Data for assessing the susceptibility of groundwater used for domestic-supply, California (ver. 2.0, July 2026)

This dataset provides groundwater age estimates for 257 wells used as domestic supply in six 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 February 2024 as part of six 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, (4) Gilroy-Hollister groundwater basin and adjacent areas outside of the basin, (5) Corning, Colusa, Yolo, and Solano subbasins of the Sacramento Valley groundwater basin, and the East Contra Costa, Tracy, Delta-Mendota, and Westside subbasins of the San Joaquin Valley groundwater basin, and (6) Eastern San Joaquin Highlands, Modesto Highlands, Turlock Highlands, Merced Highlands, Yosemite Valley, Yosemite Highlands, Madera Highlands, and Kings Highlands groundwater units. 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. First Published: April 2025 Version 2.0: July 2026