Refining the Conceptual Model for Flow in the Edwards Aquifer—Characterizing the Role of Fractures and Conduits in the Balcones Fault Zone Segment

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Publication Date

January 2004

Abstract

The Balcones Fault Zone Edwards aquifer of South Texas exhibits multimodal permeability. High matrix porosity and permeability are overshadowed by high permeability developed in structurally influenced karstic conduit systems. High permeability is developed in the confined part of the aquifer at depths greater than 4,000 feet below sea level, as well as in the unconfined zone. Optimization of use of this heavily subscribed aquifer requires accurate quantification and realistic mapping of the relationships between the limestone matrix, which stores most of the water, and the conduit system, which transmits water into, through, and out of the aquifer. This balance between storage and drainage is a key variable needed for predicting sustainability of flow during periods of low recharge and heavy use. The purpose of this study is to interpret and integrate a selection of the existing diverse data to better characterize the conduit system. Data sets selected to feed into this interpretation include (1) water-level data, (2) structural information, (3) cave maps, (4) existing water-chemistry data, and (5) well hydrographs. It is intended that analysis of the existing data will supply hypotheses that will then be tested using field-based followup studies. These might include injected tracers; high frequency, closely spaced water level monitoring; conductivity monitoring; and well testing to better define flow systems. A regionally extensive system of high-permeability zones is defined by broad troughs in the potentiometric surface in the confined zone. Indications of connections of the confined aquifer to the recharge zone are weakly defined by troughs in the available water-level data. Intersection of the Edwards/Trinity merged potentiometric surface maps with base Edwards structure maps to show that the Edwards Formation is saturated over only a part of the Edwards outcrop zone. This fact suggests that recharge flows through the Edwards into the upper beds of the underlying Glen Rose Formation before returning to the

Keywords

Balcones Fault Zone, Edwards Aquifer, Permeability

Document Type

Article

Identifier

SFS0072602_00001

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