Quality of water in the Edwards aquifer, central Travis County, Texas




St. Clair, Ann Elizabeth

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The Rollingwood area in central Travis County, Texas, is the principal zone of recharge to the Edwards limestone aquifer. As a result of extensive suburban development in the past 25 years, the number of septic-tank systems in the area has greatly increased. However, thin soils and fractured bedrock may limit the effectiveness of waste renovation in these systems. In order to determine if effluent from septic tanks has affected the quality of ground water in the Edwards aquifer, samples from wells in the area were analyzed for several constituents that may indicate contamination by septic tanks - nitrate, ammonium, chloride, phosphate, organic carbon, and total coliform bacteria. Concentrations of these parameters are similar to background concentrations in water from the Edwards, indicating that suburban development has not resulted in detectable degradation of the quality of water in the aquifer. During the study, water quality did not vary significantly areally, temporally, or as a result of heavy rainfall. The lack of degradation of water quality probably is due to (1) the thickness of the unsaturated zone (greater than 110 feet) through which effluent must travel to reach the water table, (2) adsorption of pollutants on insoluble residue in solution zones and on walls of fractures in the unsaturated zone, and (3) dilution and dispersion of contaminants in the saturated zone of the aquifer. Chemical analyses of water from Barton Creek, Barton Springs, the Edwards aquifer, and the Colorado River indicate that discharge from the aquifer at Barton Springs may be a mixture of recharge from both Barton Creek and the Colorado River. Chemically, water from wells in the Edwards is similar to water in Barton Creek, indicating that the creek is a principal source of recharge to the aquifer. Variations in the composition of water from Barton Springs correlate with the rate of discharge at the springs. When spring discharge is high, the composition of water in the springs resembles that of water from Barton Creek and wells in the Edwards. At times of low discharge at the springs - when rainfall, streamflow, and therefore, recharge from Barton Creek are low--chemistry of water at Barton Springs resembles that of Colorado River water.