Electrical resistivity tomography at Installation Restoration Site 1119 at Camp Pendleton, San Diego County, California, 2024

Marine Corps Base (MCB) Camp Pendleton is an approximately 125,000 acre facility located in coastal southern California between the Los Angeles and San Diego metropolises. Established in 1942, historical military activities on the MCB Camp Pendleton have resulted in the release of environmental contaminants at numerous sites. To mitigate the risk to human health and the environment at these sites on MCB Camp Pendleton and other installations, the Department of the Navy established the Installation Restoration Program (IRP). Installation Restoration (IR) Site 1119 is a contaminated location identified by the IRP for remedial action near the southern border of the MCB Camp Pendleton in the Upper Ysidora subbasin of the Santa Margarita River Watershed. The boundaries of Site 1119 are defined as a plume of trichloroethylene (TCE) in groundwater exceeding the maximum contaminant level of five micrograms per liter (Parson and Tidewater Inc., 2015). TCE has been detected in a nearby production well outside of the current extent of Site 1119 (Stetson Engineers, 2022). The U.S. Geological Survey (USGS) and Naval Facilities Engineering Systems Command (NAVFAC) have entered into a cooperative agreement to refine conceptual models of the hydrogeology of IR Site 1119. As part of these efforts, two electrical resistivity tomography (ERT) surveys were done in the vicinity of the production well PW 2604. Electrical resistivity Tomography (ERT) is a direct current geophysical method that is used to estimate the subsurface distribution of the electrical resistivity (measured in ohm-meters; ohm-m) of a material, and is based on the assumption that measured electric potentials (voltages) near current carrying electrodes are influenced by the electrical resistivities of the underlying material (Zohdy and others, 1974; Loke, 2000). ERT is a popular technique for subsurface investigations because it is based on simple physical principles and for its efficient data acquisition (Dahlin and Zhou, 2004). A combination of the Dipole-Dipole and Strong Gradient arrays was used for this survey and combined to create an optimized dataset (Stummer and others, 2004). The Dipole-Dipole array type yields a high precision dataset, particularly of vertical structures, but can exhibit lower signal to noise ratios (Dahlin and Zhou, 2004; Binley and Kemna, 2005), while the Strong Gradient array provides more complete spatial coverage, and high signal to noise ratio with increased acquisition efficiency (Dahlin and Zhou, 2004; Dahlin and Zhou, 2005, Advanced Geosciences Inc., 2009).