This data release contains groundwater-quality data for three parameters of interest (arsenic, manganese, and pH) and well information for sample sites for aquifers in the conterminous U.S. Water-quality data and well information were derived from a dataset compiled from three sources: the U.S. Geological Survey (USGS) National Water Information System (NWIS), the U.S. Environmental Protection Agency (USEPA) Safe Drinking Water Information System (SDWIS), and numerous agencies and organizations at the state, regional, and local level. The data compilation of the National Water Quality Program’s groundwater assessment team is an internal dataset informally referred to as the National Groundwater Aggregation (NGA). The current study of groundwater quality in the conterminous U.S. augments data compiled by others globally. Only geochemical parameters of interest (arsenic, manganese, pH) from wells in the national groundwater aggregation are presented—data from springs were not used. A table of site information includes attributes for each well, such as the state, water use code, depth, open interval (if available) and aquifer (if available). The provider of the water-quality data and well information in also in this table.