The Sulphur Bank Mercury Mine in Lake County, California, was the site of historical mercury mining from 1880s through 1973.Since 1990, the mine area has been a Superfund site managed by the U.S. Environmental Protection Agency (USEPA). As part of a Remedial Investigation, the USEPA and the U.S. Geological Survey (USGS) are collaborating on a groundwater investigation in a part of the Sulphur Bank mine site known as the Waste Rock Dam (WRD) area, which lies between Herman Impoundment (a flooded mine pit) and Clear Lake. 33 groundwater wells were sampled in the WRD area and adjacent areas of the mine site by USEPA and their consultants in six sampling events during 2022-23. Because of low water levels due to drought conditions in 2022, some wells could not be sampled. Surface water was sampled from Herman Impoundment and Clear Lake during all six sampling events, and surface water from the Northwest Pit was sampled during one event. This data release includes field and laboratory data for 295 environmental samples and 42 QA samples collected during these sampling events. Field parameters include water temperature, specific conductance, dissolved oxygen, turbidity, pH, and oxidation-reduction potential. Laboratory analyses include major cations, trace metals, mercury, anions (sulfate, chloride, bromide, and fluoride), alkalinity, forms of nitrogen (nitrate plus nitrite, and ammonium), and stable isotopes of hydrogen and oxygen in water, and of sulfur and oxygen in aqueous sulfate.