Perched-water analysis related to deep vadose zone contaminant transport and impact to groundwater

• The perched-water system at Hanford can be analyzed using 1-D simulations.
• The perching-layer conductivity is considerably smaller than what has been assumed.
• Perched water will persist with an uranium groundwater discharge of up to 6 kg/yr.
• Removal of perched water will lower the total groundwater mass discharge.
• Removal of perched water will only moderately decrease groundwater concentrations.

SummaryA series of calculations and model predictions were used to evaluate the controls on perched-water conditions and constraints on perching occurrence, persistence, and potential impact on groundwater contamination. These simulations considered perched-water conditions that have been observed in the vadose zone above a fine-grained layer located just a few meters above the water table beneath the B-Complex Tank Farms area at the Hanford Site. The perched water, containing elevated concentrations of uranium and technetium-99, is important to consider in evaluating the future flux of contaminated water into the groundwater. A study was conducted to examine the perched-water conditions and quantitatively evaluate (1) factors that control perching behavior, (2) contaminant flux toward groundwater, and, (3) associated groundwater impact. Based on the current vertical transport pathways and large areal extent of the perched system, the evaluation was conducted using a one-dimensional analysis. Steady-state analytical calculations showed that the perching-layer hydraulic conductivity is likely to be up to two orders of magnitude lower than the value obtained from Hanford site material property estimates. Numerical flow and transport simulations provided both steady-state and transient system estimates of water and contaminant behavior and were used to further refine the range of conditions consistent with current observations of perched water height and to provide estimates of future water and contaminant flux to groundwater. Near-term removal of perched water by pumping can decrease the total contaminant mass that will discharge to the groundwater, but will have only a moderate effect on the near-term discharge rate and corresponding contaminant concentration in groundwater. Combining pumping with a decrease in the recharge rate will be most effective in minimizing the impact to groundwater. These results provide a framework for constraining the behavior of perched aquifer systems especially related to impacts on contaminant transport.