USING SR ISOTOPES TO TRACE MUNICIPAL WATER INPUTS TO WATERSHEDS IN THE ST. LOUIS, MISSOURI, REGION

Urban land use can cause considerable water quality and quantity issues in watersheds. Municipal waters (drinking water and wastewater) conveyed through urban landscapes can enter streams through infrastructure leaks, sewer overflows, and over-irrigation. Understanding the locations and timing of municipal water inputs to streams is critical for remediation efforts. However, differentiating municipal waters from natural water sources is often challenging because of similarities between the endmembers or changes to the tracers of interest along the flowpath. The use of naturally occurring Sr isotope compositions can reduce these uncertainties because of the relatively conservative nature of Sr in solution and negligible fractionation of Sr isotopes at low temperatures. Thus, we test whether ⁸⁷Sr/⁸⁶Sr values can be used to constrain municipal water inputs to watersheds along a land use gradient (impervious area: 2-47%) in the St. Louis, Missouri, region, where local lithology consists of carbonates and shales. Samples of streams (n = 14), soil (n = 4), the local drinking water source (Missouri River; n = 1), treated drinking water (n = 2), and treated wastewater (n = 1) were analyzed for elemental composition (e.g., Ca, Mg, Cl, F, Sr) and ⁸⁷Sr/⁸⁶Sr values. The ⁸⁷Sr/⁸⁶Sr values are nearly identical (0.7090 ± 0.0001) among municipal water types, indicating consistent Sr isotope chemistry for this endmember. Our most rural stream sample, theoretically representing the natural water source endmember, has a ⁸⁷Sr/⁸⁶Sr value of 0.7094, while the most urban stream sample has a lower value of 0.7088. Some stream ⁸⁷Sr/⁸⁶Sr values (0.7082-0.7102) fall outside the endmember range. Soil Sr isotope values (0.7088-0.7108) are generally close to, but slightly higher than, nearby streams. Preliminary results suggest that the natural water source ⁸⁷Sr/⁸⁶Sr composition varies for each catchment and is likely influenced by lithology. Nevertheless, in general, carbonate-hosted watersheds exhibit higher Sr isotope values and shale-hosted watersheds have lower Sr isotope values that both converge towards the municipal water endmember composition with increasing urbanization. While the municipal water ⁸⁷Sr/⁸⁶Sr value is well-constrained, ongoing work aims to resolve lithologic controls on the natural water endmember.