Search for: All records

Abstract Developing sustainable urban systems is a fundamental societal challenge for the 21st century, and central Texas faces particularly synergistic challenges of a rapidly growing urban population and a projected increasingly drought-prone climate. To assess the history of urbanization impacts on watersheds here, we analyzed 51 cores from bald cypress trees in paired urban and rural watersheds in Austin, Texas. We find a significant contrast between rural and urbanized watersheds. In the rural watershed, tree-ring-width growth histories (“chronologies”) from 1844–2018 significantly and positively correlate (p < 0.01) with (1) one another, and (2) regional instrumental and proxy records of drought. In the urbanized watershed, by contrast, chronologies weakly correlate with one another, with instrumental records of drought, and with the rural chronologies and regional records. Relatively weak drought limitations to urban tree growth are consistent with the significant present-day transfer of municipal water from urban infrastructure by leakage and irrigation to the natural hydrologic system. We infer a significant, long-term contribution from infrastructure to baseflow in urbanized watersheds. In contrast to the common negative impacts of ‘urban stream syndrome’, such sustained baseflow in watersheds with impaired or failing infrastructure may be an unintended positive consequence for stream ecosystems, as a mitigation against projected extended 21st-century droughts. Additionally, riparian trees may serve as a proxy for past impacts of urbanization on natural streams, which may inform sustainable urban development. 

Austin, Texas is among the most rapidly urbanizing regions in the U.S., posing challenges to the resilience of its water resources. Geochemical differences between stream water from relatively pristine (rural) and impacted (urban) watersheds indicate several distinct controls on stream water compositions, including extent of urbanization, extent of failure of the city’s municipal water infrastructure, and differences in bedrock composition and permeability. We focus here on the largely unstudied evolution of municipal water once it leaves the infrastructure and enters the natural hydrologic system as groundwater and/or surface water. We use the distinct Sr isotope values (⁸⁷Sr/⁸⁶Sr) and other compositional differences between municipal waters, natural stream and spring water, limestone bedrock, and soils as tracers of the sources of and processes by which four Austin-area streams and springs acquire their dissolved constituents. These processes include 1) fluid-mixing between municipal and natural surface water and groundwater, 2) multiple mineral-solution reactions, including dissolution and water-rock interaction (WRI) processes of precipitation, incongruent dissolution, and recrystallization, and 3) varying groundwater residence times. Stream water in two urbanized watersheds have high⁸⁷Sr/⁸⁶Sr values and ion compositions close to values for municipal water, whereas stream and spring water in two rural watersheds have compositions close to natural stream water. Urbanized stream water compositions can be accounted for by models of municipal water contributions followed by dissolution of bedrock minerals. By contrast, rural stream water compositions are consistent with a model sequence of dissolution followed by extensive WRI with limestone. The results of this study indicate significant contributions to streams from the municipal infrastructure. We find that the evolution of this municipal water in the natural hydrologic system comprises multiple fluid-mixing processes and mineral-solution reactions, which are influenced by differences in bedrock geology. This composite evolution advances our understanding of the complexities of “Urban Stream Syndrome”.