AN EVOLVING CONCEPTUAL MODEL OF THE MIDDLE TRINITY AQUIFER, HAYS COUNTY, CENTRAL TEXAS

The Trinity Aquifer is an important water supply of central Texas and provides baseflows to streams that recharge the Edwards Aquifer. Hydrostratigraphy and structure are vertically and laterally complex, producing highly heterogeneous and anisotropic aquifer properties. Previous conceptual models in Hays County do not fully characterize this system under high-magnitude pumping. We characterize the Middle Trinity as two interconnected aquifer zones: 1) Hill Country Middle Trinity to the west, and 2) Balcones Fault Zone (BFZ) Middle Trinity to the east.

The Hill Country Middle Trinity Aquifer zone has areas similar to the karstic Edwards Aquifer. Recharge occurs through discrete karst features and losing streams, and diffusely through permeable rock outcrop. Matrix, fracture, and karst permeability are all present, and natural discharge occurs at major springs. Aqueous geochemistry is variable and generally fresh. Water ages range from modern to relatively old (tritium present; 50-100% modern carbon). Where present and unfractured, the carbonate Hensel and localized portions of the Upper Glen Rose are semi-confining units. BFZ faults may act locally as hydrologic barriers, while relay-ramp structures provide lateral continuity of geologic units and groundwater flow eastward between the two zones.

The BFZ Middle Trinity Aquifer zone is deeply confined by faulting and contains matrix and fracture permeability, with karstic features observed in boreholes. Geochemistry is often dominated by evaporite mineralogy in the Glen Rose Fm., commonly brackish (=/> 1,000 mg/L), and older (no tritium; <50% modern carbon) groundwater. Natural areas of discharge for this deep water are unknown. Discharge may be diffuse deeper into the basin, or upwards along faults. Some lateral inter-aquifer flow likely occurs into the Edwards Aquifer, which includes the upper-most 200 ft of the Upper Glen Rose, and varies spatially and temporally. The Hensel is locally confining to the underlying Cow Creek. Portions of the lower-most Upper Glen Rose and upper-most Lower Glen Rose contain evaporite minerals and are confining units between the Middle Trinity and the overlying Edwards Aquifer.

Ongoing studies will help refine this conceptual model critical to the development of a numerical model and ultimately management of the resource.