Biogeochemical Data for Mercury and other Constituents in Surface Sediment and Deep Cores from the Hells Canyon Reservoir Complex, Idaho and Oregon 2014-2018

The Hells Canyon Complex (HCC) is comprised of a series of three consecutive reservoirs (Brownlee, Oxbow, and Hells Canyon) along a 145 km reach of the Snake River bordered by Idaho to the east and Oregon to the west. Due to concerns regarding mercury (Hg) contamination within the HCC, in cooperation with Idaho Power Company, the U.S. Geological Survey has been leading an investigation into the sources, transport, microbial transformations, chemical speciation and bioaccumulation of Hg in this complex river-reservoir environment. This data release is focused on the sediment component of this larger effort and includes data collected between 2014 and 2018 from shallow surface sediment (greater than or equal to top 5 cm) and from sectioned deep cores ranging from 34 cm to 112 cm in length. While the majority of the data was collected from within the HCC, some limited additional data is included from a number of riverine bed sediment sites located in the watershed upstream of the HCC and from two additional reservoirs on the Boise River sampled as control sites (Anderson Ranch and Lucky Peak reservoirs). For shallow surface sediment, the list of mercury associated analytes reported in this dataset includes total mercury, methylmercury, and inorganic reactive mercury concentrations; as well as rate constants and calculated potential rates associated with microbial methylmercury production and degradation, derived from stable isotope amendment bottle incubations. The list of non-mercury analytes includes field measurements of temperature, oxidation-reduction potential and pH; percent dry weight; percent loss on ignition; wet bulk density; porosity; total reduced sulfur; and iron speciation. For a sub-set of the shallow sediment sampled there is additional data for: percent organic carbon (with isotopic 13C values), percent nitrogen (with isotopic 15N values), and grain size (percent greater than 64 um). Of the eight sampling events conducted within the HCC, sediment porewater samples were collected from sediment-water interface cores on three occasions to estimate benthic-pelagic diffusive fluxes of a number of mercury and non-mercury constituents. The list of porewater analytes determined and reported herein include: total mercury, methylmercury, ortho-phosphate, ammonium, sulfide, dissolved organic carbon, dissolved organic matter specific ultra-violet absorption at 254 nm (SUVA254), acetate, sulfate, thiosulfate, nitrate, and elemental analysis of Al, As, B, Br, Ca, Cd, Cl, Cu, F, Fe, K, Mg, Mn, Mo, Na, Ni, P, Pb, Se, Si (as SiO2), U, and Zn. Of the eleven deep cores collected and sub-sectioned, all sediment samples were assayed for total mercury and percent loss on ignition, nine were assayed for methylmercury, and two were assayed for dry particle density and environmental radionuclides (137Cs, 210Pb and 226Ra) to assess sedimentation rates. This data release includes five data tables given both as Excel (*.xlsx) and machine readable 'comma-separated values' format (*.csv): 1) 'T1_Data.Dictionary_HCC.2014-18', the Data Dictionary, which provides definitions and details related to the other four data tables and includes analytical methods citations; 2) 'T2_Shallow sed bulk_HCC.2014-18', the shallow (bulk) sediment dataset; 3) 'T3_Shallow sed porewater_HCC.2014-18', the shallow sediment porewater dataset; 4) 'T4_Deep cores_HCC.2014-18', the deep core profile dataset; and 5) 'T5_QA_HCC.2014-18', quality assurance data summary for all major analyses. In addition, file 'HCC 2014-18.kmz' provides the sampling locations in a machine-readable geospatial file format (*.kmz).