Samples were collected from Clear Lake, California from 2019-2022 by the U.S. Geological Survey (USGS) California Water Science Center, the USGS Forest and Rangeland Ecosystem Science Center (FRESC), the US Environmental Protection Agency, and EA Engineering Science and Technology Inc. to assess the extent and cycling of legacy mercury (Hg) contamination from Sulphur Bank Mine. Samples of waste rock, sediments, groundwater, and biological tissue (zooplankton, bluegill, smallmouth bass, Mississippi silversides, and largemouth bass) were analyzed for Hg stable isotopes by the U.S. Geological Survey Mercury Research Laboratory. Mercury concentrations for sediments, waste rocks, and groundwaters are included within this data release and information for biological Hg concentrations can be found in the companion data release (https://doi.org/10.5066/P96912PN).

Clear Lake is a 180 km2 freshwater lake located approximately 120 km northwest of Sacramento in the California Coast Range. The lake is a popular sport-fishing destination, however, there are fish consumption advisories associated with mercury (Hg) contamination for several species. The lake has three arms: a large open-water region to the northwest (Upper Arm), a smaller and narrower region to the southeast (Lower Arm), and the smallest and narrowest region to the east (Oaks Arm). The Sulfur Bank Mercury Mine (SBMM), located on the eastern shore of the Oaks Arm, was mined by underground methods starting in the 1870s and then open-pit methods during the 1920s to 1950s. Since 1992, the SBMM has been a Superfund site managed by the U.S. Environmental Protection Agency. This former Hg mining area is thought to be the dominant contributor of Hg to Clear Lake and is the focus of continued remediation to minimize Hg impacts to Clear Lake. The SBMM area is an active geothermal area with springs and gas fumaroles. Clear Lake is eutrophic to hypereutrophic, based on elevated concentrations of nutrients (various forms of nitrogen, N, and phosphorus, P) that cause excessive algal blooms and periodic fish kills. There are current studies underway (by the University of California, Davis, UCD) to improve the understanding of nutrient cycling within the lake. The UCD group is developing a three-dimensional hydrodynamic computer model (https://resources.ca.gov/-/media/CNRA-Website/Files/Initiatives/Blue-Ribbon-Committee-for-the-Rehabilitation-of-Clear-Lake/2022-BRC-Annual-Report-_20230110_DraftFinal.pdf ) of the lake that simulates periodic stratification of temperature and dissolved oxygen as well as nutrient dynamics. An effort is planned during 2023-2025 to add mercury cycling to the 3-D model. Monitoring and modeling efforts are being implemented to improve understanding of the ways that nutrient dynamics affect the cycling, transport and bioaccumulation of Hg, which remains poorly understood. This data release documents the results of water-quality sampling during June 2020 through June 2022. It includes three data tables given as machine readable 'comma-separated values' format (*.csv): 1) ‘T1_CL_DataDictionary’, the Data Dictionary, which provides definitions and details related to the other two data tables and includes citations of analytical methods; 2) ‘T2_CL_Discrete_SW’, the discrete-sample surface-water dataset including concentration data for Hg species (including total mercury and methylmercury in dissolved and particulate forms) and nutrients (including several forms of N and P); and 3) ‘T3_CL_QA_SW’, a quality assurance data summary for the discrete water samples. In addition, file ‘Clear Lake Coordinates_SW 2020-22_052324’ provides the sampling locations in a machine-readable geospatial file format (*.kmz).

Clear Lake is a 180 km2 freshwater lake located approximately 120 km northwest of Sacramento in the California Coast Range. The lake is a popular sport-fishing destination, however, there are fish consumption advisories associated with mercury (Hg) contamination for several species. The lake has three arms: a large open-water region to the northwest (Upper Arm), a smaller and narrower region to the southeast (Lower Arm), and the smallest and narrowest region to the east (Oaks Arm). The Sulfur Bank Mercury Mine (SBMM), located on the eastern shore of the Oaks Arm, was mined by underground methods starting in the 1870s and then open-pit methods during the 1920s to 1950s. Since 1992, the SBMM has been a Superfund site managed by the U.S. Environmental Protection Agency. This former Hg mining area is thought to be the dominant contributor of Hg to Clear Lake and is the focus of continued remediation to minimize Hg impacts to Clear Lake. The SBMM area is an active geothermal area with springs and gas fumaroles. Clear Lake is eutrophic to hypereutrophic, based on elevated concentrations of nutrients (various forms of nitrogen, N, and phosphorus, P) that cause excessive algal blooms and periodic fish kills. There are current studies underway (by the University of California, Davis, UCD) to improve the understanding of nutrient cycling within the lake. The UCD group is developing a three-dimensional hydrodynamic computer model (https://resources.ca.gov/-/media/CNRA-Website/Files/Initiatives/Blue-Ribbon-Committee-for-the-Rehabilitation-of-Clear-Lake/2022-BRC-Annual-Report-_20230110_DraftFinal.pdf ) of the lake that simulates periodic stratification of temperature and dissolved oxygen as well as nutrient dynamics. An effort is planned during 2023-2025 to add mercury cycling to the 3-D model. Monitoring and modeling efforts are being implemented to improve understanding of the ways that nutrient dynamics affect the cycling, transport and bioaccumulation of Hg, which remains poorly understood. This data release documents the results of water-quality sampling during June 2020 through June 2022. It includes three data tables given as machine readable 'comma-separated values' format (*.csv): 1) ‘T1_CL_DataDictionary’, the Data Dictionary, which provides definitions and details related to the other two data tables and includes citations of analytical methods; 2) ‘T2_CL_Discrete_SW’, the discrete-sample surface-water dataset including concentration data for Hg species (including total mercury and methylmercury in dissolved and particulate forms) and nutrients (including several forms of N and P); and 3) ‘T3_CL_QA_SW’, a quality assurance data summary for the discrete water samples. In addition, file ‘Clear Lake Coordinates_SW 2020-22_052324’ provides the sampling locations in a machine-readable geospatial file format (*.kmz).

This data release includes mercury concentrations and mercury stable isotope measurements measured in sediments and biological tissues collected from the Saint Louis River located in Minnesota. Sediments and biota were collected by the U.S. Geological Survey, U.S. Environmental Protection Agency, U.S. Army Corps of Engineers, and federal contractors (Battelle) from 2017-2021. Collection regions included nearshore zones within the main estuary, remedial sites within the lower river, and upstream reservoir sites. Sediments were analyzed for total mercury, methylmercury, and mercury stable isotopes by the U.S. Geological Survey Mercury Research Laboratory (MRL, Madison, Wisconsin). Biological samples were analyzed for total and methylmercury by the MRL or the Contaminant Ecology Research Laboratory (CERL, Corvallis, Oregon); mercury stable isotopes were analyzed by the MRL.

This data release includes mercury concentrations and mercury stable isotope measurements measured in sediments and biological tissues collected from the Saint Louis River located in Minnesota. Sediments and biota were collected by the U.S. Geological Survey, U.S. Environmental Protection Agency, U.S. Army Corps of Engineers, and federal contractors (Battelle) from 2017-2021. Collection regions included nearshore zones within the main estuary, remedial sites within the lower river, and upstream reservoir sites. Sediments were analyzed for total mercury, methylmercury, and mercury stable isotopes by the U.S. Geological Survey Mercury Research Laboratory (MRL, Madison, Wisconsin). Biological samples were analyzed for total and methylmercury by the MRL or the Contaminant Ecology Research Laboratory (CERL, Corvallis, Oregon); mercury stable isotopes were analyzed by the MRL.

Comma-separated values (.csv) file containing data related to mercury in fishes collected from Clear Lake, California.

Comma-separated values (.csv) file containing data related to mercury in fishes collected from Clear Lake, California.

Comma-separated values (.csv) file containing data related to mercury in biota collected from Clear Lake California.

Comma-separated values (.csv) file containing data related to mercury in biota collected from Clear Lake California.

San Francisco Bay, California is considered a mercury-impaired watershed. Elevated concentrations of mercury are found in water and sediment as well as fish and estuarine birds. Sources of mercury to the watershed since 1845 include sediment-associated mercury from mercury mining, mercury losses from gold amalgamation activities in mines of the Sierra Nevada, aerial deposition of mercury from global and regional emissions to air, and the direct discharge of mercury to Bay waters associated with the urbanization and industrialization of the estuary. We assessed historical trends in mercury bioaccumulation by measuring mercury concentrations in feathers of the endangered California Ridgway’s rail (formerly California Clapper Rail) using museum specimens. We developed a structural equation model to attribute variation in historical mercury bioaccumulation in rails to sources of mercury, and estimated the toxicological consequences of extreme mercury exposure to rails from known correlations between feather and blood mercury concentrations.