Improving the Conceptual Model for Flow in the Edwards Aquifer

Susan D. Hovorka, Geary M. Schindel, Jean-Philippe Nicot, Steven R. H. Worthington, Steven B. Johnson, Adrien Lindley
Bureau of Economic Geology, University of Texas at Austin, Texas 

Karstic dissolution in the recharge zone of the Edwards aquifer has a strong influence on landscape development, captured steamflow, and formation of numerous caves in the unconfined aquifer. However, the importance and relevance of dissolution in the deep confined aquifer has been poorly understood and the subject of debate. Within the confined part of the Edwards aquifer, recent investigations have documented the role of conduits to subsurface depths greater than 700 meters. Conduits are defined as features that focus flow along regionally interconnected large aperture openings.

We recognize conduits where they serve as localized high transmissivity drain systems that create troughs in the potentiometric surface and/or bring water, undersaturated with respect to calcite, deep into the aquifer. Travel times of several orders of magnitude faster than possible though the matrix porosity document conduit flow. Very large transmissivities can be quantified by analysis of well hydrographs. Structure has a strong influence on the location of conduits that document the significance of heterogeneitys in the creation of conduit flow, although chemical and thermal gradients also play a role in focusing dissolution. Using these tools, a conceptual model for the distribution of conduits in the confined aquifer has been created for use in numerical models. Key areas for future research include better understanding of the quantitative relationships between matrix and conduit storage.

AAPG Search and Discovery Article #90032©2004 GCAGS 54th Annual Convention, San Antonio, Texas, October 10-12, 2004