Predictive Reactive Transport Modeling at a Proposed Uranium In Situ Recovery Site with a General Data Collection Guide

Restoration of uranium in situ recovery (ISR) sites to predevelopment conditions is often very difficult. Future downgradient groundwater geochemistry can be evaluated using reactive transport modeling coupled with appropriate data collection. U.S. regulatory requirements specify that the geochemistry at the aquifer exemption boundary should never be affected, but compliance with this regulation has not been monitored at previous ISR sites. At the Dewey Burdock site near Edgemont, SD, USA, a change in groundwater flow direction created a scenario in which the oxidized side of a U roll-front deposit is downgradient of the ore zone. This increases the potential for future U transport, since conventional understanding of U geochemistry is that the reduced side provides more natural attenuation. Reactive transport modeling using U sorption parameters from batch sorption tests provides a predictive tool for future U transport. Prediction variations were tested using two different samples, considering different reaction assumptions and possible pH measurement errors. The results indicate a large range in U transport predictions, with high sensitivity to sorption parameters due to sample heterogeneity, pH, and the presence or absence of calcite. While the sample data set for these initial predictions was limited, the results highlight the need for additional calibration points and a thorough understanding of rock/water interactions in the downgradient zone. We provide a general data collection guide for steps in evaluating downgradient transport at future U ISR sites. These steps include core sampling in the downgradient and restored zones, along with batch sorption and column testing with restored and background groundwater in contact with the restored zone solid phase. Final reactive transport modeling will rely on high-quality calibration data from batch and column testing (plus any available field testing), but thorough site evaluation will also require appropriate long-term monitoring.