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Geologic framework and hydrogeologic characteristics of the Edwards Aquifer recharge zone, Bexar County, Texas

Water-Resources Investigations Report 95-4030

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Abstract

In Bexar County, residential and commercial development on the Edwards aquifer recharge zone is increasing. The aquifer possibly can be contaminated by spills, leakage of hazardous materials, or runoff from the rapidly developing urban areas that surround, or are built on, the intensely faulted and fractured, karstic limestone outcrops characteristic of the recharge zone. Furthermore, some of the hydrogeologic subdivisions that compose the Edwards aquifer have greater effective porosity than others. The areas where the most porous subdivisions crop out might provide efficient avenues for contaminants to enter the aquifer. The Edwards aquifer recharge zone has relatively large permeability resulting, in part, from the development or redistribution of secondary porosity. Lithology, stratigraphy, diagenesis, and karstification account for the effective porosity and permeability in the Edwards aquifer outcrop. Karst features that greatly enhance effective porosity in the outcrop area include sinkholes and caves. Hydrogeologic subdivision VI, the Kirschberg evaporite member, appears to be the most porous and permeable subdivision within the Kainer Formation. Hydrogeologic subdivision III, the leached and collapsed members, undivided, is the most porous and permeable subdivision within the Person Formation. Hydrogeologic subdivision II, the cyclic and marine members,.undivided, is moderately permeable, with both fabric- and not-fabric-selective porosity. The faults in northern Bexar County are part of the Balcones fault zone. Although most of the faults in this area trend northeast, a smaller set of cross-faults trend northwest. Generally, the faults are en echelon and normal, with the downthrown blocks typically toward the coast.

Additional Publication Details

Publication type:
Report
Publication Subtype:
USGS Numbered Series
Title:
Geologic framework and hydrogeologic characteristics of the Edwards Aquifer recharge zone, Bexar County, Texas
Series title:
Water-Resources Investigations Report
Series number:
95-4030
Edition:
-
Year Published:
1995
Language:
ENGLISH
Publisher:
U.S. Geological Survey ; U.S.G.S. Earth Science Information Center, Open-File Reports Section [distributor],
Description:
iii, 8 p. :maps (1 col.) ;28 cm.