| Popis: |
Austin, Texas is experiencing rapid urban development, posing challenges to the resilience of water resources. Geochemical differences between stream water from relatively pristine rural vs. impacted urban watersheds in the Austin area indicate several distinct controls on stream water composition. These include differences in the composition and permeability of watershed bedrock, extent of urbanization, and the varying degrees of failure within the City of Austin’s municipal water infrastructure. Significant losses of municipal water from infrastructure is common to most cities, yet little is known about the evolution of such water once it enters the natural hydrologic system, and the present study focuses on this evolution. Austin draws municipal water from the Colorado River, which drains a terrain comprised of multiple rock types/ages having relatively high Sr isotope ratios (⁸⁷Sr/⁸⁶Sr), compared with those of the Cretaceous limestone bedrock and natural stream water of Austin’s watersheds. This compositional distinction between municipal water/wastewater, local natural stream water, and bedrock is used as a tracer of the sources of and processes by which seven Austin-area streams acquire their dissolved constituents. These include fluid mixing between municipal water and natural stream water, water-rock interaction (WRI) processes such as dissolution, precipitation, and recrystallization, and varying groundwater residence times. Stream water in Waller and Shoal Creeks, Austin’s most extensively urbanized watersheds, have high ⁸⁷Sr/⁸⁶Sr values and geochemical compositions closer to values for municipal water than to values for streams from the more rural watersheds (e.g., Onion, Barton and Bull Creeks). The compositions of stream waters from the urbanized watersheds can largely be accounted for by models of fluid mixing between natural and municipal endmembers. Additionally, the Waller and Shoal urban stream waters are less chemically evolved, with lower Sr/Ca values, than stream waters from the more rural watersheds. Waller and Shoal Creek water compositions can be accounted for by limited WRI via dissolution of these watersheds’ Austin Chalk limestone bedrock. Stream water from the other watersheds can be modeled by more extensive WRI via both dissolution and recrystallization of the Glen Rose and Edwards limestone bedrock of those watersheds. The consistently limited WRI reflected in Waller Creek may be a consequence of fracture flow in the chalk, whereas more extensive WRI common to the more rural watersheds may be facilitated by higher matrix permeability and inferred longer residence time in the Glen Rose and Edwards. These results indicate that the geochemical evolution of municipal water, once transmitted into the natural system, is influenced by multiple fluid-mixing and WRI processes that reflect subtle but distinguishable differences in watershed geology. These differences also have implications for potential differences in contaminant transport in these watersheds. |
