10 Texas Water Research Network: A Statewide Initiative to Integrate and Advance Water Research from Inception to Application II

Monday, 13 March 2017
1:30 PM - 5:30 PM
Grand Ballroom A

TRACING CHANGES IN URBANIZING SOILS DUE TO IRRIGATION WITH MUNICIPAL WATER

Paper number: 10-5
Program: Theme Sessions
Time: 2:35 PM – 2:55 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 49, No. 1

doi: 10.1130/abs/2017SC-289469

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