URANIUM CONTAMINATION IN MAJOR US AQUIFERS: CORRELATION BETWEEN NITRATE AND URANIUM

Uranium (U) has been identified as a natural contaminant exceeding the US EPA MCL (30 μg L^-1) in ground water throughout the US. This contamination is particularly significant in the High Plains and Central Valley Aquifers, two of the largest aquifers in the US. These aquifers provide drinking water for up to 82% of inhabitants. Alkalinity increases have been traditionally attributed to mobilization of geogenic U. However, alkalinity alone cannot explain mobilization of U in these aquifers when U is buried as a reduced U(IV) sedimentary mineral. As such, oxidation of solid-phase reduced U(IV) to mobile U(VI) species is required. Nitrate, the most common groundwater contaminant, can serve as an oxidant of reduced U(IV) via biotic and abiotic mechanisms. Spatial interpolation of 414,999 groundwater-sampling activities at 139,311 testing locations predicted regions where nitrate and U contamination co-occur in the High Plains and Central Valley Aquifers. Based on these interpolations, 1.9 million people are estimated to live within 1-km of wells where U is predicted to exceed the MCL. U concentrations in groundwater ranges up to 5,400 ug L^-1 and nitrate concentrations range up to 1,894 mg L^-1, with mean concentrations of 11.93 ug L^-1, and 8.82 mg L^-1, respectively. Shallow wells (<100ft, 30.5m) revealed statistically significant (p=0.0010; p<0.0001) higher concentrations of U and nitrate and accounted for 79.2% of U and 64.8% of nitrate MCL exceedances. Direct correlation analysis further demonstrated that nitrate is significantly (p<0.0001) correlated to U contamination (Spearman’s rho [ρ]) of up to ρ=0.32 in the High Plains Aquifer and up to ρ=0.60 in the Central Valley Aquifer. These findings demonstrate the potential relationship between U mobilization and nitrate concentrations. As such it is important to begin to consider mechanism in which nitrate could drive U contamination, such as direct and indirect nitrate mediated U(IV) oxidation. Together these mechanisms combined with increasing alkalinity could lead to significant further groundwater contamination with U and a deeper understanding of these mechanisms is needed.