Effect of Vertical Aquifer Heterogeneity on the Efficiency of Aquifer Storage and Recovery Projects

摘要

The San Diego Formation is a Pliocene through Quaternary age aquifer occupying the southwestern-most part of San Diego County. Though the formation is widespread, thicknesses of up to 2600 ft exist only in the South San Diego Embayment, bounded on the east by the La Nacion and Sweetwater faults, on the south by the U.S./Mexico border, and the west by San Diego Bay and the Pacific Ocean, and on the north by Mission Valley and the San Diego River. Native water quality in the aquifer is poor, and the aquifer is vertically and laterally heterogeneous. Aquifer testing, combined with vertical borehole flowmeter measurements show that hydraulic conductivities in screened, permeable, intervals range from less than 1 ft/day to 250 ft/day, averaging 40 ft/day. These large variations in hydraulic conductivity occur over 5 ft intervals. Numerical simulation of seasonal aquifer storage and recovery (ASR) under a regional hydraulic gradient indicates that the measured vertical heterogeneity markedly effects the efficiency of ASR. Water quality in the injection/production well rapidly degrades when injection ceases and pumping starts for the case of vertically heterogeneous permeability. In contrast, if an average, homogeneous, permeability is assumed, the salinity of pumped water remains close to that of the injected water salinity for a significant time before breakthrough of the native saline water. These results demonstrate that detailed characterization of the vertical permeability distribution is critical to the evaluation of ASR projects.