Multidisciplinary Studies of the Edwards Aquifer and Adjacent Trinity Aquifer of South-Central Texas

A five-year USGS project, funded primarily by the National Cooperative Geologic Mapping Program (NCGMP), has partnered with federal, state, and municipal agencies to conduct geologic mapping, hydrologic studies, and geophysical surveys of the Edwards and southern Trinity aquifers in south-central Texas. This multidisciplinary effort has successfully produced results (http://esp.cr.usgs.gov/info/edwards/index.html) to help characterize the geohydrologic framework of two critically important ground-water systems: (1) the Edwards aquifer extending from south of Austin to west of San Antonio and (2) the southern fringe of the Trinity aquifer in the Texas Hill Country west and south of Austin. Digital geologic map compilations and subsurface well data have provided the foundations for constructing 3-D EarthVision TM geologic models of the Edwards aquifer recharge and catchment areas. An airborne magnetic survey of western Medina and eastern Uvalde counties identified numerous subsurface volcanic intrusives that potentially affect ground-water pathways through the western portion of the Edwards aquifer. Ground and airborne geophysical surveys have provided critical data on fault morphology and displacement, potential areas of karst development, and the geohydrologic properties of water-bearing units. Noble gas geochemistry exploited helium-3/tritium as excellent ground-water tracers to determine the ages and origins of ground water in Uvalde County and along the Edwards saline-fresh-water interface. Additional geologic mapping of Kinney and Bexar Counties, infiltration potential modeling of Comal County, and geochronologic age dating of volcanic intrusives from Bracketville to Austin are near completion. A project synthesis with URLs to all project studies was recently released as USGS Fact Sheet available online at URL: (http://pubs.usgs.gov/fs/2006/3145).