REDEFINING THE HYDROSTRATIGRAPHY OF THE EDWARDS AND TRINITY AQUIFERS IN THE BALCONES FAULT ZONE, HAYS AND TRAVIS COUNTIES, CENTRAL TEXAS

Recent studies suggest some units of the Upper and Lower Glen Rose limestones serve as an aquitard separating the Edwards and Middle Trinity Aquifers in the Balcones Fault Zone (BFZ) of Hays and Travis Counties, central Texas. Furthermore, those studies indicate that the uppermost portion of the Upper Glen Rose is in hydrologic communication with, and part of, the Edwards Aquifer. Evidence for hydrologic communication between the Edwards and uppermost Upper Glen Rose, and hydrologic separation of the Edwards and Middle Trinity Aquifers by units of the Upper and Lower Glen Rose, is seen in geochemical, permeability, and potentiometric data from two multiport monitor wells, three Edwards/Middle Trinity well pairs, and other wells in the area.

Potentiometric data from three well pairs and a multiport well show that potentiometric elevations are consistently higher in the Edwards Aquifer than in the Middle Trinity Aquifer. Some of these head differences are as much as 80 ft. Potentiometric surfaces for the Edwards and Middle Trinity Aquifers, constructed based on data from semi-synoptic water-level measurements, also show that the pattern of consistently higher heads in the Edwards is present over the western portion of the BFZ. In the eastern portion of the BFZ, another multiport well indicates that during wet periods, heads in the Edwards Aquifer are higher than in the Middle Trinity Aquifer, but the heads reverse during periods of drought. Heads in some low permeability units of the Upper and Lower Glen Rose change less than 3 ft from wet to dry conditions while heads in the Edwards change more than 40 ft and heads in the Middle Trinity change about 10 ft.

Potentiometric data, in addition to geochemical and permeability data indicate that there is hydrologic isolation between the Edwards and Middle Trinity Aquifers, and that the uppermost Upper Glen Rose is in communication with the Edwards Aquifer. Faults in the BFZ do not appear to provide any significant pathway for vertical flow between the Edwards and Middle Trinity Aquifers. Existing stratigraphic nomenclature, based on descriptions of these units in outcrop, is not adequate for identifying hydrogeologic units.