Surface-Water Geochemistry of Mercury, Methylmercury, Nutrients, and other Constituents in Clear Lake, Lake County, California, July 2019

Clear Lake is a 180 km2 freshwater lake located in the California Coast Range, approximately 120 km northwest of Sacramento. The lake supports a wide variety of fish and bird species and is a very popular sport-fishing destination. However, fish consumption advisories associated with mercury (Hg) contamination exist for several popular recreational species. The lake is comprised of three main regions including a large open-water region to the northwest (North Arm), a smaller and narrower region to the southeast (South Arm), and the smallest and narrowest region to the east (Oaks Arm). The Sulfur Bank Mercury Mine (SBMM), located on the south shore of the Oaks Arm, was mined by both open-pit and underground methods (intermittently active from the 1870s until 1957), and is now a U.S. Environmental Protection Agency (USEPA) Superfund site. This former Hg mining area represents a dominant contributor of Hg to the adjacent lake and is the focus of continued clean-up efforts to minimize Hg impacts to Clear Lake. The SBMM area is also host to active hot springs and gas fumaroles. Clear Lake is classified as eutrophic to hypereutrophic, based on high levels 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 Univ. of California, Davis, UCD) to improve the understanding of nutrient cycling within the lake. The way in which nutrient dynamics affect the cycling, transport and bioaccumulation of Hg remains poorly understood. Because of the expense and human resources required to conduct traditional water-quality studies, and the knowledge that water quality may vary significantly on an hourly or daily time scale in some locations, there is a need to develop more spatially and temporally robust monitoring programs to study both mercury and nutrients within Clear Lake and elsewhere. Recent advances in deployable in-situ electrochemical and optical sensors, as well as remote-sensing approaches, offer an opportunity to collect critical water-quality data at high temporal and spatial resolution to a degree previously unobtainable. However, these approaches require a detailed examination of the relationships between the constituent of concern (for example, the concentration of suspended sediment or various Hg species) and the electrochemical or optical properties of water for which the current class of sensors are best suited (for example, turbidity, dissolved organic carbon, or algal concentration). This preliminary study of particulate and filtered Hg species (including total mercury and methylmercury) and non-Hg water-column constituents was designed to lay the groundwork for developing a more robust Hg monitoring program for Clear Lake. This data release documents the results from a single (2-day) sampling event during July 2019 that focused on the collection of two types of surface-water data: a) discrete samples collected with a Van Dorn style sampler for a suite of Hg and non-Hg water constituents collected near the surface (1 m depth) and near bottom (approximately 1 m off of the bottom); and b) continuous vertical profiles collected in-situ with a water-quality sonde (electrochemical / optical probe data). Five of the nine discrete sample locations were co-located with fixed monitoring stations (moorings) being used by UCD for the study of nutrients within Clear Lake. Sampling included all 3 lake regions, including the Upper Arm (3 discrete-sample sites and 9 vertical-profile sites), the Lower Arm (3 discrete-sample sites and 9 vertical-profile sites), and the Oaks Arm (3 discrete-sample sites and 3 vertical-profile sites). This data release includes four data tables given both as Excel (*.xlsx) and machine readable 'comma-separated values' format (*.csv): 1) 'T1_Data.Dictionary_CL_07.2019', the Data Dictionary, which provides definitions and details related to the other three data tables and includes citations of analytical methods; 2) 'T2_Discrete.SW_CL_07.2019', 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); 3) 'T3_Vertical_Profiles.SW_CL_07.2019', the water-quality sonde vertical-profile dataset; and 4) 'T4_QA.SW_CL_07.2019', a quality assurance data summary for the discrete water samples. In addition, file 'SITES_CL_07.2019' provides the sampling locations in a machine-readable geospatial file format (*.kmz).