This digital dataset contains historical geochemical and other information for 580 samples of groundwater from 12 wells located within the Placerita and Newhall Oil Fields in Los Angeles County, southern California. The sampled wells include 5 monitoring wells (Dataset IDs 1-5) associated with a groundwater remediation site, 6 water-supply wells (Dataset IDs 6-11) used to supply groundwater in support of oil production, and 1 well (Dataset ID 12) constructed as an observation well to monitor water-disposal operations. The groundwater remediation site wells represent a subset of a much larger number of monitoring wells that were selected for inclusion in this dataset because they were perforated within the regional groundwater aquifer, and the geochemistry data for these wells include the greatest variety of constituents available in the data source. The numerical water chemistry data, well locations, and well construction information for Dataset IDs 1-5 were compiled from PDFs (Portable Document Format) documents located on the California Department of Toxic Substances Control EnviroStor (DTSC-EnviroStor) website. Water chemistry data for the remaining wells were compiled from a combination of scanned laboratory analysis reports available from the California Geologic Energy Management Division (CalGEM) (Dataset IDs 6-9), analytical reports available as PDFs located on the State Water Resources Control Board GeoTracker (SWRCB-GT) website (Dataset ID 10), and analytical reports located within well history files in CalGEM's online Well Finder (WF) database (Dataset IDs 11-12). The availability of location and well construction information for Dataset IDs 6-12 varied by site. A combination of scanned laboratory analysis reports, CalGEM WF well history files, and California Department of Water Resources Well Completion Reports (CDWR-WCR) were the primary sources of location and well construction information for 5 wells (Dataset IDs 7-10, and 12) and 3 wells (Dataset IDs 8, 10, and 12), respectively. Approximate locations for Dataset IDs 7 and 9 represent meridian, township, range, and section (MTRS) centroids. Google Earth was used to determine the approximate location of Dataset ID 8 based on information from the WCR for that well. No location or well construction information was found for Dataset IDs 6 and 11. Data were manually compiled into two separate files described as follows: 1) a summary data file that includes well identifiers, location, construction, the number of chemistry samples, the period of record, specific sample dates for each site, and an inventory of which constituent groups were sampled on each date; and 2) a data file of geochemistry analyses for selected constituents classified into one of the following groups: water-quality indicators, major and minor ions, nutrients, trace elements, naturally occurring radioactive material (NORM), volatile organic compounds (VOCs), and miscellaneous organics compounds. Ion (charge) balance calculations and percent error of these calculations were included for samples having a complete suite of major ion analyses. Analytical method, reporting level, reporting level type, and supplemental notes were included where available or pertinent. A data dictionary was created to describe the geochemistry data file and is provided with this data release.