Conduits and Turbulent Flow in the Edwards Aquifer

Author Steve Worthington
Year 2003
Description Description of the Edwards Aquifer Balcones Fault Zone as a typical karst aquifer and recommendations for locations of conduits in the MODFLOW model
Publisher Worthington Groundwater
Location Edwards Aquifer, Balcones Fault Zone
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The purpose of this study was to address the existence of aquifer-scale conduit networks in the Edwards Aquifer and how they could be incorporated in the model of the aquifer constructed by the U.S.G.S. The study was completed in April 2004 by Stephen Worthington of Worthington Groundwater, Toronto, Ontario, Canada. (Report is dated December 2003.)

This report has recommendations for locations of conduits in the Edwards Aquifer Balcones Fault Zone based on tests conducted from a literature review.

[Excerpts from the report:]
Over the period 2000-2003 the US Geological Survey developed a new numerical model to simulate flow in the San Antonio segment of the Edwards Aquifer. In the early stages of this process the question arose as how to best incorporate into the model the high permeability found in some areas of the aquifer. It has generally been recognized that the high permeability in the Edwards Aquifer is due to dissolution of the limestone bedrock (Maclay, 1995), and Halihan et al. (2000) concluded that “conduits … control regional-scale permeabilities and have turbulent flow” (Halihan et al., 2000, p. 129). If the location and characteristics of conduits could be either predicted on theoretical evidence or determined from aquifer measurements then this would offer the possibility of incorporating the conduits into the model and thereby produce a more accurate representation of the aquifer. This would be especially true if the conduits are organized into an aquifer-scale network. On the other hand the possibility was raised that conduits might only be local features, and that the aquifer might behave like a porous medium at a large scale. This raised three important questions:

a) What are the characteristics of aquifer-scale conduit networks?

b) What tests can be applied to determine whether there are aquifer-scale conduit networks in the Edwards Aquifer?

c) where might such conduit networks be located and how could they be incorporated in the USGS numerical model?…

The median lag between rainfall and maximum water level was 60 days, but there is much variability….[There were] two distinct responses in the aquifer. A few wells in the recharge zone together with Comal and San Marcos Springs show a rapid and large response, with peak levels being reached within a few days of the storm. The peak responses in the springs are driven by high heads in the conduits that feed the springs, with the pressure pulse from the increased water levels in the recharge zone being transmitted quickly to the springs. Conversely, most wells show a much slower and smaller response, with peak water levels being attained after about two months. These wells are not located on conduits and their response is attenuated by the much lower permeability in the matrix and smaller fractures in the aquifer….

Comparison between the Edwards Aquifer and other karst aquifers shows that the scaling effect is similar (Figure 11). The implication is that the Edwards is similarly karstified to other karst aquifers and thus has a well-developed conduit network….

Each of the six tests provides evidence that flow in the Edwards Aquifer is more like an ideal karst aquifer than an ideal porous medium aquifer. This strongly supports the concept that there is an integrated network of conduits connecting the major sinking streams with the springs…

[The conclusion] recommended location of conduits for the USGS MODFLOW model.