From Bureau of Economic Geology, The University of Texas at Austin (
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Bureau Seminar, April 16, 2004

Hydrochemical Variability in the Plateau Aquifer System
Edwards Plateau, Texas

Seay Nance


.The Edwards-Trinity (Plateau) aquifer system extends over 24,000 sq mi (62,000 sq km) in a sub-humid to semi-arid Edwards Plateau region of west-central and western Texas. Over most of its area the Plateau system is composed of two mineralogically and hydrochemically distinct aquifer intervals: a lower interval of hydraulically confined Antlers Formation (Trinity Group) quartz-dominated conglomerate and sandstone with thin limestone interbeds, and an upper interval of generally unconfined Fredericksburg/Washita Division (approx. "Edwards") carbonates composed of limestone, dolomite, and minor gypsum. The system is stratigraphically bounded at its base by Paleozoic and Lower Mesozoic strata, and at its top by Neogene-to-recent siliciclastic sedimentary rocks and cover sands. Rainfall provides most recharge to the system by percolation through the Edwards and Ogallala intervals. Hydrochemical "species" (TDS, pH, ions, etc) and hydrochemical relationships (ionic ratios, etc) show systematic distributions in the Plateau aquifer system. Dissolved ionic species originate from meteoric recharge, water-rock interactions with siliciclastics, carbonates, and sulfates in the aquifer matrix, and upward-directed cross-formational flow.

This presentation presents several aspects of an ongoing dissertation project and focuses on observations from maps of inter- and intra-aquifer hydrochemical variations and some potential uses of the information to interpret aquifer dynamics. Issues to be addressed include end-member groundwater mixing, identification of preferred recharge areas, estimation of recharge volumes, and effects of possible cation-exchange reactions on Plateau aquifer hydrochemistry.