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Prepared Ih Cooperation with The GEOLOGIC FRAMEWORK AND HYDROGEOLOGIC CHARACTERISTICS OF THE EDWARDS AQUIFER OUTCROP (BARTON SPRINGS SEGMENT), NORTHEASTERN HAYS AND SOUTHWESTERN TRAVIS COUNTIES, TEXAS u.s. GEOLOGICAL SURVEY Water:Resources nve:stig~3.tiol ... Prepared ih cooperation with the" " BARTON SPRINGS/EDWARDS AQUIFER CONSERVATION DISTRICT and the '. TEXAS WATER DEVELOPMENT BOARD - ...- •....•- .. ---.--................_---_ .. --- GEOLOGIC FRAMEWORK AND HYDROGEOLOGIC CHARACTERISTICS OF THE EDWARDS AQUIFER OUTCROP (BARTON SPRINGS SEGMENT), NORTHEASTERN HAYS AND SOUTHWESTERN TRAVIS COUNTIES, TEXAS By Ted A. Small, John A. Hanson, and Nico M. Hauwert U.S. GEOLOGICAL SURVEY Water-Resources Investigations 96-4306 - . --......- .. .,-.'"~-.",':- ..... ~' .. _·.'\c. -,.~;-: . ,-. -."~ .:. ..,.- ..... -.~ Prepared in cooperation with the BARTON SPRINGS/EDWARDS AQUIFER CONSERVATION DISTRICT and the TEXAS WATER DEVELOPMENT BOARD Austin, Texas 1996 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 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 8011 Cameron Rd. Box 25286 Austin, TX 78754-3898 Denver, CO 80225--0286 ii CONTENTS Abstract ............................................................................................................................................................................... Introduction Methods of Investigation .......................................................................................................................................... 3 Acknowledgments .................................................................................................................................................... 5 Geologic Framework ......................................................................................................................................................... ... 5 General Features ..... ... ..... ............... ................................... ....... ................................................................................. 5 Stratigraphy...................... ............ ................................................ ........................... .................................................. 5 Hydrogeologic Characteristics ......................... .................................................................................................................... 9 General Features ........................... ..... .............. ....... ....................... ......... ....................... .... .... ...... ........... ... ............ ... 9 Porosity and Permeability ... ...................................................................................................................................... 9 Summary .............................................................................................................................................................................. II References Cited ................................................................................................................................................................... II PLATE (Plate is in pocket) l. Map showing hydrogeologic subdivisions of the Edwards aquifer outcrop (Barton Springs segment), northeastern Hays and southwestern Travis Counties, Texas FIGURES I. Map showing location of the study area ............................................................................................................... 2 2. Generalized hydrogeologic section A-A' of the Edwards aquifer (Barton Springs segment), northeastern Hays and southwestern Travis Counties, Texas .................................................................... ................................ 6 TABLES I. 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 ........... 4 2. Description of composite stratigraphic section of the Edwards aquifer outcrop (Barton Springs segment), Travis County, Texas .................................................................................................................. ........................... 13 VERTICAL DATUM Sea Level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929----a geodetic datum derived from a general adjust­ ment of the first-order level nets of the United States and Canada, formerly called Sea Level Datum of 1929. CONTENTS iii Geologic Framework and Hydrogeologic Characteristics of the Edwards Aquifer Outcrop (Barton Springs Segment), Northeastern Hays and Southwestern Travis Counties, Texas 1 2 By Ted A. Small1, John A. Hanson , and Nico M. Hauwert Abstract equivalent, the basal nodular member is used instead of the Walnut Formation for this report. The hydrogeologic subdivisions within the Essentially all of hydrogeologic subdivision II, Barton Springs segment of the Edwards aquifer which is about 70 feet thick in Hays County, is outcrop in northeastern Hays and southwestern missing in Travis County. Travis Counties generally are porous and perme­ In the Barton Springs segment of the able. The most porous and permeable appear to be Edwards aquifer, the aquifer probably is most vul­ hydrogeologic subdivision VI, the Kirschberg nerable to surface contamination in the rapidly evaporite member of the Kainer Formation; and urbanizing areas on the Edwards aquifer outcrop. hydrogeologic subdivision III, the leached and col­ Contamination can result from spills or leakage of lapsed members, undivided, of the Person Forma­ hazardous materials; or runoff on the intensely tion. Hydrogeologic subdivision II, the cyclic and faulted and fractured, karstic limestone outcrops marine members, undivided, of the Person Forma­ characteristic of the recharge zone. tion, also is quite porous and permeable in Hays County. The porosity of the rocks in the Edwards INTRODUCTION aquifer outcrop is related to depositional or diage­ The Barton Springs segment of the Edwards aqui­ netic elements along specific stratigraphic horizons fer (fig. I) comprises rocks of the Lower Cretaceous (fabric selective) and to dissolution and structural Kainer and Person Formations of the Edwards Group elements that can occur in any lithostratigraphic (Rose, 1972) and the overlying Georgetown Formation. horizon (not fabric selective). Permeability The Edwards aquifer is one of the most permeable and depends on the physical properties of the rock such productive carbonate aquifers in the Nation. The Barton 2 as pore size, shape, distribution, fissuring, dissolu­ Springs segment includes about 155 square miles (mi ), tion, and interconnection of pores and vugs. is hydrologically independent from the Edwards aquifer in the San Antonio area (Slade and others, 1985), and The Edwards aquifer rocks that crop out in like the Edwards aquifer in the San Antonio area, is a the Barton Springs segment of the Edwards aquifer dissolution-modified, faulted limestone aquifer (Buszka generally have the same lithologic characteristics and others, 1990). as the Edwards aquifer rocks that crop out in Comal The northern boundary ofthe study area is the and southwestern Hays Counties. However, in the Colorado River (fig. 1), and the southern boundary is northeastern part ofthe segment in Travis County, Lone Man Creek and the Blanco River. The Barton the rock unit that is apparently equivalent to the Springs segment of the Edwards aquifer is the source of basal nodular member of the Kainer Formation is water for Barton Springs and is the major source of called the Walnut Formation. Because the units water for more than 30,000 people in northeastern Hays appear to be stratigraphically and lithologically and southwestern Travis Counties (Slade and others, 1985). IU.s. Geological Survey. According to Senger and Kreitler (1984), 2Barton Springs/Edwards Aquifer Conservation District. recharge to the Edwards aquifer in the Barton Springs • Abstract EXPLANATION o STUDY AREA LZl EDWARDS AQUIFER OUTCROP 10 MILES I Figure 1. Location of the study area. 2 Geologic Framework and Hydrogeologic Characteristics of the Edwards Aquifer Outcrop (Barton Springs Segment), Northeastern Hays and Southwestern Travis Counties, Texas segment predominantly occurs along five major creeks: Methods of Investigation Barton, Williamson, Slaughter, Bear, and Onion (fig. I). These five major creeks and other smaller creeks and The hydrogeologic subdivisions (table I) of the streams cross the Edwards aquifer outcrop (the recharge Edwards aquifer modified from Maclay and Small zone) in the Ba1cones fault zone and lose much, ifnot (1976) and the stratigraphic nomenclature of Rose all, of their flow to faults, fractures, sinkholes, and caves (1972) for the Edwards Group on the San Marcos plat­ in the outcrop. After entering the aquifer, the water gen­ form (fig. I) were used to map the Edwards aquifer out­ erally moves northeast to points of discharge, predomi­ crop in the Barton Springs segment. The carbonate-rock nantly Barton Springs. classification system of Dunham (1962) was used for the lithologic descriptions. The sedimentary carbonate The rugged, scenic, limestone hills of the classification system of Choquette and Pray (1970) was Edwards aquifer outcrop, locally known
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