Examining the Effects of Urbanization on Soil Chemistry Through Sr Isotopes in the Austin, TX Area

In urbanizing landscapes, soils may be altered by processes such as municipal water irrigation, which has implications for the study of urban environmental contaminants in soils and the streams to which the soil provide dissolved constituents. However, it is challenging to directly trace sources of dissolved ions to streams and the impacts on soils. We examine these changes by using Sr isotopes as natural tracers. In Austin, TX, municipal water has relatively high Sr isotope values (87Sr/86Sr of ~0.7089 to 0.7090, similar to the Colorado River source), whereas natural stream water has a lower 87Sr/86Sr value (87Sr/86Sr of ~0.7076, reflecting values for Cretaceous limestone bedrock). Streams from urbanized watersheds have intermediate Sr isotope values, indicating contributions to streams from municipal water, soils, or both. We evaluate the role of soils through isotope analysis of the exchangeable Sr in soils covering a range of soils types, soil horizons, and extents of urbanization. Results indicate that soils 87Sr/86Sr is not controlled by natural variations in soil type or horizon, but rather by the extent to which the soil is irrigated with municipal water. Irrigated soils 1) have higher 87Sr/86Sr (0.7086-0.7091) than unirrigated soils (0.7078- 0.7087), and 2) are within or slightly lower than the range of values for municipal water in the study area (0.7090-0.7091). This indicates that irrigation shifts soils towards municipal water values. Applying Sr isotopes in hydrologic systems may help trace urban environmental contaminants