GEOHYDROLOGIC MAPPING OF THE EDWARDS AQUIFER RECHARGE ZONE IN KINNEY AND MEDINA COUNTIES, SOUTH-CENTRAL TEXAS

Cretaceous lithostratigraphic and hydrostratigraphic units in a 1558-km2 area (nine 7.5-minute quadrangles) of the Maverick Basin in south-central Texas were mapped at 1:24,000 scale to provide part of the three-dimensional framework for Edwards aquifer studies. The mapped rock units are composed chiefly of carbonate reefal detritus and back-reef carbonate muds that accumulated on the southern edge of the continental shelf during Albian and Cenomanian time (~112 to 94 Ma). The recharge zone of the Edwards aquifer, from base to top, consists of the West Nueces, McKnight, and Salmon Peak Formations. The units crop out in northern Kinney County and dip generally southward at 1-5 degrees to become a confined aquifer overlain by the Del Rio Clay and younger rock units.

Rock formations, sinkholes, exposed collapse structures, mafic igneous bosses and dikes, and east-northeast-trending normal faults are shown in the Balcones Fault Zone and adjacent areas north and south. The map provides information to help evaluate recharge potential, storage and flow-pathway development, and to aid ongoing 3-D geologic and ground-water modeling efforts. Pantea's work includes 3-D models of the recharge zone in a part of Medina County. The models utilize well data, helicopter electromagnetic surveys, and detailed geologic mapping. The models are interactive, rotatable computer 3-D images of plausible geologic interpretations.

Investor-backed development interests have publicized estimates of annual recharge of the aquifer in Kinney County that significantly differ from those of a local governmental agency which regulates the long-term usage of the aquifer. Differing interpretations of sustainable yield will need to be compared and debated to support approval/denial of specific aquifer-development plans. Ground-water modeling based on detailed geologic maps and hydrologic studies provides more objective estimates of aquifer storage, leading to more credible predictions of sustainable groundwater development.