Groundwater salinity and the effects of produced water disposal in the Lost Hills–Belridge oil fields, Kern County, California

Increased oil and gas production in many areas has led to concerns over the effects these activities may be having on nearby groundwater quality. In this study, we determine the lateral and vertical extent of groundwater with less than 10,000 mg/L total dissolved solids near the Lost Hills–Belridge oil fields in northwestern Kern County, California, and document evidence of impacts by produced water disposal within the Tulare aquifer and overlying alluvium, the primary protected aquifers in the area. The depth at which groundwater salinity surpasses 10,000 mg/L ranges from150m (500 ft) in the northwestern part of the study area to 490–550 m (1600–1800 ft) in the south and east, respectively, as determined by geophysical log analysis and lab analysis of produced water samples. Comparison of logs from replacement wells with logs from their older counterparts shows relatively higher-resistivity intervals representing the vadose zone or fresher groundwater being replaced by intervals with much lower resistivity because of infiltration of brines from surface disposal ponds and injection of brines into disposal wells. The effect of the surface ponds is confined to the alluvial aquifer—the underlying Tulare aquifer is largely protected by a regional clay layer at the base of the alluvium. Sand layers affected by injection of produced waters in nearby disposal wells commonly exhibit log resistivity profiles that change from high resistivity in their upper parts to low resistivity near the base because of stratification by gravity segregation of the denser brines within each affected sand. The effects of produced water injection are mainly evident within the Tulare Formation and can be noted as far as 550 m (1800 ft) from the main group of disposal wells located along the east flank of South Belridge. AUTHORS Janice M. Gillespie ~ California Water Science Center, US Geological Survey (USGS), Sacramento, California; jmgillespie@usgs.gov Jan Gillespie received her B.S. degree in geology from Bemidji State University, Minnesota, her M.S. from South Dakota School of Mines and Technology, and her Ph.D. from the University of Wyoming. Formerly a petroleum and hydrogeology professor in the Department of Geosciences at California State University, Bakersfield, and a petroleum geologist in the San Joaquin Valley of California, she is now a research scientist for the USGS’s regional aquifer monitoring project for California SB4 (the Well Stimulation Bill) delineating protected aquifers near oil producing areas. Tracy A. Davis ~ California Water Science Center, USGS, San Diego, California; tadavis@usgs.gov Tracy Davis received her B.S. degree in earth sciences with emphasis on geochemistry from the University of California, San Diego. She began her career in hydrology at the USGS in 2007 studying groundwater quality of aquifers used for public supply. Her current research focuses on areas of oil and gas development and characterizing risks to California’s groundwater resources. Michael J. Stephens ~ California Water Science Center, USGS, Sacramento, California; mjstephens@usgs.gov Michael Stephens received his B.S. degree in geology from the University of Southern Indiana and his M.S. degree in geology from California State University, Sacramento. He currently works on the California Oil, Gas, and Groundwater Project at the USGS. His research is focused on modeling groundwater salinity, aquifer architecture, and the controls on salinity distributions. Lyndsay B. Ball ~ Geology, Geophysics, and Geochemistry Science Center, USGS, Denver, Colorado; lbball@usgs.gov Lyndsay Ball received her B.S. degree in environmental science from Virginia Tech and Copyright ©2019. The American Association of Petroleum Geologists/Division of Environmental Geosciences. All rights reserved. Gold Open Access. This paper is published under the terms of the CC-BY license. Manuscript received October 30, 2018; provisional acceptance March 20, 2019; revised manuscript received March 27, 2019; final acceptance June 20, 2019. DOI:10.1306/eg.02271918009 Environmental Geosciences, v. 26, no. 3 (September 2019), pp. 73–96 73