Electrical resistivity tomography in the Air Force Research Laboratory NE Groundwater Area, Edwards Air Force Base, California 2020

Edwards Air Force Base (EAFB) is located about eight kilometers (km) northeast of the city of Lancaster, California. In 1990 EAFB was placed on the U.S. Environmental Protection Agency's National priorities list due to the presence of contaminated soil and groundwater. The base was divided into Operable Units (OU) based on location and similar contaminant types (Lahontan Staff Report, 2010). The Air Force Research Laboratory (AFRL) was established in the 1950's, and contains facilities for research, development, testing, and evaluation of rocket propulsion systems (Lahontan Staff Report, 2010; AECOM, 2014), and has been in operation under various names since. Past activities at rocket test stands, evaporation ponds, burn pits, catch basins and leaking waste collection tanks has contaminated the groundwater in the AFRL area (AECOM, 2014). The AFRL occupies roughly 125 square miles (mi^2) on Leuhman Ridge and surrounding areas and is part of OU 4 and 9, roughly 5 kilometers (km) south of the community of Boron. 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, 2006, Advanced Geosciences Inc., 2009). The U.S. Geological Survey (USGS) and Air Force Civil Engineering Center (AFCEC) have entered into a cooperative agreement to refine conceptual models of the geology in the NE AFRL. As part of these efforts, two electrical resistivity tomography (ERT) surveys were done in the vicinity of the Leuhman and Spring Faults identified by Dibblee (1960) to better determine the position of these faults. ERT profile AFRL7 was done in the vicinity of the Leuhman Fault, and profile AFRL8 was done in the vicinity of the Spring Fault.