Regional Distribution of Permeability in the Edwards Aquifer
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Distribution of permeability in the Edwards aquifer of South Texas was quantified using aquifer tests, core and outcrop examination, geologic mapping, and wireline log analysis. Extensive cave systems, fractures and faults of the Balcones Fault Zone, and highly porous carbonate matrix are recognized as contributors to the multimodal porosity and permeability structure of the aquifer. Karst porosity of 1 to 3 percent was measured using image analysis of photomosaics of outcrops. Preferential dissolution of dolomitic facies, recognized both in outcrop and subsurface, controls the stratigraphic distribution of conduits. Normal faults can decrease cross-fault transmissivity by decreasing the thickness and continuity of permeable zones. Fracture zones adjacent to faults increase fault-parallel transmissivity. Relay ramps between en echelon faults maintain the continuity of the aquifer strata through faulted regions. Hydraulic conductivities based on an Edwards-specific relationship between specific capacity and transmissivity vary by nearly eight orders of magnitude, from 10-3 to 105 ft/day. Low values are typical or the Edwards outcrop, and high values are typical of the deepest parts of the confined aquifer. Matrix permeability calculated from porosity-permeability transforms overlaps only the "low" end of this range (10-3 to 10 ft/day) but shows the same trend of higher permeability in the deep part of the aquifer. The observed permeability distribution is interpreted to be the result of dolomite dissolution near the downdip limit of the aquifer because of saline and fresh water mixing.