Abstract

Faults of the Balcones fault system exert important controls on the groundwater hydrology of the Edwards and Trinity Aquifers, including the following: (i) faults juxtapose permeable and relatively impermeable hydrogeologic units, (ii) the normal fault system causes structural thinning of the Edwards and Trinity Aquifer strata, and (iii) faults provide potential pathways for infiltration of water into the groundwater systems and for lateral and vertical movement of groundwater. We present examples of these structural geologic controls on aquifer properties using data and observations from the Helotes 7.5-minute quadrangle and the Hidden Valley fault exposed in Canyon Lake Gorge. Geologic framework modeling of the Helotes quadrangle illustrates the strong potential for direct communication between the Edwards Group and Glen Rose Formation in this area. The 100+ m displacement of the Haby Crossing fault is responsible for dropping the Edwards Aquifer from hilltop exposures north of the fault to mostly buried (confined) on the south side of the fault. Consequently, the area designated as Edwards Aquifer recharge zone is at its narrowest in this part of the Balcones Fault Zone. The Hidden Valley fault has an estimated 60-70 m of throw (vertical component of displacement) along the approximately 800 m of Upper Glen Rose Limestone exposure at Canyon Lake Gorge. Water ponds on the fault zone in some places, sinks into the fault along other stretches, and discharges laterally from the fault zone in yet another. These examples of locally high permeability of an exhumed fault show the importance of map-scale faults for groundwater flow. They also cast doubt on interpretations that such faults would act as barriers to across-fault flow in cases where such faults juxtapose Glen Rose Limestone with Glen Rose Limestone or the more permeable Edwards Limestone with Glen Rose Limestone. High permeability zones under and near stream channels may serve as fast pathways of communication from the Trinity to Edwards Aquifers. Direct observations of rapid loss of stream flow into streambed caves in the Glen Rose Limestone during runoff events in areas where Trinity Aquifer wells have high salinity suggest that water chemistry must vary considerably over local areas in the Trinity Aquifer. Interpretations that rely solely on water chemistry would not reflect this rapid movement of young, low salinity groundwater moving from the Trinity to the Edwards Aquifer, raising questions about groundwater modeling based on geochemistry alone. Potentiometric contours suggest that there is lateral continuity of the Trinity to Edwards Aquifers and the potentiometric gradient is normal to the Haby Crossing fault. Tracer testing in north San Antonio also indicates that water can move rapidly across faults from the Trinity to Edwards Aquifer. Structural geologic analyses provide the basis for future, more detailed investigations of groundwater levels, multiwell pumping (drawdown) tests, tracer studies, and geochemical investigations. These analyses could provide new insight into potential groundwater communication between the Edwards and Trinity Aquifers in the Helotes Quadrangle and other nearby areas in northwest San Antonio.