Regional and local controls on groundwater salinity in California’s southwestern San Joaquin Valley, United States: Insights from airborne electromagnetic surveys

Abstract The San Joaquin Valley is one of the most intensive oil and gas and agricultural production regions of the United States, yet the regional groundwater salinity distribution is not well defined, particularly near the western margin of the valley where salinity is highly variable. Airborne electromagnetic (AEM) surveys were conducted to improve the understanding of the three-dimensional variations in groundwater salinity in the upper 300 m. A salinity mapping approach representing geophysical and interpretational uncertainty was used to predict the probability of the occurrence of three salinity categories commonly used in water resource management (fresh, total dissolved solids (TDS) < 3000 mg/L; brackish, TDS 3000–10,000 mg/L; saline, TDS > 10,000 mg/L). The salinity mapping identifies regions of fresh and brackish groundwater near oil fields, suggests potential gaps in groundwater monitoring, and supports improved fresh and brackish groundwater resource availability estimates. Shallow saline regions are common near historical lake beds and marsh areas, with abrupt transitions between saline and fresh/ brackish groundwater in both historical and modern hydrogeologic contexts. The common occurrence of a shallow saline groundwater inversion implies that reliance on single-depth salinity threshold maps may lead to overestimation of the availability of fresh and brackish groundwater. Some parts of these saline areas can be correlated to chemical indicators of oil-field water, likely resulting from historical surface disposal of produced water.