INVESTIGATION OF GROUNDWATER FLOW TO WATER SUPPLY WELLS NEAR A CONTAMINATION PLUME: INSTALLATION RESTORATION SITE 1119, MARINE CORPS BASE CAMP PENDLETON, CALIFORNIA

Installation Restoration (IR) Site 1119 is a contaminated location identified for remedial action near the southern border of Marine Corps Base (MCB) MCB Camp Pendleton in the Upper Ysidora subbasin of the Santa Margarita River watershed in southern California. The boundaries of the site are defined as a plume of trichloroethylene (TCE) in groundwater exceeding the federal maximum contaminant level (MCL) of 5 micrograms per liter. The recent detection of and subsequent increase in TCE at a nearby production well has raised concerns regarding the future viability of this well as a drinking water source. The U.S. Geological Survey has entered into an agreement with Naval Facilities Engineering Systems Command (NAVFAC) to conduct a study utilizing multiple lines of investigation to gain a better understanding of groundwater flow and contaminant transport from IR Site 1119 to nearby production wells. The study approach consists of 1) collection of tracer pulse flow logs and depth-dependent water quality samples from production wells, 2) collection of borehole geophysical logs, vertical and horizontal point velocity measurements, and water quality samples from monitoring wells under different production well pumping scenarios, and 3) collection of Electrical Resistivity Tomography (ERT) survey data from the area between IR Site 1119 and nearby production wells. Data collection completed for this study as of June 2024 includes vertical and horizontal flow measurements from monitor wells, water quality samples from production and monitor wells, and two ERT survey lines. Preliminary results indicate that 1) groundwater flow directions and velocities in and near IR Site 1119 respond rapidly to changes in production well pumping configurations, 2) a laterally extensive low resistivity layer occurs at ~40-50 feet below land surface in the area between IR Site 1119 and the production wells, and 3) a strong gradient in basic water quality parameters exists between shallower and deeper portions of the aquifer in and near IR Site 1119 under both ambient and pumped conditions. Continued data collection and analysis will facilitate a more detailed understanding of groundwater flow and contaminant transport between IR Site 1119 and nearby production wells, which will enable optimization of production well operations to maintain a water supply within regulatory contaminant limits.