AN EXAMPLE OF DETAILED GEOLOGIC FIELD MAPPING TO UNDERSTAND COMPLEX GEOLOGY AND HYDROGEOLOGY FOR PREDICTING CONTAMINANT MIGRATION, BOX CANYON LANDFILL, MARINE CORPS BASE CAMP PENDLETON, CALIFORNIA

The Box Canyon Landfill consists of 28 acres of municipal waste and minor industrial waste that were deposited from 1974 to 1984 to nearly fill a small canyon on Marine Corps Base Camp Pendleton, San Diego County (CA). Surface and groundwater from Box Canyon flow into the Santa Margarita River Valley, the primary drinking water source for the Marines stationed at Camp Pendleton. Although the landfill had been investigated using conventional techniques including drilling, soil sampling, groundwater sampling, and contaminant assessment, a more detailed geologic field study was performed to better predict contaminant migration. A detailed geologic map was prepared based on fundamental skills acquired in an introductory field mapping class, to more fully develop geologic and hydrogeologic conceptual models for the landfill. This mapping identified that Box Canyon lies within an area of complex structural deformation resulting from regional crustal extension. Newly identified normal faults in the landfill area have meters to tens of meters of offset, and include the Hillside fault, the Box Canyon fault, and a western segment of the Box Canyon fault. These faults are synthetic and antithetic normal faults related to a major detachment fault system that underlies the landfill at depth. Both the San Onofre Breccia and a Tertiary sand and gravel unit are genetically related to these linked normal faults. Movement along these faults and synorogenic deposition of the sediments into accommodation basins created separate and unique hydrogeologic systems. Groundwater in these systems, or compartments, occurs at dramatically different elevations, flows at different velocities and has striking geochemical contrasts. These observations are caused by differences in the hydraulic conductivity between the San Onofre Breccia and the unconsolidated alluvium, fault seal development along fault traces, and different mineral compositions of the units. Our basic geologic field mapping resulted in a much better understanding of the complex and sophisticated regional, stratigraphic, and three-dimensional hydrogeologic setting than did standard site investigation approaches, and ultimately provided key information on probable contaminant migration from the landfill.