This data release documents two Microsoft Excel tables; one contains data for understanding groundwater ages in the Spanish Valley watershed, and one that describe the data fields. Mean ages and age distributions from 19 groundwater samples were estimated in support of an evaluation of the groundwater resources of the Spanish Valley watershed (Masbruch and others, 2019). Individual groundwater well and spring vulnerability to land-surface contamination and changes in hydraulic conditions (for example, water extraction or reduced recharge) can be assessed using environmental tracer-based groundwater age. The detailed interpretation of groundwater age reported here supplements the apparent tracer ages of Masbruch and others (2019). Multiple age tracers sampled in groundwater were fit using TracerLPM (Jurgens and others, 2012), with working knowledge of the well dimensions, hydrogeology, and geochemistry, to assign a unique age distribution. The age distributions describes the relative contributions of flow-paths of differing age and the extent of flow-path mixing in the sample. Concentrations of tritium (3H), tritiogenic-He (3Hetrit), chlorofluorocarbons (CFC-11, -12, -113), sulphur hexafloride (SF6), and geochemically corrected carbon-14 (14C) were fit by optimizing age distribution parameters. Details of 3Hetrit calculation, 14C geochemical correction, and noble gas based estimates of conditions which were used to correct CFCs and SF6 are described in Masbruch and others (2019). Lumped parameter models (LPM) are described for each sample by the mean age, LPM name, model parameters, and in the the case of binary mixing models (BMM) the mixing fraction and the mean age of the old component. Final mean age for BMMs is calculated as (Mean Age 1 * Fraction) + (Mean Age 2 * (1 – Fraction)).

This data release documents two Microsoft Excel tables that contain data for understanding tracer concentrations and groundwater age in the Columbia Plateau aquifer system. Results for geochemical correction of carbon-14, and lumped parameter modeling of groundwater age for the sample network (VPFS, vertical flow path study) are described. Geochemical carbon-14 correction results (RFG) describe geochemical correction of carbon-14 in dissolved inorganic carbon (DIC) for groundwater age dating. Datasets includes measured water parameters and chemistry, model parameter inputs, and final corrected carbon-14 in DIC. Geochemical correction was completed using the revised Fontes and Granier model of Han and Plummer (2013). Mean age and age distribution results (TracerLPM) contain final models of groundwater age by calibration of lumped parameter models to tracer concentrations (Jurgens and others, 2012). Please see the processing steps below for additional details on the results presented in this table.

This data release documents two Microsoft Excel tables that contain data for understanding tracer concentrations and groundwater age in the Columbia Plateau aquifer system. Results for geochemical correction of carbon-14, and lumped parameter modeling of groundwater age for the sample network (VPFS, vertical flow path study) are described. Geochemical carbon-14 correction results (RFG) describe geochemical correction of carbon-14 in dissolved inorganic carbon (DIC) for groundwater age dating. Datasets includes measured water parameters and chemistry, model parameter inputs, and final corrected carbon-14 in DIC. Geochemical correction was completed using the revised Fontes and Granier model of Han and Plummer (2013). Mean age and age distribution results (TracerLPM) contain final models of groundwater age by calibration of lumped parameter models to tracer concentrations (Jurgens and others, 2012). Please see the processing steps below for additional details on the results presented in this table.

This data release contains site information and potential explanatory factor data for 1,899 groundwater sites. These sites were used to assess groundwater quality in aquifers used for domestic and public drinking water supply in the southeastern San Joaquin Valley. The southeastern San Joaquin Valley (SESJV) study unit consists of five study areas whose boundaries are defined by the eponymous California Department of Water Resources groundwater subbasins of the San Joaquin Valley groundwater basin: Madera-Chowchilla, Kings, Kaweah, Tule, and Tulare Lake. The sites consist of 198 wells representing the domestic-supply aquifer and 1,701 wells representing the public-supply aquifer. The domestic-supply aquifer wells were sampled in 2013-2015 by the USGS for either the California Groundwater Ambient Monitoring and Assessment Program Priority Basin Project (GAMA-PBP) or the USGS National Water Quality Assessment project (NAWQA). The public-supply aquifer wells were either sampled by the USGS for the GAMA-PBP in 2005-2018 or have water-quality data in the California State Water Resources Control Board Division of Drinking Water (SWRCB-DDW) public database. The data types in this data release include site identification and location, well construction and lithology data, land use characteristics, groundwater age and oxidation-reduction classifications and aridity indices. Not all sites have data for all fields. Water-quality data for the sites are available from U.S. Geological Survey (2023), and California State Water Resources Control Board Division of Drinking Water (2023). The study design and the assessment results are presented in Burow and others (2024).

This data release describes one (1) Microsoft Excel table of lumped parameter models of groundwater age for groundwater discharging to the South Loup River, Nebraska. The table (LPMAgeResults) includes final models of groundwater age and metrics by calibration of lumped parameter models to tracer concentrations using TracerLPM software (Jurgens and others, 2012). Interpreted results presented here were used to guide hydrologic interpretations of groundwater sources and flow paths of groundwater discharging to the South Loup River, NE.

This data release describes one (1) Microsoft Excel table of lumped parameter models of groundwater age for groundwater discharging to the South Loup River, Nebraska. The table (LPMAgeResults) includes final models of groundwater age and metrics by calibration of lumped parameter models to tracer concentrations using TracerLPM software (Jurgens and others, 2012). Interpreted results presented here were used to guide hydrologic interpretations of groundwater sources and flow paths of groundwater discharging to the South Loup River, NE.

Groundwater samples were collected from 60 public supply wells in the Colorado Plateaus principal aquifer. Water quality evaluations of groundwater for drinking water at public supply depths were made with the purpose of summarizing the current quality of source water (that is, untreated water) from public supply wells using two types of assessments; (1) status: an assessment that describes the current quality of the groundwater resource, and (2) understanding: an evaluation of the natural and human factors affecting the quality of groundwater, including an explanation of statistically significant associations between water quality and selected explanatory factors. To provide context for water-quality data, constituent concentrations of untreated groundwater are compared with available water-quality benchmarks Federal regulatory benchmarks for protecting human health (maximum contaminant levels [MCLs]; U.S. Environmental Protection Agency [USEPA] primary drinking water regulations; U.S. Environmental Protection Agency, 2018a) are used for this evaluation. Additionally, non-regulatory human-health benchmarks (health-based screening levels [HBSLs]; Norman and others, 2018; U.S. Geological Survey, 2018); and federal non-regulatory benchmarks for nuisance chemicals (USEPA secondary maximum contaminant levels [SMCLs]; U.S. Environmental Protection Agency, 2018b) are used. This report considers benchmarks in the context of health-based (MCLs and HBSLs) and non-health based (SMCLs) benchmarks. This sampling approach uses an equal-area grid design (Belitz and others, 2010) which allows for the estimation of the proportion of high, moderate, or low concentrations relative to federal water-quality benchmarks of selected constituents over the entire area of the aquifer. Tables included in this data release: Table 1. Identification, location, and construction information for wells sampled for the U.S. Geological Survey National Water-Quality Assessment Project, Colorado Plateaus principal aquifer, June 2013 through December 2017. Table 2. Constituent primary uses and sources; analytical schedules and sampling period; USGS parameter codes; comparison thresholds and reporting levels wells sampled for the for the U.S. Geological Survey National Water-Quality Assessment Project, Colorado Plateaus principal aquifer, June 2013 through December 2017. Table 3. Water-quality indicators in groundwater samples collected by the for the U.S. Geological Survey National Water-Quality Assessment Project, Colorado Plateaus principal aquifer, June 2013 through December 2017. [Table code definitions: NC, not collected; <, less than] Table 4. Nutrients and dissolved organic carbon in groundwater samples collected by the U.S. Geological Survey National Water-Quality Assessment Project, Colorado Plateaus principal aquifer, June 2013 through December 2017. [Table code definitions: --, less than minimum laboratory reporting level] Table 5. Major and minor ions in groundwater samples collected by the U.S. Geological Survey National Water-Quality Assessment Project, Colorado Plateaus principal aquifer, June 2013 through December 2017. [Table code definitions: --, less than minimum laboratory reporting level; E, estimated] Table 6. Trace elements in groundwater samples collected by the U.S. Geological Survey National Water-Quality Assessment Project, Colorado Plateaus principal aquifer, June 2013 through December 2017. [Table code definitions: NC, not collected; --, less than minimum laboratory reporting level] Table 7. Radionuclides in groundwater samples collected by the U.S. Geological Survey National Water-Quality Assessment Project, Colorado Plateaus principal aquifer, June 2013 through December 2017. [Table code definitions: --, less than minimum laboratory reporting level] Table 8. Volatile organic compounds (VOCs) in groundwater samples collected by the U.S. Geological Survey National Water-Quality Assessment Project, Colorado Plateaus principal aquifer,

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 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 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.