USING STRONTIUM ISOTOPES TO TRACE SOURCES OF BASE FLOW IN URBAN WATERSHEDS, CENTRAL TEXAS

Increasing urbanization of watersheds in the contributing and recharge zones of the Edwards aquifer in central Texas is a concern because of the impact it may have on the quantity and quality of recharge to the aquifer. Increasing “impervious” cover can reduce infiltration of precipitation, but increased recharge occurs from water main and sewer leakage and the infiltration of excess irrigation. Imported surface water supplies in the study area have higher ⁸⁷Sr/⁸⁶Sr values than groundwater due to a difference in geology between the surface water catchment and the study area (Cretaceous limestones). This allows Sr isotopes to serve as a tracer of urban recharge in the base flow of urban streams that may recharge the aquifer. Mean ⁸⁷Sr/⁸⁶Sr values of the urban end member and non-urbanized streams are 0.7093 and 0.7081, respectively, while base flow samples from urban streams range from 0.7080 to 0.7087. This indicates that urban stream base flow is a mixture of both end members, with higher values being indicative of more urban recharge in base flow. ⁸⁷Sr/⁸⁶Sr values of streams is positively correlated (r² = 0.75) to the percent urban land use in each watershed. ⁸⁷Sr/⁸⁶Sr values of each end member and urban stream base flow change temporally, due to changes in the inputs of Sr to each end member. The changes in urban water are due to fluctuating inputs of surface water to water supply reservoirs; those in groundwater in the study area are due to varying inputs of Sr to groundwater by direct, indirect, and localized recharge. Previous studies show that during wet periods and higher groundwater levels, flow through limestone is predominantly through conduits; residence times are relatively short and ⁸⁷Sr/⁸⁶Sr values are dominated by soil properties (0.7088). During drier periods and lower groundwater levels, flow through limestone is through more diffuse flow paths so that with longer residence times, groundwater acquires a ⁸⁷Sr/⁸⁶Sr signature similar to limestone (0.7076). Understanding how components of base flow are affected by urbanization in the contributing and recharge zones of the Edwards aquifer could prove useful in quantifying future changes in surface and groundwater resources in central Texas.