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Description
Aquifer characterization of the fractured crystalline rock aquifer along a segment of the Elsinore fault zone, in the vicinity of Agua Tibia Mountain, San Diego California, was conducted through fracture analysis, aquifer testing, air photographic interpretation, geochemical, and geophysical methods. Data from five outcrops and seven wells revealed two main clusters of fracture data: 1) a major set at about N71°E, 68°S, related to antithetic faulting and stress relief, and 2) a minor set at about N23°W, 71°W, related to the Elsinore fault zone. Photo-lineaments correlated well to fracture data. Geochemical analysis of 24 waters were separated into two groups based on the relative concentrations of anions and cations. High sodium waters are probably due to the processes of precipitation, dissolution, and cation exchange. Isotopic and geothermometric analyses indicates that these waters have not evolved through evaporation, nor are they influenced by thermal systems. Chemical and field data strongly suggest that calcite, and clays are the principal constitutes that fill fractures. The condition of plutonic rocks is highly altered due to weathering and intense fracturing, which increases clay formation. Hydraulic conductivities range from 4.9 ft/day (12.4 cm/day) to 7xl0-5 ft/day (1.78x10-4) and do not appear to be a function of plutonic rock type. The lowest values are associated with faults. The distribution of accumulated fractures thickness with depth is complex and highly fractured zones are encountered at several depths. There is no obvious decrease in relative fracture frequency with depth. The relationship between cumulative fracture thickness and hydraulic conductivity is poor, possibly stemming from mineral precipitation and poor hydraulic fracture connectivity. Simple fracture analysis may therefore be a misleading approach to estimate hydraulic conductivity. Cascading water, substantiated from aquifer and fracture analyses, indicates that shallow fractures are the main providers of water and suggests some decrease in hydraulic conductivity with depth. The water elevation contours seem to be controlled by topography, and indicate that the Elsinore fault is not a significant barrier to, nor a permeable pathway for, groundwater flow. VLF, magnetics, and dipole-dipole geophysical methods all detected the faults, which are associated with low hydraulic conductivity and low well yield. However, the geophysical methods were unable to differentiate between clean -and clay filled fractures. Wells on or near photo-lineaments appear to have higher values of hydraulic conductivity and well yield, though extreme heterogeneity could make a well miss the benefits of a fracture associated with a photo-lineament. VLF and magnetics correlated well for faults and moderately for fractures. VLF and dipole-dipole correlated well. Dipole-dipole is more sensitive and provides more information on structure and apparent dip than VLF. VLF is a more efficient method. The correlation between geophysical methods and photo-lineaments was moderate, in part due to the limited spatial overlap of the geophysical methods and the photo-lineaments. VLF had the best correlation. Some structures were detected through geophysical methods and substantiated by well bore fracture data, but were not manifested as photo-lineaments, possibly due to masking by Quaternary alluvial sediments.
