IDENTIFICATION OF GROUNDWATER RECHARGE SOURCES CONTRIBUTING TO URBAN STREAM BASE FLOW IN AUSTIN, TEXAS

Previous investigations have determined municipal water supply is a substantial groundwater recharge source to the Edwards Aquifer in central Texas, resulting from urbanization in the aquifer’s contributing and recharge zones. Knowledge of how groundwater recharge sources change with urbanization is essential for sustainable management of water resources. Austin municipal water ⁸⁷Sr/⁸⁶Sr value ranges (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). This may allow Sr isotopes to serve as a geochemical tracer of municipal water supply in urban stream base flow (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. Also, some rural streams with no Austin municipal water inputs to their watersheds have ⁸⁷Sr/⁸⁶Sr values within the range of highly urbanized Austin streams. Austin area soil leachates have ⁸⁷Sr/⁸⁶Sr values ranging to higher than those of municipal water (0.7083 – 0.7125) and spatial variability in their distribution and temporal variability in their Sr contributions to groundwater may result in these naturally high stream base flow ⁸⁷Sr/⁸⁶Sr values. This variability makes the identification and quantification of municipal water as a base flow source unreliable in some instances. This Sr isotope variation can, however, help distinguish recharge sources to urban stream base flow. Temporal variability in climatic conditions and resulting changes in effective moisture produce distinct groundwater ⁸⁷Sr/⁸⁶Sr and trace element ratio (Mg:Ca) variations and similar natural variations are found in stream base flow. Furthermore, municipal water inputs to urban watersheds also vary seasonally. This causes changing proportions of natural vs. municipal water sources in base flow of small urban streams. These ⁸⁷Sr/⁸⁶Sr trends/variations are distinct based on the recharge source and provide a better constraint on groundwater recharge sources than ⁸⁷Sr/⁸⁶Sr values alone.