High concentrations of uranium were detected in samples from wells used for domestic drinking water supplies in the San Joaquin Valley. Of 163 domestic wells sampled by the California Groundwater Ambient Monitoring and Assessment Program Priority Basin Project (GAMA-PBP) and the National Water Quality Program (NWQP) in 2008-2015, 26 percent had uranium concentrations greater than the U.S. Environmental Protection Agency maximum contaminant level (EPA MCL) of 30 µg/L, with 13 percent of the wells having uranium concentration between 100 µg/L and 450 µg/L. To evaluate the potential anthropogenic and geologic causes of these anomalously high uranium concentrations, Rosen and others (2019) compiled water quality and ancillary data for 450 samples collected between 1993 and 2018 from 257 primarily domestic or public drinking water supply wells sampled by the USGS for GAMA-PBP or NWQP studies. Water-quality data compiled from the USGS NWIS database includes: field water-quality parameters (dissolved oxygen and pH), concentrations of major ions, trace elements, and nutrients, and tritium activities. Groundwater age and oxidation-reduction status classifications were derived from the water-quality data, and equilibrium saturation indices for minerals of interest were calculated from the water-quality data using PHREEQC. Ancillary data compiled for each well site include: well construction information, land use characteristics in 2001 and geologic characteristics. Rosen and others (2019) used graphical and spatial relations, statistical correlations, and principle component analysis to evaluate changes in uranium concentrations over time and infer processes responsible for occurrence of elevated uranium concentrations. They conclude that the process previously identified by Jurgens and others (2010) is responsible for a large part of the observed patterns of increasing uranium concentrations and occurrence of uranium concentrations greater than the EPA MCL - increased bicarbonate concentrations in recharge used for agricultural irrigation causes uranium sorbed on San Joaquin Valley sediments derived from Sierra Nevada granitic rocks to be become soluble. Rosen and others (2019) infer that in addition to solubility enhanced by bicarbonate, the highest uranium concentrations - which were found in the historic discharge zone at the distal end of regional groundwater flow system in the San Joaquin Valley - likely also reflect dissolution of reduced uranium minerals by the more oxic modern recharge water. All of the water-quality and ancillary data used by Rosen and others (2019) are presented in this Data Release.

Rosen, M.R., Burow, K.R., and Fram, M.S., 2019 , Anthropogenic and geologic causes of anomalously high uranium concentrations in groundwater used for drinking water supply in the southeastern San Joaquin Valley, California: Journal of Hydrology, v. 577, ppg. 12409, https://doi.org/10.1016/j.jhydrol.2019.124009.

Jurgens, B.C., Fram, M.S., Belitz, K., Burow, K.R., and Landon, M.K., 2010, Effects of groundwater development on uranium: Central Valley, California, USA: Ground Water, v. 48, p. 913-928, https://doi.org/10.111/j.1745-6584.2009.00635.x.

First Posted August 8, 2019
Revised December 19, 2019, ver 1.1