Data for Analysis of Decadal Trends in the Quality of Groundwater Used for Public Drinking-Water Supply in California, 2004-2023, California GAMA Priority Basin Project

This data release documents datasets supporting analysis of decadal trends in the quality of groundwater resources used for public drinking-water supply across California. The analysis primarily utilizes samples collected by the U.S. Geological Survey's Groundwater Ambient Monitoring and Assessment Priority Basin Project (GAMA-PBP) during 2004-2023 as part of a cooperative research program with the California State Water Resources Control Board. Decadal changes in groundwater quality were assessed for a total of 145 water-quality constituents and geochemical indicators with more than 300 paired decadal samples at up to 444 study sites located throughout California. Pairs of samples at each study site were selected based on elapsed time between samples (as close to a decade as possible within a range of 7 to 15.5 years) for each considered constituent. A preliminary analysis of detection frequencies for 104 organic constituents (volatile organic compounds and pesticide constituents) was made to select those with sufficient detections for subsequent trends analysis. Significance of constituent changes between paired samples (initial and decadal) were determined using concentration-based log-log models developed from analysis of field replicates taken by the GAMA-PBP during 2004-2024. Directional changes (increase or decrease) for individual constituents were computed statewide or within subdivisions of study sites aggregated by geography or land-use (network groups) using the Wilcoxon signed-rank test with the Pratt modification for zeros. Correlation analysis was used on decadal differences (decadal minus initial concentrations) to identify constituents that more commonly changed with each other and relations of water-quality changes to those of geochemical indicators.

This data release contains 12 tables formatted as tab-delimited text files. The Tbl01_Sites.txt table contains attributes of the 444 study sites used to assess decadal trends in this study. The Tbl02_Constituents.txt table contains attributes of the 145 water-quality constituents and geochemical indicators assessed in this study. The Tbl03_Organic_Analyses.txt file contains decadal sample pairs for 104 organic constituents used to assess detection frequencies. The Tbl04_Organic_Detection_Frequencies.txt table summarizes detection frequency results for 104 organic constituents. The Tbl05_Alkalinity_Comparison.txt table contains comparisons of field and laboratory measured alkalinity values for 612 samples collected by the GAMA-PBP during 2005-2023. The Tbl06_Method_Codes.txt table contains descriptions of the analytical methods used to evaluate decadal trends for constituents assessed in this study. The Tbl07_Replicates.txt contains field replicate data collected by the GAMA-PBP during 2004-2024 for 52 constituents analyzed for step trends in this study. The Tbl08_Replicate_Model_Coefficients.txt table contains log-log model coefficients for 49 constituents with appropriate data to develop log-transformed regression models. The Tbl09_Decadal_Comparison_Samples.txt file contains sample pairs used to assess decadal trends with confidence intervals for initial and decadal samples computed from log-log models. The Tbl10_Decadal_Comparison_Summary.txt file summarizes statistical analyses for 54 water-quality constituents and geochemical indicators with sufficient data to compute decadal trends. The Tbl11_Inorganic_Cluster_Input.txt table shows the input dataset of decadal differences for inorganic water-quality constituents and geochemical indicators used for hierarchical cluster analysis of Spearman's correlation coefficients. The Tbl12_Constituent_Change_Correlations.txt table summarizes correlation analysis results for decadal differences of 17 water-quality constituents to those of 8 geochemical indicators.