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Geologic Framework and Hydrogeologic Characteristics of the Edwards Aquifer Outcrop (Barton Springs Segment), Northeastern Hays and Southwestern Travis Counties, Texas

By Ted A. Small1 , John A. Hanson1 , and Nico M. Hauwert2

1 U.S. Geological Survey.
2 Barton Springs/Edwards Aquifer Conservation District.

U.S. Geological Survey
Water-Resources Investigations Report 964306

In cooperation with the Barton Springs/Edwards Aquifer Conservation District
and the Texas Water Development Board


Abstract

The hydrogeologic subdivisions within the Barton Springs segment of the Edwards aquifer outcrop in northeastern Hays and southwestern Travis Counties generally are porous and permeable. The most porous and permeable appear to be hydrogeologic subdivision VI, the Kirschberg evaporite member of the Kainer Formation; and hydrogeologic subdivision III, the leached and collapsed members, undivided, of the Person Formation. Hydrogeologic subdivision II, the cyclic and marine members, undivided, of the Person Formation, also is quite porous and permeable in Hays County. The porosity of the rocks in the Edwards aquifer outcrop is related to depositional or diagenetic elements along specific stratigraphic horizons (fabric selective) and to dissolution and structural elements that can occur in any lithostratigraphic horizon (not fabric selective). Permeability depends on the physical properties of the rock such as pore size, shape, distribution, fissuring, dissolution, and interconnection of pores and vugs.

The Edwards aquifer rocks that crop out in the Barton Springs segment of the Edwards aquifer generally have the same lithologic characteristics as the Edwards aquifer rocks that crop out in Comal and southwestern Hays Counties. However, in the northeastern part of the segment in Travis County, the rock unit that is apparently equivalent to the basal nodular member of the Kainer Formation is called the Walnut Formation. Because the units appear to be stratigraphically and lithologically equivalent, the basal nodular member is used instead of the Walnut Formation for this report. Essentially all of hydrogeologic subdivision II, which is about 70 feet thick in Hays County, is missing in Travis County.

In the Barton Springs segment of the Edwards aquifer, the aquifer probably is most vulnerable to surface contamination in the rapidly urbanizing areas on the Edwards aquifer outcrop. Contamination can result from spills or leakage of hazardous materials; or runoff on the intensely faulted and fractured, karstic limestone outcrops characteristic of the recharge zone.

Table of Contents

Abstract

Introduction

Methods of Investigation

Acknowledgments

Geologic Framework

General Features

Stratigraphy

Hydrogeologic Characteristics

General Features

Porosity and Permeability

Summary

References Cited

Plate

1.   Map showing hydrogeologic subdivisions of the Edwards aquifer outcrop (Barton Springs segment), northeastern Hays and southwestern Travis Counties, Texas

Figures

1.   Map showing location of the study area
2.   Generalized hydrogeologic section A–A' of the Edwards aquifer (Barton Springs segment), northeastern Hays and southwestern Travis Counties, Texas

Tables

1.   Summary of the lithologic and hydrologic properties of the hydrogeologic subdivisions of the Edwards aquifer outcrop (Barton Springs segment), northeastern Hays and southwestern Travis Counties, Texas
2.   Description of composite stratigraphic section of the Edwards aquifer outcrop (Barton Springs segment), Travis County, Texas

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U.S. Department of the Interior, U.S. Geological Survey
Persistent URL: http://pubs.water.usgs.gov/wri964306
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Last modified: Friday, October 06 2006, 04:21:10 PM
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