The initial development, calibration, and initial application of a three-dimensional model of groundwater flow and contaminant transport are important first steps in developing a tool for evaluating the impacts of large-scale site plume transport. Such a tool can also be used to evaluate the effects of remediating Hanford Site groundwater and to design long-term monitoring strategies for the Site. The following summary and conclusions are based on the initial development and calibration of the three-dimensional flow model for the Hanford Site and the examination of results of transport models for selected contaminant plumes.
The three-dimensional flow and transport model is based on the CFEST code and is able to simulate the major aquifer features needed to assess large-scale plume migration and transport. The subregion modeling capability of CFEST allows flexibility for evaluating smaller-scale plumes and related phenomena.
Previous calibration efforts reported in Wurstner et al. (1995) were improved with recalibration of the two-dimensional model that considered constraints based on measured transmissivities. This two-dimensional calibration, combined with knowledge of the three-dimensional hydrogeologic layers defined in the model, provides the technical basis for developing the three-dimensional model both by considering the previous two-dimensional calibration and the updated geologic and hydrologic conceptualization of the unconfined aquifer system.
The three-dimensional flow model provides good general agreement with 1979 conditions and reproduces the transient response of the aquifer reasonably well. Model simulations of projected reductions in artificial discharges at the Site showed that over about a 300-year period, the water table will decline significantly and return to near pre-Hanford water-table conditions estimated for 1944. Over this period, model results show that 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. The model results 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.
Results of the three-dimensional transport modeling resulted in the following conclusions regarding specific contaminants:
In general, the results of iodine-129, technetium-99, and uranium transport analyses with the three-dimensional model are also in agreement with those of Chiaramonte et al. (1996). Both models predicted the same general movement and shape for each of the simulated plumes from source locations to points of discharge along the Columbia River.
For iodine-129 in particular, maximum concentrations in both models were found to discharge to the Columbia River in the vicinity of the Old Hanford townsite. However, as in the case of tritium, results of the three-dimensional model indicated
For technetium-99, concentration levels in the technetium-99 plume declined below regulatory levels of concern before the plume moved out of the 200-Area plateau. For uranium, transport results show the plume remains largely within the 200-Area plateau. The overall trends of technetium-99 and uranium transport resulting from this analysis were very consistent with technetium-99 and uranium simulation results done by Chiaramonte et al. (1996).
As in the case of tritium, these differences 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 contaminant in the unconfined aquifer.
Potential water-quality impacts were examined by evaluating predictions of tritium concentrations at drinking-water supply well locations on the Hanford Site (Figure 6.1). Two principal drinking water supplies are 1) main and auxiliary supply wells located at the Fast Flux Test Facility (FFTF) in the 400 Area (wells 499-S0-7, 499-S0-8, and 499-S1-8J), and 2) the City of Richland wells in the North Richland well field. Other drinking-water supplies of concern include one well at Yakima Barricade and two wells at the Washington Public Power Supply System (WPPSS) that are used as backup for a surface-water supply (wells 699-13-1A and 699-13-1B). Other water-supply wells potentially impacted include 1) two wells at B Plant (299-E15 and 299-E28-11) and one well at the AY/AZ Tank farms (299-E26-6) currently being used for emergency cooling water in the 200-East Area, 2) one well at the Hanford Patrol Training Center, and 3) one well being used for aquatic studies in the 300 Area (well 399-4-12). Following is a brief assessment of the impacts of tritium at all of these water-supply well systems at the Hanford Site.
400 Area Water Supply. The water supply for the 400 Area is provided by wells completed in the unconfined aquifer. Detection of tritium in the initial supply wells (499-S0-7 and 499-S0-8), which are completed near the top of the aquifer, led to the development of an additional well in the lower part of the aquifer in 1985 (well 499-S1-8J) to reduce tritium concentration levels to below the 4-mrem/yr effective dose equivalent standard. This deeper well is the primary supply well, and well 499-S0-7 is used as a backup supply. Well 499-S0-8 is maintained for emergency use.
Currently, the water supply wells at FFTF produce groundwater containing tritium concentrations ranging from 11,000 to 38,000 pCi/L. Model results suggest that the shallow auxiliary wells at the FFTF will continue to be impacted by the tritium plume originating from the 200-East Area for the next 10 to 20 years (Figures 6.2 and 6.3). Tritium levels at this location are expected to remain above the 20,000-pCi/L level until sometime before 2020 (Figure 6.4). After that time, tritium will continue to decline to below 500 pCi/L between the years 2070 and 2080.
North Richland Well Field. Currently, tritium is detected in wells at the North Richland well field at levels essentially equivalent to those in the Columbia River at the Richland Pumphouse (Dirkes and Hanf 1996). Model results suggest that tritium concentrations now found in the 300 Area exceeding 2000 pCi/L will not reach the North Richland well field.
Water Supplies at WPPSS. Currently, backup water supply wells at WPPSS are sampled quarterly by the WPPSS personnel for selected radionuclides including tritium, and less frequently for nitrates and volatile organic compounds. Tritium is being detected at levels between 1000 and 2000 pCi/L. Model results suggest that tritium levels at these well locations will potentially be above this level over the next 5 to 10 years as the tritium plume slowly migrates eastward. However, modeling results also suggest that tritium levels will decline to below 20,000 pCi/L sometime before 2020 (Figure 6.4) and will decline to well below 2000 pCi/L after about 60 to 70 years.
Water Supply at the Hanford Training Center. Currently, no tritium attributable to the plume originating from the 200-East Area has been measured in the water supply well at the Hanford Training Center. Model results suggest that tritium concentrations now found in the 300 Area exceeding 2000 pCi/L will not reach this supply well.
Water Supply in 300 Area. Currently, tritium attributable to the plume originating from the 200-East area has been measured in the water supply well in the 300 Area used for aquatic studies. Model results suggest that tritium concentrations now found in the 300 Area exceeding 3000 pCi/L will decline at this location over the next 10 to 20 years.
Water Supplies in the 200-East Area. Currently, the two wells at B Plant (299-E15 and 299-E28-11) and the one well at the AY/AZ Tank farms (299-E26-6) currently being used for emergency cooling water in the 200-East Area are being impacted by tritium. Tritium levels in the vicinity of B-Plant wells currently range from 12,000 to 20,000 pCi/L. Model results suggest that tritium levels in B-Plant will likely remain at this level for the next several years but will decline below levels of concern within 10 to 20 years. Tritium levels in wells in the vicinity of the AY/AZ Tank farms currently range from about 3000 to 5000 pCi/L. Model results suggest that tritium levels in the AY/AZ Tank farm area will likely remain at this level for the next 10 to 20 years.
Simulated levels of iodine-129 suggest that only water supplies in the 200-East Area could potentially be impacted. The two wells near B-Plant and the one well near the AX/AZ Tank farms currently contain iodine-129 levels below 1 pCi/L, but wells near the AX/AZ Tank farms could potentially produce higher levels of iodine as the current iodine-129 plume migrates eastward from the 200-East Area. Model-predicted levels of iodine-129 shown in Figure 6.5 suggest that within 20 to 30 years, iodine levels in excess of 1 pCi/L currently in the 200-East Area would be found about halfway to the Columbia River by 2030. The iodine-129 plume in the 200-West Area will be expected to migrate slowly toward the 200-East Area, but model results (Figure 6.5) suggest that levels exceeding 1 pCi/L would not reach the 200-East Area within 30 years.
Potential water-quality impacts projected by the three-dimensional model simulations
of technetium-99, uranium, and strontium-99 were examined by evaluating predicted
concentrations of these constituents at drinking-water supply well locations
on the Hanford Site. None of the identified water supplies on the Site will
be impacted by significant concentrations of any of these constituents. Levels
of technetium-99, uranium, and strontium predicted for future conditions show
no impact on identified water supplies in the 200-East Area near B plant and
the AY/AZ Tank farm area.
