ASSESSING POTENTIAL EFFECTS OF OIL AND GAS ACTIVITY ON GROUNDWATER QUALITY OF AQUIFERS NEAR AND OVERLYING THE ELK HILLS AND NORTH COLES LEVEE OIL FIELDS, SAN JOAQUIN VALLEY, CALIFORNIA

California’s San Joaquin Valley (SJV) is one of the top oil and gas producing regions in the United States and has a long history of oil and gas production dating to the late 1800s. The proximity of oil and gas activities to groundwater resources utilized in the SJV for agricultural, domestic, and municipal water supply has led to recent studies assessing groundwater quality near oil fields and potential pathways between oil and gas sources and groundwater. Here, we investigate chemical and isotopic tracer data from new and historical groundwater samples collected near Elk Hills and North Coles Levee Oil Fields—southwest SJV Oil Fields classified as high priority for monitoring by the California State Water Resources Control Board Oil and Gas Regional Monitoring Program (RMP)­—to assess if thermogenic gas or saline water from oil producing formations have mixed with groundwater. Ion concentrations and stable isotopes of water indicated water-rock interactions and mixing of groundwater recharged from Kern River, managed aquifer recharge, and Coast Range precipitation sources were major controls on groundwater quality. RMP samples collected from 34 wells had trace or non-detectable concentrations of methane and dissolved hydrocarbons that indicated little or no migration of thermogenic gases into sampled wells from natural processes or leakage from oil and gas wells. In contrast, historical data showed increasing Cl and B concentrations in water wells downgradient of injection wells used to dispose of produced water in the southern part of Elk Hills Oil Field. Mixing models were consistent with the water wells containing up to 15 % saline water from oil producing formations. Shallow groundwater along the western margin of Buena Vista Lake Bed had elevated concentrations of Cl, B, and SO₄ that geochemical models indicated could be explained by water-rock interactions involving gypsum in combination with either (1) evaporative cycles associated with ephemeral Buena Vista Lake or (2) legacy disposal of produced water in upgradient ephemeral drainages. In summary, the analysis of chemical and isotopic tracer data suggests oil and gas development activities have not led to widespread presence of oil and gas compounds in groundwater near and overlying Elk Hills and North Coles Levee Oil Fields, with some local exceptions.