Sr Isotope Dendrochemistry in Urban Riparian Trees as a Temporal Record of Urbanization Impacts on Stream-Water Chemistry

Our understanding of the temporal evolution of anthropogenic impacts on streams from increasing watershed urbanization is limited by the absence of archived, historical stream-water samples. Riparian trees that derive a large portion of their water from streams may provide a historical proxy by recording stream-water chemical signatures. Preservation of stream-water elemental concentrations in the bole of streamside trees is complicated by biologic fractionation. ⁸⁷Sr/⁸⁶Sr is not affected by biologic fractionation and, therefore, may provide a reliable stream-water signature. In Austin, Texas, ⁸⁷Sr/⁸⁶Sr has been determined for multiple environmental reservoirs, including: dust, soil, rain, limestone bedrock, stream water, municipal water, and waste water. ⁸⁷Sr/⁸⁶Sr of municipal (0.7089) and waste water (0.7084) is considerably different than that of natural stream water (0.7079). Stream-water Sr isotope compositions correlate highly to the degree of urbanization for six watersheds, most likely due to a greater municipal and waste-water influx. Streamside trees have the potential to record within their annual rings the Sr isotope ratio of the stream water, and therefore provide a historic record of changing stream water chemistry due to urbanization. In one urban watershed, present-day ⁸⁷Sr/⁸⁶Sr values in young branches from 18 trees (baldcypress - Taxodium distichum and black willow - Salix nigra) increase downstream by 0.0005, which parallels a downstream increase of ⁸⁷Sr/⁸⁶Sr in stream water. This supports the potential of Sr as a dendrochemical record of stream water. A Sr isotope time series over the past 50 years, constructed from baldcypress and cedar elm (Ulmus crassifolia), suggests that stream-water ⁸⁷Sr/⁸⁶Sr has increased by 0.0002 over this period. We infer that this temporal trend is due to an increased influx of municipal and waste water into the urban watershed. In contrast, trees from a largely unurbanized watershed have an unchanging Sr isotope composition over the last 80 years.