Geochemical and Depositional Controls on Uranium Concentrations in Groundwater in the Eastern San Joaquin Valley and Tulare Basin, California

Uranium concentrations in groundwater in California’s Eastern San Joaquin Valley and Tulare Basin violate drinking water standards in many wells and show temporal variation. Much of the uranium enters groundwater by desorbing from sediment surfaces and forming calcium-uranyl-carbonato complexes. For most wells with at least 4.5 ug/L uranium, at least 95% or aqueous uranium was calculated to be in these complexes. A surface complexation model parameterized by Mahoney et al., 2009, shows that both calcium and bicarbonate in groundwater can increase uranium desorption from ferrihydrite surfaces. Uranium concentrations in groundwater show an increasing temporal trend near Modesto, in the northern portion of the study area, but often do not show an increasing trend in other areas with increasing calcium, bicarbonate alkalinity, and total dissolved solids concentrations in groundwater. Uranium levels are highest in the western portion of the Eastern San Joaquin Valley. This may be linked to sediment deposition patterns, where more uranium is stored in and on sediments in the western, distal portion of fluvial fans.

A method for determining “potentially mobile uranium fraction”, or the ease of uranium desorption, from ferrihydrite surfaces by bicarbonate and calcium, is developed and applied. This method uses the Mahoney et al. (2009) parameterized diffuse-layer surface complexation model to approximate the concentration of uranium that would be adsorbed to ferrihydrite to achieve the observed uranium, calcium, and bicarbonate concentrations at equilibrium. This uranium concentration is then divided by 0.01 mmol/L uranium to determine the potentially mobile uranium fraction value. The potentially mobile uranium fraction values calculated for 346 wells in the study area increase from east to west, showing more uranium susceptible to desorption from sediment surfaces in the more distal portions of fluvial fans. Groundwater with high salinity levels had lower potentially mobile uranium fraction values than groundwater with low salinity levels, likely because uranium has been leached from sediment surfaces in the more saline groundwater.

In the Tuolumne and Stanislaus River fluvial fans, near the city of Modesto, uranium concentrations in groundwater are high because of elevated bicarbonate alkalinity and calcium concentrations, despite low potentially mobile uranium fraction values in groundwater from many wells. In the Kings River Fluvial Fan, groundwater with high salinity levels show anomalously high potentially mobile uranium fraction values. These high uranium levels may signal that other processes, like uranium(IV) oxidation by oxic or nitrate-rich groundwater, are releasing uranium in the Kings and San Joaquin River basins.

Aquifer depths with low salinity in the western portion of the Eastern San Joaquin Valley and Tulare Basin are particularly vulnerable to high uranium concentrations if calcium and bicarbonate alkalinity concentrations increase. Limiting calcium and bicarbonate alkalinity levels in groundwater is the best way to prevent increases in uranium concentrations in many portions these regions.