RELATIONS OF GROUNDWATER QUALITY TO OIL DEVELOPMENT IN DIFFERENT HYDROGEOLOGIC SETTINGS – INITIAL RESULTS FROM CALIFORNIA REGIONAL GROUNDWATER MONITORING NEAR OIL FIELDS (Invited Presentation)

In cooperation with the California State Water Resources Control Board’s Regional Monitoring Program (RMP) of Water Quality in Areas of Oil and Gas Production, the U.S. Geological Survey has been assessing: 1) the location and characteristics of protected groundwater in proximity to oil production; 2) evidence of oil and gas fluids mixing with protected groundwater, and, if present, determining the processes potentially responsible; and 3) how oil production has affected groundwater quality relative to other processes. A comparison of results to date from the eastside and westside of the San Joaquin Valley indicates that hydrogeologic setting influences the occurrence of oil-field fluids in groundwater. In the Fruitvale study area on the eastside, groundwater above oil-bearing zones is heavily used, with vertical separation of water and oil well perforations > 670 meters (m). This area has relatively high recharge from the Kern River that determines regional groundwater quality. Oil-field fluids were infrequently detected as minor local deviations from regional conditions. These detections sometimes occurred in water wells near areas of high produced water injection and high density of oil wells, some of which may provide pathways for oil-field gases and solutes to reach groundwater. On the westside, protected groundwater is primarily east (and downgradient) of the Lost Hills, South and North Belridge oil fields. Adjacent to these fields, many groundwater samples from wells currently used for irrigation or industrial supply showed no evidence of mixing with oil-field fluids, but some samples indicated mixing with produced waters, likely from historic disposal in surface ponds. There are currently limited data to assess lateral movement of produced water injected for disposal into adjacent groundwater. Within the fields, multiple lines of geochemical evidence indicated overlying groundwater is mixing with oil-field fluids. This result may be expected considering the vertical (< 140 m) and lateral proximity of sampled wells to oil-bearing formations and production activities. Conceptual models for relations of the occurrence of oil-field fluids in groundwater with hydrogeologic setting and potential risk factors are being evaluated in additional RMP study areas.