The USGS first identified uranium as a potential issue in California's groundwater during studies conducted in the Modesto area funded by the USGS National Water Quality Assessment Program (Jurgens and others, 2008). Subsequently, the GAMA program undertook a broader analysis of uranium concentrations across the State. That study (Jurgens and others, 2010) identified where uranium was a problem and perhaps more importantly, explained why concentrations were increasing over time.

Succinctly, plants naturally release bicarbonate from their roots into groundwater as they grow (Figure 1). Concentrations of bicarbonate in shallow groundwater increase during the summer irrigation season, and groundwater pumping draws this water deeper into the aquifer. The uranium is leached from natural soils by this high-carbonate water, leading to higher concentrations of dissolved uranium.

A joint GAMA/ NAWQA study (Rosen and others, 2019) further refined our understanding of the issue, ruling out nitrate as a contributing factor and providing more explanation of how granitic soils in alluvial fans and long-term groundwater flow paths are responsible for the observed geographical distributions of uranium concentrations.

These processes have also contributed to an increased in salinity in the same area because higher bicarbonate concentrations increase weathering of silicate rock. (Hansen, Jurgens, and Fram, 2018).