Surface geophysics investigations at Edwards Air Force Research Laboratory, Antelope Valley, California, 2018

The Air Force Research Laboratory (AFRL) is about 7 kilometers southwest of Boron, California, and covers 320 square kilometers of Edwards Air Force Base. The AFRL consists of 12 facilities for testing full-size rocket engines, engine components, and liquid and solid propellants. The historical release of contaminants from rocket test stands, evaporation ponds, burn pits, catch basins, and leaking waste-collection tanks has contaminated groundwater in the AFRL. Groundwater aquifers near the AFRL are mostly restricted to fractured granitic bedrock, but previous studies indicate that groundwater and associated contaminants have moved into alluvium to the north and northwest. The U.S. Geological Survey (USGS) and the U.S. Air Force entered into a cooperative agreement to refine the understanding of the bedrock-alluvial aquifer transition zone downgradient from the AFRL. As part of that effort, surface geophysical data were collected to: (1) assess changes in the depth to bedrock with increasing distance from the AFRL; (2) to provide information on shallow geologic structures near the AFRL; and (3) to assess the presence of any faults that could present partial barriers to groundwater flow. The surface geophysical methods collected northwest of the AFRL in 2018 were electrical resistivity tomography (ERT), horizontal-to-vertical spectral ratio (HVSR) passive seismic, and time-domain electromagnetic (TEM).