THE IMPACTS OF URBANIZATION ON SOIL: APPLICATIONS OF STRONTIUM ISOTOPES

Urban areas accompanied by human activity disturb the natural landscape upon which they develop, disrupting important pedogenic processes and the ecosystem services they provide. In urban environments, quantifying the degree of alteration to soil physical and chemical properties is critical for understanding ecosystem resiliency, especially as climate patterns continue to shift. The source-tracing capabilities of strontium isotopes (⁸⁷Sr/⁸⁶Sr) can be applied to understanding the influence of urbanization on soils. We use ⁸⁷Sr/⁸⁶Sr values of natural (e.g., soil, bedrock, groundwater) and anthropogenic (e.g., municipal water) sources to evaluate the impact of urbanization on soils in eight Austin, Texas, watersheds that span a wide range of urbanization (8–95% developed land). We compare soils by naturally-varying parameters (e.g., mineralogy, thickness, soil type, watershed) and anthropogenic parameters (e.g., irrigation systems, soil amendments, fertilization). Among natural parameters examined, there is a strong positive correlation between soil thickness and ⁸⁷Sr/⁸⁶Sr value. This is likely due to the relatively low ⁸⁷Sr/⁸⁶Sr value of the region's Cretaceous limestone bedrock (0.7078). Thin soils (~20 cm) will have more carbonate bedrock-derived strontium (with a low ⁸⁷Sr/⁸⁶Sr value) compared to thick soils (~50 cm). This isotope-thickness relationship is observed in Austin-area soils that are not irrigated (m = 158 [⁸⁷Sr/⁸⁶Sr]/m; R² = 0.83), and in previous central Texas studies. In contrast, this relationship is not observed among Austin-area irrigated soils (m = 9.14 [⁸⁷Sr/⁸⁶Sr]/m; R² = 0.03). For all eight watersheds, most soil ⁸⁷Sr/⁸⁶Sr values range between those for limestone bedrock and municipal supply water (0.7091), indicating soil interaction with municipal supply water either through infrastructure leakage and/or irrigation. Soils irrigated with municipal water have elevated ⁸⁷Sr/⁸⁶Sr values relative to unirrigated soils in seven of the eight watersheds. These results are consistent with municipal water resetting natural soil ⁸⁷Sr/⁸⁶Sr via ion exchange processes, which further demonstrates the use of strontium isotopes as a natural tracer to assess the overprint of urbanization on natural soil characteristics.