Water chemistry data for samples collected at groundwater and surface-water sites in the San Ardo Oil Field study area, March 2019–May 2021, Monterey County, California

The California State Water Resources Control Board (State Water Board) initiated the Oil and Gas Regional Monitoring Program (RMP) to assess effects of oil and gas development on groundwater designated for any beneficial use. The U.S. Geological Survey (USGS) is the technical lead in conducting the RMP through the California Oil, Gas, and Groundwater (COGG) Program, working in cooperation with the State Water Board, and in partnership with other State and local agencies. The USGS collected and analyzed groundwater (GW) and surface water (SW) for the San Ardo Oil Field study area, the area within the San Ardo Oil Field administrative boundary and the surrounding five-kilometer buffer zone, in Monterey County, California. Eighteen groundwater, two stream, and two spring samples were collected from March 2019 through May 2021. Groundwater samples were collected from one public-supply well, five monitoring wells, two irrigation wells, one livestock well, five oil-field water supply wells, and four domestic wells. Samples were collected using established water data-collection protocols and procedures and analyzed for (1) water-quality indicators, (2) major and minor ions, (3) nutrients,  (4) trace elements, (5) volatile organic compounds (VOCs), (6) naturally-occurring radioactive material (radium isotopes), (7) geochemical and age-dating tracers, (8) dissolved organic carbon (DOC), (9) dissolved standard and hydrocarbon gases (methane through hexane), and (10) dissolved noble gases and atmospheric gases. In total, 185 constituents, water-quality indicators, or properties are included in this data release. Quality-control (QC) samples were used to assess the quality of laboratory results.