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.