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.

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.

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.

Mendenhall and others (1916) published depth-to-water data compiled in 1910 for all known wells in the San Joaquin Valley, California. Data for the 3,429 wells having depth-to-water of greater than zero feet were used to construct an interpolated depth-to-water surface for the entire SJV. This map represents groundwater levels in approximately 1910, prior to extensive development of the groundwater system. A depth-to-water contour map with contour lines of 5, 10, 15, and 25 meters below land surface was then drawn from this raster surface.

Mendenhall and others (1916) published depth-to-water data compiled in 1910 for all known wells in the San Joaquin Valley, California. Data for the 3,429 wells having depth-to-water of greater than zero feet were used to construct an interpolated depth-to-water surface for the entire SJV. This map represents groundwater levels in approximately 1910, prior to extensive development of the groundwater system. A depth-to-water contour map with contour lines of 5, 10, 15, and 25 meters below land surface was then drawn from this raster surface.

Well construction data for 11,917 domestic and 2,390 public-supply wells in the Central Valley were compiled as part of the U.S. Geological Survey (USGS) National Water Quality Assessment Project (NAWQA) and California State Water Resources Control Board (SWRCB) Groundwater Ambient Monitoring and Assessment Program Priority Basin Project (GAMA-PBP). Data were compiled for wells reported in the USGS National Water Information System (NWIS) database and from well information reported to the SWRCB Department of Drinking Water (SWRCB-DDW). Driller’s log data were transcribed from scanned images of well completion reports filed with California Department of Water Resources (DWR). The wells reported in this data release were filtered by water use to select domestic and public-supply wells and omit other water uses. The compilation was then assumed to be representative of the total population of domestic and public-supply wells in the Central Valley. The wells in the compilation were constructed between 1911 and 2008 but are not grouped or separated by date. The data were used to produce two point data sets containing well location and construction information (depth from land surface to the top and bottom of the well screen, hereafter well-screen tops and bottoms; and screen length), and 12 interpolated GIS raster surfaces created by using Empirical Bayesian Kriging on a 1600 by 1600 meter (1 square-mile) grid. The tables are also included in csv format. The 12 rasters comprise predicted values for well screen tops and bottoms and their 10th and 90th quantile values. The interpolated surfaces may also be used to calculate volumes of water-supply in the Central Valley defined by the well-screen tops and bottoms.

Well construction data for 11,917 domestic and 2,390 public-supply wells in the Central Valley were compiled as part of the U.S. Geological Survey (USGS) National Water Quality Assessment Project (NAWQA) and California State Water Resources Control Board (SWRCB) Groundwater Ambient Monitoring and Assessment Program Priority Basin Project (GAMA-PBP). Data were compiled for wells reported in the USGS National Water Information System (NWIS) database and from well information reported to the SWRCB Department of Drinking Water (SWRCB-DDW). Driller’s log data were transcribed from scanned images of well completion reports filed with California Department of Water Resources (DWR). The wells reported in this data release were filtered by water use to select domestic and public-supply wells and omit other water uses. The compilation was then assumed to be representative of the total population of domestic and public-supply wells in the Central Valley. The wells in the compilation were constructed between 1911 and 2008 but are not grouped or separated by date. The data were used to produce two point data sets containing well location and construction information (depth from land surface to the top and bottom of the well screen, hereafter well-screen tops and bottoms; and screen length), and 12 interpolated GIS raster surfaces created by using Empirical Bayesian Kriging on a 1600 by 1600 meter (1 square-mile) grid. The tables are also included in csv format. The 12 rasters comprise predicted values for well screen tops and bottoms and their 10th and 90th quantile values. The interpolated surfaces may also be used to calculate volumes of water-supply in the Central Valley defined by the well-screen tops and bottoms.

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.