Hydrogeology and assessment of the effect of oil-production activities in the Midway Valley area, western Kern County, California

The southwestern San Joaquin Valley, California includes oil fields and oil-field water disposal facilities, (ponds and injection wells). The Tulare Formation and overlying alluvium comprise the main aquifers in the study area and are commonly used for produced water disposal. Water quality in the aquifers is naturally brackish (total dissolved solids (TDS) 3,000-10,000 mg/L) across most of the area. Geophysical logs are useful in determining salinity variations within aquifers including the depth at which TDS>10,000 mg/L, often used as a criterion for evaluating underground injection permits. The depth to water with 10,000 mg/L TDS ranges from 366 m in the northwest to ~ 1,500 m in the southeast. Water table elevations from porosity logs and groundwater wells indicate the water table slopes south-southeast, showing predominant groundwater flow direction from oil field disposal areas toward better quality groundwater east of the oil fields. However, groundwater velocities <60 meter/year indicate it could take over a century to affect fresher groundwater to the east. Electric log data show formation resistivity near some disposal facilities has decreased over time, indicating that the salinity of the brackish aquifer has increased due to disposal of saline produced water. Evidence of increased salinity over time in water saturated sand intervals >1.5 km from disposal facilities may be caused by mechanical failures and/or incomplete borehole seals in poorly constructed or abandoned wellbores prevalent throughout the study area. 1 U.S. Geological Survey, California Water Science Center, San Diego, CA 92101 2 U.S. Geological Survey, Geology Geophysics and Geochemistry Water Science Center, Lakewood, CO 80225 3 U.S.