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

David A. Ferrill^,1, Darrell W. Sims^,1, Deborah J. Waiting^,1, Alan P. Morris^,2, Nathan M. Franklin^,3 and Alvin L. Schultz^,4

¹ CNWRA, Southwest Research Institute®, San Antonio, Texas, 78238-5166, USA
² Department of Earth and Environmental Science, University of Texas, San Antonio, Texas 78249, USA
³ CNWRA, Southwest Research Institute®, San Antonio, Texas, 78238-5166, USA
⁴ 1241 Milam Building, 115 East Travis Street, San Antonio, Texas 78205, USA

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.

Key Words: Edwards Aquifer • Edwards Group • Balcones fault zone • faults and faulting • normal faults

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