FY00 Initial Assessments for S SX Field Investigation Report (FIR):

Transcription

1 PNWD-3111 FY00 Initial Assessments for S SX Field Investigation Report (FIR): Simulations of Contaminant Migration with Surface Barriers M. D. White M. Oostrom M. D. Williams July 2001 Prepared for CH2M Hill Hanford Group, Inc. under Solicitation Tank Farm Modeling Battelle Agreement Battelle, Pacific Northwest Division Richland, Washington 99352

2 Summary In support of CH2M Hill Hanford Group, Inc. s (CHG) preparation of a Field Investigative Report (FIR) for the Hanford Site Single-Shell Tank Waste Management Area (WMA) S SX, a suite of numerical simulations of flow and solute transport were executed to predict the performance of surface barriers for reducing long-term risks from potential groundwater contamination at the S SX WMA. The scope and parametric data for these simulations were defined by a modeling data package provided by CHG. This report documents the simulation of fourteen cases involving two-dimensional cross-sections through the S SX WMA and one three-dimensional domain of a single tank (quarter symmetry) within the S SX WMA. The suite of twodimensional simulations were used to investigate the impact of surface barriers, waterline leaks, clastic dikes, nonuniform inventories, inventories displaced toward the water table, concentration dependent density and viscosity for the transporting fluid (i.e., water), and meteoric recharge. The three-dimensional simulation was used to investigate the impact of dimensionality on the numerical predictions. Four transported solutes were considered: technetium-99 (Tc 99), cesium-137 (Cs 137), nitrate (NO 3 ), and chromium (Cr). The large quantity of simulation data makes its reproduction in numerical, graphical or visual form impractical. Therefore, selected results are presented in this report, with a majority of the data being archived in electronic form. The two principal objectives of this work were to conduct the simulations and analyses using an open scientific approach and to provide modeling results that could be verified and repeated. In partial fulfillment of these objectives, the source coding for the STOMP simulator, ancillary utilities coding, input files, simulation output files, and converted result files have been archived in electronic form, with sufficient detail to repeat the calculations reported herein. All simulations comprised steady-flow and transient components, where flow fields developed from the steady-flow component were used to initialize the transient simulation. Steady-flow initial conditions were developed by simulating from a unit hydraulic gradient condition to a steady-flow condition, dictated by the initial meteoric recharge at the surface, water table elevation, water table gradient, no flux vertical boundaries, soil-type zonations and hydrologic properties and location of impermeable tanks. From these starting conditions, transient simulations of solute transport were executed for a 1000-year period (i.e., year 2000 to 3000) that involved changes in the flow fields in response to the application of surface barriers, water-line leaks, or soluteconcentration dependent density and viscosity. The physical domains for the twodimensional simulations were east-west sections across the S SX WMA boundary. These domains were discretized with grid resolutions of m (1.75 ft) in the horizontal direction and m (1.5 ft) in the vertical direction, yielding 42,900- to 48,516-node grids. The simulations involving a clastic dike used grid refinement to resolve the clastic dike, yielding 50,232-node grids. The physical domain for the threedimensional simulation was a quarter section of tank SX 108. This domain was modeled at the same grid resolution as the two-dimensional simulations, yielding a 119,422-node grid. Execution times for these simulations varied from 20 to 120 hours, with the longest executions occurring for the three-dimensional simulations and the i

3 density and viscosity dependent two-dimensional simulations. Mass balance errors over the 1000-year simulation period for the solute species ranged between 2.06 x 10-7 and 5.94 x 10-5 percent for pressure and temperature dependent density and viscosity, and between 2.19 x 10-2 and 9.83 x 10-2 percent for those simulations involving nitrateconcentration-dependent density and viscosity. A principal objective of this investigation was to evaluate the effectiveness of interim barriers to the infiltration of meteoric water (from winter precipitation and snowmelt) on the migration of contaminants from previous leak sources. Sources of contamination at the S SX WMA include releases from tanks SX 108, SX 115, and S 104 and ancillary equipment, and crib 216-S-25. To assess the impact of surface barriers, water-leaks, clastic dikes, fluid properties, and meteoric recharge on the migration of known contaminant distributions, fourteen suites of two-dimensional simulations were executed. The reference suite of simulations (Base Case) considered the migration of contaminants from field estimates of concentration distributions through the vadose zone and groundwater to the S SX WMA Boundary with no interim barriers but a closure barrier by the year Contaminant concentrations at the S SX WMA Boundary were then translated to three additional compliance points (i.e., 200W Fence, Exclusion Boundary, and Columbia River). Simulation results for the base case scenario predicted arrivals of peak concentrations of the contaminant Tc 99 in the following sequence: S SX WMA Boundary, year 2049; 200W Fence, year 2183; Exclusion Boundary, year 2344; and Columbia River, year Arrival times for peak concentrations of the contaminants Cr and NO 3 were similar as those for Tc 99, with the variations being primarily due to the initial inventory distributions. The contaminant Cs 137, because of its retardation factor and radioactive decay, was undetected the S SX WMA Boundary by the year An index of the simulation cases is shown in Table 1, and a summary of the results are provided in Tables 2 through 4, showing the peak time and concentration, for each simulation case and compliance point for the mobile three species (i.e., Tc 99, Cr, NO 3 ), respectively. The impact of an interim surface barrier was investigated by altering the basecase simulation to include an interim barrier installed by the year 2010 (Case 2 versus Case 1). Results from these simulations showed that, whereas, the peak arrival time at the S SX WMA Boundary was delayed by 15 to 18 years with the interim barrier, the peak concentrations were reduced to 30.3%, 10.0%, and 42.9% for Tc 99, Cr, and NO 3, respectively. Peak concentrations, translated to the remote compliance points were similarly reduced. Earlier application of the interim barrier will further delay and reduce the peak arrival times and concentrations at the compliance points. The impact of a soil-saturating water-line leak was investigated by altering the base-case simulation to include a water-line leak near the dome of tank SX 115. Two levels of water-line leaks were simulated: 25 kgal over a 5-day period (Case 3) and 200 kgal over a 5-day period (Case 13). For the 25-kgal leak, simulation results showed this quantity of water to be sufficient to saturate the soil between tanks SX 114 and SX 115, but this plume of saturated water rapidly diffused as it migrated down through the vadose zone, having negligible impact below the Plio-Pleistocene layer. Peak ii

4 Case No. Description Table 1. Case Descriptions Interim Barrier Inventory Distribution Meteoric Recharge 1 Base Case (No Action Alternative) no uniform 100 mm/yr 2 Barrier Alternative yes uniform 100 mm/yr 3 Water-Line Leak (25 kgal) no uniform 100 mm/yr 4 Clastic Dike no uniform 100 mm/yr 5 Nonuniform Inventory no nonuniform 100 mm/yr 6 Nonuniform Inventory with Barrier yes nonuniform 100 mm/yr 7 Displaced Nonuniform Inventory no displaced 100 mm/yr 8 Density and Viscosity Effects no uniform 100 mm/yr 9 Base Case with 50 mm/yr Recharge no uniform 50 mm/yr 10 Base Case with 30 mm/yr Recharge no uniform 30 mm/yr 11 Base Case with 10 mm/yr Recharge no uniform 10 mm/yr 12 Alternative Inventory no alternate 100 mm/yr 13 Water-Line Leak (200 kgal) no uniform 100 mm/yr 14 Cropped Inventory no cropped 10 mm/yr Tc 99 Conc. Table 2. Streamtube Analysis Summary for Tc 99 (DWL 900 pci/l) S SX WMA 200W Fence Exclusion Boundary Columbia River (pci/l) Time Conc. Time Conc. Time Conc. Time Conc. Case x x x x 10 3 Case x x x x 10 3 Case x x x x 10 3 Case x x x x 10 3 Case x x x x 10 3 Case x x x x 10 3 Case x x x x 10 4 Case x x x x 10 3 Case x x x x 10 3 Case x x x x 10 3 Case x x x x 10 2 Case x x x x 10 3 Case x x x x 10 3 Case x x x x Peak concentrations arrive after 3000 years. iii

5 Cr Conc. Table 3. Streamtube Analysis Summary for Cr (DWL 50 μg/l) S SX WMA 200W Fence Exclusion Boundary Columbia River (μg/l) Time Conc. Time Conc. Time Conc. Time Conc. Case x x x x 10 2 Case x x Case x x x x 10 2 Case x x x x 10 2 Case x x x x 10 2 Case x x x Case x x x x 10 2 Case x x x x 10 2 Case x x x x 10 2 Case x x Case x Case x x x x 10 2 Case x x x x 10 2 Case x NO 3 Conc. Table 4. Streamtube Analysis Summary for NO 3 (DWL 45,000 μg/l) S SX WMA 200W Fence Exclusion Boundary Columbia River (μg/l) Time Conc. Time Conc. Time Conc. Time Conc. Case x x x x 10 4 Case x x x x 10 3 Case x x x x 10 4 Case x x x x 10 4 Case x x x x 10 4 Case x x x x 10 3 Case x x x x 10 4 Case x x x x 10 4 Case x x x x 10 3 Case x x x x 10 3 Case x x x x 10 2 Case x x x x 10 4 Case x x x x 10 4 Case x x x x Peak concentrations arrive after 3000 years. iv

6 concentrations and arrival times at the four compliance points were essentially unchanged from the base-case simulations, demonstrating that the single-event waterline leak had negligible impact on the migration of contaminants from the S SX WMA. For the 200-kgal leak, simulation results showed leak water migrating over the tank domes and descending rapidly to the groundwater. As with the 25-kgal leak, arrival times were shorten, but shape and peak values of the concentration breakthrough curves at the S-SX WMA Boundary, were only slightly altered. Clastic dikes, near-vertical geologic features filled with unconsolidated sediments, have been postulated to form a polygonal pattern in the vicinity of the S SX WMA. The influence of a single clastic dike, situated between tanks SX 108 and SX 109, that extended vertically from the base of the tanks to the top of the Plio- Pleistocene unit was investigated as an alteration to the base-case simulation. This suite of simulations used an altered grid for cross section SX DD compared with the base case simulation, where grid refinement was used to resolve the clastic dike. Peak concentrations and arrival times for Tc 99 for the clastic dike simulations at the four compliance points were nearly identical to those for the base case, demonstrating the negligible influence of a single clastic dike situated outside initial contaminant inventory domain. The influence of initial contaminant inventory distributions were investigated by considered three alterations to the base-case distributions on contaminants. In each of these alterations the integrated quantity of contaminant mass was conserved (i.e., held equal to the values reported in the modeling data package [Khaleel et al. 2001]. In the first alteration (nonuniform distribution) solute mass was concentrated in the region between tanks. In the second alteration (displaced-nonuniform distribution) solute mass was concentrated in the region between tanks and displaced toward the water table. In the third alteration (alternate distribution) the solute mass was uniformly distribution over a prescribed circular area (e.g., tank bottom surface). Compared against the uniform distribution the nonuniform simulations showed little change in peak arrival times of the contaminants at the four compliance points, primarily because of the homogenizing affect of the Plio-Pleistocene layer. For example peak concentrations of Tc 99 for the nonuniform distribution at the S SX WMA Boundary, was only 108% of that for the uniform distribution. This behavior was observed with and without the interim barrier. The displaced-nonuniform inventory showed markedly earlier peak arrival times and concentrations for Tc 99 at the four compliance points. Peak concentrations of Tc 99 at the S SX WMA Boundary were 232% of those for the uniform distribution. For the alternate distribution, Tc-99 concentrations at the S SX WMA Boundary were nearly unchanged, although the Cr solute with its shallower initial distribution showed variations in the peak concentration. The inventory distribution simulations indicated that errors in areal distributions of solutes resulted in greater differences in solute concentration at the first compliance point for shallower initial distributions, but that differences ranged from 9.7% to 81.6%. The base-case simulations considered the concentration of transported solutes as having no influence on the properties of the carrier fluid (i.e., the infinitely dilute assumption). At the field-measured concentrations, however, the density and viscosity v

7 of water is dependent on the concentration of dissolved solutes, in particular NaNO 3. To investigate the influence of concentration dependent fluid properties, a suite of simulations were executed that varied from the base case in that the aqueous-phase density and viscosity were considered to be a function of the NaNO 3 concentration, as determined by the concentration of dissolved NO 3 specie. Results from these simulations showed slightly earlier arrival times of the peak concentrations and slightly higher peak concentrations for Tc 99 (e.g., 105% of the base-case concentration at the S SX WMA Boundary) at the four compliance points. The Cr and NO 3 breakthrough curve results showed similar trends. These results indicate that density and viscosity affects have negligible impact on the migration of contaminants through the vadose zone beneath the S SX WMA, partially due to the influence of the Plio-Pleistocene layer. To investigate the impact meteoric recharge on contaminant transport, a series of simulations were executed, using uniform inventory distributions and no interim barriers, that differed in surface recharge (100, 50, 30, and 10 mm/yr) for the first forty years. After the initial forty-year period the closure barrier was assumed to control the surface recharge. The initial flow field for these simulations was the steady-flow condition for the meteoric recharge rate and fixed groundwater conditions (i.e., water table elevation and gradient). Reduced meteoric recharges delayed peak arrival times and reduced peak concentrations at the compliance points, yielding more uniform breakthrough curves. The exception, however, was the 10 mm/yr meteoric recharge rate, where arrival times were delayed beyond 1000 years. In addition to this series of simulations cropped inventories were simulated at a meteoric recharge of 10 mm/yr. For these simulations, the solute inventories were cropped below a depth of m (127.4 ft). The importance of vertical distribution of inventory was demonstrated in these results. A three-dimensional simulation of the SX-108 tank (modeled in quarter symmetry) was executed to provide a quantitative comparison against the twodimensional cross-section simulations. To isolate the influence of domain dimensionality, the geology of the three-dimensional domain was developed by extrapolating the east-west cross-section geology in the north-south direction. In addition the horizontal and vertical grid spacings were unaltered from the base case simulations. The three-dimensional simulation was executed following the base-case scenario (i.e., no interim barrier, no water-line leaks, and a closure barrier by 2040). The resulting flow fields and solute migration results at the water table show close agreement compared against the two-dimensional base-case simulations. Limited three-dimension simulation results indicate that errors associated with analyzing the S SX WMA using two-dimensional cross-sections are minimal for the Base Case. This report documents the simulation process, from converting the modeling data to input format to retrieving archived results. The report is divided into sections that generally follow the overall simulation procedures. First the investigation objectives are summarized, followed by a listing of the numerical simulations that were executed. The next section describes the process of converting the data provided in the modeling data package into input files for the STOMP simulator. Much of this discussion relies on the reader having access to the STOMP guide documents and focuses on the correlation between the modeling data package and STOMP input cards. This section also includes descriptions of converting geologic cross sections into two- vi

8 and three-dimensional soil distribution maps and converting initial inventory data into distributions of dissolved contaminant concentrations. To meet the modeling specifications four, previously unavailable features (i.e., capabilities) were implemented into the STOMP simulator: 1) solute-concentration-dependent density and viscosity, 2) saturation-dependent permeability anisotropy (i.e., Polmann model), 3) solute-soildependent enhanced macrodispersivity, and 4) Courant number limiter (i.e., multiple transport time stepping). Implementation of these capabilities into the STOMP simulator is described. The next section provides a short summary of the code compilation and execution, which included two platforms: 1) workstations operating under UNIX and personal computers operating under Windows NT. Simulation results are described in summary format, supported with line plots and color-scale images. The result sections begin with descriptions of the techniques and utility programs used to convert the simulation results from the conventional STOMP output format to those forms reported in this document. Results are then presented for each of the cases starting with the coupled vadose-zone and unconfined aquifer simulations and followed with the translation of those results for contaminant transport to the remote compliance points through streamtube modeling. The primary emphasis in reporting results was to provide a straightforward summary of the simulations and streamtube modeling using tables, plots and color-scaled images. The most significant finding was that groundwater concentrations predicted for Tc-99 and Cr exceeded the drinking water standard at the WMA S-SX boundary. For the base case and interim barrier cases, predicted concentrations of Tc-99 in the groundwater exceeded the drinking water standard at all compliance points. vii

9 Table of Contents Summary... iii Table of Contents...viii List of Tables... xi List of Figures... xiii 1.0 Introduction Case Descriptions Base Case (No Action Alternative) Barrier Alternative and No Water-Line Leak No Barrier and Water-Line Leak (25 kgal/5 days) No Barrier and Clastic Dikes Nonuniform Inventory Distribution and No Barrier Nonuniform Inventory Distribution and Barrier Location of Inventory Distribution and No Barrier Density and Viscosity Effects Base Case w/ 50 mm/yr Meteoric Recharge Base Case w/ 30 mm/yr Meteoric Recharge Base Case w/ 10 mm/yr Meteoric Recharge Alternate Inventory No Barrier and Water-Line Leak (200 kgal/5 days) Cropped Inventory Three-Dimensional Base Case (No Action Alternative) Technical Approach Overview Modeling Data Package Recharge Estimates Vadose Zone Flow and Transport Properties Stochastic Model for Macroscopic Anisotropy Clastic Dike Infilling Material Properties Bulk Density and Retardation Coefficient viii

10 Table of Contents (contd) Diffusivity Macrodispersivity Unit Dose Factors Input File Generation Input File Zonation File Inventory File(s) Variable-Diameter Distribution Fixed-Diameter Distribution Inventory Distribution Maps Implemented Features Concentration Dependent Viscosity and Density Polmann Anisotropy Model Enhanced Macrodispersivity Courant Number Limiter STOMP Execution Result Translation Streamtube Modeling Simulation Results Base Case (No Action Alternative) Barrier Alternative and No Water-Line Leak No Barrier and Water-Line Leak (25 kgal) No Barrier and Clastic Dikes Nonuniform Inventory Distribution and No Barrier Nonuniform Inventory Distribution and Barrier Location of Inventory Distribution and No Barrier Density and Viscosity Effects Base Case (50 mm/yr Meteoric Recharge) Base Case (30 mm/yr Meteoric Recharge) Base Case (10 mm/yr Meteoric Recharge) Alternate Inventory Distribution ix

11 Table of Contents (contd) 4.13 No Barrier and Water-Line Leak (200 kgal) Cropped Inventory Distribution (10 mm/yr Meteoric Recharge) Three-Dimensional Simulations Solute Mass Balance Streamtube Modeling Results Electronic Files Source Coding Geology Initial Inventory Steady-Flow Simulations Coupled Vadose Zone and Unconfined Aquifer Modeling Streamtube Modeling References Appendix A. STOMP Input File and Data Conversion...A.1 Appendix B. Vadose Zone and Unconfined Aquifer Modeling Results... B.1 Appendix C. Recharge Sensitivity Modeling Results...C.1 x

12 List of Tables Table Time Estimates for Interim and Closure Barriers at the S and SX Tank Farms and Corresponding Recharge Estimates Table Composite van Genuchten Mualem Parameters for Various Strata at the S SX Tank Farms Table Macroscopic Anisotropy Parameters Based on Polmann Equations for Strata at the S-SX WMA Table Effective Hydraulic Parameters for Clastic Dike Infilling Materials Table Effective Parameter Estimates for the Product of Bulk Density and Retardation Coefficient for Cs 137 at S SX WMA Table Non-reactive Macrodispersivity Estimates for Strata at S SX WMA Table Unit Dose Factors for Cs 137 and Tc 99 [Rittmann 1999] Table Inventory File Naming Convention Table Initial Inventory Distribution Schedule Table Distances and Travel Times from S SX WMA Table Streamtube Characteristics Table Streamtube Modeling Properties Table Peak Concentrations and Arrival Times at the First Compliance Point (i.e., S SX WMA Boundary) for Case Table Mean Aqueous-Phase Saturation Table Peak Concentrations and Arrival Times at the First Compliance Point (i.e., S SX WMA Boundary) for Case Table Peak Concentrations and Arrival Times at the First Compliance Point (i.e., S SX WMA Boundary) for Case Table Peak Concentrations and Arrival Times at the First Compliance Point (i.e., S SX WMA Boundary) for Case Table Peak Concentrations and Arrival Times at the First Compliance Point (i.e., S SX WMA Boundary) for Case xi

13 List of Tables (contd) Table Peak Concentrations and Arrival Times at the First Compliance Point (i.e., S SX WMA Boundary) for Case Table Peak Concentrations and Arrival Times at the First Compliance Point (i.e., S SX WMA Boundary) for Case Table Peak Concentrations and Arrival Times at the First Compliance Point (i.e., S SX WMA Boundary) for Case Table Peak Concentrations and Arrival Times at the First Compliance Point (i.e., S SX WMA Boundary) for Case Table Mean Aqueous Saturation Table Peak Concentrations and Arrival Times at the First Compliance Point (i.e., S SX WMA Boundary) for Case Table Mean Aqueous Saturation Table Peak Concentrations and Arrival Times at the First Compliance Point (i.e., S SX WMA Boundary) for Case Table Mean Aqueous Saturation Table Peak Concentrations and Arrival Times at the First Compliance Point (i.e., S SX WMA Boundary) for Case Table Peak Concentrations and Arrival Times at the First Compliance Point (i.e., S SX WMA Boundary) for Case Table Peak Concentrations and Arrival Times at the First Compliance Point (i.e., S SX WMA Boundary) for Case Table Solute Mass Balance Errors Table 5.1. Distances and Travel Times from S-SX WMA Table 5.2. Streamtube Characteristics Table 5.3. Streamtube Analysis Summary for Tc 99 (Groundwater Limit 900 pci/l) Table 5.4. Streamtube Analysis Summary for Cr (Groundwater Limit 50 μg/l) xii

14 List of Tables (contd) Table 5.5. Streamtube Analysis Summary for NO 3 (Groundwater Limit 45,000 μg/l) Table Source Code Directory Table Rock/Soil and Inactive-Node Distribution Files Table Initial Inventory Distribution Files Table Steady-Flow Initial Condition Files Table Coupled Vadose Zone and Unconfined Aquifer Modeling Files Table Streamtube Modeling Files xiii

15 List of Figures Figure Rock/Soil Zonation for Cross Section SX DD Figure Rock/Soil Zonation for Cross Section SX FF Figure Rock/Soil Zonation for Cross Section S CC Figure Rock/Soil Zonation for Cross Section SX DD w/ Clastic Dike Figure Translation Geometry Figure Aqueous-Phase Saturation at 2000 (steady-flow conditions) for Cross Section SX DD (SX 107, 108, 109) Figure Aqueous-Phase Saturation at 2050 (0.1 mm/yr) for Cross Section SX DD (SX 107, 108, 109) Figure Aqueous-Phase Saturation at 2540 (0.1 mm/yr) for Cross Section SX DD (SX 107, 108, 109) Figure Aqueous-Phase Saturation at 3000 (3.5 mm/yr) for Cross Section SX DD (SX 107, 108, 109) Figure Aqueous-Phase Saturation at 2000 plus 5 days Cross Section SX FF (SX 113, 114, 115) Figure Aqueous-Phase Saturation at 2000 plus 10 days Cross Section SX FF (SX 113, 114, 115) Figure Aqueous-Phase Saturation at 2000 plus 25 days Cross Section SX FF (SX 113, 114, 115) Figure Aqueous-Phase Saturation at 2000 plus 50 days Cross Section SX FF (SX 113, 114, 115) Figure Aqueous-Phase Saturation at 2000 plus 183 days Cross Section SX FF (SX 113, 114, 115) Figure Aqueous-Phase Saturation at 2000 plus 365 days Cross Section SX FF (SX 113, 114, 115) Figure Tc 99 Aqueous Concentration at 2001 Cross Section SX FF (SX 113, 114, 115) xiv

16 List of Figures (contd) Figure Tc 99 Aqueous Concentration at 2040 Cross Section SX FF (SX 113, 114, 115) Figure Aqueous-Phase Saturation at 2000 (50 mm/yr) Cross Section SX DD (SX 107, 108, 109) Figure Aqueous-Phase Saturation at 2000 (30 mm/yr) Cross Section SX DD (SX 107, 108, 109) Figure Aqueous-Phase Saturation at 2000 (10 mm/yr) Cross Section SX DD (SX 107, 108, 109) Figure Aqueous-Phase Saturation at 2000 plus 5 days Cross Section SX FF (SX 113, 114, 115) Figure Aqueous-Phase Saturation at 2000 plus 10 days Cross Section SX FF (SX 113, 114, 115) Figure Aqueous-Phase Saturation at 2000 plus 25 days Cross Section SX FF (SX 113, 114, 115) Figure Aqueous-Phase Saturation at 2000 plus 50 days Cross Section SX FF (SX 113, 114, 115) Figure Aqueous-Phase Saturation at 2000 plus 183 days Cross Section SX FF (SX 113, 114, 115) Figure Aqueous-Phase Saturation at 2000 plus 365 days Cross Section SX FF (SX 113, 114, 115) Figure Tc 99 Aqueous Concentration at 2001 Cross Section SX FF (SX 113, 114, 115) Figure Tc 99 Aqueous Concentration at 2040 Cross Section SX FF (SX 113, 114, 115) xv

17 1.0 Introduction The U.S. Department of Energy (DOE) is charged with the cleanup of sites where the subsurface environment has been contaminated with dangerous constituents. Included among these sites are four of the seven Hanford Site Single-Shell Tank (SST) Waste Management Areas (WMAs) referred to as S SX, B BX-BY, T and TX TY. In keeping with its charge, the DOE has initiated a corrective action program that will address the impacts of previous and potential future leaks and releases of wastes from tanks at these four WMAs on the vadose zone environment. The Tank Farm Vadose Zone Project, a component of DOE s overall corrective action program, has begun a series of field investigations at the S SX WMA, which are necessitated by the Hanford Tri-Party Agreement (TPA M ). By January 2002, DOE is required to submit to the Washington State Department of Ecology, under the Hanford Tri-Party Agreement (TPA Milestone M T01), a Field Investigative Report (FIR) for WMA S SX. Included in this report, will be consideration of interim corrective measures (ICMs), such as surface covers or barriers [Haass 1999]. These considerations shall investigate, through numerical simulation, the performance of proposed ICMs with respect to their impact on reducing long-term risks from potential groundwater contamination at the S SX WMA. A series of numerical simulations were conducted to evaluate the performance of ICMs such as surface barriers, for reducing long-term human health risks from potential groundwater contamination at the S-SX WMA. The specific objectives of the numerical assessment are to quantify the risks posed by past tank releases if no interim corrective measures are implemented, and determine the degree to which implementation of selected ICMs will decrease the risks posed by past tank releases. The assessments of this investigation specifically focus on the impacts to groundwater resources (i.e., the concentration of contaminants in the groundwater) and long-term risk to human health associated with groundwater use. The evaluations consider the extent of contamination currently within the vadose zone, movement of contaminants through the vadose zone to the groundwater, and movement of contaminants through the groundwater to points of compliance. By providing quantitative comparisons between ICM concepts, the results from this evaluation may impact current operations or future decisions regarding the retrieval of tank waste and closure of the S-SX WMA. This report documents initial investigations performed via numerical simulation of contaminant migration beneath the S SX WMA and the calculation of associated risks at point(s) of compliance. The scope and data required to perform these numerical simulations are documented in the modeling data package [Khaleel et al. 2000], provided by CH2M Hill Hanford Group, Inc. The numerical simulations reported herein where executed with the STOMP simulator [White and Oostrom 2000a,b] and consider the distribution of contaminants presently in the vadose zone, the migration of those contaminants to the groundwater under the influence of surface barriers, the further migration of contaminants through the groundwater to the point(s) of compliance and the types of human activities at the point(s) of compliance. As specified by the data package, four contaminant species (Cs 137, Tc 99, Cr, and NO 3 ), representing the range of mobile and immobile constituents, were considered in these 1.1

18 migration and risk analyses. All but one series of simulations reported were executed on two-dimensional grids, that represented cross sections traversing three single-shell tanks within the S SX WMA. Initial contaminant inventories were developed for each cross section in accordance with the modeling data package. The remaining simulation used a three-dimensional grid that modeled a quarter section of a single. Grid resolutions for all simulations were m (1.75 ft) horizontally and m (1.5 ft) vertically, except for a one simulation that used finer grid spacings in the region surrounding a clastic dike feature, which was modeled as an inclusion in the site geology. All simulations were executed assuming isothermal conditions, but with pressure dependent liquid density. The vadose zone was modeled as an aqueous-gas porous media system, where transport through the gas phase was neglected. All but one simulation used the infinite dilution assumption for coupling fluid flow and contaminant transport. The one simulation not following this assumption, coupled aqueous phase density with the concentration of a single contaminant, NaNO 3. The principal objective for these investigations was to execute the simulations specified in the modeling data package, using proven, scientifically based, computational software and report the generated results. To promote an open exchange of scientific knowledge and ideas the software used to generate the reported results, has a readable source code that will be made available, upon request, to the Government and its contractors. To ensure the capability to repeat these simulations in the future, the source coding, input files and output files have been stored in electronic form and will also be made available to the Government and its contractors. Although Battelle, Pacific Northwest Division maintains copyright on the STOMP simulator the Government retains a paid-up non-exclusive, irrevocable worldwide license to reproduce, prepare derivative works, perform publicly and display publicly by or for the Government, including the right to distribute to other Government contractors. Numerical simulation of contaminant migration through the vadose zone and unconfined aquifer beneath the S SX WMA required the conversion information in the modeling data package into electronic input that could be interpreted by the STOMP simulator, execution of the software, and translation of the simulation output into graphical form for reporting. This report documents these three steps and summarizes the simulation results for each of the twenty-three cases executed. Inventory estimates were considered a critical parameter of these analyses and uncertainties in the initial inventory were considered. Inventory estimates used to develop initial distributions used the recently collected data on Tc-99 and other constituents from the Hanford Site borehole , borehole 299-W23-19, near tank SX-113, and the MACTEC-ERS Cs-137 plume distributions [Goodman 2000]. A principal assumption of the investigation is that no additionally contaminant releases will occur in the future, and that water-line leaks from the existing piping in the S-SX WMA have been addressed and resolved. Two simulation cases, however, did consider water-line leaks in the vicinity of tank SX-115. For simulations that considered surface barriers, it was assumed that interim barriers became effective in the year 2010 and closure barriers in the year Surface barriers were expected to significantly reduce the infiltration of meteoric water and delay the arrival of contaminants at the water table. The curved surface of the tank dome, impermeable tank wall, and water shedding by these surfaces were modeled in 1.2

19 these simulations. Because of this water shedding, soils between tank surfaces were predicted to have elevated water contents compared with those sediments located outside tank region. The sedimentary soils were assumed to have moisture-dependent anisotropy, where the ratio of horizontal to vertical relative permeability was defined to be a function of the soil saturation. Except for the three-dimensional case, two-dimensional cross-sections were used to simulate fluid flow and contaminant transport through the vadose zone and groundwater. Three representations of west-east cross sections through the S-SX Tank Farm were considered: 1) cross section (SX-DD ) through tanks SX 107, 108, 109; 2) cross section (SX-FF ) through tanks SX 113, 114, 115; and 3) cross section (S-CC ) through tanks S 104, 105, 106. Note that in the discussions and images of this report these cross sections are referred to either by tank sequence or cross-section designation. For example, cross-section designation SX-DD refers to the cross section through tanks SX 107, 108, and 109. Each transient simulation was preceded by a steady-flow simulation to establish initial conditions. Steady-flow conditions were established using a constant surface recharge of meteoric water and fixing the water table gradient across the cross section. No solute transport was considered during the steady-flow simulations. Transient simulations involved both fluid flow and solute transport. The fluid flow transient started with the steady-flow conditions flow field and responded to changes in meteoric recharge, caused by barrier emplacements. Two simulations additionally considered water-line leaks and another simulation considered variation in the fluid density and viscosity with nitrate concentration. The water-table hydraulic gradient remained fixed throughout the transient simulation. The transient solute transport simulations conducted for each cross section generated a breakthrough curve (BTC) (i.e., plot of dissolved solute concentration versus time) at the western boundary (i.e., groundwater outflow region) of the S-SX WMA. The temporal and spatial distribution of the BTCs by cross section were recognized and the principle of superposition was used to generate a composite BTC across the S-SX WMA compliance boundary. An analytical streamline approach was used to translate the BTCs from the S-SX WMA boundary compliance points to the three remote compliance points. Solute concentrations at the compliance points were converted into dose estimates using conversion factors [Khaleel et al. 2000]. Fluid flow within the vadose zone was described using the Richard s equation; whereas, contaminant transport was described using the conventional advectivedispersive transport equation with an equilibrium linear sorption coefficient formulation (i.e., K d formulation). Stratigraphic information for the three representative cross sections was based on studies by Johnson et al. [1999] and Price and Fecht [1976a, b]. These cross sections include dipping strata and when combined with the Polmann model [Polmann 1990] for anisotropy in relative permeability for unsaturated soils allows the simulator to model the enhanced spreading at the fine-grained to coarse-grained interfaces and the increased down-slope movement of water along these interfaces. Modeling parameters used to describe soil-moisture retention, phase relative permeability, saturated hydraulic conductivity (intrinsic permeability), and bulk density (porosity), for individual stratum were based on data collected from 200 Area 1.3

20 soils [Khaleel et al. 2000]. For each stratum (soil type) defined on the cross-section stratigraphy, the small-scale laboratory measurements were scaled spatially upward to obtain equivalent horizontal and vertical unsaturated hydraulic conductivities as a function of mean tension [Khaleel et al. 2000]. This scaling technique yields a mathematical expression describing macroscopic anisotropy in the unsaturated hydraulic conductivity as a function of mean tension for each stratum. Arithmetic averaging of van Genuchten parameters [van Genuchten 1980] were used to define the soil-moisture retention function for each stratum. Where multiple soil samples were unavailable for a given stratum, data was used from soil samples taken from the same stratum, but from sites in the vicinity of the S-SX WMA. When available hydraulic properties were determined from laboratory-measurements of both soil-moisture retention and unsaturated hydraulic conductivity. This approach avoided extrapolating unsaturated hydraulic conductivities [van Genuchten 1980; Mualem 1976] to dry conditions, based on a saturated conductivity estimate [Khaleel et al. 1995]. To reflect field conditions, laboratory data additionally were corrected for the presence of any gravel fraction in the sediment samples [Khaleel and Relyea 1997]. In keeping with the approach taken for modeling fluid flow, solute transport properties for bulk density, diffusivity, and dispersivity were specified for each stratum. Contaminant mobility was defined through an equilibrium linear sorption coefficient which was specified uniquely for each combination of solute and stratum. The geochemical environment beneath the S-SX WMA is highly disturbed from it natural state, because the fluid leaking from the underground storage tanks changed the geochemistry. Available data suggests that the most severe changes in the geochemical environment have occurred in the soils underlying and surrounding tanks SX 107, 108, and 109. Uncertainty remains about the linear sorption coefficient and the applicability of a linear-sorption model for Cs-137, whose sorption has been shown to depend on the concentration of competing ions. There is little doubt, however, that the linear sorption coefficient (i.e., K d ) for Tc-99 is close to 0 ml/g in Hanford sediments. This low K d, coupled with its long half-life (i.e., 2.03 x 10 5 yr) allows Tc 99 to migrate long distances in both the vadose zone and groundwater, posing a threat to groundwater quality for a long time. Initial conditions for soil-moisture (expressed in terms of capillary pressure) and solute concentration were needed to initiate these transient flow and transport simulations. Initial soil-moisture was established through steady-flow simulations. Steady-flow simulations are pseudo-transient simulations that proceed from some soilmoisture distribution with constant boundary conditions until steady-flow conditions are reached. In establishing the steady-flow soil-moisture profiles, constant recharge was used as the boundary condition along the ground surface and a constant west to east hydraulic gradient was imposed for the groundwater. Vertical surfaces were considered zero-flux boundaries. Initial conditions for solute concentrations were based on contaminant concentrations and integrals of the inventory estimates for Cs 137, Tc 99, NO 3 and Cr [Khaleel et al 2000] and five different distribution models: 1) uniform, 2) nonuniform, 3) displaced nonuniform, 4) alternate, and 5) cropped. A variety of distribution models were used to investigate the influence of initial solute distributions on solute concentrations at the S-SX WMA boundary compliance point. 1.4

21 As noted above, two-dimensional west-to-east cross sections through the S-SX WMA were used for modeling fluid flow and solute transport. The simulation domain extended horizontally to include the WMA boundaries and vertically from the ground surface to roughly 6 m (20 ft) below the water table, located at a depth of about 65 m (213 ft) below ground surface. The geologic strata were assumed to be continuous across the cross section, but not of constant thickness. Sloped interfaces for geologic units were included based on data in Johnson et al. [1999] and Price and Fecht [1976a,b]. To account for the residual effects of tank leaks, concentration dependent density and viscosity of the leaked fluid were considered in a set of simulations. The effects of concentration dependent density and viscosity, however, are somewhat offsetting. Elevated fluid density with increased solute concentration was expected to enhance the vertical mobility of the contaminated plume, but increased viscosity with increased solute concentration reduces the mobility of the plume. Clastic dikes were included in the modeling series as a sensitivity analysis. These geologic features occur throughout the 200 West Area on the Hanford Site, and have been hypothesized as potential pathways for vertical transport of radionuclides, thus explaining the deep migration of contaminants at the SX tank farm. Hydrologic property data on the clastic dike infilling material were based on Khaleel [1999]. Variable grid spacing was used to model features such as the presence of clastic dikes. 1.5