TRACING CHANGES IN URBANIZING SOILS DUE TO IRRIGATION WITH MUNICIPAL WATER

Urban soils may be altered by municipal water irrigation, having implications for the study of urban environmental contaminants in soils and the streams to which they provide dissolved constituents. We examine these changes using Sr isotopes as natural tracers. In Austin, TX, municipal water has relatively high Sr isotope values (⁸⁷Sr/⁸⁶Sr of ~0.7089-0.7090), similar to its Colorado River source, whereas natural stream water has a lower ⁸⁷Sr/⁸⁶Sr value (⁸⁷Sr/⁸⁶Sr of ~0.7077), reflecting values for Cretaceous limestone bedrock (Christian et al. 2011). Streams in 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 (i.e., in leachates) in soils representing a range of soils types and horizons and extents of urbanization. Data indicates soil ⁸⁷Sr/⁸⁶Sr is not controlled by natural variations in soil type or horizon, but the extent of soil irrigation. 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 values for municipal water (0.7091-0.7095), indicating irrigation shifts soils towards municipal water values. Different soil horizons from the same irrigated sample site differ by 0.00002, within analytical uncertainty, and both soil horizons have a Sr isotope value similar to that of municipal water, indicating total resetting of exchangeable Sr by irrigation at these sites. Three unirrigated samples were soaked in municipal water for 48 hours and 2 weeks. Differences in ⁸⁷Sr/⁸⁶Sr values of untreated samples and soaked samples are within analytical uncertainty, suggesting longer time scales are needed to reset the exchangeable Sr. Two splits of a soil sample from an irrigate site were taken, one leached with deionized water (DI) and the other with ammonium acetate. The ⁸⁷Sr/⁸⁶Sr value of the split leached with DI was significantly greater than the split leached with ammonium acetate suggesting ammonium acetate extracts exchangeable Sr while DI extracts exchangeable Sr and dissolves salts precipitated from municipal water. Results indicate municipal water rather than soil isotopic variation is the primary control on elevated ⁸⁷Sr/⁸⁶Sr values of urban streams.