Geologic Controls on Interaction Between the Edwards and Trinity Aquifers, Balcones Fault System, Texas

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

Geologic Controls on Interaction Between the Edwards and Trinity Aquifers, Balcones Fault System, Texas

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

Structural framework of the Edwards Aquifer recharge zone in south-central Texas

The Edwards Aquifer, the major source of water for many communities in central Texas, is threatened by population growth and development over its recharge zone. The location of the recharge and confined zones and the flow paths of the aquifer are controlled by the structure of and deformation processes within the Balcones fault system, a major system of predominantly down-to-the-southeast normal faults. We investigate the geologic structure of the Edwards Aquifer to assess the large-scale aquifer architecture, analyze fault offset and stratigraphic juxtaposition relationships, evaluate fault-zone deformation and dissolution and fault-system architecture, and investigate fault-block deformation and scaling of small-scale (intrablock) normal faults. Characterization of fault displacement shows a pattern of aquifer thinning that is likely to influence fault-block communication and flow paths. Flow-path constriction may be exacerbated by increased fault-segment connectivity associated with large fault displacements. Also, increased fault-zone deformation associated with larger-displacement faults is likely to further influence hydrologic properties. Overall, faulting is expected to produce strong permeability anisotropy such that maximum permeability is subhorizontal and parallel to fault-bedding intersections. At all scales, aquifer permeability is either unchanged or enhanced parallel to faults and in many cases decreased perpendicular to faults