California Oil, Gas, and Groundwater Program

Publication: Conference and Public Meeting Abstracts or Presentations


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Ball, L. B., Gillespie, J. M., Davis, T. A., Minsley, B. and Landon, M. K.


California State Water Resources Control Board Stakeholder Meeting, Sacramento, California, June 12, 2018


The U.S. Geological Survey is conducting an interdisciplinary study of groundwater quality near oil and gas fields in California as part of the Oil and Gas Regional Groundwater Monitoring Program authorized by California Senate Bill 4 and implemented by the California State Water Resources Control Board. A primary objective of the regional monitoring program is to map relatively fresh protected groundwater (total dissolved solids (TDS) < 10,000 milligrams per liter) near oil and gas fields that could support beneficial use. Airborne electromagnetic (AEM) surveys are being used to develop a three-dimensional understanding of subsurface geology that can affect groundwater movement and quality near oil fields and to map protected groundwater. Due to the dense infrastructure within the fields, AEM surveys focus on the less-developed and agricultural areas adjacent to the field boundaries. In October 2016, areas hydrologically down-gradient (east) of the Lost Hills, North Belridge, and South Belridge oil fields along the southwestern margin of the San Joaquin Valley were surveyed. By leveraging ground truth from borehole geophysical logs and water sample data, quantitative AEM-derived maps of groundwater salinity categories across the surveyed areas were created alongside qualitative interpretations.

AEM methods measure subsurface resistivity, which is sensitive to the geologic material, fluid saturation and salinity, and subsurface temperature. Using known geology and salinity observations from independent borehole and water-quality data from wells, AEM-derived resistivity can be used to interpret changes in salinity by building relationships specific to the study area. TDS measurements from wells in close proximity to the AEM flight lines and with both known construction and historically stable TDS values/ recent samples are being used to define the regional salinity-to-resistivity relationship. This interpretative process results in three-dimensional maps of the likelihood of (a) fresh and (b) brackish-to saline-groundwater categories.

Results suggest that shallow (< 1 kilometer) native groundwater salinity varies widely throughout the San Joaquin Valley as a function of the geologic setting. On the west side of the valley, groundwater TDS tends towards brackish-to-saline conditions in marine sedimentary units, which shallowly occur in a series of anticlines, while relatively fresh groundwater can occur in the terrestrial formations and the overlying alluvium. Recharge from surface-water diversions, agricultural land use, and historical produced-water disposal have led to local and regional secondary effects on groundwater quality. At a regional scale, extensive clay units, particularly the Corcoran clay equivalent, appear to control shallow groundwater movement, as evidenced by abrupt transitions in salinity between subsurface layers as seen in the 3D maps.