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

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Principal Investigators
David A. Ferrill
Darrell W. Sims
Deborah J. Waiting
Alan P. Morris
Nathan M. Franklin
Ronald T. Green
John A. Stamatakos

Inclusive Dates: 10/01/00 - 04/01/02

Background - Water for the city of San Antonio and the surrounding area comes from wells pumping groundwater from the Edwards Aquifer and, recently, the Trinity Aquifer. San Antonio residents are well aware of growing concerns about the quantity and quality of water available, and potential restrictions resulting from competing demands on this resource by agricultural, residential, and industrial users. San Antonio is the second fastest growing major city in the United States, and recent acceleration in development in the Edwards Aquifer recharge zone and the up-gradient catchment areas is having detrimental effects on the quantity and quality of water being recharged. Groundwater recharge and flow in the Edwards Aquifer are controlled by the geometry of the faulted Edwards Aquifer and adjacent strata. Water enters the aquifer (recharges) by means of solution features controlled by faults and fractures exposed at the Earth's surface. Flow through the aquifer is accommodated by faults, fractures, and solution conduits below the surface. Both the recharge and flow capabilities have been enhanced by dissolution of the Edwards limestone by natural rain and ground water. The dissolution process has been most efficient in areas of high fault and fracture density. Thus, there is a strong linkage between geologic structure and rapid pathways for ground water recharge and flow in the Edwards Aquifer. The geometry and porosity and permeability architecture of the exposed (recharge zone) and confined (artesian) parts of the Edwards Aquifer are strongly influenced by faulting of the Balcones fault system over the last 30 million years. This deformation controls both large- and small-scale geometry of the aquifer, location of flow barriers and pathways, and ease of flow along flow paths.

Approach - In this project, the research team evaluated the distribution and characteristics of fault block deformation features in a selected area of the Edwards Aquifer Recharge Zone. The team analyzed mapped faults in a representative part of the Edwards Aquifer Recharge Zone in the San Antonio area using the CNWRA-developed 3DStress™ program to 1) interpret the ancient stress field that produced the primary faulting in the Balcones fault system, 2) identify mapped large faults that are in anomalous orientations and that may have been mismapped, 3) identify faults that appear to transfer displacement between larger faults and may be indicative of locally intense deformation, and 4) calculate estimated slip directions on faults and use the calculated slip directions to predict areas of increased potential deformation. Using existing mapping, well data, and new site-specific data, the team developed an EarthVision™ three-dimensional (3D) computer model of a selected portion of the recharge zone. The model provides a 3D representation of the fault system and fault block geometry and serves as a framework for geometric kinematic assessment of deformation within the fault blocks. This 3D modeling uses a new fault-block deformation analysis technique developed at the CNWRA. Based on this 3D modeling, including calculated fault slip directions from the 3DStress™ analysis, expected fault block strains throughout the 3D model were computed. Field observations of the natural deformation style were used in the modeled area of the recharge zone and surrounding areas to calibrate the computed deformation pattern. The result was a calibrated geometric-kinematic model of fault block deformation that can be applied to the entire Edwards Aquifer to help identify areas most likely to develop recharge features. These results are of interest to local water authorities and planning groups.

Accomplishments - This project resulted in two manuscripts for peer-reviewed journals, which summarize the work conducted. The first manuscript, titled "Structural Controls on the Edwards Aquifer Recharge Zone," is in review for the Geological Society of America Bulletin. The results include analyses at a wide range of scales of structural geological characteristics on the Edwards Aquifer in north San Antonio, Texas, that are likely to influence groundwater recharge and subsurface flow. The second manuscript, titled "Dilational Normal Faults" was published in the February 2003 issue of the Journal of Structural Geology. This article provides an explanation for localized fault control on lateral groundwater flow paths, including the development of subhorizontal, fault-parallel dissolution channels in the Edwards Aquifer. During the project, the team interfaced with various organizations involved in management of the Edwards Aquifer, including the Edwards Aquifer Authority, San Antonio Water System, United States Geological Survey, and the Texas Bureau of Economic Geology. Each of these organizations provided important data and insight into the analysis of the Edwards Aquifer. The team has made technical presentations and given project updates at two Edwards Aquifer Technical Advisory Group meetings, a U.S. Geological Survey organized "Edwards-Trinity Fracture Workshop" and an EarthVision user's meeting. In addition, team members gave a presentation titled "Origin of Dilational Normal Faults and Implications for Fluid Flow and Mineralization" at the fall 2002 meeting of the American Geophysical Union.

In performing this work, the research team demonstrated application of 3DStress™ and new normal-fault-block deformation analysis techniques to aquifer characterization. These results also are transferable to the south and east of the Balcones fault system, where the Edwards Group limestones are important oil and gas reservoirs. As an interesting note, fault rock in one of the faults that the team studied in detail in Martin Marietta's Beckman quarry contains devolitalized oil, indicating migration and trapping of oil in the fault zone. Team members anticipate that the success of this project will enable SwRI to attract additional outside funding for related studies of the Edwards Aquifer, and of other fractured aquifers and reservoirs elsewhere in the world.

New insights into fault and fracture influences on recharge and subsurface permeability in the Edwards Aquifer have emerged by combining geologic framework modeling (a), field investigations (b), and 3DStress™ analysis (c).

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