GROUNDWATER QUALITY IN AQUIFERS NEAR AND OVERLYING THE ELK HILLS AND NORTH COLES LEVEE OIL FIELDS, SAN JOAQUIN VALLEY, CALIFORNIA

High densities of oil/gas wells and large volumes of injected water led to the area of the Elk Hills/North Coles Levee Oil Fields being selected for analysis of groundwater quality as part of the California State Water Resources Control Board’s oil and gas Regional Monitoring Program. Both new and historical data from groundwater and oil-field water/gas samples were used to evaluate the major processes controlling groundwater quality. Water from most wells showed minimal or no evidence of potential oil-field water/gas constituents based on (1) low or undetectable concentrations of dissolved hydrocarbon compounds and (2) comparison of inorganic chemical concentrations in hypothetical mixing models that include oil-field water as a saline endmember. Hierarchical clustering of major ion data from over 150 groundwater wells and 25 oil and gas wells identified two major groundwater groups (A and B), and five smaller oil field groups differentiated by oil reservoir pools. The group A cluster contained Ca-HCO3 or Ca-Na-HCO3 groundwaters of low TDS (< 300 mg/L) and δ¹⁸O and δ²H values consistent with recharge originating from Sierra Nevada snowmelt. Cl/Br and tritium differentiated group A recharge sources into old (pre-1950s) water from the Kern River and modern (post-1950s) water from artificial recharge operations. One group A sample had five dissolved hydrocarbon detections, including toluene and xylenes, at low concentrations ranging from 0.01 to 4.9 µg/L but no other potential oil-field water/gas constituents. The group B cluster typically contained old Na-SO4 or Na-Cl groundwaters of moderate TDS (up to 5,000 mg/L) that are common on the west side of the San Joaquin Valley. One group B sample had a low-level toluene detection (0.49 µg/L) and ion ratios consistent with mixing of groundwater and small fractions of oil field water. In summary, the data indicate that oil and gas activities/sources have had either minimal or no effects on groundwater quality in the area of sampled wells. However, the number of wells available to sample in proximity to potential oil/gas sources, particularly for the Elk Hills field, was limited. The sampled wells may not yet be suitable for detecting possible water-quality changes related to oil and gas activities sources considering travel times in an aquifer that largely contains old water.