IDENTIFICATION OF MUNICIPAL WATER SOURCES CONTRIBUTING TO URBAN STREAMFLOW IN AUSTIN, TX

Previous investigations have determined that municipal water can provide a substantial surface water and groundwater recharge source for the Edwards aquifer in central Texas. Knowledge of how water inputs to urban watersheds change with urbanization is essential for sustainable water resources management. The range for ⁸⁷Sr/⁸⁶Sr values for Austin municipal water (0.7086- 0.7094) are distinct from those of naturally occurring phreatic groundwater (0.7078) and stream discharge in many rural watersheds (0.7077- 0.7084). These differences demonstrate the potential for Sr isotopes to serve as a tracer of municipal water inputs to urban streamflow (Christian et al. 2011). While highly urbanized Austin streams have elevated ⁸⁷Sr/⁸⁶Sr values (0.7085- 0.7088) relative to some rural streams, a number of urban streams and springs have ⁸⁷Sr/⁸⁶Sr values higher than those of municipal water. Soil-exchangeable Sr in the region, having ⁸⁷Sr/⁸⁶Sr values that range higher (0.7083- 0.7125) than those of municipal water, is the likely source. Spatial variability in the distribution of high ⁸⁷Sr/⁸⁶Sr soil and temporal variability in soil-exchangeable Sr contributions to groundwater may result in naturally high streamflow ⁸⁷Sr/⁸⁶Sr values, making the identification and quantification of municipal water as a streamflow source unreliable in some instances. Temporal variability in climatic conditions and resulting changes in effective moisture can result in distinct natural groundwater ⁸⁷Sr/⁸⁶Sr and trace element ratio (Mg/Ca) variations, due to differences in overall groundwater residence times and water-rock interaction. We observe these temporal geochemical variations in some Austin-area streams. Unlike natural water sources, municipal water inputs to urban watersheds peak during the summer (and drought) when natural recharge inputs (precipitation) are minimal or nonexistent. Thus, proportions of natural vs. municipal water sources in the streamflow of some highly urbanized streams vary seasonally, resulting in distinct ⁸⁷Sr/⁸⁶Sr and Mg/Ca temporal trends, based on the recharge source.

Christian, L.N., Banner, J.L., Mack, L.E., 2011, Sr isotopes as tracers of anthropogenic influences on stream water in the Austin, Texas, area: Chemical Geology, v. 282, p. 84-97.