This data release contains groundwater-quality data for nitrate and chloride, and well information for sample sites for aquifers in the Great Lakes Basin within the United States. Water-quality data and well information were derived from a dataset compiled from the U.S. Geological Survey (USGS) National Water Information System and numerous agencies and organizations at the state, regional, and local level. The data presented in this data release are data pulled from the USGS National Water Quality Program groundwater assessment team’s internal dataset, informally referred to as the National Groundwater aggregation (NGA). Data collected in the Great Lakes Basin by the U.S. and Canada augment the NGA. Specifically, only geochemical parameters of interest (nitrate and chloride) from wells in the national groundwater aggregation are presented in this data release, while data from springs were not used.