Identification and quantification of municipal water sources contributing to urban streamflow in the Austin, Texas area

Previous studies have shown that municipal water can provide a substantial surface water and groundwater recharge source for the Edwards aquifer in central Texas. Knowledge of how water sources to urban watersheds change with urbanization is essential for sustainable water resource management. The range for 87Sr/86Sr values for Austin municipal water (0.7086–0.7094) is distinct from that of naturally occurring phreatic groundwater (0.7076-0.7079) and stream discharge in many rural watersheds (0.7077– 0.7084). Many streams in urbanized Austin watersheds have elevated 87Sr/86Sr values (0.7085–0.7088) relative to these rural streams. These differences demonstrate the potential for Sr isotopes to serve as a tracer of municipal water inputs to urban streamflow. A few urban streams and springs, however, have 87Sr/86Sr values higher than those of municipal water. Soil is the likely source of these elevated values. 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 using Sr isotopes unreliable in some instances. Temporal variability in climatic conditions and resulting changes in effective moisture can result in distinct natural groundwater 87Sr/86Sr and Mg/Ca ratio variations, due to differences in overall groundwater residence times and water-rock interaction. Unlike natural water sources, municipal water inputs to urban watersheds peak during the summer (and periods of 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. In some urban watersheds, municipal water appears to be a significant streamflow component during dry periods. However, temporal variation in natural Sr inputs to vadose and phreatic groundwater may result in the overestimation of municipal water contributions to streamflow and groundwater recharge during relatively wet periods.