Abstract

Groundwater quality near the Elk Hills and North Coles Levee oil fields is classified as high priority for monitoring by the State Water Resources Control Boards Regional Monitoring Program (RMP) because of the high densities of oil and gas wells in the oil fields and the large volumes of fluids injected for enhanced oil recovery and water disposal. The RMP objectives include 1) determining the hydrogeologic relations between oil and gas activities and protected groundwater, and 2) determining whether or not there is evidence of fluid migration and water-quality changes related to oil and gas activities. To meet these objectives, the U.S. Geological Survey (USGS; from 2017 to 2019) compiled and evaluated information from the oil fields and surrounding areas including historical groundwater quality data, water and oil well construction data, and oil field injection and development data. In addition, the USGS collected water samples from 32 groundwater wells and one groundwater banking pond and analyzed them for a suite of constituents selected to assess whether groundwater quality could be influenced by oil and gas activities. Sampled wells had depths ranging from 32 to 1140 feet (ft) and included 26 monitoring wells, four irrigation wells, and two public supply wells. The area studied by the RMP includes both the Elk Hills and North Coles Levee oil fields, which cover about 45 and 18 square miles, respectively, as well as a three-mile buffer around the oil fields (Figure 1). The oil fields are located in the southern part of the San Joaquin Valley, about 20 miles west of Bakersfield, California.

Sample data from most wells and historical data with more limited analytes showed no evidence of oil and gas fluids in groundwater. Water from three of the sampled wells (Figure 1) showed evidence of mixing with small volumes of oil and gas fluids, based on co-occurrence of dissolved hydrocarbon compounds, dissolved organic carbon, total dissolved solids (TDS), and/or ratios of inorganic ions. Water from well COLN-06 had unexpectedly high TDS for the location of the well in the groundwater flow system, ion ratios consistent with mixing of groundwater and produced water, and a dissolved hydrocarbon compound detection (toluene, 0.49 micrograms per liter [g/L]). Water from well COLN-09 had five detections of dissolved hydrocarbon compounds, including toluene and xylenes, at low concentrations ranging from 0.01 to 4.9 g/L but no other indicators of oil and gas fluids. Tritium and carbon-14, which are used to estimate the time since water recharged the aquifer, indicate groundwater from wells COLN-06 and COLN-09 was recharged prior to 1950, suggesting that the source of oil and gas fluids is a leaky well or some other subsurface pathway rather than infiltration of modern (post-1950) water as would be expected if these fluids were derived from activities at the land surface. Well COLN-06 is located downgradient and within 1,600 ft of 7 injection and 11 production wells, with 25 million barrels of water injected and 11 million barrels of water produced from 1977-2015, suggesting possible sources for mixing with oil and gas fluids. Well COLN-09 is not located within 1,600 ft of injection or production wells, which suggests oil and gas fluids would have had to migrate over a larger lateral distance or that the detections were unrelated to oil and gas wells. Water from well ELKH-07, near the dry Buena Vista lakebed margin, had a dissolved hydrocarbon compound detection (toluene, 2.8 g/L), high dissolved organic carbon (11 mg/L) compared to water from other wells sampled in the area, and TDS measuring 50,600 mg/L, which is about 30,000 mg/L higher than the TDS of water from other wells sampled in the area. Ion ratios show that the high TDS values of shallow groundwater near the Buena Vista lake margin are likely related to evaporative concentration of water and/or water-rock interactions. Tritium and carbon-14 indicate that water from well ELKH-07 contains a mixture of modern (post-1950) and pre-modern (pre-1950) water, suggesting that oil and gas fluids could be related to infiltration associated with activities at the land surface. Historical oil development practices sometimes released produced waters to stream drainage networks, which drain to the Buena Vista lake where this well is located. Other factors that may affect groundwater quality near the Elk Hills and North Coles Levee oil fields include groundwater banking operations, irrigation return flows, and declining water levels.