Title: Bedrock geology and hydrostratigraphy of the Edwards and Trinity aquifers within the Driftwood and Wimberley 7.5-minute quadrangles, Hays and Comal Counties, Texas By Allan K. Clark and Robert R. Morris 2017 Scientific Investigations Map 3386 ABSTRACT: The Edwards and Trinity aquifers are major sources of water in south- central Texas and are both classified as major aquifers by the State of Texas. The population in Hays and Comal Counties is rapidly growing, increasing demands on the area’s water resources. To help effectively manage the water resources in the area, refined maps and descriptions of the geologic structures and hydrostratigraphic units of the aquifers are needed. This report presents the detailed 1:24,000 bedrock hydrostratigraphic map as well as names and descriptions of the geologic and hydrostratigraphic units of the Driftwood and Wimberley 7.5-minute quadrangles in Hays and Comal Counties, Tex. Hydrostratigraphically, the rocks exposed in the study area represent a section of the upper confining unit to the Edwards aquifer, the Edwards aquifer, the upper zone of the Trinity aquifer, and the middle zone of the Trinity aquifer. In the study area, the Edwards aquifer is composed of the Georgetown Formation and the rocks forming the Edwards Group. The Trinity aquifer is composed of the rocks forming the Trinity Group. The Edwards and Trinity aquifers are karstic with high secondary porosity along bedding and fractures. The Del Rio Clay is a confining unit above the Edwards aquifer and does not supply appreciable amounts of water to wells in the study area. The hydrologic connection between the Edwards and Trinity aquifers and the various hydrostratigraphic units is complex because the aquifer system is a combination of the original Cretaceous depositional environment, bioturbation, primary and secondary porosity, diagenesis, and fracturing of the area from Miocene faulting. All of these factors have resulted in development of modified porosity, permeability, and transmissivity within and between the aquifers. Faulting produced highly fractured areas which allowed for rapid infiltration of water and subsequently formed solutionally enhanced fractures, bedding planes, channels, and caves that are highly permeable and transmissive. The juxtaposition of the aquifers and hydrostratigraphic units due to of faulting has resulted in areas of interconnectedness between the Edwards and Trinity aquifers and the various hydrostratigraphic units that form the aquifers. DISCLAIMERS: This database, identified as SIM3386, has been approved for release by the U.S. Geological Survey (USGS). Although this database has been subjected to rigorous review and is substantially complete, the USGS reserves the right to revise the data pursuant to further analysis and review. Furthermore, it is released on condition that neither the USGS nor the United States Government may be held liable for any damages resulting from its authorized or unauthorized use. Although these data have been processed successfully on a computer system at the U.S. Geological Survey, no warranty, expressed or implied is made regarding the display or utility of the data on any other system, or for general or scientific purposes, nor shall the act of distribution constitute such warranty. The USGS or the U.S. Government shall not be held liable for improper or incorrect use of the data described and/or contained herein. Any use of trade, product or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this information product, for the most part, is in the public domain, it also may contain copyrighted material as noted in the text. Permission to reproduce copyrighted items for other than personal use must be secured from the copyright owner. REPORT FILES: The pamphlet and map for this report are located at and can be downloaded via the Web from at the U.S. Geological Survey publications website at https://doi.org/10.3133/sim3386 List of files: 00ReadMe.txt sim3386_pamphlet.pdf sim3386_pamphlet_table1.pdf sim3386_map.pdf DATA FILES: The database can be downloaded via the Web from ScienceBase http: https://doi.org/10.5066/F76D5RXQ ArcInfo export files, ArcView shapefiles, and an ArcGIS geodatabase may be extracted from a zip-compressed file (SIM3386GIS.zip). The database contact is: Diana E. Pedraza 210-691-9237 dpedraza@usgs.gov U.S. Geological Survey 5563 Dezavala Road San Antonio, TX 78249 The data were compiled from field mapping of the hydrostratigraphic units was accomplished on an Apple iPad2 containing geospatially registered 7.5-minute USGS topographic maps and utilizing integrated global positioning system (GPS). The field data were transferred to Esri ArcMap (Esri, 2014), quality checked, and then used to produce the study area’s hydrostratigraphic map. Lithologic and hydrostratigraphic descriptions are from field observations and previous investigations. This geospatial database consists of ArcInfo export files (.e00), ArcView shapefiles, and ArcGIS geodatabase files contained in a zip archive. See the list below. Please refer to the file SIM3386.met for detailed metadata documentation for this geospatial database. List of files: 00ReadMe.txt SIM3386_map.pdf SIM3386.met (.xml) bprhbasr_1983.tif bprhbasr_1983.tfw bprhbasr_1983.prj SIM3386_GIS.zip: shapefiles directory: Geo_Polys.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) polygon Geo_Lines.shp (.dbf/.prj/.sbn/.sbx/.shx.xml/.shx) line SIM3386_GIS.gdb directory (contains GIS data as geodatabase feature classes) HOW TO OBTAIN THE DIGITAL MAP AND PAMPHLET FILES: The map sheet and accompanying pamphlet can be obtained via the Internet from the U.S. Geological Survey publications website. Go to the web page at https://doi.org/10.3133/sim3386/ and follow the directions to download the files. The main product is a Portable Document Format (.pdf) map, which requires Adobe Acrobat for viewing. This Portable Document Format (PDF) file (sim3371_map.pdf) can be downloaded, from which paper copies may be printed. Acrobat software runs on a variety of systems, and is available for download free of charge from Adobe at http://www.adobe.com. Publication Reference: Clark, A.K., and Morris, R.R., 2017, Bedrock geology and hydrostratigraphy of the Edwards and Trinity aquifers within the Driftwood and Wimberley 7.5-minute quadrangles, Hays and Comal Counties, Texas: U.S. Geological Survey Scientific Investigations Map 3386, pamphlet ## p., 1 sheet, scale 1:24,000, https://doi.org/10.3133/sim3386. HOW TO OBTAIN THE DIGITAL GIS FILES: The digital files constituting the geologic map database of this report can be obtained via the Internet from ScienceBase: ScienceBase link: https://doi.org/10.5066/F76D5RXQ ScienceBase Reference: Clark, A.K., Pedraza, D.E., and Morris, R.R., 2017, Data release of the bedrock geology and hydrostratigraphy of the Edwards and Trinity aquifers within the Driftwood and Wimberley 7.5-minute quadrangles, Hays and Comal Counties, Texas: U.S. Geological Survey data release, https://doi.org/10.5066/F76D5RXQ. This map is offered as an online-only, digital publication. Users should be aware that, because of differences in rendering processes and pixel resolution, some slight distortion of scale may occur when viewing it on a computer screen or when printing it on an electronic plotter, even when it is viewed or printed at its intended publication scale. ArcReader may be downloaded free of charge from http://www.esri.com/software/arcgis/arcreader/download.htm for viewing additional files included in this report.