Groundwater-quality data were collected from 748 wells as part of the National Water-Quality Assessment Project of the U.S. Geological Survey National Water-Quality Program from May 2012 through December 2013. 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, and radionuclides. These groundwater quality data are tabulated in a U.S. Geological Survey Data Series Report DS-997 which is available at http://dx.doi.org/10.3133/ds997 and in this data release. Quality-control samples also were collected; data from blank and replicate quality-control samples are included in the related report (DS-997) and this data release. This compressed file contains 28 files of groundwater-quality data in ASCII text tab-delimited format and 28 corresponding metadata in xml format for wells sampled for the U.S. Geological Survey National Water-Quality Assessment Project, May 2012 through December 2013.