Mercury and associated constituents in water and sediment of the Snake and Salmon rivers (Idaho, Oregon, Washington)

This dataset includes concentrations of mercury and related constituents in surface water and sediment from the middle and lower Snake River (Idaho, Oregon, Washington), Salmon River (Idaho), Clearwater River (Idaho), and many key tributaries from November 2022 through December 2024. Data were collected to better understand mercury sources, biogeochemical transformation processes, spatial and temporal trends, and exposure to humans and aquatic organisms. This dataset combines results from two studies related by geography and biogeochemical processes: the Hells Canyon Mercury Study and the Salmon River/Lower Snake River Mercury Study. The Hells Canyon Mercury Study is a collaboration between Idaho Power Company, Idaho Department of Environmental Quality, the University of California at Davis, and the U.S. Geological Survey to evaluate mercury dynamics and long-term trends in the Snake River through the Hells Canyon reservoir complex. Data from this study includes water chemistry results from (a) biweekly samples at four sites upstream, within, and immediately downstream of the Hells Canyon reservoir complex; and (b) annually at multiple sites and depths in Brownlee Reservoir during its maximum stratification in late summer. These data build upon previous results published by Poulin and others (2023) spanning 2014 to 2022. The Salmon River/Lower Snake River Mercury Study is a collaboration between Nez Perce Tribe, Idaho Power Company, and the U.S. Geological Survey to provide baseline mercury data for water and sediment throughout the Salmon River Basin and in the lower Snake River (i.e., the Snake River below the Hells Canyon Complex and above its confluence with the Columbia River) and Clearwater River. Data for this study include water and sediment chemistry results from 40 locations sampled at varying frequencies. All water samples were analyzed for filter-passing total mercury and methylmercury, particulate total and methylmercury, dissolved organic carbon (DOC), dissolved organic matter (DOM) composition (ultraviolet absorbance at the wavelength 254 nanometers), and total suspended solids (TSS). Samples from the Snake River upstream and through the Hells Canyon Complex were also analyzed for particulate organic carbon (POC) and stable isotopic composition; particulate nitrogen (PN) and stable isotopic composition; inorganic anions (chloride, nitrate, sulfate); and additional optical characterization of DOM. Sediment samples were analyzed for total and methylmercury. There are 3 files (*.csv) provided in this product: Table_1_Sites.csv - site information Table_2_Data_dictionary.csv - data dictionary detailing terms used in Table 3 Table_3_Results.csv - Surface water and sediment chemistry results

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