Evaluation and Design of a Potential Tracer Test for the Ringold Upper Mud Aquifer at the 100-H Area of the Hanford Site - 22401

A groundwater tracer study is being evaluated for the Ringold Upper Mud (RUM) semi-confined aquifer in the 100-H Area at Hanford, where Cr(VI) concentrations are highest and more accurate hydrologic information is needed for optimization of the pump-and-treat (P and T) remedy. A technical evaluation was performed to determine the feasibility, optimal design, and monitoring strategy of a tracer test in the RUM aquifer that can provide needed hydrologic characterization without impacting P and T operations and remedial performance. Performing this detailed 'pre-field' evaluation will help provide more realistic expectations and successful test outcomes. The evaluation included electrical resistivity tomography (ERT) characterization at the site, numerical flow and transport (F and T) modeling, synthetic time-lapse ERT simulations, and laboratory experiments. The evaluation considered scenarios involving a pulsed injection of a potassium bromide (KBr) groundwater tracer at varying concentrations and volumes in a single injection well with capture (recovery) using a nearby P and T extraction well. F and T simulations predicted tracer breakthrough in the extraction well at detectable concentrations within a reasonable timeframe for monitoring. Laboratory column experiment results support the conclusion that a KBr tracer test can be designed to avoid performance impacts to the SIR-700 resin, used in the P and T system to remove Cr(VI), by staying within a moderate range of injection volumes and concentrations. Synthetic time-lapse ERT simulations indicate surface-based ERT imaging can spatially resolve subsurface tracer transport. However, for quantitative analysis of tracer transport using time-lapse ERT, relatively high tracer injection volumes and/or concentrations are likely needed. Given the integrated evaluation results, a convergent-flow tracer test involving a moderate KBr tracer injection concentration and volume [10 g/L and 37,854 L (10,000 gal)] and monitoring with time-lapse ERT to supplement traditional breakthrough curve analysis is recommended. (authors)