Bennett, Peter A. Oldham, Nicole A. Estrada, Christian L. Alpers, Charles N. 20260417 spreadsheet https://waterdata.usgs.gov/ U.S. Geological Survey https://doi.org/10.5066/P14N6RB3 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 Sulphur 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 1990, the SBMM has been a Superfund site managed by the U.S. Environmental Protection Agency (EPA). 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 (2025) studies underway [collaborations between EPA, the U.S. Geological Survey (USGS), and the University of California, Davis (UCD)] to improve the understanding of nutrient and mercury cycling within the lake. 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. The UCD part of the project is focused on developing a three-dimensional hydrodynamic computer model of the lake that simulates periodic stratification of temperature and dissolved oxygen, nutrient dynamics, and mercury cycling. The USGS part of the project is focused on monitoring of water quality conditions at locations in the lake and development of sensor-based methodologies for monitoring constituents of concern at relevant spatial and temporal scales. This approach requires 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, algal concentration, or fluorescent properties of dissolved organic matter). The USGS data monitoring includes continuous data collected from moored sondes deployed at two sites, vertical profiles collected with sondes at multiple sites, and water quality samples collected at selected sites and depths. The vertical profiles collected with a YSI EXO2 multiparameter sonde is the subject of this data release. This data release documents the results of water-quality vertical profiles during June 2020 through May 2025. It includes two 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 data table and includes citations of analytical methods; 2) T2_CL_Vertical_Profiles, the vertical profile surface-water dataset including water-quality parameters. Vertical profiles were collected at 17 sites in Clear Lake, with most sites profiled once or twice. Two of the sites in the Oaks arm have moored sondes collecting continuous data. Laboratory analytical results from water quality samples collected at selected depths in the vertical profiles are presented in Agee and others (2024). Additional vertical profiles have been collected by the USGS within Clear Lake in order to inform conservation and management decisions concerning the Clear Lake Hitch (Wulff and others, 2019). This study of water-quality parameters within the vertical column was designed to provide data that will be useful in developing a more robust Hg monitoring program for Clear Lake. The period of record for the current version of this data product encompasses the 2020-2025 study period. The product is designed such that it can be subsequently updated as new data becomes available annually. 20200604 20250528 ground condition Complete As needed -122.8470 -122.6722 39.0940 38.9614 ISO 19115 Topic Category inlandWaters USGS Thesaurus surface water quality water quality water chemistry mercury mercury contamination sulfur suspended material (water) nutrient content (water) bioaccumulation Marine Realms Information Bank (MRIB) keywords water quality mercury analysis mercury cycle lake Alexandria Digital Library Feature Type Thesaurus springs (hydrographic) mine sites USGS Metadata Identifier USGS:68a7926ad4be02616520f820 Common Geographic Areas Lake County Clear Lake California No access constraints. Please see 'Distribution Information' for details. These data are marked with a Creative Common CC0 1.0 Universal License. These data are in the public domain and do not have any use constraints. Users are advised to read the dataset's metadata thoroughly to understand appropriate use and data limitations. It is requested that the suggested citation for this USGS data release be included in any publications that reference or utilize these data. Peter A Bennett USGS - SOUTHWEST REGION Hydrologic Technician mailing and physical

California Water Science Center, California District Office - Placer Hall

Sacramento CA 95819 916-802-0526 pabennet@usgs.gov The project originates in the USGS California Water Science Center (CAWSC), Sacramento, with funding through an agreement with the U.S. Environmental Protection Agency. Microsoft Environment, Excel Version 2506 Xylem Environmental Solutions 2024 Revision L publication Accessed 3/15/2026 https://www.ysi.com/File%20Library/Documents/Manuals/EXO-User-Manual-Web.pdf U.S. Geological Survey 20250612 Version 1.0: November 28, 2023; Version 1.1: June 12, 2025 publication Techniques and Methods 9-A6.8 Reston, VA U.S. Geological Survey https://doi.org/10.3133/tm9A6.8 Jennifer L Agee Charles N Alpers Mark C Marvin-DiPasquale Jacob A Fleck Le Kieu Evangelos Kakouros Peter Bennett Patrick T Watanabe Daryna Sushch 20240926 dataset https://www.sciencebase.gov U.S. Geological Survey U.S. Geological Survey data release https://doi.org/10.5066/p9achwlj Marissa L Wulff Dennis A Steinke Matthew J Young Justin K Clause Frederick V Feyrer 2019 dataset https://www.sciencebase.gov U.S. Geological Survey U.S. Geological Survey data release https://doi.org/10.5066/p9l40jg3 Richard J. Wagner Robert W. Boulger Carolyn J. Oblinger Brett A. Smith 2006 publication n/a US Geological Survey https://doi.org/10.3133/tm1D3 United States Geological Survey 1994 2026 dataset https://www.sciencebase.gov U.S. Geological Survey U.S. Geological Survey, 2026, USGS Water Data for the Nation: U.S. Geological Survey National Water Information System database https://doi.org/10.5066/f7p55kjn No formal attribute accuracy tests were conducted however attributes have been reviewed to ensure accuracy. No formal logical accuracy tests were conducted. Tables have been checked for duplicate and omitted data. Reported data values fall within expected ranges. Data set is considered complete for the information presented, as described in the abstract. Users are advised to read the rest of the metadata record carefully for additional details. No formal positional accuracy tests were conducted No formal positional accuracy tests were conducted Water-quality field data were collected by the USGS California Water Science Center (CA-WSC). A YSI EXO2 multiparameter sonde (Xylem Environmental Solutions, 2024), attached to a handheld user interface via field cable, was used to record all water-quality data. The sonde was calibrated and maintained in accordance with methods described in Guidelines and standard procedures for continuous water-quality monitors: Station operation, record computation, and data reporting (Wagner and others, 2006). The multiparameter sonde was lowered on a secured line to alleviate tension on the cable and lowered through the water column at 0.5 meter (m) intervals. At each depth interval, measurements were logged on the handheld unit generating a vertical profile. The logged data from each profile was then downloaded onto a Universal Serial Bus (USB) flash drive and transferred to a field laptop within one day. The vertical profiles data, accessible through this data release, were not rounded and are reported as provided by the YSI EXO 2 multiparameter sonde. Post sampling quality assurance/quality control (QA/QC) was conducted as follows: spreadsheets were exported from the EXO2 sondes in .xlsx format. Each vertical profile was examined with a visual scan to filter out unreasonable values (e.g., negative depths) or flag values below the Reporting Level (RL). Water-quality samples were collected at each of the 17 locations during the collection of vertical profiles. These samples are indicated in the T2_CL_Vertical_Profiles table. The analytical results for the water-quality samples are available in Water Data for the Nation (WDFN; U.S. Geological Survey, 2026) and some are also reported in Agee and others (2024). The field parameters associated with the water-quality samples in WDFN were collected separately from these vertical profiles and as such times, depths, and values may differ. 20260304 Peter Bennett USGS - SOUTHWEST REGION Hydrologic Technician mailing and physical

California Water Science Center,California District Office - Placer Hall

Sacramento CA 95819 916-802-0526 pabennet@usgs.gov T1_CL_DataDictionary.csv (T1_CL_DataDictionary) Data dictionary providing information about entities in T2_CL_Vertical_Profiles.csv. All information in this data dictionary is also provided in the Entity and Attribute Information section for T2. Producer Defined Table Number Table sequence identity Producer Defined 2 2 Table Name Name of the table. Producer Defined Name of the table. Table Column Alphanumeric character indicating column. Producer Defined Alphanumeric character indicating column. Column Headings Contains the header information of columns. Producer Defined Contains the header information of columns. Column Description Contains a description of the type of information within the columns. Producer Defined Contains a description of the type of information within the columns. Extended Description Additional information for items within the Column Description. Producer Defined -- No Data Producer defined Additional information for items within the Column Description. Unit Abbreviation Lists the unit abbreviations used within the table. Producer Defined -- No Data Producer defined Lists the unit abbreviations used within the table. Unit Definition Defines the units used within the table. Producer Defined -- No Data Producer defined Defines the units used within the table. Additional Column Note Describes data source. Producer Defined -- No Data Producer defined Describes data source. T2_CL_Vertical_Profiles.csv (T2_CL_Vertical_Profiles) Vertical profile water-quality data collected at Clear Lake, California. Producer Defined NWIS_SiteID USGS National Water Information System (NWIS) Site Identifier. A unique alphanumeric code string used to identify each field site sampled. Producer Defined USGS National Water Information System (NWIS) Site Identifier. A unique alphanumeric code string used to identify each field site sampled. Station_Name Name of the field site monitoring station where samples were collected. Producer Defined Name of the field site monitoring station where samples were collected. CollectionDate_(mmddyyyy) Sample collection date in mmddyyyy (month, day, year) format. Producer Defined 6/4/2020 5/28/2025 CollectionTime_(hhmmss) Sample collection time in hhmmss (hours, minutes, seconds) format and Pacific Standard Time. Producer Defined Sample collection time in hhmmss (hours, minutes, seconds) format and Pacific Standard Time. ClearLakeArm Name of the arm or area of Clear Lake where sample was collected. Producer Defined Name of the arm or area of Clear Lake where sample was collected. Field_ID An alphanumeric identifier or code assigned in the field and used to identify the field sampling collection location. Producer Defined An alphanumeric identifier or code assigned in the field and used to identify the field sampling collection location. Latitude North American Datum of 1983 (NAD 83) Decimal latitudinal coordinate location value for where sample was collected. Producer Defined 38.961425 39.093994 decimal degrees 0.000001 Longitude North American Datum of 1983 (NAD 83) Decimal longitudinal coordinate location value for where sample was collected. Producer Defined -122.846986 -122.672189 decimal degrees 0.000001 EXO2_Field_Temp_(deg_C) Field temperature in degrees Celsius (C) measured using a Yellow Springs Instruments (YSI) EXO2 Multi-Parameter Water Quality Sonde (Xylem Environmental Solutions, 2024). This measured value corresponds to USGS NWIS the Parameter Code 00010, and method code THM01. Producer Defined 8.646 28.344 degrees Celsius (C) 0.001 EXO2_Field_SpC_(µS_per_cm_at_25_deg_C) Field specific conductivity in microsiemens per centimeter at 25 degrees Celsius (µS/cm at 25 deg C). Measured using a Yellow Springs Instruments (YSI) EXO2 Multi-Parameter Water Quality Sonde (Xylem Environmental Solutions, 2024). This measured value corresponds to USGS NWIS Parameter Code 00095, method code SC001. Producer Defined -- No Data Producer defined 276.2 425.7 microsiemens per centimeter at 25 degrees Celsius (µS/cm at 25 deg C) 0.1 EXO2_Field_Air Pressure_(mmHg) Field barometric pressure in millimeters of mercury (mm Hg) measured using a Yellow Springs Instruments (YSI) EXO2 Multi-Parameter Water Quality Sonde (Xylem Environmental Solutions, 2024). This measured value corresponds to USGS NWIS the Parameter Code 00025, method code BAROM. Producer Defined -- No Data Producer defined 719.2 735.5 millimeters of mercury (mm Hg) 0.1 EXO2_Field_DO_(mg/L) Field dissolved oxygen measurement measured using a Yellow Springs Instruments (YSI) EXO2 Multi-Parameter Water Quality Sonde (Xylem Environmental Solutions, 2024). This measured value corresponds to USGS NWIS Parameter Code 00300, method code LUMIN. Producer Defined -- No Data Producer defined -0.07 15.96 milligram per liter (mg/L) 0.01 EXO2_Field_DO_QA Field dissolved oxygen, quality assurance Producer Defined -- No Data Producer defined <RL Value below reporting level of 0.1 mg/L Producer defined EXO2_Field_DO_per_Sat Field dissolved oxygen percent of saturation measurement measured using a Yellow Springs Instruments (YSI) EXO2 Multi-Parameter Water Quality Sonde (Xylem Environmental Solutions, 2024). Producer Defined -- No Data Producer defined -0.8 195.6 percent (%) 0.1 EXO2_Field_DO_Saturation_QA Field dissolved oxygen percent of saturation, quality assurance Producer Defined -- No Data Producer defined <RL Value below reporting level of 1% saturation Producer defined EXO2_Field_pH_(std_units) Field pH measurement measured using a Yellow Springs Instruments (YSI) EXO2 Multi-Parameter Water Quality Sonde (Xylem Environmental Solutions, 2024). This measured value corresponds to USGS NWIS Parameter Code 00400, method code PROBE. Producer Defined -- No Data Producer defined 6.12 9.3 standard units (std units) 0.01 EXO2_Field_Turb_(FNU) Field turbidity measurement measured using a Yellow Springs Instruments (YSI) EXO2 Multi-Parameter Water Quality Sonde (Xylem Environmental Solutions, 2024). This measured value corresponds to USGS NWIS Parameter Code 63680, method code TS213. Producer Defined -- No Data Producer defined 0.35 69.68 formazin nephelometric turbidity unit (FNU) 0.01 EXO2_Field_Chla_(RFU) Field chlorophyll measurement measured using a Yellow Springs Instruments (YSI) EXO2 Multi-Parameter Water Quality Sonde (Xylem Environmental Solutions, 2024). This measured value corresponds to USGS NWIS Parameter Code 32315. Producer Defined -- No Data Producer defined 0.15 17.48 relative fluorescence units (RFU) 0.001 EXO2_Field_fDOM_(QSE) Field fluorescent dissolved organic matter measurement measured using a Yellow Springs Instruments (YSI) EXO2 Multi-Parameter Water Quality Sonde (Xylem Environmental Solutions, 2024). This measured value corresponds to USGS NWIS Parameter Code 32295, method code FL017 Producer Defined -- No Data Producer defined 11.58 38.70 quinine sulfate equivalents (QSE) 0.01 EXO2_Field_WaterDepth_(m) Field water depth measurement measured using a Yellow Springs Instruments (YSI) EXO2 Multi-Parameter Water Quality Sonde (Xylem Environmental Solutions, 2024). This measured value corresponds to USGS NWIS Parameter Code 72178. Producer Defined 0.048 19.136 meters (m) 0.001 Associated_Water-quality_Sample Vertical profile reading has a paired water-quality sample collected at that date and site. The field parameters associated with the water-quality sample were collected separately from the vertical profile and as such times, depths, and values may differ. Producer Defined Y This vertical profile reading has a paired water-quality sample collected at that date and site. The field parameters associated with the water-quality sample were collected separately from the vertical profile and as such times, depths, and values may differ. Producer defined -- This vertical profile reading does not have a paired water quality sample collected at that site, date, and depth. Producer defined This data product includes two data tables (*.csv) 1) T1_CL_DataDictionary - Data dictionary providing information about entities in T2_CL_Vertical_Profiles.csv. All information in this data dictionary is also provided in the Entity and Attribute Information section for T2. 2) T2_CL_Vertical_Profiles - Vertical profile water-quality data collected at Clear Lake, California N/A U.S. Geological Survey - ScienceBase GS ScienceBase mailing address

Denver Federal Center

Building 810

Mail Stop 302

Denver CO 80225 United States 1-888-275-8747 sciencebase@usgs.gov Unless otherwise stated, all data, metadata and related materials are considered to satisfy the quality standards relative to the purpose for which the data were collected. Although these data and associated metadata have been reviewed for accuracy and completeness and approved for release by the U.S. Geological Survey (USGS), no warranty expressed or implied is made regarding the display or utility of the data for other purposes, nor on all computer systems, nor shall the act of distribution constitute any such warranty. Digital Data https://doi.org/10.5066/P14N6RB3 None 20260417 Peter A Bennett USGS - SOUTHWEST REGION Hydrologic Technician mailing and physical

California Water Science Center, California District Office - Placer Hall

Sacramento CA 95819 916-802-0526 pabennet@usgs.gov FGDC Content Standard for Digital Geospatial Metadata FGDC-STD-001-1998