APPROACHES TO QUANTIFYING FAULT-CONTROLLED PERMEABILITY AND FLOW CHARACTERISTICS OF THE EDWARDS AND TRINITY AQUIFERS

The Edwards and Trinity Aquifers in south-central Texas are limestone-dominated systems where faults play an important role in determining locations for recharge, subsurface flow paths, and discharge of groundwater. As the primary water source for San Antonio, the Edwards Aquifer represents an important resource that must be better understood from a geologic and hydrologic perspective to provide the basis for sound decisions regarding water management. While much work has been done to better characterize the structural framework of the Edwards Aquifer system, efforts to date have focused primarily on assessing the large-scale aquifer architecture, including orientations and offsets of faults, and to a lesser extent details of fault zone and intrablock deformation that are important contributors to overall aquifer flow characteristics. Opportunities remain for a more complete integration of structural geology analyses with hydrologic studies. Integration of this type is important for careful planning of future studies to maximize understanding of these complex aquifer systems. A variety of approaches are available for quantifying fault controls on aquifer permeability and other parameters important for understanding fluid flow. Among these are (i) tracer tests using single- or multi-well configurations, caves, and springs, (ii) long-term monitoring of water levels, (iii) geochemical analyses of aquifer formations and water samples, (iv) integration of structural geometries (e.g., fault and fracture orientations) with past and present stress-state information, (v) field-scale tracer and aquifer pump tests across mapped faults and faults zones, and (vi) laboratory testing and experiments of fault zone rocks. In this presentation we discuss these approaches and provide case examples of studies that have been completed. We conclude with suggestions for future work that would further enhance our understanding of the hydrogeology of the Edwards and Trinity Aquifer systems.