TRACING THE IMPACT OF MUNICIPAL WATER IN URBANIZING STREAMS AND SOILS

The geochemistry of streams and soils may be altered by urbanization processes, such as land use change, increased irrigation, increased storm water runoff, and increased municipal water runoff. However, it is challenging to directly trace sources of dissolved ions to streams and the impacts on soils. In this study, we use Sr isotopes to distinguish between municipal water (high ⁸⁷Sr/⁸⁶Sr of ~0.7089 to 0.7090 from the Colorado River) and natural stream water (SW) (⁸⁷Sr/⁸⁶Sr of ~0.7077 from Cretaceous limestone bedrock) in Austin, TX. Previous studies have found SW in more urbanized Austin watersheds have high ⁸⁷Sr/⁸⁶Sr relative to SW in less urbanized watersheds, suggesting two alternative hypotheses: 1) municipal water from leaky infrastructure and irrigation constitute a significant fraction of SW in more urbanized watersheds, or 2) a positive correspondence between soil and SW ⁸⁷Sr/⁸⁶Sr in a given watershed reflects the natural variability in ⁸⁷Sr/⁸⁶Sr values in soils in controlling SW Sr. To test these hypotheses, we examine Sr isotopes in soils, water, and bedrock in a single watershed having a strong spatial gradient in urbanization, and control for factors such as irrigation, land use change, and soil type. Results show irrigated soils have higher ⁸⁷Sr/⁸⁶Sr (0.7085-0.7091) than unirrigated soils (0.7078- 0.7084). ⁸⁷Sr/⁸⁶Sr values of irrigated soils are within or slightly lower than values for municipal water (0.7091-0.7095). These results indicate that irrigation shifts soils towards municipal water values, and thus natural soil Sr isotope variability (i.e., hypothesis 2) is not a significant control on SW Sr. Applying Sr isotopes in hydrologic systems may help trace urban environmental contaminants.