclusion from these investigations are provided in Volume Five, Appendix K.
F.4.4 MODELING ASSUMPTIONS AND UNCERTAINTIES
This appendix provides the basis of potential groundwater impacts associated with each of the TWRS alternatives. Developing the groundwater assessments provided in this appendix required several assumptions to uncertainties of some of the data. The major assumptions and uncertainties are related to either the natural system (i.e., an understanding and ability to assign vadose zone and aquifer parameter values) or uncertainties inherent to the assessment approach.
The most important assumptions and uncertainties are as follows:
- The rates of infiltration into natural ground and through a cap;
- Distribution coefficient (Kd) of contaminants;
- Uncertainty in future groundwater flow direction due to decay of groundwater mounds onsite;
- Uncertainty in future groundwater flow direction and vadose zone thickness due to climate change;
- Uncertainty in vadose zone transport due to use of one-dimensional flow and transport simulation; and
- Uncertainty due to calculation of releases during retrieval.
The basis for these assumptions and their potential impact on the alternatives is provided in Volume Five, Appendix K.
F.4.5 CONCEPTUAL GROUNDWATER CUMULATIVE IMPACTS
This section addresses potential cumulative groundwater impacts of other past and projected future waste disposal activities. The activities that may have a cumulative impact on the TWRS alternatives are as follows :
- Past-practice waste disposed of to the ground as liquid;
- Past leaks from waste tanks;
- Past-practice waste disposed of to the ground as solid;
- Solid low-level radioactive waste to be disposed of in the Environmental Restoration Disposal Facility (ERDF);
- Solid low-level radioactive waste to be disposed of in the 200 West Area burial grounds; and
- Solid low-level radioactive waste to be disposed of in the US Ecology burial grounds.
These activities result in both near-term and long-term groundwater impacts. The near-term impacts are in response to past-practice liquid waste disposal to the ground. Large volumes, over 1.29E+12 L (3.40E+11 gal) in the 200 Areas, containing radionuclides and hazardous chemicals have been discharged to the ground surface or subsurface since 1944 (Wodrich 1991). Long-term groundwater impacts are associated with 1) leaching of solid waste disposed of to the ground in the 200 Areas and on the Central Plateau (Wood et al. 1995), and with 2) the relatively low-volume leaks from the waste tanks, as compared with volumes discharged to cribs and ponds. It is assumed that all of these disposal activities, except for the past-practice liquid disposal, would have some cumulative impact with respect to the TWRS activities. Quantitative information, such as would be developed for a performance assessment, on the fate of current contaminant plumes resulting from past-practice liquid waste disposal is not available; however, the following discussion suggests these contaminants will not interact with groundwater contaminant plumes associated with the TWRS alternatives.
Potential cumulative impacts with respect to contaminants C-14, I-129, Tc-99, and uranium are provided in the following sections for each of the solid waste disposal facilities. These contaminants were chosen for comparison because they have high mobility in the Hanford vadose zone and groundwater, have been routinely monitored in the groundwater, and have been identified as contributing much of the tank waste-related risk.
F.4.5.1 Past-Practice Liquid Waste Disposal
Liquid waste disposal has resulted in extensive groundwater contamination in the 200 Areas as well as downgradient toward the Columbia River. Information on specific contaminants disposed of to ground surface or subsurface is limited to only a few key constituents including nitrate and radionuclides with half-lives greater than 10 years and in quantities large enough to be of concern in waste disposal and cleanup (Wodrich 1991). These radionuclides are Sr-90/Y-90, Cs-137, Tc-99, I-129, uranium, Am-241, and plutonium. Table F.4.5.1 provides a comparison of the inventories estimated for the past-practice liquid and solid waste disposal, past waste tank leaks, and TWRS tank waste. Quantitative estimates of contaminant concentration in groundwater with an acceptable degree of uncertainty from past-practice liquid waste disposal activities are not possible using available information. Key information that is not available includes definition of the multiple source terms (e.g., waste volume, contaminant concentration, release duration) and residual waste remaining in the vadose zone. A semi-quantitative approach coupled with some qualitative assumptions is used because of these limitations.
The past-practice liquid waste disposal impacts on groundwater are believed to be ongoing and would be greatly reduced by the time the TWRS alternatives would potentially impact groundwater. Thus, they are considered near-term impacts. These conclusions are based on several assumptions and on observations of groundwater contaminant concentration trends discussed later in this section. The assumptions are as follows:
- Present groundwater contaminants, concentration levels, and distribution in the 200 Areas and downgradient are a result of the past-practice liquid disposal in the 200 Areas.
- All liquid waste disposal to the ground at previously used waste disposal facilities (e.g., cribs, trenches, drains, and reverse wells) has been stopped or will be stopped by the year 2000.
- There will be no new ground disposal of radioactive or hazardous chemical-containing liquids, except for tritium.
- The remediation alternative for the past-practice liquid waste disposal sites will be installation of caps by the year 2005.
- Less mobile contaminants in the past-practice liquid waste may contribute to the cumulative impact but are not considered at this time.
Given these assumptions, the present concentrations of highly mobile contaminants in groundwater such as tritium, Tc-99, I-129, nitrate, and to a lesser extent, uranium currently would be experiencing a large reduction in concentration that would continue for less than 10 years, followed by many years where the contaminant concentration in groundwater diminishes at a much slower rate. Change in uranium concentrations in well 299-W19-18 is an example of this process. This well is located in the 200 West Area adjacent to the inactive 216-U-1 and 216-U-2 cribs. The uranium concentration in this well has reduced at a uniform rate of approximately 3,000 µg/L for a 2 to 3-year period since remediation of the cribs in 1988 (Woodruff-Hanf 1993). By the end of 1992, the uranium concentration in this well was approximately 750 µg/L and the rate of reduction had dropped to approximately 80 ug/L/yr. The rate of concentration reduction is expected to continue to decline but at a very slow rate such that the uranium concentration at this well would appear to become constant at some low level. This level is not known and is assumed to be inconsequential by the time contaminants from the tanks arrive at the groundwater. This early reduction in concentration also is observed for tritium in observation well 699-24-33 (Woodruff-Hanf 1993).
In the performance assessment for the low-level waste burial grounds in the 200 West Area (Wood et al. 1995), it is concluded that mixing of the present day plume with that from the burial grounds is unlikely. These burial grounds include disposal sites with and without caps, thus times to peak groundwater contaminant concentrations range from approximately 125 to 1,000 years from present. The performance assessment presents the following discussion to support this conclusion. First, the particle velocity in the unconfined aquifer, on the order of 10 m/yr, would result in the migration of the present plume a few hundred meters over a few decades (Wood et al. 1995). Secondly, additional plume generation is unlikely because liquid discharge nearly has ceased, and it is likely that only very small quantities of the mobile radionuclides such as Tc-99, C-14, and I-129 remain in the present soil column. Other less mobile radionuclides are present in the soil column. They are believed to be short-lived (Wood et al. 1995) and would decay to inconsequential quantities before reaching the unconfined aquifer.
Of all the TWRS alternatives, the No Action and Long-Term Management alternatives have the earliest potential groundwater impact. First arrival of contaminants to the groundwater has been estimated to occur at about 140 years for these alternatives. Estimated first arrival of contaminants to groundwater for the other alternatives ranges from approximately 1,070 years for the ex situ alternatives to 2,330 years for the in situ alternatives. Cumulative impacts with respect to past-practice liquid disposal likely would be very low for the ex situ and in situ alternatives and, with a larger degree of uncertainty, is assumed to be very low for the No Action and Long-Term Management alternatives.
F.4.5.2 Past Leaks From the Single-Shell Tanks
Liquid waste from past tank leaks has resulted in vadose zone contamination beneath the leaking tanks and may be impacting the groundwater in the vicinity of the tanks. Potential groundwater impacts are currently being investigated as part of Resource Conservation and Recovery Act (RCRA) Groundwater Assessments for the T Farm Waste Management Area and will be ongoing soon for the S-SX and B-BX-BY Waste Management Areas.
Past SST tank leaks are considered to result in long-term groundwater impacts (compared to impacts from liquid waste disposal discussed in Section F.4.5.1) because the leak volume was, for the most part, insufficient to immediately flush the contaminants all the way through the vadose zone and into the underlying groundwater. Under current conditions (e.g., no cap over the tanks), impacts to the underlying groundwater are expected to occur over a period similar to that predicted for the No Action alternative, which is approximately 300 years. Groundwater impacts from past tank leaks would be expected to begin soon and may already be occurring because contaminants from the leaks are likely distributed vertically in the vadose zone from the tank bottoms to near the water table. A bounding approach is used to the extent practicable to estimate potential impacts from past waste tank leaks. The leak volume is taken as the upper range of the cumulative leak volume as provided in the inventory and surveillance reports (Hanlon 1996). The release to the groundwater is assumed to be analogous to release to the groundwater in the No Action alternative. Provided in the following discussion are the estimated leak volume, radioisotope content of the leaks, and the potential impact of the leaks on groundwater.
Leak monitoring is ongoing for the 177 waste tanks, and reports on waste inventory and surveillance are released monthly and quarterly. The report for the month ending February 29, 1996 (Hanlon 1996) indicates that 67 of the 149 SSTs are assumed leakers. There are no reported leaks from the 28 DSTs. The tank identification number, date tank was declared leaker, estimated leak volume, estimated activity of leak, and date the tank was stabilized are provided in Table F.4.5.2. The range of leak volume is from approximately 1,300 L (350 gal) from tank 241-C-204 in the 200 East Area to 436,000 L (115,000 gal) from tank 241-T-106 in the 200 West Area. Total leak volume from all 67 assumed leakers ranges from 2.30E+06 to 3.4E+06 L (600,000 to 900,000 gal). Interim stabilization has been completed on all but five assumed leaking tanks.
The monthly tank waste and surveillance reports (Hanlon 1996) provide a estimated range of Cs-137 associated with the waste tank leaks. However, quantitative estimates of radioisotopes such as Tc-99, C-14, I-129, and U-238 in the liquids that leaked from the waste tanks are not available. The activities of Tc-99 and uranium (assumed to be U-238) isotopes that would have been released with the tank leaks were estimated based on the total and isotopic activity of liquid waste disposal in the 200 Areas (including waste tank leaks) (Wodrich 1991). These estimates, provided in Table F.4.5.2, include an upward adjustment to account for the upper bound of leak volume of 3.4E+06 L (900,000 gal). (Hanlon 1996). The amount or activity of nitrate and C-14, respectively, was estimated based on the 3.4E+06 L (900,000 gal) cumulative leak volume and the concentration of each contaminant in the tanks as shown on Table F.2.2.14. Nitrate is assumed to have a concentration of 3.6E+02 g/L in all the tanks. The concentration of C-14 varies from tank to tank, therefore, the maximum concentration in any one SST source area was used, which was 6.74E-05 g/L in source area 2ESS. The estimated past leak quantities for these constituents are provided in Table F.4.5.2.
The potential impacts to groundwater are provided for waste tank leaks in terms of maximum potential concentration of four critical isotopes. These estimated values are provided in Table F.4.5.3. The estimated maximum concentrations of the selected contaminants in groundwater range from approximately 1 percent (for nitrate) to 25 percent (for U-238) of the maximum predicted values for the No Action alternative.
F.4.5.3 Past-Practice Solid Waste Disposal
Quantitative estimates of contaminant concentration in groundwater with an acceptable degree of uncertainty from past-practice solid waste disposal activities is not possible with the present available information. As with past-practice liquid waste disposal, key information not available includes definition of the multiple source terms (e.g., waste volume, contaminant concentration, release duration). A semi-quantitative approach is used because of these limitations.
The approach is based on the premise that the potential impacts from the In Situ Fill and Cap alternative can be used as an analog for estimating impacts from past-practice solid waste disposal. This estimate is conservative given the following major assumptions.
- The remediation alternative for the past-practice solid waste disposal sites will be installation of caps by the year 2005.
- The inventory of past-practice solid waste is in proportion to the distribution of waste in the tanks.
Contaminants from past-practice solid waste disposal would be expected to reach the groundwater at approximately the same time as contaminants from the In Situ Fill and Cap alternative, given the previous assumptions. Based on the ratio of estimated past-practice solid waste disposed to waste in tanks for C-14 and uranium (Table F.4.5.2), a factor of 1.2 is used to adjust the calculated groundwater concentrations upward from the In Situ Fill and Cap alternative. This is a semi-quantitative approximation of the potential impacts of the past-practice solid waste disposal. Table F.4.5.3 provides the potential maximum groundwater concentrations for Tc-99, I-129, C-14, and uranium. Maximum groundwater impacts of the past-practice solid waste disposal activities would occur at approximately 5,000 years based on the In Situ Fill and Cap alternative analog.
F.4.5.4 Solid Low-Level Radioactive Waste Disposal in the Environmental Restoration Disposal Facility
The proposed ERDF is a deep-lined trench disposal facility for the waste generated by the Comprehensive Environmental Response, Compensation, and Liability Act of 1980 cleanup activities. The ERDF will be located adjacent to the southwest corner of the 200 West Area. The waste will be disposed of primarily in bulk noncontainerized form and is anticipated to consist primarily of contaminated soils and concrete rubble (Wood et al. 1995a). There are currently two principal documents that provide calculated groundwater dose information: the Remedial Investigation and Feasibility Report for the Environmental Restoration Disposal Facility (DOE 1994h) and the Environmental Restoration Disposal Facility Performance Assessment (Wood et al. 1995a). The performance assessment (PA) was used herein as the basis for potential ERDF inventory and groundwater contaminant concentrations because the approach taken in the PA is similar to that used to calculate groundwater impacts from the TWRS waste tanks.
The projected ERDF inventory for Tc-99, I-129, C-14, and uranium is provided in Table F.4.5.2 (Wood et al. 1995a). The PA provides calculated groundwater drinking dose estimates for these radionuclides based on consumption of 730 L/yr (193 gal/yr). The maximum groundwater concentration for these radionuclides is calculated from the maximum dose using the drinking water consumption rate assumed in the PA and the DOE internal dose factor (Wood et al. 1995a).
Using Tc-99 as an example, the maximum groundwater concentration is calculated as follows. The reported maximum drinking water dose is 0.007 mrem/yr (Wood et al. 1995a) and the DOE internal dose factor for Tc-99 is 1.3E-06 mrem/pCi (Wood et al. 1995a). The maximum groundwater concentration of Tc-99 is calculated by dividing the reported maximum dose of 0.007 mrem/yr by the consumption rate of 730 L/yr (193 gal/yr) and the internal dose factor of 1.3E-06 mrem/pCi. This results in a maximum Tc-99 concentration in groundwater of 7.38 pCi/L. This maximum concentration would occur at approximately 1,500 years from present, assuming a Kd of zero and infiltration rate of 0.5 cm/yr (0.2 in./yr) (Wood et al. 1995a). Calculated maximum groundwater concentrations for Tc-99, I-129, C-14, and uranium are provided in Table F.4.5.3.
F.4.5.5 Solid Low-Level Radioactive Waste Disposal in the 200 West Burial Grounds
The 200 West low-level waste burial grounds consist of shallow (5 to 10 m deep [16 to 33 ft]), unlined trenches of variable widths (3 to 10 m wide [10 to 33 ft]), and lengths (50 to 100 m long [160 to 330 ft]). Potential groundwater impacts have been calculated in the Performance Assessment for the Disposal of Low-Level Waste in the 200 West Area Burial Grounds (Wood et al. 1995). This performance assessment examines the potential groundwater impacts from disposal of waste in two different facility types. The first, called a Category 1 waste facility, is assumed to have no functional barriers (e.g., cap) and is intended to contain very low concentrations of radionuclides. The other facility is called a Category 3 waste facility and is assumed to have a cap that controls infiltration to the same degree as the natural soil and vegetative system (Wood et al. 1995). Radionuclide inventory for each waste category is provided in Table F.4.5.2.
The maximum groundwater contaminant concentration for Tc-99, I-129, C-14, and uranium was calculated as described in Section F.4.5.3 and is provided in Table F.4.5.3.
F.4.5.6 Solid Low-Level Radioactive Waste Disposal in the US Ecology Burial Grounds
The US Ecology Burial Grounds is a commercial low-level waste disposal facility located on the Central Plateau just southwest of the 200 East Area and approximately 4 km (2.5 mi) east of the 200 West Area. Radionuclide inventory and maximum groundwater concentrations for Tc-99, I-129, C-14, and uranium were estimated for the US Ecology site at closure (Jacobs 1996). These values are based on preliminary estimates of future solid radioactive waste emplacement at the site. The estimates assume closure of the facility in about the year 2063. The inventory and maximum groundwater concentrations are provided in Tables F.4.5.2. and F.4.5.3, respectively.
F.4.6 Groundwater Impacts for Nominal Case
The preparation of the groundwater impacts assessment required numerous assumptions concerning not only the subsurface conditions that affect fate and transport through the vadose zone and unconfined aquifer but also the contents of the waste tanks and the release of waste during remediation. Bounding assumptions were used that would result in calculations of impacts than would be conservative compared to impact results based on average or nominal assumptions. This section provides calculated groundwater impacts for nominal estimates of waste tank releases for a scenario modified from the Ex Situ Intermediate Separations alternative. All other approaches and assumptions relative to fate and transport in the vadose zone and groundwater are the same as were used for calculating the groundwater impacts for the Ex Situ Intermediate Separation alternative summarized in Section F.3.5.
F.4.6.1 Nominal Case Source Term
The source term for this scenario is a result of releases from SSTs during waste retrieval, releases from the residuals in SSTs and DSTs, and releases from the LAW vaults. Only the long-term mobile risk contributing contaminants are considered for this scenario. These contaminants are I-129, C-14, Tc-99, and U-238. The grouping of these contaminants is the same for the base case Ex Situ Intermediate alternative scenario except for Np-237. The base-case Ex Situ Intermediate Separations alternative includes Np-237 with the above group of long-term mobile risk contributing contaminants. There is a large uncertainty surrounding the mobility of Np-237 in the Hanford Site vadose zone and unconfined aquifer and for the bounding impact analyses, it was conservatively placed in Kd group 1 (Kd = 0), which means that Np-237 would move at the same rate as the water in the vadose zone and underlying aquifer. For the nominal case scenario, Np-237 is assumed to have a Kd of 1 mL/g. In the following, a discussion of each of the three potential sources for the nominal case scenario is provided.
Released During Waste Retrieval
As with the bounding scenario for the Ex Situ Intermediate Separations alternative, retrieval releases only occur from the SSTs. The DSTs are assumed to have no releases during retrieval. Retrieval occurs over a 15-year period and the work is assumed to be ongoing at all eight of the source areas during this period. The infiltration scenario is the same as that assumed for the Ex Situ Intermediate Separations alternative, where it would decrease to 0.5 cm/yr (1.36E-05 m/day) for a 29-year period (15-year period of waste retrieval followed by a construction period or 14-years) from 5.0 cm/yr (1.36E-04 m/day). Infiltration through the Hanford Barrier at the end of construction is assumed to be 0.05 cm/yr (1.36E-06 m/day) for a 1,000-year period. It is assumed to double to 0.10 cm/yr (2.74E-06 m/day) after the 1,000-year period and remain at that level for the remainder of the period of interest.
The assumed release volume of 15,000 L (4,000 gal) per SST is retained for this scenario. For the nominal case, the contaminant concentration in the retrieval releases is two-thirds of the concentrations assumed for the Ex Situ Intermediate Separations alternative. The contaminants released during retrieval, their estimated mass and concentrations are provided in Tables F.4.6.1 and F.4.6.2. For this analysis, only the long-term risk contributors Tc-99, I-129, C-14, and U-238 are considered.
Releases from Waste Tank Residuals
The bounding scenario for the Ex Situ Intermediate Separations alternative incorporates an assumption that 1 percent of initial total tank waste remains after retrieval as a residual. This assumption does not account for recovery of the more soluble constituents during hydraulic retrieval. For the nominal case scenario, mobile soluble constituents in the residual inventory for the base case Ex Situ Intermediate Separations alternative are reduced, based on sludge wash factors reported in WHC-EP-0616. The residual inventory, concentration, and duration of release for the long-term risk contributors are provided in Table F.4.6.3, F.4.6.4, and F.4.6.5, respectively.
Releases from the LAW vaults
The releases from the LAW vaults for this scenario have not been modified from the bounding Ex Situ Intermediate Separations alternative because their contribution to overall risk in very small. The LAW vault inventory and initial contaminant concentrations are provided in Table F.2.2.6 and F.2.2.19, respectively.
F.4.6.2 Calculated Impacts for the Nominal Case Scenario
The calculated maximum contaminant concentrations from tank sources (i.e., waste released during retrieval from SSTs and residual waste released from SSTs and DSTs) are provided in Table F.4.6.5. Maximum calculated contaminant concentrations from the LAW vault sources are the same as were calculated for the Ex Situ Intermediate Separation Alternative LAW vault sources (Table 3.5.2).
The calculated concentrations from tank sources are lower than calculated concentrations for the Ex Situ Intermediate Separations alternative tank sources (Table F.3.5.1), as would be expected. Absent from this scenario is the impact of Np-237 because with a Kd of 1 mL/g, its movement in the vadose zone is sufficiently retarded such that it does not reach the unconfined aquifer within the 10,000-year period of interest. Provided in Volume Three, Appendix D, are the calculated risk values based on the nominal case groundwater concentrations.
Figure F.3.3.6 Predicted Carbon-14 Concentrations in Groundwater at 2,500 Years for the In Situ Fill and Cap Alternative
Figure F.3.8.3 Predicted Uranium-238 Concentrations in Groundwater at 5,000 Years for the Ex Situ/In Situ Combination 1 Alternative (Tank Sources Only)
Figure F.3.8.9 Predicted Technetium-99 Concentrations in Groundwater at 10,000 Years for the Ex Situ/In Situ Combination 1 Alternative (LAW Vault Sources Only)
Figure F.3.8.10 Predicted Uranium-238 Concentrations in Groundwater at 10,000 Years for the Ex Situ/In Situ Combination 1 Alternative (LAW Vault Sources Only)
Figure F.3.9.9 Predicted Technetium-99 Concentrations in Groundwater at 10,000 Years for the Ex Situ/In Situ Combination 2 Alternative (LAW Vault Sources Only)
Figure F.4.3.1 Comparison of Groundwater Elevations Predicted by VAM2D and CFEST
Figure F.4.3.3 Predicted Tritium Concentrations in Groundwater
Table F.3.1.1 Maximum Concentrations Calculated for the No Action Alternative
Table F.3.2.1 Maximum Concentrations Calculated for the Long-Term Management Alternative
Table F.3.3.1 Maximum Concentrations Calculated for the In Situ Fill and Cap Alternative
Table F.3.4.1 Maximum Concentrations Calculated for the In Situ Vitrification Alternative
Table F.4.5.1 Summary of Tank Leak Estimates from Single-Shell Tanks
Table F.4.6.1 Inventory of Contaminants Released During Retrieval - Nominal Case (Tank Waste)
Table F.4.6.2 Concentration of Contaminants Released During Retrieval For Nominal Case (Tank Waste)
Table F.4.6.3 Inventory of Residual Contaminants Released - Nominal Case
Table F.4.6.4 Concentration of Residual Tank Waste Releases - Nominal Case
Table F.4.6.5 Contaminant Releases Modeled for Nominal Case
Table F.4.6.6 Maximum Concentration Calculated for the Nominal Case - Tank Sources
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