A three-dimensional numerical model of groundwater flow and transport, based on the Coupled Fluid Energy, and Solute Transport (CFEST) code, was developed for the Hanford Site to support the Hanford Groundwater Project (HGWP), managed by Pacific Northwest National Laboratory.(A) The model was developed to increase the understanding and better forecast the migration of several contaminant plumes being monitored by the HGWP, and to support the Hanford Site Composite Analysis for low-level waste disposal in the 200-Area Plateau.
Recent modeling efforts have focused on continued refinement of an initial version of the three-dimensional model developed in 1995 and its application to simulate future transport of selected contaminant plumes in the aquifer system. This version of the model was updated using a more current version of the CFEST code called CFEST96.
Prior to conducting simulations of contaminant transport with the three-dimensional model, a previous steady-state, two-dimensional model of the unconfined aquifer system was recalibrated to 1979 water-table conditions with a statistical inverse method implemented in the CFEST-INV computer code. The results of the recalibration were used to refine the three-dimensional conceptual model and to calibrate it with a conceptualization that preserves the two-dimensional hydraulic properties and knowledge of the aquifer's three-dimensional properties for the same 1979 water-table conditions. The transient behavior of the three-dimensional flow model was also calibrated by adjusting model storage properties (specific yield) until transient water-table predictions approximated observed water-table elevations between 1979 and 1996.
Following the steady-state and transient-state calibrations, the three-dimensional model was applied to predict the future response of the water table to postulated changes in Hanford operations. Over about a 300-year period following elimination of wastewater discharges to the ground at the Site, the water table predicted by the model declines significantly and returns to near pre-Hanford water-table conditions that were estimated to exist in 1944. Model results show that over this period, the water table will drop as much as about 11 m in the 200-West Area and 10 m in the 200-East Area near B Pond. Model results of steady-state simulations compared favorably with overall water-table conditions estimated prior to Hanford Operations in 1944, except in two areas: 1) the area west of the 200-Area plateau, where higher predicted hydraulic heads reflect boundary conditions that consider the effect of increased irrigation from areas upgradient of the modeled region; and 2) the area north of Richland, where the model considered the hydraulic effect of the North Richland well field.
Flow modeling results also suggest that as water levels drop in the vicinity of central areas in the model where the basalt crops out above the water table, the saturated thickness of the unconfined aquifer greatly decreases and the aquifer may actually dry out. This thinning/drying of the aquifer is predicted to occur in the area between Gable Butte and the outcrop south of Gable Mountain, resulting in the northern area of the unconfined aquifer becoming hydrologically separated from the area south of Gable Mountain and Gable Butte. Therefore, flow from the 200-West Area and the northern half of the 200-East Area, which currently migrates through the gap between Gable Butte and Gable Mountain, will be effectively cut off in the next 200 to 300 years. In time, the overall water table (including groundwater mounds near the 200-East and -West Areas) will decline, and groundwater movement from the 200-Area plateau will shift to a dominantly west-to-easterly pattern of flow toward points of discharge along the Columbia River between the Old Hanford townsite and the Washington Public Power Supply System facility.
The FY 1997 effort includes three-dimensional model simulations of the existing tritium, iodine-129, technetium-99, uranium, and strontium-90 plumes originating from the 200-Area plateau. Each of the transport simulations was based on the predicted future transient-flow conditions and a high-resolution, finite-element grid was designed to resolve transport calculations in the areas of current and future contamination. The following conclusions were reached based on the simulations:
Tritium - Simulations of the tritium plume originating from the 200 Area predicted that the 200-East Area plume will continue to discharge along the Columbia River between the Old Hanford townsite and an area north of the 300 Area over a period of about 90 to 100 years until it completely discharges into the river and/or decays below detection limits. The tritium plume from the 200-West Area is predicted to migrate beneath the 200-East Area as it is reduced by dispersion and decay.
Iodine-129 - Predictions of the iodine-129 plume beneath the 200-East Area indicated that the plume will migrate toward and discharge into the Columbia River over a period of about 600 years. Over the period of migration, iodine-129 concentrations that are predicted to discharge into the river will decline slightly by the processes of dispersion and sorption. However, initial concentrations will not fall significantly below current levels because of the radionuclide's long half-life. The iodine-129 plume from the 200-West Area is predicted to migrate toward the 200-East Area as it is reduced by dispersion and sorption.
Technetium-99 - Transport modeling of the technetium-99 plume indicated that the current technetium-99 plumes in the 200-East and the 200-West Areas will continue to slowly migrate laterally from source locations in the 200 Areas to the Columbia River. The plumes in the 200-West Area decline to levels below regulatory concern as they migrate eastward toward the 200-East Area. The technetium-99 plume from the northern part of the 200-East Area is predicted to continue migrating northward through the gap between Gable Butte and Gable Mountain toward the Columbia River. However, concentration levels of the simulated plumes decline to below drinking water standards (900 pCi/L) over a period of 100 years by the processes of dilution by infiltration and plume dispersion.
Uranium - Simulations of the uranium plume in the 200 Areas predicted that the current uranium plumes will not migrate significantly from current source locations in the 200 Area because uranium is adsorbed with a distribution coefficient (Kd) of 3.0 ml/g. Concentration levels of the simulated plumes in the
200 Area will decline below regulatory concern over the next 50 years by the processes of dilution by infiltrating rainwater and plume dispersion. During the simulation period, the uranium plume largely remained within the exclusion and buffer areas of the 200-Area plateau.
Strontium-90 - Transport modeling of the strontium-90 plume in the 200-East Area predicted that the strontium-90 plume will likely migrate very little from its current location within the 200 Area because strontium-90 sorbs strongly to Hanford sediments (Kd = 20 ml/g). Concentration levels of the simulated plume in the buffer area surrounding the 200 Area will decline over the next 100 years to below regulatory concern by processes of radioactive decay and plume dispersion as the plume migrates from the current location. The strontium-90 plume is predicted to remain largely within the 200-Area plateau.
In general, the results of transport analyses of tritium, iodine-129, technetium-99, and uranium with the three-dimensional model were in agreement with past site-wide modeling results obtained by Chiaramonte et al. (1996) in support of the Environmental Restoration Program. However, the current three-dimensional model has resulted in higher estimates of peak concentrations at shallow depths and less vertical migration than was predicted in previous site-wide model applications. These discrepancies are attributable to differences in basic assumptions made about the hydrogeologic framework and the horizontal and vertical discretization used in each model. The differences in assumptions resulting from each modeling approach affect lateral and vertical distributions of predicted hydraulic heads and contaminants in the unconfined aquifer.
The near-term impacts of existing plumes to on-site drinking water supplies were examined by evaluating predicted concentration levels in the vicinity of identified on-site water supplies over the next 30 years. Projected future levels of tritium suggest that water supply wells in the 400 Area at the Fast Flux Test Facility (FFTF) and emergency water supply wells in the 200-East Area will continue to be impacted by the tritium plume originating from the 200-East Area for the next 10 to 20 years. Tritium levels at well locations in the 400 Area and the 200-East Area are expected to remain above the 20,000-pCi/L level until sometime between 2010 and 2020. After that time, tritium will continue to decline to below 500 pCi/L at some time between the years 2070 and 2080. Model results suggest that tritium concentrations now found in the 300 Area in excess of 2,000 pCi/L will not reach the North Richland well field.
Transport analyses suggest that only water supplies in the 200-East Area could potentially be impacted by elevated levels of iodine-129. Model-predicted levels of iodine-129 suggest that within 20 to 30 years iodine levels in excess of 1 pCi/L that are currently in the 200-East Area will remain about halfway to the Columbia River. The iodine-129 plumes in the 200-West Area will be expected to migrate slowly toward the 200-East Area but model results (Figure 6.4) suggest that levels in excess of 1 pCi/L will not reach the 200-East Area within 30 years.
Projected future levels of technetium-99, uranium, and strontium-90 show that none of the identified water supplies on the Hanford Site, including those in the 200-East Area near B Plant and AY/AZ Tank Farm, will be impacted by future transport of these contaminants.