GEOCHEMICAL INDICATORS OF URBAN STREAMWATER CONTAMINATION IN THE BULL CREEK WATERSHED, AUSTIN, TX

The degradation of water quality as a result of urbanization is a major challenge in the field of water sustainability. We apply the isotopic composition of dissolved Sr (⁸⁷Sr/⁸⁶Sr) as a proxy for urban water contamination in the tributaries to Bull Creek, in Austin, Texas. The Bull Creek watershed has undergone rapid urbanization over the past thirty years. The creek drains into the Colorado River at Lake Austin, a principal source of Austin’s municipal water supply. The watershed also partially lies within the recharge zone of the northern segment of the Edwards Aquifer, which provides water for other rapidly growing municipalities in central Texas (i.e. Georgetown, Pflugerville, and Round Rock). Previous studies in Austin have demonstrated strong correlations between ⁸⁷Sr/⁸⁶Sr and both elevated levels of dissolved anthropogenic ions (e.g. F^-) and various physical indicators of urbanization (e.g. median structure age) when examining average stream water concentrations from multiple watersheds in the Austin area (Christian et al., 2011). The influx of municipal water into natural surface streams is inferred as the ⁸⁷Sr/⁸⁶Sr in urbanized streams falls between relatively high values for municipal water and the lower values found in local streams sourcing from non-urbanized catchments in the Austin area. The contrast in ⁸⁷Sr/⁸⁶Sr is a result of differences in the sources of dissolved ions; local Austin streams are underlain by karstified Cretaceous limestone, whereas the Colorado River passes over Precambrian granite terrain upstream of Austin. Soil weathering is also a potential source of high ⁸⁷Sr/⁸⁶Sr in streamwater and can complicate mixing models that compare natural stream and municipal water endmembers. Results for the Bull Creek watershed reveal notable differences between urbanized and non-urbanized watersheds in dissolved F^-, Na⁺, and K⁺. Streamwater ⁸⁷Sr/⁸⁶Sr from urbanized Bull Creek tributaries (0.7078 ≤ ⁸⁷Sr/⁸⁶Sr ≤ 0.7084) falls in the range between non-urbanized tributaries (⁸⁷Sr/⁸⁶Sr = 0.7078) and local tap water (⁸⁷Sr/⁸⁶Sr = 0.7092). Mixing models that compare Bull Creek streamwater to Austin area wastewater indicate that wastewater mixing of 20%-50% into natural streamwater may account for nearly 80% of urban tributaries under low flow conditions.