Paper No. 70-4
Presentation Time: 8:45 AM
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.