Clear Lake, located within the Coast Range west of California’s Central Valley, is the largest natural freshwater lake contained fully within the state and geologically is considered to be the oldest lake in North America. Clear Lake is popular for recreation and provides critical habitat to a wide variety of fish and bird species. Water quality in Clear Lake is degraded by both by mercury contamination and harmful algal blooms (HABs). The mercury contamination is largely associated with the Sulfur Bank Mercury Mine (SBMM), located on the eastern shore of the Oaks Arm in the northeastern portion of the Lake. The mine was operated intermittently from the 1870s until 1957 and is now a U.S. Environmental Protection Agency (USEPA) Superfund site. Mercury contamination from SBMM and its biogeochemical mercury cycling within the lake are the focus of research being conducted by the U.S. Geological Survey (USGS) in cooperation with USEPA. The HABs are associate with excessive nutrient eutrophication within the lake; nutrient cycling and the distribution of oxic and anoxic conditions within the water column are active areas of investigation by researchers at the University of California at Davis (UCD). This product represents the results of a collaboration between USGS and UCD scientists which focuses on analyses of mercury speciation in shallow sediment cores from Clear Lake that were experimentally manipulated with respect to overlying water redox conditions.
Twenty-four shallow sediment cores (up to 25 cm deep) were initially collected by the UCD research team as part of their research efforts that focused on measuring nitrogen and phosphorus flux from the sediment to the overlying water under experimentally controlled oxic and anoxic conditions. The cores were collected in sets of four per site and were sampled from six unique sites around the lake, including: three from the northwestern area (Upper Arm,UA), one from the southeastern area (Lower Arm, LA), one from the northeastern area (Oaks Arm, OA), and one from the ‘narrows’ (NR) that hydrologically connects the UA and the OA. The cores were incubated for 30 days under laboratory controlled oxic or anoxic overlying water conditions (two cores per site per treatment) for the purposes of the UCD nutrient flux experiments. Upon completion of these experiments, each core was vertically sectioned into 2-cm or 5-cm intervals and preserved for additional analyses. A sample split for each sediment interval was collected, preserved frozen, and shipped to the USGS research group for the analyses presented in this data release product. The results of the UCD nutrient flux experiments are not presented as part of this product and will be published elsewhere by the UCD team. The USGS researchers subsequently sub-sampled and analyzed the uppermost sediment layer (top 0-2 cm interval) from all 24 previously incubated cores for the following constituents: total mercury, methylmercury, reactive divalent inorganic mercury, acid-extractable ferrous iron, poorly crystalline (amorphous) ferric iron, crystalline ferric iron, total reduced sulfur, organic content (as determined by loss on ignition), and percent dry weight. In addition, four cores (one each from the UA, LA, OA, and NR areas) were selected for analysis of total mercury, organic content, and percent dry weight at all depth intervals sampled. Further details regarding initial core collection, incubation conditions and analyses conducted by the USGS are provided in the metadata section of this product. This product includes three data tables given as machine readable 'comma-separated values' format (.csv): 1) ‘T1_Data_Dictionary_CL_SED_2019’, the Data Dictionary, which provides definitions and details related to the other two data tables and includes citations of analytical methods; 2) ‘T2_Data_CL_SED_2019’, the primary sediment results dataset for the constituents described above; and 3) ‘T3_QA_CL_SED_2019’, a summary of quality assurance data for the analysis of the constituents described above. In addition, the file ‘SITES_CL.SED-2019’ provides the sampling locations in a machine-readable geospatial file format (.kmz), and the similarly titled *.jpg file is a flat image showing the locations of the six sampling sites.