Groundwater age distribution and susceptibility to natural and anthropogenic contaminants were assessed for selected principal aquifers of the Western United States: the Central Valley aquifer system (CVAL), the Basin and Range basin-fill aquifers (BNRF), the Rio Grande aquifer system (RIOG), the High Plains aquifer (HPAQ), the Columbia Plateau basaltic-rock aquifers (CLPT), and the Colorado Plateaus aquifers (COPL). Groundwater ages were estimated by calibration of environmental tracers (tritium, tritiogenic helium-3, chlorofluorocarbons, sulfur hexafluoride, carbon-14 and radiogenic helium-4) to lumped parameter models (LPMs) for 1,353 samples from 1,182 sample locations. Groundwater samples were collected from wells (mainly drinking-water) in the CVAL between 2004 and 2018 as part of the California State Water Resources Control Board Groundwater Ambient Monitoring and Assessment Priority Basin Project (GAMA-PBP) and the National Water-Quality Assessment (NAWQA) Project; and in the BNRF in 2013, the RIOG in 2014 and 2015, the HPAQ between 2014 and 2017, the CPLT in 2016, and the COPL in 2017 as part of NAWQA. Table 1 reports the primary results of this assessment and it contains condensed results from dissolved gas modeling and calculated environmental tracer concentrations; results of the tritium age classification, susceptibility index, and mean groundwater age of each sample in this assessment; and water level and well construction information for some wells. Calibrated lumped parameter models provide the optimal mean age and mixing parameter(s) used to compute the distribution of ages that explain the measured tracer concentrations in a sample. Tables 2 and 3 provide results in support of Table 1. Table 2 reports detailed results for the calibration of dissolved gas models to neon, argon, krypton, xenon, and nitrogen. Calibrated dissolved gas models provide the optimal water temperature, excess air, entrapped air, fractionation of gases, and excess nitrogen gas (mainly from denitrification) that explain the measured dissolved gases in a sample. Table 3 reports measured concentrations and the detailed calculations of environmental tracer concentrations derived from the dissolved gas modeling results in Table 2. Calculated concentrations of environmental tracers that can be used in groundwater age calculations are the dry air mixing ratio of sulfur hexafluoride or chlorofluorocarbons, tritiogenic helium-3, which is the concentration of helium-3 from the decay of tritium, and radiogenic helium-4, which is the amount of helium generated from the decay of uranium and thorium in aquifer sediments. In addition to these three tables, two ancillary tables are included to provide more detailed information about the fields and the abbreviations used in tables 1-3.

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 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.

The availability of groundwater-quality data along with geophysical information for relatively deep wells (wells generally more than 300 feet deep) containing saline water (dissolved-solids concentrations greater than 2,000 milligrams per liter)) is limited throughout the state of Texas. Water-quality samples are important for calibrating estimates of groundwater salinity derived from geophysical well logs. Water-quality samples and geophysical logs were collected from a total of 12 wells completed in selected aquifers (Trinity, Edwards-Trinity (Plateau), Carrizo-Wilcox, Sparta, and Yegua-Jackson) in Texas.

The availability of groundwater-quality data along with geophysical information for relatively deep wells (wells generally more than 300 feet deep) containing saline water (dissolved-solids concentrations greater than 2,000 milligrams per liter)) is limited throughout the state of Texas. Water-quality samples are important for calibrating estimates of groundwater salinity derived from geophysical well logs. Water-quality samples and geophysical logs were collected from a total of 12 wells completed in selected aquifers (Trinity, Edwards-Trinity (Plateau), Carrizo-Wilcox, Sparta, and Yegua-Jackson) in Texas.