Stream and Spring Water Evolution in a Rapidly Urbanizing Watershed, Austin, TX

Quantifying urban development impacts on fresh water quality and quantity is critical, especially as growing populations concentrate in urban centers and with climate change projections of increased hydrologic extremes. We investigate geochemical processes through which municipal supply and waste water, carbonate bedrock, and soils impact stream and spring water compositions within the Bull Creek watershed (Austin, Texas). This watershed exhibits a sharp geographic divide between urban and rural land. Urban and rural waters were assessed to quantify relative influences of municipal water on stream and spring water elemental compositions and Sr-87/Sr-86 values. Higher Sr-87/Sr-86 for samples from urban sites relative to rural sites can be accounted for by two processes: (1) water leakage from municipal infrastructure and/or irrigation or (2) ion exchange as precipitation infiltrates through soils with varying Sr-87/Sr-86. Irrigated soils have higher Sr-87/Sr-86 than unirrigated soils, indicating that irrigated municipal water resets soil compositions and that process (1) is a dominant driver of urban stream and spring water evolution. Geochemical modeling results indicate that urban waters consist of 50% to 95% municipal water. Geochemical modeling further demonstrates the evolution of municipal water as it infiltrates as groundwater and undergoes water-rock interaction. These results are compared with groundwater compositions on a regional scale to infer local flow paths and relative groundwater residences times of municipal water. This study provides a geochemical modeling framework that quantifies both the significance of municipal water on urban stream water and soil compositions and the role of municipal water within urbanized watersheds and aquifers. Key Points We document the geochemical evolution of infiltrating municipal water (via leakage and/or irrigation) within a semiurbanized watershed Geochemical modeling results indicate stream and spring water consists of 50-90% municipal water within urban areas Our geochemical modeling approach estimates subsurface flow paths and relative groundwater residence times of infiltrating municipal water