Permeability Structure of the Edwards Aquifer, South Texas: Implications for Aquifer Management

This report describes the three-dimensional distribution of hydraulic conductivity in the Edwards aquifer from Kinney to Comal Counties, Texas. Quantitative and interpretive assessment of the three-dimensional distribution of permeability in the prolific, complex, and heterogeneous Edwards fractured karstic aquifer leads to improved understanding of the dynamic behavior of the aquifer and identifies areas where information is lacking. The study area comprises the Kinney to Comal County section of the aquifer, which supplies the city of San Antonio, Comal and San Marcos Springs, and agriculture. Techniques used in this study include analysis of specific-capacity tests; reinterpretation of research-well results; stratigraphic and porosity interpretation of wireline logs; core, core-plug, and thin-section analysis; subsurface and outcrop mapping; and quantitative analysis of outcrop analogs of the aquifer. Permeability in the Edwards aquifer varies over eight orders of magnitude, and it is multimodal. Multimodal permeability distribution implies that the fastest moving water can travel many times faster than the largest volume of water. This distribution is important to aquifer-usage assessment involving drawdown, traveltimes, or effective porosity. Structurally influenced cave systems contribute the highest hydraulic conductivities (101 to 106 ft/d), solution-enhanced fractures and stratigraphically controlled karst contribute intermediate values, and the porous carbonate matrixcontributes hydraulic conductivities of 10-3 to10 1 ft/d. Because the interface between the aquifer and saline zone downdip is complex in three dimensions, the risk of water-quality reduction in times of lowered water level needs to be assessed separately ineach part of the aquifer. In the narrow north part of the aquifer, the interface is locally fault bounded. In the central part, the deepest penetration of fresh water may be related to high transmissivity on the San Marcos Platform edge. In the west part of the aquifer, structural influences on brine leakage complicate the fresh-saline interface. Permeability distribution is related to aquifer genesis. Dolomite alteration and dissolution driven by mixing of saline and fresh water have had a strong effect on permeability development in the confined aquifer. The unconfined aquifer in the north part of the study area has a two-order-of-magnitude lower average permeability than does the confined aquifer. The observed aquifer permeability distribution can improve assessments of various spring-flow scenarios. In the west part of the aquifer, geologic data suggest that permeability structure is related to multistage dissolution of evaporites. In this area, most permeability is developed in the fractured upper part of the Edwards Group above the evaporite section, accounting for steeper head gradients in this area. This west-to-east aquifer heterogeneity, which could cause variable responses to drawdown, may be relevant in any attempt to balance or regulate western agricultural or eastern municipal usage.
Susan D. Hovorka
Robert E. Mace
Edward W. Collins

Hovorka, S. D., Mace, R. E., and Collins, E. W., 1998, Permeability Structure of the Edwards Aquifer, South Texas: Implications for Aquifer Management: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 250, 55 p.

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The University of Texas at Austin, Bureau of Economic Geology
Report of Investigation

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