GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 324-19
Presentation Time: 9:00 AM-6:30 PM

TRACING THE IMPACT OF MUNICIPAL WATER IN URBANIZING STREAMS AND SOILS


YAZBEK, Lindsey D., Geosciences, Trinity University, One Trinity Place, San Antonio, TX 78212, BANNER, Jay L., Jackson School of Geosciences, University of Texas at Austin, 1 University Station C1100, Austin, TX 78712 and SENISON, Jeffrey J., Halliburton, Houston, TX 77072, lyazbek@trinity.edu

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.