This dataset provides groundwater age estimates for 203 wells used as public- and domestic-supply in two selected areas of California. Groundwater ages were estimated by calibration of environmental tracers (tritium, tritiogenic helium-3, sulfur hexafluoride, carbon-14 and radiogenic helium-4) to lumped parameter models (LPMs). 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. Groundwater samples were collected between March 2006 and October 2022 as part of four studies done for the California Groundwater Ambient Monitoring and Assessment Priority Basin Project (GAMA-PBP): (1) Central Eastside San Joaquin Basin public-supply, (2) South Coast Interior Basins - Gilroy public-supply, (3) Modesto, Turlock, and Merced subbasins of the San Joaquin Valley groundwater basin domestic-supply, and (4) Gilroy-Hollister groundwater basin and adjacent areas outside of the basin domestic-supply. Table 1 reports the primary results of this assessment including mean groundwater age, linear recharge rate, groundwater age classification based on tritium, condensed results from dissolved gas modeling, and calculated environmental tracer concentrations. Tables 2, 3, and 4 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, tritiogenic helium-3, which is the concentration of helium-3 from the decay of tritium, and radiogenic helium-4. Table 4 reports information used to calculate carbon-14 dilution for use in groundwater age calculations. In addition to these five tables, two ancillary tables (Table 5 and Table 6) are included to provide more detailed information about the fields and the abbreviations used in Tables 1-4.

This data release documents five tables and one geographic information systems shapefile feature used to evaluate groundwater quality and concentration trends for groundwater basins in the Gilroy-Hollister Valley and northern San Joaquin Valley, California. This dataset provides a framework for evaluating groundwater quality data at the spatial scale of groundwater basins and the temporal scale of 5-year intervals. These spatial and temporal scales were selected because the California Sustainable Management Act (SGMA) program includes local Groundwater Sustainability Plans (GSPs) with 5-year review cycles for approved basins (California Department of Water Resources, 2025). This dataset presents a proposed method for water-quality evaluations and is not intended to supersede datasets or results presented in existing GSPs. Groundwater quality data were downloaded from the California State Water Resources Control Boards - Groundwater Information System Data and Download Page (SWRCB, 2024), which compiles local, state, and federal agencies and is commonly used as a data source for GSPs. Groundwater quality data were selected for five groundwater basins: Gilroy-Hollister Valley - Llagas Area (3-003.01), Gilroy-Hollister Valley - North San Benito (3-003.05), San Joaquin Valley - Modesto (5-022.02), San Joaquin Valley - Turlock (5-022.03), San Joaquin Valley - Merced (5-022.04). Groundwater-quality data were evaluated against state and federal water quality benchmarks used for drinking water. Each groundwater basin was divided into 15 or 20 equal-area grid cells that were used to spatially weight detection frequencies above benchmarks and identify areas where groundwater quality and trends may be more prevalent than other areas. This data release evaluated detection frequencies of constituents above benchmarks in wells, in cells, and in the basin. This data release also computed groundwater quality trends in municipal and domestic wells by comparing concentrations in the previous two five-year time periods (2014-2018; 2019-2023) and by computing monotonic concentration trends in municipal wells. Results from this effort may identify constituents and areas needing additional monitoring to assess groundwater quality conditions and trends in a groundwater basin. Methods for calculating spatial weighting of concentrations and the statistical tests for trends are based on common techniques and recently published work (Belitz and others, 2010; Jurgens and others, 2019; Haugen and others, 2021). Results of the water quality characteristics and trends are summarized in table 1. Table 2 is a list of all constituents that were above a federal or state water-quality benchmark in at least one of the groundwater basins and an evaluation of reporting levels among the different projects that analyzed each constituent. Table 3 is a list of the maximum value at a well for each groundwater quality constituent in each five-year time-period. Table 4 is a count of wells in cells that are high, moderate, or low for each constituent analyzed. Table 5 is a detailed report on the statistical results of the three trend methods used in this data release. Geospatial data of the gridded groundwater basins is included in a GIS shapefile.

This dataset provides groundwater age estimates for 175 wells used as domestic supply in four selected areas of California. Groundwater ages were estimated by calibration of environmental tracers (tritium, tritiogenic helium-3, sulfur hexafluoride, carbon-14 and radiogenic helium-4) to lumped parameter models (LPMs). 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. Groundwater samples were collected from domestic or small-system wells between September 2020 and October 2022 as part of four studies done for the California Groundwater Ambient Monitoring and Assessment Priority Basin Project (GAMA-PBP): (1) Modesto, Turlock, and Merced subbasins of the San Joaquin Valley groundwater basin, (2) Butte, Sutter, and Yuba subbasins of the Sacramento Valley groundwater basin and adjacent areas on the western slope of the Sierra Nevada, (3) Kern County subbasin of the San Joaquin Valley groundwater basin, and (4) Gilroy-Hollister groundwater basin and adjacent areas outside of the basin. Table 1 reports the primary results of this assessment including mean groundwater age, linear recharge rate, groundwater age classification based on tritium, condensed results from dissolved gas modeling, and calculated environmental tracer concentrations. Tables 2, 3, and 4 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, tritiogenic helium-3, which is the concentration of helium-3 from the decay of tritium, and radiogenic helium-4. Table 4 reports information used to calculate carbon-14 dilution for use in groundwater age calculations. In addition to these four tables, two ancillary tables (Table 5 and Table 6) are included to provide more detailed information about the fields and the abbreviations used in Tables 1-4.

This report provides a full digitization of historic groundwater-quality and depth-to-water data from Mendenhall and others (1916) Water Supply Paper 398, “Ground Water in San Joaquin Valley, California” in a modern format suitable for further analysis of California’s water supply resources. Included are geochemical data for over 400 wells collected by Mendenhall in the fall of 1910, as well as depth-to-water and well construction information from over 4000 wells compiled by his team from over 15 years of well surveys throughout the San Joaquin Valley. Additionally, these data provide geospatial and geochemical data for sampled wells in California's San Joaquin Valley (SJV) in support of the publication: Hansen, J.A., Jurgens, B.C, Fram, M.S., Quantifying Anthropogenic Contributions to Century-Scale Groundwater Salinity Changes, San Joaquin Valley, California, USA, Science of the Total Environment, vol. XX, no. X, pp. XX-XX, 2018.

This report provides a full digitization of historic groundwater-quality and depth-to-water data from Mendenhall and others (1916) Water Supply Paper 398, “Ground Water in San Joaquin Valley, California” in a modern format suitable for further analysis of California’s water supply resources. Included are geochemical data for over 400 wells collected by Mendenhall in the fall of 1910, as well as depth-to-water and well construction information from over 4000 wells compiled by his team from over 15 years of well surveys throughout the San Joaquin Valley. Additionally, these data provide geospatial and geochemical data for sampled wells in California's San Joaquin Valley (SJV) in support of the publication: Hansen, J.A., Jurgens, B.C, Fram, M.S., Quantifying Anthropogenic Contributions to Century-Scale Groundwater Salinity Changes, San Joaquin Valley, California, USA, Science of the Total Environment, vol. XX, no. X, pp. XX-XX, 2018.

Groundwater-quality data collected between 1993 and 2015 were compiled from the U.S. Geological Survey (USGS) National Water Information System (NWIS) database for 722 wells in the San Joaquin Valley (SJV). Groundwater-quality data retrieved included lab analyses of complete major ion data (calcium, magnesium, sodium, potassium, chloride, sulfate, nitrate, alkalinity, bicarbonate, carbonate, silica, and TDS) for 613 samples, and an additional 109 samples with measured values of specific conductance. Most of these wells were sampled as part of the California Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project or the USGS National Water Quality Assessment (NAWQA) Program. In addition to GW quality data, the dataset includes well depths, measured or interpolated water levels, summary land-use information, and a tritium-based groundwater age classification. Each well was assigned to a geospatial grid cell in one of six SJV regions (https://www.sciencebase.gov/catalog/item/5892423ee4b072a7ac145e06). These data support the following publication: Hansen, J.A., Jurgens, B.C, Fram, M.S., Quantifying Anthropogenic Contributions to Century-Scale Groundwater Salinity Changes, San Joaquin Valley, California, USA: Science of the Total Environment, vol. XX, no. X, pp. XX-XX, 2018.

Groundwater-quality data collected between 1993 and 2015 were compiled from the U.S. Geological Survey (USGS) National Water Information System (NWIS) database for 722 wells in the San Joaquin Valley (SJV). Groundwater-quality data retrieved included lab analyses of complete major ion data (calcium, magnesium, sodium, potassium, chloride, sulfate, nitrate, alkalinity, bicarbonate, carbonate, silica, and TDS) for 613 samples, and an additional 109 samples with measured values of specific conductance. Most of these wells were sampled as part of the California Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project or the USGS National Water Quality Assessment (NAWQA) Program. In addition to GW quality data, the dataset includes well depths, measured or interpolated water levels, summary land-use information, and a tritium-based groundwater age classification. Each well was assigned to a geospatial grid cell in one of six SJV regions (https://www.sciencebase.gov/catalog/item/5892423ee4b072a7ac145e06). These data support the following publication: Hansen, J.A., Jurgens, B.C, Fram, M.S., Quantifying Anthropogenic Contributions to Century-Scale Groundwater Salinity Changes, San Joaquin Valley, California, USA: Science of the Total Environment, vol. XX, no. X, pp. XX-XX, 2018.

Mendenhall and others (1916) assessed groundwater resources in California's San Joaquin Valley in 1910 to estimate the availability of groundwater of suitable quality for agricultural, industrial, and drinking water supplies. They inventoried nearly all existing wells, compiled depth-to-water at 4,002 wells, and collected water-quality data at 485 wells. Samples were collected from 114 wells for laboratory analysis of total dissolved solids (TDS), chloride, sulfate, bicarbonate, carbonate, calcium, magnesium, sodium+potassium, and silica (Mendenhall and others. 1916; Dole, 1909). Field assays were used to measure TDS, chloride, sulfate, bicarbonate, carbonate, and total hardness in samples from 371 wells (Mendenhall and others. 1916; Leighton, 1905). Samples from 32 wells were analyzed using both laboratory and field assay methods. These data have been transcribed into a modern database format for use in future groundwater research.

Mendenhall and others (1916) assessed groundwater resources in California's San Joaquin Valley in 1910 to estimate the availability of groundwater of suitable quality for agricultural, industrial, and drinking water supplies. They inventoried nearly all existing wells, compiled depth-to-water at 4,002 wells, and collected water-quality data at 485 wells. Samples were collected from 114 wells for laboratory analysis of total dissolved solids (TDS), chloride, sulfate, bicarbonate, carbonate, calcium, magnesium, sodium+potassium, and silica (Mendenhall and others. 1916; Dole, 1909). Field assays were used to measure TDS, chloride, sulfate, bicarbonate, carbonate, and total hardness in samples from 371 wells (Mendenhall and others. 1916; Leighton, 1905). Samples from 32 wells were analyzed using both laboratory and field assay methods. These data have been transcribed into a modern database format for use in future groundwater research.

The U.S. Geological Survey collected groundwater samples from 33 wells used for domestic and small system drinking water supplies in Santa Clara and San Benito Counties, California in 2022. The wells were sampled for the Gilroy Hollister basin and surrounding areas 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 includes the Llagas and North San Benito subbasins of the Gilroy-Hollister Valley groundwater basin and surrounding areas. The study unit was divided into 40 grid cells and one or two domestic or small system well was sampled to represent 28 of the grid cells; the remaining 12 grid cells has no accessible wells. Groundwater levels were measured in 17 of 33 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.