From July to November 2019, 48 environmental samples and four quality assurance and quality control samples were collected throughout the Lake Champlain and Susquehanna River Basins. Samples were collected from 10 production wells and nine domestic wells in the Lake Champlain Basin, and 14 production wells and 15 domestic wells in the Susquehanna River Basin. Of the total wells sampled, 24 were completed in bedrock and 24 were completed in sand and gravel. Approximately 20 percent of samples were collected from wells that previously have been sampled as part of this study in 2005, 2009, and 2014. Groundwater samples were collected prior to any treatment or filtration systems, from existing domestic and production wells equipped with permanently installed pumps. Samples were collected and processed using standard USGS methods and were analyzed for 364 constituents including physicochemical properties, dissolved gases, major ions, nutrients, trace elements, pesticides, volatile organic compounds (VOCs), radiological activity, and indicator bacteria. All analytes are listed in the data release, along with the associated method detection and reporting limits, some of which changed over time. About 73 percent of constituents analyzed were not detected at concentrations greater than the method detection limit. Samples were analyzed at the USGS National Water Quality Laboratory (NWQL) in Denver, Colorado, and other certified laboratories following published methods. Quality assurance and quality control samples were collected and assessed in each of the sampled basins. In the Susquehanna River Basin, a replicate was collected at well SY1363, and in the Lake Champlain Basin a replicate was collected at well W1616, and both were analyzed for all constituents. An equipment blank and field blank were also analyzed for all constituents. The VOC 1,2,4-Trimethylbenzene was detected in the blank samples and all the groundwater samples, and contamination is suspected from the scintillation liquid in the radon sampling kits. Due to the potential for contamination bias, results for 1,2,4-Trimethylbenzene are not included.