DISTRIBUTION OF SALINE GROUNDWATER ALONG THE SOUTHWESTERN MARGIN OF CALIFORNIA'S SAN JOAQUIN VALLEY MAPPED WITH AIRBORNE ELECTROMAGNETICS

In California’s San Joaquin Valley, historical and on-going oil and gas development has raised questions about the potential impacts to groundwater quality, including the introduction of relatively high salinity produced water to shallow aquifers. Relatively few groundwater wells, particularly in the western San Joaquin Valley where elevated shallow salinity has limited historical groundwater development, hinder traditional salinity mapping approaches that rely on groundwater sampling alone. To address this data gap, high-resolution airborne electromagnetic (AEM) surveys are being conducted adjacent to a number of oil fields along the southwestern margin of the valley as part of the California State Water Resources Control Board Oil and Gas Regional Monitoring Program and the U.S. Geological Survey’s California Oil Gas and Groundwater Program. These surveys provide spatially comprehensive information about the resistivity structure of the upper 300 meters, which is sensitive to a combination of salinity and geologic properties. A probabilistic salinity mapping approach has been used with available local water quality observations and Bayesian inversion of the AEM data to create 3D maps of fresh, saline, and brackish groundwater while quantifying joint uncertainty inherited from the geophysical data and interpretational relations. The regional mapping results reveal complex and geologically controlled salinity distributions where background total dissolved solids concentrations range from less than 1,000 to more than 50,000 milligrams per liter. These results provide an improved understanding of the regional hydrogeologic setting and associated background salinity. Where independent water quality analyses indicate the presence of produced water, the AEM-derived mapping provides insights into the potential extent of associated saline anomalies and can help identify areas for future groundwater quality sampling.