Mobilization of Naturally Occurring Uranium to Water Supply Wells, Southern Tucson Basin

Uranium concentrations exceeding the federal MCL were encountered in water during routine sampling from two community supply wells located south of Tucson, Arizona. The wells are situated adjacent to an area of recently developed agricultural fields on the western margin of the Tucson basin. A study conducted to evaluate the situation and compliance alternatives included extensive examination of the geological and hydrogeological setting and geochemical characterization of groundwater to determine the source of elevated uranium concentrations and appropriate measures to deal with the problem.

Trace metal composition of alluvial basin-fill sediments is poorly known, but various uraniferous sources are present in the area and uranium content in surficial materials is known to typically be about 5 mg/kg and range up to about 22 mg/kg. Uranium is naturally occurring in the alluvial sediments and groundwater underlying the area. Uranium solubility in groundwater depends strongly on the presence of potential complexing agents in solution, such as carbonate, phosphate and sulfate, as well as on the pH and oxidation-reduction conditions. Uranium is most mobile in oxic and alkaline groundwater where it forms highly soluble uranyl-carbonate complexes. The association of elevated uranium concentrations in groundwater with nitrate has been widely noted and a variety of mechanisms are known by which nitrate can directly or indirectly result in potential uranium mobilization. Evaluation of the hydrogeology and groundwater chemistry, including geochemical modeling using PHREEQC, indicate that the source of the observed uranium concentrations in the water supply wells is most likely due to increased concentrations of bicarbonate and calcium associated with irrigation return flow from the adjacent agricultural fields that has resulted in desorption of uranium from aquifer solids through formation of a highly soluble calcium-uranyl-carbonate complex. The area of elevated uranium concentrations appears to be restricted to the area impacted by irrigation return flows.