THE IMPORTANCE OF FRACTURE CHARACTERIZATION FOR UNDERSTANDING RECHARGE, FLOW, AND CONTAMINANT TRANSPORT IN THE EDWARDS AQUIFER, SOUTH-CENTRAL TEXAS

The Edwards aquifer of south-central Texas, considered one of the most productive carbonate aquifers in the United States, lies within and adjacent to the Balcones fault zone. The geology of the recharge area is structurally complex because of the numerous, normal, high-angle faults that trend on average to the northeast, are downthrown to the southeast, and linked by scattered cross-faults trending southeast. The en echelon fault system contains many relay ramp structures, where varying extensional forces between faults have increased the porosity through fracturing. Mapping of fracture orientations assists in the determination of localized directional permeability in the near-surface and in the identification of structural features, such as cross-faults, relay ramps and other deformation, that may have generated ground-water flowpath anisotropy.

The lack of natural filtration and diffusion in fractured carbonate aquifers reduces the effective response for managing sources of contamination to the ground-water supply. The interconnected fractures are zones of high transmissivity, especially when numerous and concentrated in the down-gradient direction; e.g. through ramps and along cross-faults. The fracture networks strongly influence the direction of flow, and in many cases, the actual flow velocity within a single saturated fracture is an order of magnitude or more than that of the regionally-observed, mean flow rates. Many small fractures can maintain full saturation with laminar flow, while large diameter conduits may never reach capacity. These concepts identifying high velocity, laminar ground-water flow, would facilitate the application of Darcy's Law to quantifying hydrologic parameters and modeling high velocity, circuitous ground-water flow paths, either locally or regionally, in a fractured carbonate aquifer located within extensional tectonic settings. Detailed fracture mapping in the Edwards aquifer recharge area will help to track paths of contaminants by: 1) determining the direction of the fracture network trends to identify the initial areas most at risk, and 2) calculating fracture flow velocities in conjunction with mean flow velocities to determine, in 2-dimensions, the initial concentration plume of any known contaminant.