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Geologic Framework and Hydrogeologic Characteristics of the Edwards Aquifer Outcrop, Hays County, Texas

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GEOLOGIC FRAMEWORK AND HYDROGEOLOGIC CHARACTERISTICS OF THE EDWARDS AQUIFER OUTCROP, HAYS COUNTY, TEXAS U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 95-4265 Prepared in cooperation with the EDWARDS UNDERGROUND WATER DISTRICT Blank Page GEOLOGIC FRAMEWORK AND HYDROGEOLOGIC CHARACTERISTICS OF THE EDWARDS AQUIFER OUTCROP, HAYS COUNTY, TEXAS By John A. Hanson and Ted A. Small U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 95–4265 Prepared in cooperation with the EDWARDS UNDERGROUND WATER DISTRICT Austin, Texas 1995 U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY Gordon P. Eaton, Acting Director Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. For additional information write to: Copies of this report can be purchased from: District Chief U.S. Geological Survey U.S. Geological Survey Branch of Information Services 8027 Cameron Rd. Box 25286 Austin, TX 78754–4733 Denver, CO 80225–0286 ii CONTENTS Abstract ................................................................................................................................................................................ 1 Introduction .......................................................................................................................................................................... 1 Methods of Investigation .......................................................................................................................................... 3 Acknowledgments .................................................................................................................................................... 3 Geologic Framework ............................................................................................................................................................ 5 General Features ....................................................................................................................................................... 5 Stratigraphy ............................................................................................................................................................... 5 Hydrogeologic Characteristics ............................................................................................................................................. 7 General Features ....................................................................................................................................................... 7 Porosity and Permeability ......................................................................................................................................... 8 Summary .............................................................................................................................................................................. 9 References Cited ................................................................................................................................................................... 9 PLATE [Plate is in pocket] 1. Map showing hydrogeologic subdivisions of the Edwards aquifer outcrop, Hays County, Texas FIGURE 1. Map showing location of the study area ............................................................................................................... 2 TABLE 1. Summary of the lithologic and hydrologic properties of the hydrogeologic subdivisions of the Edwards aquifer outcrop, Hays County, Texas ................................................................................................................... 4 CONTENTS iii Blank Page Geologic Framework and Hydrogeologic Characteristics of the Edwards Aquifer Outcrop, Hays County, Texas By John A. Hanson and Ted A. Small Abstract INTRODUCTION All of the hydrogeologic subdivisions within Located in the Lower Cretaceous Kainer and the Edwards aquifer outcrop in Hays County have Person Formations of the Edwards Group (Rose, 1972) some porosity and permeability. The most porous and the overlying Georgetown Formation, the Edwards and permeable appear to be hydrogeologic sub- aquifer is one of the most permeable and productive car- division VI, the Kirschberg evaporite member of bonate aquifers in the nation. The dissolution-modified, faulted limestone aquifer (Buszka and others, 1990) is the Kainer Formation; hydrogeologic subdivision the sole source of public water supply for San Antonio III, the leached and collapsed members, undivided; and is the major source of water for Hays County. and hydrogeologic subdivision II, the cyclic and Most recharge to the Edwards aquifer occurs in marine members, undivided, of the Person Forma- counties immediately west of Bexar County (fig. 1). tion. The two types of porosity in the Edwards Streams cross Edwards aquifer outcrops (the recharge aquifer outcrop are fabric selective, which is zone) in the Balcones fault zone and lose much, if not related to depositional or diagenetic elements and all, of their flow to faults, fractures, sinkholes, and typically exists in specific stratigraphic horizons; caves in the outcrop. After entering the aquifer, the and not fabric selective, which can exist in any water moves east to points of discharge in Bexar County lithostratigraphic horizon. Permeability, the capac- (mostly municipal wells) and then northeast, parallel or ity of porous rock to transmit water, depends on the almost parallel to the northeast-trending Balcones faults physical properties of the rock such as size, shape, into Comal and Hays Counties, where it is discharged and distribution of pores, and fissuring and dissolu- by wells and springs. tion. Additional recharge to the Edwards aquifer is from Edwards aquifer outcrops in the Balcones fault Two faults, San Marcos Springs and zone in northern Bexar County and southern Comal and Mustang Branch, completely, or almost com- Hays Counties. The rugged, scenic limestone hills of the pletely, offset the Edwards aquifer by juxtaposing Edwards aquifer outcrops are the site of rapidly Edwards aquifer limestone against nearly imper- encroaching residential and commercial development. meable upper confining units along parts of their Kipp and others (1993, p. 1) report a substantial traces across Hays County. These faults are increase in development activities over the Edwards thought to be barriers, or partial barriers, to ground- recharge zone during 1992–93. They also note that water flow where the beds are juxtaposed. increased development brings a greater threat of con- tamination to the Edwards aquifer. The aquifer could be In Hays County, the Edwards aquifer proba- contaminated from spills or leakage of hazardous mate- bly is most vulnerable to surface contamination in rials; or runoff from the rapidly developing urban areas the rapidly urbanizing areas on the Edwards aquifer that surround, or are built on, the intensely faulted and outcrop. Contamination can result from spills or fractured, karstic limestone outcrops characteristic of leakage of hazardous materials; or runoff on the the recharge zone. Furthermore, some of the hydrogeo- intensely faulted and fractured, karstic limestone logic subdivisions that compose the Edwards aquifer outcrops characteristic of the recharge zone. have greater effective porosity and permeability than Abstract 1 98o 07’30" HAYS COUNTY o 30 15’ rton Ba C re 98o ek 98o 15’ Cr eek n io n O 97 o52’30" 30o 07’30" C EdwardsPlateau y Cre pr e k ess C O re io n NC e On o A k 94 45’ L B R I V E R 30o Plain R San Marcos Platform E V Base modified from University of Texas, I Bureau of Economic Geology, 1979, R 1981a, b, 1983 Coastal O Purga C to ry N C A ree L o k 29 52’30" B San Marcos Gulf EXPLANATION STUDY AREA EDWARDS AQUIFER OUTCROP 0 5 10 MILES TEXAS San Antonio HAYS COUNTY NE O COMAL Z COUNTY T L U BEXAR A COUNTY F S E N C O BAL Figure 1. Location of the study area. 2 Geologic Framework and Hydrogeologic Characteristics of the Edwards Aquifer Outcrop, Hays County, Texas others, and in areas where they crop out, might provide member of the Kainer Formation, which caps several efficient avenues for contaminants to enter the aquifer. hills in southwestern, north-central, and northeastern According to Buszka (1987, p. 2), "Carbonate aquifers Hays County, can be identified on aerial photographs by such as the Edwards are readily susceptible to ground- a pattern of concentric rings of sparse vegetation grow- water contamination where the presence of pollutants ing on the differentially weathered limestone. The coincides with the outcrop of the aquifer." In Hays regional dense member of the Person Formation can be County, the Edwards aquifer probably is most vulnera- ble to surface contamination in the rapidly urbanizing recognized on aerial photographs as small, light-to- areas on the Edwards aquifer outcrop. almost-white areas. The U.S. Geological Survey, in cooperation with Well logs and geologic map data were compiled the Edwards Underground Water District, mapped the and used in mapping the hydrogeologic subdivisions Edwards aquifer outcrop and described its hydrogeo- of the Edwards aquifer in the study area. The thick- logic characteristics (porosity and permeability) to doc- nesses of the hydrogeologic subdivisions that compose ument conditions pertinent to movement and the Edwards aquifer were determined from well logs in contamination of ground water. This report describes and adjacent to the aquifer outcrop in Hays County. the
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