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

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Geologic Framework and Hydrogeologic Characteristics of the Edwards Aquifer Outcrop, Comal County, Texas GEOLOGIC FRAMEWORK AND HYDROGEOLOGIC CHARACTERISTICS OF THE EDWARDS AQUIFER OUTCROP, COMAL COUNTY, TEXAS U.S. GEOLOGICAL SURVEY Prepared in cooperation with the EDWARDS UNDERGROUND WATER DISTRICT Cover photograph: One of the spring oritices at Coma! Springs discharging a combined flow of 336 cubic feet per second, March IYSl4. Photograph by Ted A. Small. GEOLOGIC FRAMEWORK AND HYDROGEOLOGIC CHARACTERISTICS OF THE EDWARDS AQUIFER OUTCROP, COMAL COUNTY, TEXAS By Ted A. Small and John A. Hanson U.S. GEOLOGICAL SURVEY Water-Resources Investigations 94-4117 Prepared in cooperation with the EDWARDS UNDERGROUND WATER DISTRICT Austin, Texas 1994 U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY Gordon P. Eaton, Director Any use of trade, product, or firm names is for descriptive purposes onfy and does not imply endorsement by the U.S. Government. For additional information write to: Copies of this report can be purchased from: U.S. Geological Survey Earth Science Information Center District Chief Open-File Reports Section U.S. Geological Survey Box 25286, Mail Stop 517 8011 Cameron Rd. Denver Federal Center Austin, TX 78754-3898 Denver, CO 80225-0046 II CONTENTS Abstract ................................................................................................................................................................................ 1 Introduction .. ... .. ....... ... .. ... .. ..... .. ... .. ................... ... ....... ......... ............ ........ .. .......... .. .......... .. ....... ........ .. ..... ....... .. ... .. ... .. ..... .... 1 Methods of Investigation .. ... .. .. ..... .. ..... ... .. .. ........ .. .. ........ .. .. ... .. ..... .. ........ .. .. ... ....... ..... .. ............... .. ..... .. ... .. .. ... ..... .... .. 2 Acknowledgments .................................................................................................................................................... 2 Geologic Framework . .... ... ..... .... ... .. ... .. .. ... .. ... ....... .. ... .. .. .......... .......... .. ............ ... ....... .. .......... .. ... .. ....... ..... ..... ..... ......... ... .. .. .. 5 General Features ........ .. ... ....... ..... .. .. ..... .......... .. ... ......... ... .. .. ........ .. ..... .. ... .. .......... .. .......... .. .. ..... ... .. .. ..... ... .. ..... .. ... .. .. .. 5 Stratigraphy............................................................................................................................................................... 5 Hydrogeologic Characteristics·............................................................................................................................................. 7 General Features . .. .. ... ....... .. ... ....... .. .......... ..... ............ .. ...................... .. ....... ............... .. ..... .. ........ ....... .. ... .............. .. .. 7 Porosity and Permeability .... .. ..... .... ... ....... .. ... .. ............... .. .. ... ............ .. .......... ....... ..... .. ..... ....... ..... ................. .. ... .. .. .. 7 Summary .............................................................................................................................................................................. 8 References Cited ... .. ... .. .. ... .. ... .. ..... .. ....... .......... .. ... .. ........................... .. ..... ............ ................. .. ................. .. ....... ... .. .............. 9 PLATE [Plate is in pocket] 1. Map showing hydrogeologic subdivisions of the Edwards aquifer outcrop, Comal County, Texas. FIGURE I. Map showing location of the study area ............................................................................................................... 3 TABLE 1. Summary of the lithologic and hydrologic properties of the hydrogeologic subdivisions of the Edwards aquifer outcrop, Co mal County, Texas .. .. .. ..... ....... ....... ................................ .. .. ........ ................... .............. ........... 4 CONVERSION FACTORS AND VERTICAL DATUM Multiply By To obtain foot (ft) 0.3048 meter foot per mile (ft/mi) 0.1894 meter per kilometer inch per year (in/yr) 25.4 millimeter per year mile (mi) 1.609 kilometer Sea level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929--a geodetic datum derived from a general adjustment of the first-order level nets of the United States and Canada, formerly called Sea Level Datum of 1929. CONTENTS Ill Geologic Framework and Hydrogeologic Characteristics of the Edwards Aquifer Outcrop, Carnal County, Texas By Ted A. Small and John A. Hanson Abstract limestone outcrops characteristic of the recharge zone. All of the hydrogeologic subdivisions within the Edwards aquifer outcrop in Comal INTRODUCTION County have some porosity and permeability. The most porous and permeable appear to be hydro­ The Edwards aquifer, located in the Lower Cre­ geologic subdivision VI, the Kirschberg evaporite taceous Kainer and Person Fonnations of the Edwards member of the Kainer Formation; hydrogeologic Group (Rose, 1972) and the overlying Georgetown Fonnation, is one of the most highly permeable and subdivision III, the leached and collapsed mem­ productive aquifers in Texas (Maclay and Small, bers, undivided; and hydrogeologic subdivision II, 1984). The dissolution-modified, faulted limestone the cyclic and marine members, undivided, of the aquifer (Buszka and others, 1990) is the sole source of Person Formation. The two types of porosity in the public-water supply for San Antonio (ninth largest city Edwards aquifer outcrop are fabric selective, in the United States) and is the major source of water which is related to depositional or diagenetic ele­ for Comal County. ments and typically exists in specific stratigraphic Most recharge to the Edwards aquifer is west of horizons; and not fabric selective, which can exist Bexar County (fig. 1). Rivers and rainfall runoff in nor­ in any lithostratigraphic horizon. Permeability, the mally dry streambeds cross Edwards aquifer outcrops capacity of porous rock to transmit water, depends (the recharge zone) in the Balcones fault zone and lose on the physical properties of the rock such as pore much, if not all, of their flow to faults, fractures, sink­ size, shape, distribution, fissuring, and dissolution. holes, and caves in the outcrop. After entering the aqui­ fer, the water moves east to points of discharge in Bexar Two faults, Comal Springs and Hueco County (mostly municipal wells) and then northeast, Springs, completely, or almost completely, offset parallel or almost parallel to the northeast -trending the Edwards aquifer along much of their respec­ Balcones faults into Comal and Hays Counties, where tive traces across Comal County. Porous and per­ it is discharged by wells and by springs. Additional meable Edwards aquifer limestone is juxtaposed recharge to the Edwards aquifer is from Edwards aqui­ against impermeable upper confining units in fer outcrops in the Balcones fault zone in northern these areas. These faults completely, or almost Bexar County and southern Co mal and Hays Counties. completely, offset the Edwards aquifer and are The rugged, scenic, limestone hills of the Edwards thought to be barriers or partial barriers to ground­ aquifer outcrops are the site of rapidly encroaching res­ water flow where the beds are juxtaposed. idential and commercial development. The aquifer pos­ sibly can be contaminated by spills, leakage of In Comal County, the Edwards aquifer is hazardous materials, or runoff from the rapidly devel­ probably most vulnerable to surface contamina­ oping urban areas that surround. or are built on, the tion in the rapidly urbanizing areas on the Edwards intensely faulted and fractured, karstic limestone out­ aquifer outcrop. Possible contamination can result crops characteristic of the recharge zone. Furthennore, from spills, leakage of hazardous materials, or run­ some of the hydrogeologic subdivisions that compose off onto the intensely faulted and fractured, karstic the Edwards aquifer are inherently more porous than Abstract 1 others, and the areas where the most porous subdivi­ centric rings formed by the sparse vegetation growing sions crop out are susceptible to transmitting contami­ on the differentially weathered limestone. The regional nants into the aquifer. According to Buszka (1987, p. dense member of the Person Formation can be recog­ 2), "carbonate aquifers, such as the Edwards, are nized on aerial photographs by small, light to almost readily susceptible to ground-water contamination white areas. where the presence of pollutants coincides with the out­ crop of the aquifer." In Comal County, the Edwards Well logs and geologic map data were collected, aquifer probably is most vulnerable to surface contam­ compiled, and used in mapping the hydrogeology of ination in the rapidly urbanizing areas on the Edwards the study area. The thicknesses of the hydrogeologic aquifer outcrop. subdivisions that compose the Edwards aquifer were The U.S. Geological Survey, in cooperation with determined from well logs in and adjacent to the aqui­ the Edwards Underground Water District, mapped the fer outcrop in Comal County. The upper member of the Edwards aquifer outcrop and described its hydrogeo­ Lower Cretaceous Glen Rose Limestone, the lower logic characteristics (porosity and permeability) to doc­ confining unit (table 1), was mapped adjacent to the ument conditions pertinent to movement
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