Geochemical evolution of ground water in the Barton Springs segment of the Edwards Aquifer

Abstract

The water quality in a karst (limestone) aquifer changes over time, making the application of traditional hydrogeologic principles difficult or impossible. This research's goal was to advance the understanding of the Barton Springs segment of the Edwards aquifer within and around Austin, Texas. This was accomplished by analyzing time-series water-quality data from long, medium, and short time scales. Analysis provided insights into direction of ground-water flow, sources of spring discharge, and mixing of geochemically distinct waters in the aquifer. The results of this research are of interest because of the aquifers role as a drinking water supply, its role as a habitat for the endangered Barton Springs salamander (Eurycea sosorum), and for its central role in creating the popular Barton Springs Pool. Twenty-six years of water-quality data were compared against contemporaneous streamflow and spring discharge rates to evaluate ground-water connection to surface-water processes. Fifteen of 26 wells in this dataset showed a correlation between these measurements. Ion ratios of Mg/Ca, SO₄/Cl, and Na/Ca showed that active ground-water processes included dilution by recently-recharged surface water, inconguent dissolution, and mixing with water from a saline zone and an underlying aquifer. Four wells were shown to intersect major flowpaths, and five wells were shown to intersect minor flowpaths. Major ion and Sr isotope data collected over two years from four karst springs (Main, Eliza, Old Mill, and Upper Barton Springs) provided insight into water flow in the aquifer. Main and Eliza were fed by ground water from the same flowpath(s) in the aquifer, as their geochemical compositions were indistinguishable. Old Mill received 4-9 percent of its water from a saline zone, as shown by elevated ion concentrations and a quantitative mixing model. Upper Spring obtained some of its water from an isolated subbasin in the aquifer, as indicated by radiogenic ⁸⁷Sr/⁸⁶Sr values measured in this subbasin. Oxygen and hydrogen isotope values indicated that ground water was well-mixed over year or longer timescales. Oxygen isotope samples collected from the springs following a rainfall event showed how stormflow recharge flows to the springs. A hydrograph separation using showed an immediate increase in spring discharge following rainfall but a 12-hour delay before storm water reached the spring. This suggested an advancing front of storm water that expelled pre-storm water from the karst conduits. Discharge of pre-storm ground water was reduced by up to 44 percent after rainfall, suggesting that stormflow pressurized the karst conduit system and reduced gradients between the aquifer matrix and conduits. Specific conductance was also an effective and inexpensive tracer of stormflow, on the basis of its strong correlation (r²=0.96) to oxygen isotope values. Resource managers and scientists may be interested in these findings, as the potential for contamination of this spring system is increased after large rainfall events