This data release contains the concentration and quality-assurance results for inorganic constituents, as well as organic compounds (volatile organic compounds [VOC], per- and polyfluoroalkyl substances [PFAS], pesticides and disinfection byproducts [DBP]) and microbiological results collected in the Little Big Horn River and residential or commercial tapwater locations in Montana. Twenty-two tapwater samples (six sourced from public supply and 14 sourced from private wells) and six surface-water samples were collected in Montana between August 8-10, 2022. Pharmaceuticals, VOC, and PFAS were analyzed at the U.S. Geological Survey (USGS) National Water Quality Laboratory (NWQL) in Denver, Colorado. Pesticides and DBP were analyzed at the USGS Organic Chemistry Research Laboratory (OCRL) in Sacramento, California. Inorganic contaminants were analyzed at the USGS Redox Chemistry Laboratory (RCL) and the Analytical Trace Element Chemistry Laboratory (ATECL), Boulder, Colorado. Cyanotoxins were analyzed at the USGS Organic Geochemistry Research Laboratory (OGRL), Lawrence, Kansas. Microbiological pathogens were analyzed by the National Institute of Health/National Institute of Allergy and Infections Disease, Rocky Mountain Laboratory, Hamilton, Montana.