Hydrogeology of Barton Springs, Austin, Texas

Senger, Rainer K.
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The major point of discharge of the Edwards aquifer between the Colorado River and the ground-water flow divide near Kyle, 15 miles south of Austin, is Barton Springs; it comprises five major springs. The Balcones Fault Zone southwest of Austin is the principal zone of recharge to the aquifer. Changes in water levels of wells in the area show good correlation with changes in spring discharge, indicating good interconnection. The potentiometric surface of the aquifer shows a shift from conditions of high flow to low flow. During low flow groundwater flow lines are concentrated in the eastern portion of the Balcones Fault Zone. Water-levels are also significantly lower. The water levels of wells in the Rollingwood area do not follow that pattern. Overall hydrologic parameters of the aquifer were estimated by applying recession curve analyses to hydrographs of the spring discharge and of water-level declines in the potentiometric surface throughout the aquifer. Additionally, a two-dimensional ground-water flow model was constructed for the northeastern part of the aquifer in order to simulate the observed water-level fluctuations in well 58-42-915. The average value of transmissivity inferred from the model agrees well with results based on the recession curve analysis. Storativity, however, differed by about one order of magnitude. The water chemistry of the springs varies also between high-flow and low-flow discharge. The concentrations of Na, Cl, SO4, and Sr increase during low flow, indicating influx from the 'bad-water' zone (water from downdip Edwards with 1000 ppm total dissolved solids or more). This inflow of water from the 'bad-water' zone during low flow is also documented by the water chemistry of well 58-50-216, approximately two miles south of Barton Springs; during dry periods there is a large increase in total dissolved solids in that well. Even though the chemical composition of Barton Springs changes with varying discharge, the general water chemistry in the Edwards limestone aquifer remains constant. The aquifer contains calcium-bicarbonate water that evolves to a sodium-sulphate water and then a sodium-chloride water as it moves dondip. In some locations, however, leakage from the Glen Rose Formation increases the sulphate and strontium concentrations. This leakage occurs along large displacements of faults, where the Edwards Formation is adjacent to the Glen Rose Formation. In addition, carbonate equilibria of selected samples from the aquifer, springs, and creeks were calculated. Creek water is saturated with respect to calcite and dolomite during conditions of approximate steady state flow. During floods after heavy rainfall the water chemistry of most of the creeks, except for Barton Creek, indicate undersaturation with respect to calcite and dolomite. Saturated spring water which occurs only during very high discharge could result because spring flow is sustained to a major part by saturated flood water from Barton Creek. The influx of highly saturated 'bad-water' appears to have little effect on the saturation state of water from Barton Springs.