Data for evaluating sustainable management of groundwater-quality and trends for groundwater basins in the Gilroy-Hollister Valley and northern San Joaquin Valley, California

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.