REFINEMENT OF THE MIDDLE TRINITY AQUIFER HYDROSTRATIGRAPHIC UNITS, SOUTH-CENTRAL TEXAS USING KEY SUBSURFACE DATA

The surface and subsurface geology of the middle Trinity aquifer in south-central Texas were studied to gain a better understanding of the hydrostratigraphy of the aquifer. A detailed study of two borehole cores obtained from wells drilled in 2001 at the Camp Stanley Storage Activity (CSSA) area near San Antonio, Texas were used to better characterize a number of hydrostratigraphic units of the middle Trinity aquifer. The usage of hydrostratigraphic terminology is inconsistent within the geologic community; there are different definitions for the term “hydrostratigraphic unit” (hsu), including: a part of an aquifer, a select part of a groundwater-availability or assessment model, or a geologic unit with distinct hydraulic properties. For this study, a hsu is defined as a body of rock with distinct hydrologic properties (such as porosity), and that is mappable over considerable distance. Two well sites, MW9-CC and MW5-LGR, were cored at the CSSA and logged by the USGS to depths of more than 470 and 450 feet, respectively. Close examination of the cores and geophysical logs has resulted in the designation of six distinct hsu’s (A through F) for the lower Glen Rose Limestone.