GEOCHEMICAL INDICATORS OF MUNICIPAL WATER INFLUX INTO STREAMWATER IN THE BULL CREEK WATERSHED, AUSTIN, TX

Municipal water loss from leaky infrastructure represents a significant challenge to sustainability. We apply the isotopic composition of dissolved Sr (⁸⁷Sr/⁸⁶Sr) as a proxy for municipal water inputs to the tributaries and springs in Bull Creek in Austin, Texas. As a result of the history of land development in Austin, some watersheds are densely urbanized while others remain closer to their undeveloped states amidst sparse rural development. Streamwater studies in Austin have demonstrated strong correlations between mean ⁸⁷Sr/⁸⁶Sr and both elevated levels of dissolved anthropogenic ions (e.g. F^-) and various physical indicators of urbanization (e.g. median structure age) in samples taken from multiple watersheds (Christian et al., 2011). Influx of municipal water is inferred as the ⁸⁷Sr/⁸⁶Sr in urbanized streams falls between the high value found in municipal water and the lower values found in local streams sourcing from non-urbanized catchments. The contrast in ⁸⁷Sr/⁸⁶Sr is a result of water-rock interaction; local Austin surface streams are underlain by karst Cretaceous limestone whereas the Colorado River (the source of Austin tap) passes over Precambrian igneous and metamorphic rocks upstream of Austin. Here we focus on the Bull Creek watershed, and find elevated levels of dissolved Na and Cl consistent among nearly all sampled urban tributaries and springs compared to their rural counterparts. Although Na and Cl reveal a general trend in streamwater degradation, ⁸⁷Sr/⁸⁶Sr mixing graphs identify specific springs and tributaries that are locally influenced by municipal water influx. ⁸⁷Sr/⁸⁶Sr vs. 1/Sr relationships show that all samples with ⁸⁷Sr/⁸⁶Sr elevated beyond a threshold fall along a mixing line between rural streamwater and municipal water endmembers, while samples with lower ⁸⁷Sr/⁸⁶Sr do not plot within the mixing region. ⁸⁷Sr/⁸⁶Sr indicates that municipal water contributes up to half of the discharge at urban sites where mixing is evident. Scaling our urbanization studies down to a single watershed demonstrates the both the applicability of Sr isotopes to fingerprinting endmembers in a multi-parameter water quality investigation, and the potential to locate leaky infrastructure in Austin, where nearly 5.9 billion gallons of tap water were lost throughout a 12 month period in 2008-2009.