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.

The U.S. Geological Survey collected groundwater samples from 95 domestic wells in Tulare and Kings Counties, California in 2014-2015. The wells were sampled for the Tulare Shallow Aquifer Study Unit of the California State Water Resources Control Board Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project’s assessment of the quality of groundwater resources used for domestic drinking water supply. Domestic wells commonly are screened at shallower depths than are public-supply wells. The Tulare Shallow Aquifer Study Unit includes the Kaweah, Tule, and Tulare Lake subbasins of the San Joaquin Valley groundwater basin and adjacent areas of the Sierra Nevada. The study unit was divided into equal area grid cells and one domestic well was sampled in each cell. Groundwater samples were analyzed for field water-quality parameters, volatile organic compounds, pesticides and pesticide degradates, nutrients, major ions and trace elements, gross alpha and gross beta particle activities, noble gases, tritium, carbon-14 in dissolved inorganic carbon, stable isotopic ratios of water and dissolved nitrate, and microbial indicators. These data support the following publication: Fram, M.S., 2017, Groundwater Quality in the Shallow Aquifers of the Tulare, Kaweah, and Tule Groundwater Basins and Adjacent Highlands areas, Southern San Joaquin Valley, California: U.S. Geological Survey Fact Sheet 2017–3001, 4 p., http://dx.doi.org/10.3133/fs20173001.

The U.S. Geological Survey collected groundwater samples from 95 domestic wells in Tulare and Kings Counties, California in 2014-2015. The wells were sampled for the Tulare Shallow Aquifer Study Unit of the California State Water Resources Control Board Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project’s assessment of the quality of groundwater resources used for domestic drinking water supply. Domestic wells commonly are screened at shallower depths than are public-supply wells. The Tulare Shallow Aquifer Study Unit includes the Kaweah, Tule, and Tulare Lake subbasins of the San Joaquin Valley groundwater basin and adjacent areas of the Sierra Nevada. The study unit was divided into equal area grid cells and one domestic well was sampled in each cell. Groundwater samples were analyzed for field water-quality parameters, volatile organic compounds, pesticides and pesticide degradates, nutrients, major ions and trace elements, gross alpha and gross beta particle activities, noble gases, tritium, carbon-14 in dissolved inorganic carbon, stable isotopic ratios of water and dissolved nitrate, and microbial indicators. These data support the following publication: Fram, M.S., 2017, Groundwater Quality in the Shallow Aquifers of the Tulare, Kaweah, and Tule Groundwater Basins and Adjacent Highlands areas, Southern San Joaquin Valley, California: U.S. Geological Survey Fact Sheet 2017–3001, 4 p., http://dx.doi.org/10.3133/fs20173001.

The U.S. Geological Survey collected groundwater samples from 44 wells used for domestic and small system drinking water supplies in the western Central Valley, California in 2023. The wells were sampled for the Western Central Valley domestic-supply aquifer study unit of the State Water Resources Control Board Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project’s assessment of the quality of groundwater resources used for domestic and small system drinking water supplies. The study unit consists of two study areas. The western Sacramento Valley study area comprises the Corning, Colusa, Yolo, and Solano subbasins of the Sacramento Valley groundwater basin, and the western San Joaquin Valley study area comprises the East Contra Costa, Tracy, Delta-Mendota, and Westside subbasins of the San Joaquin Valley groundwater basin. The study unit was divided into 49 grid cells and one domestic or small system well was sampled to represent 44 of the grid cells; the remaining 5 grid cells has no accessible wells. Groundwater levels were measured in 28 of 44 wells. Table 1 contains sample and site information. Groundwater samples were analyzed for field water-quality parameters, microbial indicators, volatile organic compounds, pesticides and pesticide degradants, nutrients, major ions and trace elements, hexavalent chromium, perchlorate, per- and polyfluoroalkyl substances (PFAS), stable isotopes, and groundwater age tracers. Results from these analyses are reported in Tables 2 through 8. Tables 9 and 10 report results from quality control samples. This data release contains two supplementary tables that define abbreviations (Table 11) and provide additional context for the information presented in Tables 1 through 10 (Table 12). A zipfile is included containing geospatial data (study unit boundaries, grid cell boundaries, site locations) as shapefiles.

The U.S. Geological Survey collected groundwater samples from 49 wells used for domestic and small system drinking water supplies and 10 monitoring wells in San Bernardino County, California in 2018. The wells were sampled for the Mojave Basin (MOBS) Shallow Aquifer Study Unit of the California State Water Resources Control Board Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project’s assessment of the quality of groundwater resources used for domestic and small system drinking water supplies. Domestic and small-system wells commonly are screened at shallower depths than public-supply wells. The MOBS study unit covered the Upper, Middle, and Lower Mojave River groundwater basins, the El Mirage groundwater basin, and part of the Harper groundwater basin [California Department of Water Resources, CA DWR basin designations 6-42, 6-41, 6-40, 6-43, 6-47] in the Desert hydrogeologic province (California Department of Water Resources, 2003; Johnson and Belitz, 2003). Wells in the MOBS study unit generally tap unconsolidated alluvial aquifers composed of sediment eroded from granitic and metamorphic rocks, with portions of aquifer material in the northeast part of the study unit eroded from volcanic rock. The MOBS study unit was divided into two study areas, the floodplain study area along the course of the Mojave River, and the regional study area covering the rest of the study unit. The study areas were further divided into equal area grid cells, with 35 grid cells of 85 square-kilometers each in the regional study area, and 15 grid cells of 29 square-kilometers in the floodplain study area. Domestic or small-system wells were sampled in 48 of the 50 grid cells. The 11 understanding wells (10 monitoring wells and 1 domestic well) were selected to provide additional data for evaluation of the effect of septic systems on groundwater resources used for domestic drinking water supply. Groundwater samples were analyzed for field water-quality parameters, volatile organic compounds, pesticides and pesticide degradates, nutrients, major ions and trace elements, hexavalent chromium, perchlorate, gross alpha and gross beta particle activities, noble gases, tritium, carbon-14 in dissolved inorganic carbon, stable isotopic ratios of water and dissolved nitrate, and microbial indicators. Groundwater levels were measured in nearly all of the sampled wells. All results are presented in this data release, except results for pesticide and pesticide degradates, sulfur hexafluoride, and noble gases.

The U.S. Geological Survey collected groundwater samples from 49 wells used for domestic and small system drinking water supplies and 10 monitoring wells in San Bernardino County, California in 2018. The wells were sampled for the Mojave Basin (MOBS) Shallow Aquifer Study Unit of the California State Water Resources Control Board Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project’s assessment of the quality of groundwater resources used for domestic and small system drinking water supplies. Domestic and small-system wells commonly are screened at shallower depths than public-supply wells. The MOBS study unit covered the Upper, Middle, and Lower Mojave River groundwater basins, the El Mirage groundwater basin, and part of the Harper groundwater basin [California Department of Water Resources, CA DWR basin designations 6-42, 6-41, 6-40, 6-43, 6-47] in the Desert hydrogeologic province (California Department of Water Resources, 2003; Johnson and Belitz, 2003). Wells in the MOBS study unit generally tap unconsolidated alluvial aquifers composed of sediment eroded from granitic and metamorphic rocks, with portions of aquifer material in the northeast part of the study unit eroded from volcanic rock. The MOBS study unit was divided into two study areas, the floodplain study area along the course of the Mojave River, and the regional study area covering the rest of the study unit. The study areas were further divided into equal area grid cells, with 35 grid cells of 85 square-kilometers each in the regional study area, and 15 grid cells of 29 square-kilometers in the floodplain study area. Domestic or small-system wells were sampled in 48 of the 50 grid cells. The 11 understanding wells (10 monitoring wells and 1 domestic well) were selected to provide additional data for evaluation of the effect of septic systems on groundwater resources used for domestic drinking water supply. Groundwater samples were analyzed for field water-quality parameters, volatile organic compounds, pesticides and pesticide degradates, nutrients, major ions and trace elements, hexavalent chromium, perchlorate, gross alpha and gross beta particle activities, noble gases, tritium, carbon-14 in dissolved inorganic carbon, stable isotopic ratios of water and dissolved nitrate, and microbial indicators. Groundwater levels were measured in nearly all of the sampled wells. All results are presented in this data release, except results for pesticide and pesticide degradates, sulfur hexafluoride, and noble gases.

The U.S. Geological Survey collected groundwater samples from 95 domestic wells in Tulare and Kings Counties, California in 2014-2015. The wells were sampled for the Tulare Shallow Aquifer Study Unit of the California State Water Resources Control Board Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project’s assessment of the quality of groundwater resources used for domestic drinking water supply. Domestic wells commonly are screened at shallower depths than are public-supply wells. The Tulare Shallow Aquifer Study Unit includes the Kaweah, Tule, and Tulare Lake subbasins of the San Joaquin Valley groundwater basin and adjacent areas of the Sierra Nevada. The study unit was divided into equal area grid cells and one domestic well was sampled in each cell. Groundwater samples were analyzed for field water-quality parameters, volatile organic compounds, pesticides and pesticide degradates, nutrients, major ions and trace elements, gross alpha and gross beta particle activities, noble gases, tritium, carbon-14 in dissolved inorganic carbon, stable isotopic ratios of water and dissolved nitrate, and microbial indicators.

The U.S. Geological Survey collected groundwater samples from 95 domestic wells in Tulare and Kings Counties, California in 2014-2015. The wells were sampled for the Tulare Shallow Aquifer Study Unit of the California State Water Resources Control Board Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project’s assessment of the quality of groundwater resources used for domestic drinking water supply. Domestic wells commonly are screened at shallower depths than are public-supply wells. The Tulare Shallow Aquifer Study Unit includes the Kaweah, Tule, and Tulare Lake subbasins of the San Joaquin Valley groundwater basin and adjacent areas of the Sierra Nevada. The study unit was divided into equal area grid cells and one domestic well was sampled in each cell. Groundwater samples were analyzed for field water-quality parameters, volatile organic compounds, pesticides and pesticide degradates, nutrients, major ions and trace elements, gross alpha and gross beta particle activities, noble gases, tritium, carbon-14 in dissolved inorganic carbon, stable isotopic ratios of water and dissolved nitrate, and microbial indicators.

The U.S. Geological Survey collected groundwater samples from 49 wells used for domestic and small system drinking water supplies in Amador, Placer, Sutter, and Sacramento Counties, California in 2017. The wells were sampled for the Sacramento Metropolitan (SacMetro) Shallow Aquifer Study Unit of the California State Water Resources Control Board Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project’s assessment of the quality of groundwater resources used for domestic and small system drinking water supplies. Domestic and small-system wells are commonly screened at shallower depths than public-supply wells. The SacMetro study unit consisted of three “study areas” corresponding to three California Department of Water Resources groundwater subbasins: the North American and South American subbasins of the Sacramento Valley groundwater basin, and the Cosumnes subbasin of the San Joaquin Valley groundwater basin. Wells in the SacMetro study unit were in alluvial aquifers composed of sediments derived from the Sierra Nevada Mountains east of the study unit. The study unit was divided into 56 approximately 58 square kilometer grid cells, and a domestic or small-system well was sampled in 49 of those cells. Groundwater samples from all 49 wells were analyzed for field water-quality parameters, volatile organic compounds, pesticides and pesticide degradates, nutrients, major ions and trace elements, arsenic speciation, chromium (VI), perchlorate, gross alpha and gross beta particle activities, sulfur hexafluoride, tritium, carbon-14 in dissolved inorganic carbon, noble gases, stable isotopic ratios of water and dissolved nitrate, and microbial indicators. Groundwater levels were measured in 44 of the 49 wells. All results are presented in this data release, except results for pesticide and pesticide degradates. In 2017, the U.S. Geological Survey National Water-Quality Assessment (NAWQA) Program sampled 22 monitoring wells located in the SacMetro study unit as part of ongoing NAWQA studies of groundwater quality in different land-use settings. Of the 22 wells, 21 were part of an urban land-use network and 1 was part of a rice land use network. Results for analyses of perchlorate, chromium (VI), and stable isotopic ratios are presented in this data release, and results for analyses of field water-quality parameters, volatile organic compounds, pesticide and pesticide degradates, nutrients, major ions and trace elements, dissolved organic carbon, and tritium will be presented in a data release to be published separately by NAWQA. This data release supports the following publication: Groundwater Quality in the Sacramento Metropolitan shallow aquifer, California: U.S. Geological Survey Open-File Report 2019-1047, 4p., https://doi.org/10.3133/ofr20191047.

The U.S. Geological Survey collected groundwater samples from 49 wells used for domestic and small system drinking water supplies in Amador, Placer, Sutter, and Sacramento Counties, California in 2017. The wells were sampled for the Sacramento Metropolitan (SacMetro) Shallow Aquifer Study Unit of the California State Water Resources Control Board Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project’s assessment of the quality of groundwater resources used for domestic and small system drinking water supplies. Domestic and small-system wells are commonly screened at shallower depths than public-supply wells. The SacMetro study unit consisted of three “study areas” corresponding to three California Department of Water Resources groundwater subbasins: the North American and South American subbasins of the Sacramento Valley groundwater basin, and the Cosumnes subbasin of the San Joaquin Valley groundwater basin. Wells in the SacMetro study unit were in alluvial aquifers composed of sediments derived from the Sierra Nevada Mountains east of the study unit. The study unit was divided into 56 approximately 58 square kilometer grid cells, and a domestic or small-system well was sampled in 49 of those cells. Groundwater samples from all 49 wells were analyzed for field water-quality parameters, volatile organic compounds, pesticides and pesticide degradates, nutrients, major ions and trace elements, arsenic speciation, chromium (VI), perchlorate, gross alpha and gross beta particle activities, sulfur hexafluoride, tritium, carbon-14 in dissolved inorganic carbon, noble gases, stable isotopic ratios of water and dissolved nitrate, and microbial indicators. Groundwater levels were measured in 44 of the 49 wells. All results are presented in this data release, except results for pesticide and pesticide degradates. In 2017, the U.S. Geological Survey National Water-Quality Assessment (NAWQA) Program sampled 22 monitoring wells located in the SacMetro study unit as part of ongoing NAWQA studies of groundwater quality in different land-use settings. Of the 22 wells, 21 were part of an urban land-use network and 1 was part of a rice land use network. Results for analyses of perchlorate, chromium (VI), and stable isotopic ratios are presented in this data release, and results for analyses of field water-quality parameters, volatile organic compounds, pesticide and pesticide degradates, nutrients, major ions and trace elements, dissolved organic carbon, and tritium will be presented in a data release to be published separately by NAWQA. This data release supports the following publication: Groundwater Quality in the Sacramento Metropolitan shallow aquifer, California: U.S. Geological Survey Open-File Report 2019-1047, 4p., https://doi.org/10.3133/ofr20191047.