Natural Uranium Contamination in Major U.S. Aquifers Linked to Nitrate

Groundwater geochemical data collected from two major U.S. aquifers, High Plains (HP) and Central Valley (CV), revealed naturally occurring groundwater uranium (U) exceeding the U.S. Environmental Protection Agency maximum contaminant level (MCL = 30 μg/L) across 22375 km² where 1.9 million people live. Analysis of geochemical parameters revealed a moderately strong correlation between U and nitrate, a common groundwater contaminant, as well as alkalinity and calcium [Spearman's rho (ρ) ≥ 0.30; p < 0.001]. Nitrate is recognized to alter U solubility by oxidative dissolution of reduced U(IV) minerals. Approximately 78% of areas where U concentrations were interpolated above the MCL were correlated to the presence of nitrate (Pearson's r ≥ 0.5; p < 0.05). Shallow groundwater was determined to be the most susceptible to co-contamination (HP, ρ = 0.46; CV, ρ = 0.52). Together, these results indicate that nitrate, a primary contaminant, should be considered as a factor leading to secondary groundwater U contamination in addition to the recognized role of alkalinity and calcium.