The California State Water Resources Control Board (State Water Board) initiated the Regional Groundwater 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 and associated quality control (QC) samples during September 2014 through October 2018. Groundwater samples collected in 2017-18 were from eight public supply wells, eight monitoring wells and one irrigation well located within the Montebello Oil Field study area, a three-mile buffer zone overlying and surrounding the Montebello Oil Field administrative boundary in Los Angeles County, California. Groundwater from four additional monitoring wells was collected in the Montebello Oil Field study area as part of the exploratory phase of the RMP; those sites were sampled in 2014. Samples were collected using established groundwater data-collection protocols and procedures. Samples were analyzed for water-quality indicators, major and minor ions; nutrients; trace elements, volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs); naturally-occurring radioactive material; geochemical and age-dating tracers; dissolved organic carbon (DOC) characteristics; dissolved standard and hydrocarbon gases (methane through hexane); and dissolved noble gases and atmospheric gases. In total, 286 constituents, water-quality indicators, or properties were measured. QC samples collected as part of the Montebello Oil Field study area include one replicate, two equipment blanks and two source-solution blanks. Additional QC samples collected from study areas for which sampling was done during December 2017 through October 2018 include three replicates, three spikes, two field blanks, and two source-solution blanks.