Flow in the San Antonio segment of the Edwards aquifer: matrix, fractures, or conduits?


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Publication Date

January 2000


Understanding water movement through fractured karst aquifers is difficult , but it is important for those dependent upon these resources. Much of central Texas depends primarily on the fractured and karstified San Antonio segment of the Edwards aquifer. The problem of scale dependent permeability makes interpretations of the aquifer difficult by limiting our ability to effectively utilize small-scale data for lar.ger scale problems. We used permeabilities measured at the small-(lab and outcrop), well-, and regional-scales to evaluate if small-scale data can be used to reproduce and interpret well-and regional-scale data. Small-scale data sets of matrix permeability, fracture aperture, and conduit size from cores and outcrops were utilized. Well-scale permeabilities were estimated from pump tests. Regional-scale permeabilities were estimated from numerical models. A modified layered aquifer model was used to calculate well-and regional-scale permeabilities from the small-scale data. Using an average regional hydraulic gradient to compute Reynolds numbers, the small-scale permeability data were used to predict the occurrence of non-linear laminar to turbulent flow. The results indicate that, in general, fractures control flow on the well-scale in the Edwards, and that many wells sample non-linear laminar to turbulent flow within the aquifer. The results also indicate that conduits are not the major contributors to well-scale permeability in the Edwards, but control regional scale permeabilities and have turbulent flow. Finally, the results indicate that pump tests would yield no measurable drawdown in approximately 15% of wells due to either fractures or conduits.