Hypogene Processes in the Balcones Fault Zone Edwards Aquifer in South-Central Texas, a New Conceptual Model to Explain Aquifer Dynamics

The Balcones Fault Zone Edwards Aquifer (Edwards Aquifer) is one of the most important karst aquifer systems in the United States. It provides water to more than 1.7 million people and is used for agricultural, municipal, industrial, and recreational purposes. The Edwards Aquifer in south-central Texas extends 400 kilometers from the Rio Grande River near Del Rio, eastward to San Antonio and then northeastward through Austin. The aquifer ranges from 10 to 60 kilometers wide and extends to a depth of 1,200 meters. The aquifer is contained in the Cretaceous Edwards Group limestone and is 150 to 300 meters thick.

Most existing aquifer models are epigenic and implicitly or explicitly assume that circulating meteoric waters formed the Edwards Aquifer. The initial karstification is assumed to have occurred along paleokarst features that originated during an initial subaerial exposure. Such paleokarst features created zones of enhanced permeabilites that allowed the deep circulation of waters and the formation of the present aquifer.

However, epigenic processes do not adequately explain many features in the Edwards Aquifer. For example, large voids that are isolated from epigenetic processes formed deep in the aquifer by mixing corrosion. Acidic water is generated by oxidation of sulfides and intrusion of CO2 from below, and possibly from hydrocarbon sources. In addition, the Edwards Aquifer displays characteristics that are difficult to accommodate in epigenic models. Examples include the wide distribution of high permeabilities, extreme depth of circulation and the extremely high yields of most wells.

Recent data and new interpretations suggest hypogenic processes have contributed to the formation of the Edwards Aquifer (Klimchouk, 2007). Klimchouk concludes that rising waters from depth are important agents of karst development and this model explains many hitherto crypic features in the Edwards Aquifer.