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

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).

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

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

Sediment cores were collected in the Cache Creek Settling Basin (CCSB), Yolo County, California, during October 2011 at 10 locations (borehole sites) and during August 2012 at 5 other locations. Total core depths ranged from approximately 4.6 to 13.7 meters (15 to 45 feet), with penetration to about 9.1 meters (30 feet) at most locations. Detailed subsampling (3-centimeter intervals) was done at total of seven locations: six along an east-west transect in the southern part of the Cache Creek Settling Basin and at one in the northern part of the basin for analyses of total mercury; organic content; and cesium-137, which was used for dating. This data release reports results of the analyses of each subsample of these sediment cores, including associated quality-assurance and quality-control data.

Sediment cores were collected in the Cache Creek Settling Basin (CCSB), Yolo County, California, during October 2011 at 10 locations (borehole sites) and during August 2012 at 5 other locations. Total core depths ranged from approximately 4.6 to 13.7 meters (15 to 45 feet), with penetration to about 9.1 meters (30 feet) at most locations. Detailed subsampling (3-centimeter intervals) was done at total of seven locations: six along an east-west transect in the southern part of the Cache Creek Settling Basin and at one in the northern part of the basin for analyses of total mercury; organic content; and cesium-137, which was used for dating. This data release reports results of the analyses of each subsample of these sediment cores, including associated quality-assurance and quality-control data.

The Cache Creek Settling Basin (CCSB) is a 13.3 km2 leveed basin located at the terminal drainage of the Cache Creek watershed, immediately NE of the town of Woodland (Yolo County), California and approximately 18 km NW of Sacramento, California. The basin was constructed by the U.S. Army Corps of Engineers (completed in 1937 and modified in 1993) for the purpose of trapping suspended sediment transported from the upper Cache Creek watershed during high-flow events, thus preventing sediment from entering the Yolo Bypass, a larger downstream floodwater conveyance and agricultural zone. In addition to trapping suspended sediment, the CCSB also traps sediment-associated mercury (Hg), which is particularly elevated in the upper watershed, owing to natural Hg deposits and associated historic Hg mining areas. The CCSB is a multi-use area containing a number of habitat types that are typical of the California Central Valley region, including: open-water (stream, canal), riparian, floodplain (woody, non-woody), and agriculture (row crop fields) habitats. This dataset includes shallow surface sediment (top 0–2 cm interval) constituent concentration data (primarily) and microbial methylmercury production potential rate data (limited) collected between April 2010 and July 2019 from the above-noted habitats to assess spatial and temporal variations in sediment geochemistry and mercury speciation within the CCSB. The dataset is presented in two sections (child pages): one has the constituent concentration and microbial rate data, and the other includes results of habitat mapping that was conducted within the CCSB for the purpose of data analysis. First posted – October 16, 2018 (available from author) Revised – July, 2021 (version 2.0)

The Cache Creek Settling Basin (CCSB) is a 13.3 km2 leveed basin located at the terminal drainage of the Cache Creek watershed, immediately NE of the town of Woodland (Yolo County), California and approximately 18 km NW of Sacramento, California. The basin was constructed by the U.S. Army Corps of Engineers (completed in 1937 and modified in 1993) for the purpose of trapping suspended sediment transported from the upper Cache Creek watershed during high-flow events, thus preventing sediment from entering the Yolo Bypass, a larger downstream floodwater conveyance and agricultural zone. In addition to trapping suspended sediment, the CCSB also traps sediment-associated mercury (Hg), which is particularly elevated in the upper watershed, owing to natural Hg deposits and associated historic Hg mining areas. The CCSB is a multi-use area containing a number of habitat types that are typical of the California Central Valley region, including: open-water (stream, canal), riparian, floodplain (woody, non-woody), and agriculture (row crop fields) habitats. This dataset includes shallow surface sediment (top 0–2 cm interval) constituent concentration data (primarily) and microbial methylmercury production potential rate data (limited) collected between April 2010 and July 2017 from the above-noted habitats to assess spatial and temporal variations in sediment geochemistry and mercury speciation within the CCSB. The dataset is presented in two sections (child pages): one has the constituent concentration and microbial rate data, and the other includes results of habitat mapping that was conducted within the CCSB for the purpose of data analysis.

The Cache Creek Settling Basin (CCSB) is a 13.3 km2 leveed basin located at the terminal drainage of the Cache Creek watershed, immediately NE of the town of Woodland (Yolo County), California and approximately 18 km NW of Sacramento, California. The basin was constructed by the U.S. Army Corps of Engineers (completed in 1937 and modified in 1993) for the purpose of trapping suspended sediment transported from the upper Cache Creek watershed during high-flow events, thus preventing sediment from entering the Yolo Bypass, a larger downstream floodwater conveyance and agricultural zone. In addition to trapping suspended sediment, the CCSB also traps sediment-associated mercury (Hg), which is particularly elevated in the upper watershed, owing to natural Hg deposits and associated historic Hg mining areas. The CCSB is a multi-use area containing a number of habitat types that are typical of the California Central Valley region, including: open-water (stream, canal), riparian, floodplain (woody, non-woody), and agriculture (row crop fields) habitats. This dataset includes shallow surface sediment (top 0–2 cm interval) constituent concentration data (primarily) and microbial methylmercury production potential rate data (limited) collected between April 2010 and July 2019 from the above-noted habitats to assess spatial and temporal variations in sediment geochemistry and mercury speciation within the CCSB. The dataset is presented in two sections (child pages): one has the constituent concentration and microbial rate data, and the other includes results of habitat mapping that was conducted within the CCSB for the purpose of data analysis. First posted – October 16, 2018 (available from author) Revised – July, 2021 (version 2.0)