CO2 LEAKAGE IMPACTS ON SHALLOW GROUNDWATER: APPLICATION OF URANIUM ISOTOPE COMPOSITION TO TRACK THE ORIGIN AND MOBILITY OF URANIUM AT A NATURAL ANALOG SITE, CHIMAYO, NM

Groundwater near Chimayo, northern New Mexico, is affected by CO₂ and saline waters that upwell along a series of regional faults. This site provides an opportunity to investigate groundwater chemistry in the presence of elevated CO₂, and can provide insight into potential impacts of CO₂ and brine migration on a drinking water aquifer. Sequential extraction experiments in aquifer sediments indicate that uranium (U) is primarily found in the exchangeable, carbonate mineral, and Mn-oxide fractions. This suggests that cation exchange/adsorption and dissolution/precipitation of calcite could be important reactions controlling U in groundwater. However, prior modeling suggests that increased U concentrations found in well waters near fault-related CO₂ release is primarily due to entrainment of U from brackish water associated with the deep CO₂. In order to understand the factors leading to U release under different CO₂ levels, U isotope data were obtained for eight groundwaters, including a CO₂-rich upwelling brine. Uranium concentrations range from 10 to 497 ppb and generally show a positive correlation with dissolved CO₂ concentration, with the notable exception of the sample with the highest U concentration. Preliminary isotopic results yield ²³⁴U/²³⁸U activity ratios (AR) that range from 1.0 to 5.9 and vary widely even in wells in close proximity to each other. The high ARs suggest a significant flux from recently weathered material, either soil or host aquifer minerals. Groundwater ²³⁸U/²³⁵U ratios fall between 137.75 and 137.95. Waters with elevated CO₂ (>20 mmol dissolved CO₂) also yield the lowest ²³⁸U/²³⁵U ratios. Hyperbolic relationships between ²³⁸U/²³⁵U and U concentration and dissolved CO₂ in most samples suggest a mixing relationship between high-CO₂ brines and low-CO₂ ground waters. This is further bolstered by an inverse relationship between ²³⁸U/²³⁵U and ⁸⁷Sr/⁸⁶Sr. We hypothesize that dissolution of U-bearing minerals under oxidizing conditions by deep, CO₂-charged brines resulted in fractionation of ²³⁸U/²³⁵U, and the resultant isotopically light fluid subsequently mixed with shallower ground waters. Additional variation in the ²³⁸U/²³⁵U ratios could have been induced by exchange and dissolution/precipitation reactions in the aquifer.