Data for decadal shifts in groundwater age detected by environmental tracers across California, USA

This data release documents seven tables containing environmental tracer data and lumped parameter model results used to evaluate temporal changes in groundwater age at 268 sites across California from 2004 to 2021. These sites are spatially distributed throughout the state and represent groundwater at depths used for public supply. Each site was sampled at least twice, with paired samples collected 8 to 12 years apart. Environmental tracer concentrations from paired samples were fit to age distribution models, commonly known as lumped parameter models, to determine whether a single age distribution could explain the set of tracer concentrations observed at a well. Model fits were evaluated using the chi-square test statistic and models with a chi-square probability greater than 5 percent were considered to have a stable groundwater age; otherwise, the site was classified as unsteady. Each paired sample was tested across a range of ages, mixture types, unsaturated zone travel times, and carbon-14 dilution factors. This analysis generated hundreds of thousands of models, providing a comprehensive initial screen to identify sites where groundwater age is unsteady. Site information, tracer measurements, and model results are presented in this data release. All tracer and water-quality data are available from the USGS Groundwater Ambient Monitoring and Assessment-Priority Basin Project website (Jurgens and others, 2018) and the USGS Water Data for the Nation database (U.S. Geological Survey, 2025). Details of the quality-control analysis, tracer data processing, and groundwater age modeling are provided in the metadata processing steps.

Table 1 contains information about each site, including location and well construction information, and whether the site had a change in age.
Table 2 contains tracer data and summary model results for 315 sample pairs analyzed in this study.
Table 3 contains detailed information on the calibrated age models, which includes the lumped parameter model name, the fitted parameters, the measured and modeled tracer concentrations, and the regional tracer histories in recharge used in the calibration process. Table 3 contains all the information necessary to reproduce the model fit of each sample using TracerLPM (Jurgens and others, 2012).
Table 4 contains the details of the air-water equilibrium model results for each sample computed in DGMETA (Jurgens and others, 2020), including the concentrations of dissolved gases.
Table 5 contains the computations of environmental tracer concentrations using the air-water equilibrium model results computed in DGMETA (Jurgens and others, 2020).
Table 6 is a data dictionary that contains a list of all the field names and their descriptions used in each tables in this data release.
Table 7 lists abbreviations and their definitions used in this data release.