SPATIAL AND STATISTICAL ANALYSIS OF URANIUM IN THE HIGH PLAINS AQUIFER IN COMPARISON TO THE CENTRAL VALLEY AQUIFER

The High Plains (HP) aquifer in the Midwestern U.S. and the Central Valley (CV) aquifer in California each supply groundwater that is used for agriculture irrigation and drinking water for millions of people. Groundwater in both aquifers contain regions of uranium exceeding the maximum contaminant level (MCL) of 30 µg/L. Previous research has found that nitrate from fertilizers can oxidize the insoluble mineral uraninite into soluble U(VI), which can subsequently form aqueous complexes (with carbonate, calcium/magnesium and carbonate, or sulfate) inhibiting reabsorption onto aquifer mineral surfaces. Our aim for this study was to use publicly available data to determine the spatial distribution of uranium and other chemicals (alkalinity, calcium, carbonate, chloride, iron, magnesium, manganese, nitrate, oxygen, sodium, sulfate) in the HP aquifer and compare it to the CV aquifer to determine the mechanisms responsible for U mobilization. Publicly available data were uploaded into ArcGIS Pro and were kriged using a machine-learning algorithm, Empirical Bayesian kriging. Data were also assessed using multivariate dimension reduction statistics including a principal component analysis and hierarchical cluster analysis. Uranium concentrations were >30 µg/L in two places in the HP aquifer (northwest Texas and northwest Nebraska) and in the Kings Groundwater Basin (KGWB) in the CV aquifer. Our analysis suggests uranium is mobilized differently depending on the hydrogeologic conditions. Along the North Platte River in the HP aquifer, uranium is mobilized by a process that does not include nitrate, but rather involves dissolved oxygen, elevated pH (>8), and high alkalinity, leading to carbonate complexation. In the KGWB and in northwest Texas, uranium appears to be oxidized by nitrate and forms calcium/magnesium complexes. All uranium hotspots were in the HP and CV aquifers were in shallow groundwater. Results from this study can expand our understanding of geochemical processes underpinning unsafe levels of uranium in groundwater and in designing targeted remediation strategies to protect groundwater.