Abstract

The Edwards Aquifer is a karst aquifer that extends along the Balcones Escarpment through 10 counties from Bell County in the northeast to Kinney County in the southwest. The aquifer has become a focus of research due to the growing demands for quality water for the City of San Antonio, Texas and other industrial and agricultural users. Specifically, the structural architecture of the Balcones Fault Zone is being studied to locate environmentally sensitive recharge areas, understand ground water flow pathways and barriers within the Edwards Aquifer, and evaluate the potential for hydrologic communication between the Trinity and Edwards Aquifers.

A three-dimensional digital geologic framework model, as used in oil and gas exploration and production, is an ideal method to represent the structural architecture of the Edwards Aquifer Recharge Zone (Balcones Fault Zone). The area of the USGS 7.5′ (7.5 minute) Castle Hills Quadrangle was selected for the initial model as it includes the Edwards Aquifer unconfined (recharge) and confined zones. The construction of an accurate 3D geologic framework model requires sufficient qualified data to supply control points in the model area. Data are available for the selected quadrangle in the form of geologic maps, roadcuts, quarry exposures, and subsurface well data. Representatives of agencies concerned with the regulation and preservation of the Edwards Aquifer (Edwards Aquifer Authority, San Antonio Water System, U.S. Geological Survey, and the Bureau of Economic Geology) provided access to their most recent data collected in the model area, in exchange for shared access to study results.

We describe a generalized workflow for creating a 3D geologic framework model based on various surface and subsurface data sources. Much of the effort required for construction of an accurate 3D geologic framework model is devoted to the collection, qualification, and preparation of data. The amount of data required depends on the complexity within and size of the model area. In the case of our model, digital data for surface expressed features (faults, geology, and topography), as well as subsurface properties (depth and thickness of hydrogeologic units), are essential for a representative model and reliable sources must be determined. If necessary, critical assumptions are made when data are lacking and the basis for those assumptions is documented.

The resulting high-resolution 3D geologic framework model is a very useful tool for representing structural architecture. The model can be expanded by adding adjacent areas, and updated as new data come available. The model can be manipulated by rotating and tilting; stripped of layers to better view fault displacement, structural framework, and structural thinning; and sliced horizontally, vertically, or in bench cuts. Expansion of this model is anticipated to depict water flow paths and barriers within the Edwards Aquifer, and to provide responsible agencies with a tool to evaluate the most environmentally sensitive aquifer recharge areas, so they may be preserved and protected against contamination.