UNRAVELLING THE COMPLEX GEOLOGY OF THE EDWARDS AQUIFER AND ADJACENT TRINITY AQUIFER OF SOUTH-CENTRAL TEXAS

A five-year USGS project, funded by the National Cooperative Geologic Mapping Program (NCGMP), has successfully used multidisciplinary science to help characterize the geologic framework of two important aquifer systems: (1) the Edwards aquifer extending from south of Austin to west of San Antonio and (2) the southern part of the Trinity aquifer in the Texas Hill Country west and south of Austin. The Edwards aquifer is one of the most productive carbonate aquifers in the United States and is the primary water source for San Antonio, the nation's 7th largest city. The southern part of the Trinity aquifer forms the catchment area for the Edwards aquifer and may contribute to the Edwards' water budget by subsurface flow across formation boundaries at considerable depths.

Digital geologic map compilations (e.g., USGS Special Investigations Map 2004-1430 and 2005-2873) and subsurface well data have provided the foundations for constructing 3-D EarthVision geologic models (e.g., USGS SI Report 2004-5226 and in press) of the Edwards aquifer recharge and catchment areas. Ground and airborne geophysical surveys (USGS Open-file Reports 2003-0226 and 2005-1158) have provided critical data on structures and properties of hydrologic units, magnitude of fault displacements, and degree of karst development. Edwards and Trinity aquifer flow paths and flow rates are known to be strongly influenced by geologic structures. One geophysical survey (USGS OFR 2002-0049 and in press) of western Medina and eastern Uvalde Counties identified a subsurface volcanic intrusive field that potentially affects both the flow paths and rates through western parts of the Edwards confined zone. In addition, noble gas geochemistry exploited helium-3/tritium as excellent ground-water tracers to determine the ages and origins of ground water along the Edwards saline-/fresh-water interface and in Uvalde County. A project synthesis with URLs to all project studies was recently published as USGS Fact Sheet FS 2006-3145 and is available online at URL: http://pubs.usgs.gov/fs/2006/3145/).