AN ASSESSMENT OF SR ISOTOPES AS AN ANTHROPOGENIC TRACER IN STREAM WATER FROM THE AUSTIN, TEXAS, AREA

Urban hydrogeologic environments are a complex arrangement of point and diffuse sources that contribute both natural and anthropogenically sourced water into surface water systems. Urban stream hydrology is further complicated by the competing effects of: (1) diminished stream water base-flow recharge as a result of reduced vadose zone infiltration due to impervious cover and (2) increased recharge from municipal and wastewater pipe infrastructure leakage and irrigation. Relative contributions from these sources are often difficult to quantify because many traditional aqueous geochemical tracers are subject to many environmental processes, which limit their effectiveness (for example, evaporation, adsorption, biological degradation). Identifying distinctive tracers that are subject to few alteration processes is critical to understanding the sources and movement pathways of modern water resources.

Sr isotopes were evaluated as an anthropogenic tracer in eight Austin, Texas-area streams in 2001 and 2002. Stream water ⁸⁷Sr/⁸⁶Sr values varied in these watersheds between ⁸⁷Sr/⁸⁶Sr = 0.70778 and 0.70918. Mean stream water ⁸⁷Sr/⁸⁶Sr values were strongly correlated (r² > 0.90) with the percent of urban development within the respective watersheds and also covaried (r² > 0.86) with several indices of anthropogenic activity (for example, median structure age). In addition, strong correlations (r² > 0.78) exist between the mean stream water ⁸⁷Sr/⁸⁶Sr values and other elements commonly associated with anthropogenic sources (F, Cl, NO₃, Na, and K).

The City of Austin obtains all of the source water for its municipal water system from the Colorado River of Texas, which contains elevated ⁸⁷Sr/⁸⁶Sr ratios (mean = 0.70910) due to stream water contributions from the Llano uplift region in central Texas. In contrast, most local stream water flows over Cretaceous-age, limestone bedrock, which contributes Sr with lower ⁸⁷Sr/⁸⁶Sr ratios (0.70742 – 0.70804). Moreover, ⁸⁷Sr/⁸⁶Sr values from deeper limestone aquifers that commonly recharge local streams through springs varied within a similar range (0.70759 – 0.70815). These two isotopically distinct sources serve as the primary mixing model end-members and can be used to estimate the relative proportions of water sources that contribute to Austin-area streams.