Hydrogeology of the Edwards Aquifer, Austin Area, Central Texas

The Edwards Formation, on the downthrown side of Mt. Bonnell fault in the Austin, Texas, area (Hays and Travis Counties), is part of the northeastern extension of the Edwards Underground Reservoir, the primary source of water in numerous counties along the Balcones Fault Zone. Recharge to the aquifer is supplied mainly by creeks that cross the Balcones Fault Zone southwest of Austin. Barton Springs is the major point of discharge. Changes in water levels of wells in the area correlate positively with changes in discharge at Barton Springs, suggesting good interconnection. The potentiometric surface of the aquifer changes significantly from high flow to low flow at Barton Springs. During low-flow conditions, ground-water flow lines converge in the eastern part of the Balcones Fault Zone. Water Ievels are also much lower (less than 30 m) and indicate flow from the "bad-water” zone (water with 1,000 mg/L TDS or more from downdip in the Edwards Formation).Water chemistry at Barton Springs also varies between high and low discharge. Concentrations of sodium, chlorine, sulfate, and strontium increase with decreasing discharge, indicating influx from the "bad-water" zone. This influx of highly saturated "bad-water” into the fresh-water aquifer theoretically results in a decrease in saturation state with respect to calcite and dolomite. The decrease in saturation state would enhance carbonate dissolution at the interface between fresh water and "bad-water" zones, thereby increasing permeabilities in this section of the aquifer. The Edwards aquifer generally contains a consistent calcium bicarbonate water. In some areas of the fresh-water section, however, leakage from the Glen Rose Formation increases the sulfate and strontium concentrations. Leakage occurs across fronts created by large displacements of faults that bring the Edwards Formation into contact with the Glen Rose Formation updip.