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,

Groundwater-quality data were collected from 983 wells as part of the National Water-Quality Assessment Project of the U.S. Geological Survey National Water-Quality Program and are included in this report. The data were collected from six types of well networks: principal aquifer study networks, which are used to assess the quality of groundwater used for public water supply; land-use study networks, which are used to assess land-use effects on shallow groundwater quality; major aquifer study networks, which are used to assess the quality of groundwater used for domestic supply; enhanced trends networks, which are used to evaluate the time scales during which groundwater quality changes; vertical flow-path study networks, which are used to evaluate changes in groundwater quality from shallow to deeper depths; and modeling support studies, which are used to provide data to support groundwater modeling. Groundwater samples were analyzed for a large number of water-quality indicators and constituents, including major ions, nutrients, trace elements, volatile organic compounds, pesticides, radionuclides, microbiological indicators, and some special interest constituents (arsenic speciation, chromium [VI] and perchlorate). Most of the data included were collected from wells that were sampled between January 2017 and December 2019. Microbiological indicator data for networks sampled in 2016 are included in this data release. These groundwater quality networks are described in a U.S. Geological Survey Data Series report DS####, which is available at https://dx.doi.org/XXXXXX, and the results are in this data release. Data for quality-control samples collected in 2017 through 2019 also are included in this data release. Data from samples collected between 2012 and 2016 are associated with networks described in previous reports in this data series (Arnold and others, 2016a and b; 2017a and b; 2018a and b; and 2020a and b). There are 24 data tables included in this data release and they are referenced as tables 1 through 14 and appendix tables 5-11 through 5-20 in the larger work citation (see supplemental information for descriptions). Two tables summarizing well depth and open interval are included in the data series report and were derived from table 1 in this data release. A separate table named DSR_2017-19_Description_of_Data_Fields.txt describes the 405 unique fields contained in the 24 data tables.

Groundwater-quality data were collected from 983 wells as part of the National Water-Quality Assessment Project of the U.S. Geological Survey National Water-Quality Program and are included in this report. The data were collected from six types of well networks: principal aquifer study networks, which are used to assess the quality of groundwater used for public water supply; land-use study networks, which are used to assess land-use effects on shallow groundwater quality; major aquifer study networks, which are used to assess the quality of groundwater used for domestic supply; enhanced trends networks, which are used to evaluate the time scales during which groundwater quality changes; vertical flow-path study networks, which are used to evaluate changes in groundwater quality from shallow to deeper depths; and modeling support studies, which are used to provide data to support groundwater modeling. Groundwater samples were analyzed for a large number of water-quality indicators and constituents, including major ions, nutrients, trace elements, volatile organic compounds, pesticides, radionuclides, microbiological indicators, and some special interest constituents (arsenic speciation, chromium [VI] and perchlorate). Most of the data included were collected from wells that were sampled between January 2017 and December 2019. Microbiological indicator data for networks sampled in 2016 are included in this data release. These groundwater quality networks are described in a U.S. Geological Survey Data Series report DS####, which is available at https://dx.doi.org/XXXXXX, and the results are in this data release. Data for quality-control samples collected in 2017 through 2019 also are included in this data release. Data from samples collected between 2012 and 2016 are associated with networks described in previous reports in this data series (Arnold and others, 2016a and b; 2017a and b; 2018a and b; and 2020a and b). There are 24 data tables included in this data release and they are referenced as tables 1 through 14 and appendix tables 5-11 through 5-20 in the larger work citation (see supplemental information for descriptions). Two tables summarizing well depth and open interval are included in the data series report and were derived from table 1 in this data release. A separate table named DSR_2017-19_Description_of_Data_Fields.txt describes the 405 unique fields contained in the 24 data tables.

Groundwater-quality data were collected from 559 wells as part of the National Water-Quality Assessment Project of the U.S. Geological Survey National Water-Quality Program from January through December 2014. The data were collected from four types of well networks: principal aquifer study networks, which assess the quality of groundwater used for public water supply; land-use study networks, which assess land-use effects on shallow groundwater quality; major aquifer study networks, which assess the quality of groundwater used for domestic supply; and enhanced trends networks, which evaluate the time scales during which groundwater quality changes. Groundwater samples were analyzed for a large number of water-quality indicators and constituents, including major ions, nutrients, trace elements, volatile organic compounds, pesticides, radionuclides, and some special interest constituents (arsenic speciation, chromium (VI) and perchlorate). These groundwater quality data are tabulated in a U.S. Geological Survey Data Series Report DS-1063 which is available at https://dx.doi.org/10.3133/ds1063 and in this data release. Quality-control samples also were collected and select data from 2012-2014 are included in the report DS1063 and this data release. Data from the environmental and QC blank and replicate samples from the 2012-2013 sampling period were presented in Arnold and others (2016). Data from spike QC samples have not previously been published Data from VOC spike QC data from May 2012-December 2014 are published in this report (see larger work citation) and data release along with an analysis of QC data where detections in field blank samples, variability in replicate samples, and recoveries in VOC spike samples are described for the entire sampling period through the date covered in this report (May 2012-December 2014). Pesticide spike samples and analysis of recoveries will be presented in a later report. There are 42 tables that are part of the larger work citation. There are 9 tables in the text of the larger work citation and 33 external tables included in this data release. The 9 tables not included in the data release are summary tables derived from some of the other 33 tables. Tables in the text include table 1, Appendix 1 tables 1-1 and 1-2; Appendix 2 table 2-1, and Appendix 4 tables 4-1 through 4-6. Tables that are external files are tables 1 through 12; Appendix 4 tables 4-7 through 4-27. This compressed file contains 33 files of groundwater-quality data in ASCII text tab-delimited format and 33 corresponding metadata in xml format for wells sampled for the U.S. Geological Survey National Water-Quality Assessment Project.

Groundwater-quality data were collected from 559 wells as part of the National Water-Quality Assessment Project of the U.S. Geological Survey National Water-Quality Program from January through December 2014. The data were collected from four types of well networks: principal aquifer study networks, which assess the quality of groundwater used for public water supply; land-use study networks, which assess land-use effects on shallow groundwater quality; major aquifer study networks, which assess the quality of groundwater used for domestic supply; and enhanced trends networks, which evaluate the time scales during which groundwater quality changes. Groundwater samples were analyzed for a large number of water-quality indicators and constituents, including major ions, nutrients, trace elements, volatile organic compounds, pesticides, radionuclides, and some special interest constituents (arsenic speciation, chromium (VI) and perchlorate). These groundwater quality data are tabulated in a U.S. Geological Survey Data Series Report DS-1063 which is available at https://dx.doi.org/10.3133/ds1063 and in this data release. Quality-control samples also were collected and select data from 2012-2014 are included in the report DS1063 and this data release. Data from the environmental and QC blank and replicate samples from the 2012-2013 sampling period were presented in Arnold and others (2016). Data from spike QC samples have not previously been published Data from VOC spike QC data from May 2012-December 2014 are published in this report (see larger work citation) and data release along with an analysis of QC data where detections in field blank samples, variability in replicate samples, and recoveries in VOC spike samples are described for the entire sampling period through the date covered in this report (May 2012-December 2014). Pesticide spike samples and analysis of recoveries will be presented in a later report. There are 42 tables that are part of the larger work citation. There are 9 tables in the text of the larger work citation and 33 external tables included in this data release. The 9 tables not included in the data release are summary tables derived from some of the other 33 tables. Tables in the text include table 1, Appendix 1 tables 1-1 and 1-2; Appendix 2 table 2-1, and Appendix 4 tables 4-1 through 4-6. Tables that are external files are tables 1 through 12; Appendix 4 tables 4-7 through 4-27. This compressed file contains 33 files of groundwater-quality data in ASCII text tab-delimited format and 33 corresponding metadata in xml format for wells sampled for the U.S. Geological Survey National Water-Quality Assessment Project.

Groundwater-quality data were collected from 502 wells as part of the National Water-Quality Assessment Project of the U.S. Geological Survey National Water-Quality Program and are included in this report. Most of the wells (500) were sampled from January through December 2015 and 2 of them were sampled in 2013. The data were collected from five types of well networks: principal aquifer study networks, which are used to assess the quality of groundwater used for public water supply; land-use study networks, which are used to assess land-use effects on shallow groundwater quality; major aquifer study networks, which are used to assess the quality of groundwater used for domestic supply; enhanced trends networks, which are used to evaluate the time scales during which groundwater quality changes; and vertical flow-path study networks, which are used to evaluate changes in groundwater quality from shallower to deeper depths. Groundwater samples were analyzed for a large number of water-quality indicators and constituents, including major ions, nutrients, trace elements, volatile organic compounds, pesticides, radionuclides, and some special interest constituents (arsenic speciation, chromium [VI] and perchlorate). These groundwater quality data are tabulated in a U.S. Geological Survey Data Series Report DS-XXXX which is available at https://dx.doi.org/XXXXXX and in this data release. Data from the environmental and QC samples from the 2012-2013 sampling period were presented in Arnold and others (2016a,b) and those from the 2014 sampling period were presented in Arnold and others (2017a,b). There are 20 data tables included in this data release and they are referenced as tables 1-12 and appendix tables 3.3-3.10 in the larger work citation. There are 25 tables that are part of the larger work citation; the 5 tables not included in the data release are summary tables derived from some of the other 20 tables. A version of table 1 is included in both the text and data release. This compressed file contains 20 files of groundwater-quality data in ASCII text tab-delimited format and 20 corresponding metadata in xml format for wells sampled for the U.S. Geological Survey National Water-Quality Assessment Project.

Groundwater-quality data were collected from 502 wells as part of the National Water-Quality Assessment Project of the U.S. Geological Survey National Water-Quality Program and are included in this report. Most of the wells (500) were sampled from January through December 2015 and 2 of them were sampled in 2013. The data were collected from five types of well networks: principal aquifer study networks, which are used to assess the quality of groundwater used for public water supply; land-use study networks, which are used to assess land-use effects on shallow groundwater quality; major aquifer study networks, which are used to assess the quality of groundwater used for domestic supply; enhanced trends networks, which are used to evaluate the time scales during which groundwater quality changes; and vertical flow-path study networks, which are used to evaluate changes in groundwater quality from shallower to deeper depths. Groundwater samples were analyzed for a large number of water-quality indicators and constituents, including major ions, nutrients, trace elements, volatile organic compounds, pesticides, radionuclides, and some special interest constituents (arsenic speciation, chromium [VI] and perchlorate). These groundwater quality data are tabulated in a U.S. Geological Survey Data Series Report DS-XXXX which is available at https://dx.doi.org/XXXXXX and in this data release. Data from the environmental and QC samples from the 2012-2013 sampling period were presented in Arnold and others (2016a,b) and those from the 2014 sampling period were presented in Arnold and others (2017a,b). There are 20 data tables included in this data release and they are referenced as tables 1-12 and appendix tables 3.3-3.10 in the larger work citation. There are 25 tables that are part of the larger work citation; the 5 tables not included in the data release are summary tables derived from some of the other 20 tables. A version of table 1 is included in both the text and data release. This compressed file contains 20 files of groundwater-quality data in ASCII text tab-delimited format and 20 corresponding metadata in xml format for wells sampled for the U.S. Geological Survey National Water-Quality Assessment Project.

Groundwater samples were collected and analyzed from 1,015 wells as part of the National Water-Quality Assessment Project of the U.S. Geological Survey National Water-Quality Program and the water-quality data and quality-control data are included in this data release. The samples were collected from three types of well networks: principal aquifer study networks, which are used to assess the quality of groundwater used for public water supply; land-use study networks, which are used to assess land-use effects on shallow groundwater quality; and major aquifer study networks, which are used to assess the quality of groundwater used for domestic supply. Groundwater samples were analyzed for a large number of water-quality indicators and constituents, including nutrients, major ions, trace elements, volatile organic compounds (VOCs), pesticides, radionuclides, and microbial indicators. Data from samples collected between 2012 and 2019 are associated with networks described in a collection of data series reports and associated data releases (Arnold and others, 2016a,b, 2017a,b, 2018a,b, 2020a,b; Kingsbury and others, 2020 and 2021). This data release includes data from networks sampled in 2019 through 2023. For some networks, certain constituent group data were not completely reviewed and released by the analyzing laboratory for all network sites in time for publication of this data release. For networks with incomplete data, no data were published for the incomplete constituent group(s). Datasets excluded from this data release because of incomplete results will be included in the earliest data release published after the dataset is complete. NOTE: While previous versions are available from the author, all the records in previous versions can be found in version 4.0. First posted - December 12, 2021 (available from author) Revised - January 27, 2023 (version 2.0: available from author) Revised - November 2, 2023 (version 3.0: available from author) Revised - April 18, 2025 (version 4.0) The compressed file (NWQP_GW_QW_DataRelease_v4.zip) contains 24 files: 23 files of groundwater-quality, quality-control data, and general information in ASCII text tab-delimited format, and one corresponding metadata file in xml format that includes descriptions of all the tables and attributes. A shapefile containing study areas for each of the sampled groundwater networks also is provided in folder NWQP_GW_QW_Network_Boundaries_v4 of this data release and is described in the metadata (Network_Boundaries_v4.zip). The 23 data files are as follows: Description_of_Data_Fields_v4.txt: Information for all constituents and ancillary information found in Tables 3 through 21. Network_Reference_List_v4.txt: References used for the description of the networks sampled by the U.S. Geological Survey (USGS) National Water-Quality Assessment (NAWQA) Project. Table_1_site_list_v4.txt: Information about wells that have environmental data. Table_2_parameters_v4.txt: Constituent primary uses and sources; laboratory analytical schedules and sampling period; USGS parameter codes (pcodes); comparison thresholds; and reporting levels. Table_3_qw_indicators_v4.txt: Water-quality indicators in groundwater samples collected by the USGS NAWQA Project. Table_4_nutrients_v4.txt: Nutrients and dissolved organic carbon in groundwater samples collected by the USGS NAWQA Project. Table_5_major_ions_v4.txt: Major and minor ions in groundwater samples collected by the USGS NAWQA Project. Table_6_trace_elements_v4.txt: Trace elements in groundwater samples collected by the USGS NAWQA Project. Table_7_vocs_v4.txt: Volatile organic compounds (VOCs) in groundwater samples collected by the USGS NAWQA Project. Table_8_pesticides_v4.txt: Pesticides in groundwater samples collected by the USGS NAWQA Project. Table_9_radchem_v4.txt: Radionuclides in groundwater samples collected by the USGS NAWQA Project. Table_10_micro_v4.txt: Microbiological indicators in groundwater samples collected by

Groundwater samples were collected and analyzed from 782 wells as part of the National Water-Quality Assessment Project of the U.S. Geological Survey National Water-Quality Program and the water-quality data and quality-control data are included in this data release. The samples were collected from three types of well networks: principal aquifer study networks, which are used to assess the quality of groundwater used for public water supply; land-use study networks, which are used to assess land-use effects on shallow groundwater quality, and major aquifer study networks, which are used to assess the quality of groundwater used for domestic supply. Groundwater samples were analyzed for a large number of water-quality indicators and constituents, including nutrients, major ions, trace elements, volatile organic compounds (VOCs), pesticides, radionuclides, and microbial indicators. Data from samples collected between 2012 and 2019 are associated with networks described in a collection of data series reports and associated data releases (Arnold and others, 2016a,b, 2017a,b, 2018a,b, 2020a,b; Kingsbury and others, 2020 and 2021). This data release includes data from networks sampled in 2019 through 2022. For some networks, certain constituent group data were not completely reviewed and released by the analyzing laboratory for all network sites in time for publication of this data release. For networks with incomplete data, no data were published for the incomplete constituent group(s). Datasets excluded from this data release because of incomplete results will be included in the earliest data release published after the dataset is complete. NOTE: While previous versions are available from the author, all the records in previous versions can be found in version 3.0. First posted - December 12, 2021 (available from author) Revised - January 27, 2023 (version 2.0: available from author) Revised - November 2, 2023 (version 3.0) The compressed file (NWQP_GW_QW_DataRelease_v3.zip) contains 24 files: 23 files of groundwater-quality, quality-control data, and general information in ASCII text tab-delimited format, and one corresponding metadata file in xml format that includes descriptions of all the tables and attributes. A shapefile containing study areas for each of the sampled groundwater networks also is provided as part of this data release and is described in the metadata (Network_Boundaries_v3.zip). The files are as follows: Description_of_Data_Field_v3.txt: Information for all constituents and ancillary information found in Tables 3 through 21. Network_Reference_List_v3.txt: References used for the description of the networks sampled by the USGS NAWQA Project. Table_1_site_list_v3.txt: Information about wells that have environmental data. Table_2_parameters_v3.txt: Constituent primary uses and sources; laboratory analytical schedules and sampling period; USGS parameter codes (pcodes); comparison thresholds; and reporting levels. Table_3_qw_indicators_v3.txt: Water-quality indicators in groundwater samples collected by the USGS NAWQA Project. Table_4_nutrients_v3.txt: Nutrients and dissolved organic carbon in groundwater samples collected by the USGS NAWQA Project. Table_5_major_ions_v3.txt: Major and minor ions in groundwater samples collected by the USGS NAWQA Project. Table_6_trace_elements_v3.txt: Trace elements in groundwater samples collected by the USGS NAWQA Project. Table_7_vocs_v3.txt: Volatile organic compounds (VOCs) in groundwater samples collected by the USGS NAWQA Project. Table_8_pesticides_v3.txt: Pesticides in groundwater samples collected by the USGS NAWQA Project. Table_9_radchem_v3.txt: Radionuclides in groundwater samples collected by the USGS NAWQA Project. Table_10_micro_v3.txt: Microbiological indicators in groundwater samples collected by the USGS NAWQA Project. Table_11_qw_ind_QC_v3.txt: Water-quality indicators in groundwater replicate samples collected by the USGS NAWQA Project.

The U.S. Geological Survey collected groundwater samples from 44 wells used for domestic and small system drinking water supplies in the western Central Valley, California in 2023. The wells were sampled for the Western Central Valley domestic-supply aquifer study unit of the State Water Resources Control Board Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project’s assessment of the quality of groundwater resources used for domestic and small system drinking water supplies. The study unit consists of two study areas. The western Sacramento Valley study area comprises the Corning, Colusa, Yolo, and Solano subbasins of the Sacramento Valley groundwater basin, and the western San Joaquin Valley study area comprises the East Contra Costa, Tracy, Delta-Mendota, and Westside subbasins of the San Joaquin Valley groundwater basin. The study unit was divided into 49 grid cells and one domestic or small system well was sampled to represent 44 of the grid cells; the remaining 5 grid cells has no accessible wells. Groundwater levels were measured in 28 of 44 wells. Table 1 contains sample and site information. Groundwater samples were analyzed for field water-quality parameters, microbial indicators, volatile organic compounds, pesticides and pesticide degradants, nutrients, major ions and trace elements, hexavalent chromium, perchlorate, per- and polyfluoroalkyl substances (PFAS), stable isotopes, and groundwater age tracers. Results from these analyses are reported in Tables 2 through 8. Tables 9 and 10 report results from quality control samples. This data release contains two supplementary tables that define abbreviations (Table 11) and provide additional context for the information presented in Tables 1 through 10 (Table 12). A zipfile is included containing geospatial data (study unit boundaries, grid cell boundaries, site locations) as shapefiles.