The Edwards aquifer is a unique regional feature of south-central Texas, and yet, no large-scale geologic map has been available that depicts its hydrostratigraphic features in a consistent manner. The complex geology of the recharge area, as defined by the Texas Commission on Environmental Quality (TCEQ), includes lithologic units assignable to the Lower Cretaceous Edwards Group, which is underlain by the Glen Rose Limestone (lower confining unit) and overlain by the Upper Cretaceous Del Rio Clay, Buda Limestone, and Eagle Ford and Austin Groups (upper confining units). The geology of the Edwards aquifer in the northeast part of the recharge area is characterized by the Kainer, Person and Georgetown Formations, which are subdivided into 8 informal hydrostratigraphic units. Hydrostratigraphic subdivision of the San Marcos Platform facies units helped show the effects these eight informal units have on flow paths and recharge potential. In the area west of San Antonio in Medina, Uvalde and Kinney Counties, significant facies changes exist across the Devils River Trend facies and into the Maverick Basin facies.
The central part of the recharge area (Devil River Trend facies) contains reefal facies lithologies of the undivided Devils River Formation and overlying Georgetown Formation. The western part of the recharge area (Maverick Basin facies) contains lithologies of deeper-water origin composed of the West Nueces, McKnight, and Salmon Peak Formations. For detailed stratigraphy of the Edwards Formation and equivalent stratigraphic units, click on Recharge Zone Geology.
In the project study area, the Edwards Group is juxtaposed against the older Glen Rose Limestone by faulting in the Balcones fault zone. The principal set of these extensional faults (faults in rocks along which there has been bed-parallel elongation) generally trends southwest to northeast and are linked by scattered cross-faults. Generally, the faults are en echelon (in step-like arrangement), normal (hanging wall has moved downward relative to foot wall), and high-angle (nearly vertical), with the downthrown blocks typically toward the southeast. This fault morphology has generally resulted in a progression from the older, lower part of the Glen Rose Limestone exposed in the northern part of the study area, to the younger, upper part of the Glen Rose Limestone and overlying Edwards Group rocks exposed in the Balcones fault zone to the south.
Map compilation efforts began in late FY-02 and a USGS 1:200K SI Map (U.S. Geological Survey Open-file Report 2004-1430) was released in late FY-05. This compilation has helped identify minor inconsistencies in fault portrayal and distribution of some units, contains an accurate San Marcos Platform/Devils River Trend transition zone, and is the first effort of its kind to digitally synthesize the geology of the Edwards recharge zone.
The Trinity aquifer forms the catchment area for the Edwards aquifer in the headwaters of streams that drain south across the Edwards Plateau, and the Trinity intercepts some surface flow above the Edwards recharge zone. General opinion is that the Trinity may also contribute to the Edwards aquifer by subsurface flow across formation boundaries. Geologic mapping of the western limits of the Edwards recharge zone and underlying Trinity aquifer rocks units is planned in FY-05 through FY-06 for substantial parts of Edwards, and Kinney Counties. This mapping will complement similar mapping by the Texas BEG for areas west of Austin.
The Edwards aquifer is structurally complex due to the numerous, normal, high-angle Balcones faults that trend on average to the northeast, are downthrown to the southeast, and linked by scattered cross-faults. Although general trends of the fault and joint patterns are known for this area, studies are needed that allow semi-quantitative input into detailed hydrologic models for the Edwards and Trinity aquifers (Sharp and Banner, 1997). Detailed mapping of Edwards fracture networks in FY-05 and -06 are providing insights into recharge vulnerability, localized areas of increased fracture porosity, and subsequent directional permeability variations of the aquifer system.
One of the ultimate goals of this subtask is to produce contamination assessment maps (sometimes referred to as vulnerability, susceptibility, and stack-unit maps) for critical parts of the Edwards recharge area. Previous GIS analyses of readily-available spatial datasets of Bexar County (see below) have produced vulnerability maps of the Edwards contributing and recharge zones. The five digital datasets used in previous vulneralbility models included: (1) hydrologic properties of Edwards Group hydrostratigraphic units, (2) structural features (faults/fractures), (3) karst features (caves/sinkholes), (4) slope analyses from 30-meter DEM surfaces, and (5) soil characteristics. Using these 5 parameters, infiltration-potential modeling of the Edwards recharge area in Comal County produce composite values that range from 0-85. These ratings are based on the effect each parameter has on precipitation/runoff entering the Edwards ground-water system.
An infiltration-potential assessment model for Comal County, an area containing critical springs and enormous population growth, includes a 10-meter DEM grid and new hydrostratigraphic digital datasets (see below). This Arc GIS model is based on the effect each feature has on precipitation/runoff entering the groundwater system. The addition of a 10-meter DEM grid allows smaller cells in the matrix to appear whereas the same polygons are missing in the 30-meter grid. Faults and fractures tend to be concentrated in the more brittle hydrostratigraphic units, particularly the dolomitic and grainstone members, and the addition of a fracture density parameter shows certain areas in the southern part of the county to have higher infiltration potential. Incorporation of basin accumulation indices, which calculates the infiltration potential of contributing (drainage) areas to slope, as well as up-to-date vegetation coverages, is included in the model.
U.S. Department of the Interior | U.S. Geological Survey
URL: http://esp.cr.usgs.gov/info/edwards/mapping.html
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Page Last Modified: Wed 12-Oct-2005 9:57:28 MDT
