PNNL

Abstract

Initial scoping calculations of the unconfined aquifer at the Hanford Site were carried out for the U.S. Bureau of Reclamation (USBR) to investigate the potential impacts on the Hanford unconfined aquifer that would result from leakage from the proposed Black Rock Reservoir to the west. Although impacts on groundwater flow and contaminant transport were quantified based on numerical simulation results, the investigation represented a qualitative assessment of the potential lateral recharge that could result in adverse effects on the aquifer. Because the magnitude of the potential leakage is unknown, hypothetical bounding calculations were performed. When a quantitative analysis of the magnitude of the potential recharge from Black Rock Reservoir is obtained, the hydrologic impacts analysis will be revisited. The analysis presented in this report represents initial bounding calculations. A maximum lateral recharge (i.e., upland flux) was determined in the first part of this study by executing steady-state flow simulations that raised the water table no higher than the elevation attained in the Central Plateau during the Hanford operational period. This metric was selected because it assumed a maximum remobilization of contaminants that existed under previous fully saturated conditions. Three steady-state flow fields were then used to analyze impacts to transient contaminant transport: a maximum recharge (27,000 acre-ft/yr), a no additional flux (365 acre-ft/yr), and an intermediate recharge case (16,000 acre-ft/yr). The transport behavior of four radionuclides was assessed for a 300 year simulation period with the three flow fields. The four radionuclides are tritium, iodine-129, technetium-99, and uranium-238. Transient flow and transport simulations were used to establish hypothetical concentration distributions in the subsurface. Using the simulated concentration distributions in 2005 as initial conditions for steady-state flow runs, simulations were executed to investigate the relative effects on contaminant transport from the increased upland fluxes. Contaminant plumes were analyzed for 1) peak concentrations and arrival times at downstream points of compliance, 2) the area of the aquifer contaminated at or above the drinking water standard (DWS), and 3) the total activity remaining in the domain at the end of the simulation. In addition to this analysis, unit source release simulations from a hypothetical tracer were executed to determine relative travel times from the Central Plateau. The results of this study showed that increases in the lateral recharge had limited impact on regional flow directions but accelerated contaminant transport. Although contaminant concentrations may have initially increased for the more mobile contaminants (tritium, technetium-99, and iodine-129), the accelerated transport caused dilution and a more rapid decline in concentrations relative to the Base Case (no additional flux). For the low-mobility uranium-238, higher lateral recharge caused increases in concentration, but these concentrations never approached the DWS. In this preliminary investigation, contaminant concentrations did not exceed the DWS study metric. With the increases in upland fluxes, more mass was transported out of the aquifer, and concentrations were diluted with respect to the base case where no additional flux was considered.