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, June 2013 through December 2017. [Table code definitions: NC, not collected; --, less than minimum laboratory reporting level] Table 9. Pesticides 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; E, estimated] Table 10. Quality control results for constituents analyzed for 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: NC, not collected; --, less than minimum laboratory reporting level] Table 11. Quality control results for constituents analyzed for majors 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 12. Quality control results for constituents analyzed for 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: --, less than minimum laboratory reporting level] Table 13. Quality control results of a replicate analyzed for 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; NC, not collected] Table 14. Quality control results for constituents analyzed for VOCs 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; NC, not collected; E, estimated] Table 15. Quality control results for constituents analyzed for pesticides 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; NC, not collected]