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Description
Marine Corps Base Camp Pendleton located in the extreme northwestern corner of San Diego County, California relies solely on groundwater for domestic needs. The Las Flores basin is the smallest of the four groundwater basins on base with a drainage area of 26.6 square miles. Currently there are 4 production wells in use in Las Flores basin. The water is principally used to supply drinking, irrigation, and wash water on the base. A municipal wastewater treatment plant, is located in the basin. The plant provides secondary level treatment of effluent. The effluent is currently discharged to percolation ponds located near the plant at the narrow constriction near the eastern end of the alluvial basin, 3 miles from the coast. The Regional Water Quality Control Board issued a Cease and Desist Order to Camp Pendleton concerning the treatment plant, for violation of its effluent quality limitations. Because of the shallow groundwater in the area of the existing ponds, rebuilding and using those ponds would not be allowed under California law. Several alternatives were explored prior to this study. Those options included relocating the ponds to the lower and middle portions of the basin, construction of an ocean outfall, and construction of a pipeline to another basin. Unfortunately several factors conspired to prevent the use of each of these options. The last alternative to be explored was the use of injection wells to inject the treated effluent into the artesian aquifer near the coast. Currently about 390 acre-feet of groundwater is extracted from the basin each year. Two-thirds of this water is returned in the form of wastewater effluent percolation. Future plans call for extraction of 1,160 acre-feet per year and disposal of 785 acre-feet per year of wastewater. The current percolation ponds will be replaced by a series of injection wells about a thousand feet from the coast. The purpose of this study was to evaluate the potential of the aquifer to receive the effluent by injection into the coastal artesian aquifer in this basin. It includes examination of potential well and aquifer plugging mechanisms and potential changes in groundwater quality. The study included the completion of five deep wells, geophysical logging (electric, gamma and flow meter), injection testing, water sample analysis, a tidal influence survey, and numerical modeling. Based on this study it is concluded that wastewater disposal in this artesian aquifer is possible. The target aquifer is sandy and extends to about 800 or 900 feet bgs. An overlying fine-grained deposit acts as an aquiclude creating the artesian conditions. This deposit varies from about 350 feet thick at the coast and tapers to nothing about 1 mile inland from the coast. There is currently no evidence of seawater intrusion within the basin. This is likely due to the minimal current extractions and the existence of the fine-grained coast deposits. Finite-element modeling indicates that the future extraction and disposal will result in minimal changes in water quality at the camp supply wells.
