Structural Controls on the Edwards Aquifer Recharge Zone, 20-9223

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Principal Investigators
David A. Ferrill
Darrell W. Sims
Alan P. Morris (Consultant)
Deborah J. Waiting
Ronald T. Green
John A. Stamatakos

Inclusive Dates: 10/01/00 - Current

Background - San Antonio's water is produced from wells drilled into the Edwards Aquifer. The aquifer is recharged along surface exposure of faults and fractures in the Edwards limestone. Flow into and through the aquifer is along faults and fractures that have been enlarged by natural dissolution. Although faults are considered in regional flow models, there has been no detailed examination of how faults and fractures control recharge of the aquifer.

Approach - The research team is evaluating the distribution and characteristics of fault block deformation features in a selected area of the Edwards Aquifer recharge zone and using the acquired knowledge to develop a calibrated model for prediction of recharge characteristics in areas of limited data. The team is analyzing mapped faults in a representative part of the Edwards Aquifer recharge zone using the CNWRA-developed 3DStress™ program to 1) interpret the ancient stress field that produced the Balcones fault system, 2) identify mapped faults that are in anomalous orientations, 3) identify faults that appear to transfer displacement between larger faults and that may indicate locally intense deformation, and 4) calculate original slip directions on faults. Existing maps, well data, and new site-specific data are used to develop an Earthvision™ three-dimensional (3D) computer model depicting the fault and fault-block system of part of the recharge zone. The model will be used for a geometric-kinematic assessment of the fault blocks. The team will use the preceding analyses to produce a quantitative evaluation of fault block deformation. This work will include a fault-block deformation analysis using a technique recently developed at CNWRA. Using the CNWRA-developed technique the expected fault block strains throughout the 3D model will be computed. Field investigations to calibrate the computed deformation pattern are being conducted. The anticipated result will be a geometric-kinematic model of fault block deformation. The model and related project results can then be applied to the entire Edwards Aquifer to identify areas most likely to contain recharge features. This model will be of interest to organizations responsible for selecting areas where recharge is most important and organizations making decisions about future development in northern San Antonio.

Accomplishments - Accomplishments to date include the 1) development of a Geographic Information System (GIS) database, 2) preliminary 3DStress™ analysis, 3) inception of 3D geologic framework modeling, 4) preparation of analytical tools for fault block strain calculation, 5) field work to characterize fault deformation in selected localities, and 6) development of a mechanistic model for formation of dilational normal faults. The team has selected key horizons for geologic framework modeling and is preparing the geologic framework model. During field work in west Texas, the team discovered faults in limestone that provide excellent analogs for dilational faults seen or inferred in the Edwards Aquifer recharge zone. Analysis of these faults led to the preparation of a manuscript entitled "Dilational Normal Faults" which has recently been accepted for publication in the Journal of Structural Geology.

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