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Simulation of flow in the Edwards Aquifer, San Antonio region, Texas, and refinement of storage and flow concepts

Robert W. Maclay·Larry F. Land
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Abstract

The Edwards aquifer is a complexly faulted, carbonate aquifer lying within the Balcones fault zone of south-central Texas. The aquifer consists of thin- to massive-bedded limestone and dolomite, most of which is in the form of mudstones and wackestones. Well-developed secondary porosity has formed in association with former erosional surfaces within the carbonate rocks, within dolomitized-burrowed tidal and evaporitic deposits, and along inclined fractures to produce an aquifer with transmissivities greater than 100 ft2/s. The aquifer is recharged mainly by streamflow losses in the outcrop area of the Edwards aquifer and is discharged by major springs located at considerable distances, as much as 150 mi, from the areas of recharge and by wells. Ground-water flow within the Edwards aquifer of the San Antonio region was simulated to investigate concepts relating to the storage and flow characteristics. The concepts of major interest were the effects of barrier faults on flow direction, water levels, springflow, and storage within the aquifer. A general-purpose, finite-difference model, modified to provide the capability of representing barrier faults, was used to simulate ground-water flow and storage in the aquifer. The approach in model development was to conduct a series of simulations beginning with a simple representation of the aquifer framework and then proceeding to subsequent representations of increasing complexity. The simulations investigated the effects of complex geologic structures and of significant changes in transmissivity, anisotropy, and storage coefficient. Initial values of transmissivity, anisotropy, and storage coefficient were estimated based on concepts developed in previous studies. Results of the simulations confirmed the original estimates of transmissivity values (greater than 100 square feet/s) in the confined zone of the aquifer between San Antonio and Comal Springs. A storage coefficient of 0.05 in the unconfined zone of the aquifer produced the best simulation of water levels and springflow. A major interpretation resulting from the simulations is that two essentially independent areas of regional flow were identified in the west and central part of the study area. Flows from the two areas converge at Comal Springs. The directions of computed flux vectors reflected the presence of major barrier faults, which locally deflect patterns of ground-water movement. The most noticeable deflection is the convergence of flow through a geologic structural opening, the Knippa gap, in eastern Uvalde County. A second significant interpretation is that ground-water flow in northeastern Bexar, Comal, and Hays Counties is diverted by barrier faults toward San Marcos Springs, a regional discharge point. Simulations showed that several barrier faults in the northwestern part of the San Antonio area had a significant effect on storage, water levels, and springflow within the Edwards aquifer.

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Robert W. Maclay, Larry F. Land. 1988. Simulation of flow in the Edwards Aquifer, San Antonio region, Texas, and refinement of storage and flow concepts. https://doi.org/10.3133/wsp2336a

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