GEOCHEMICAL EVOLUTION OF MUNICIPAL WATER IN THE NATURAL HYDROLOGIC SYSTEM

Austin, Texas is experiencing rapid urban development, posing challenges to watershed resilience. Geochemical differences between stream water from relatively pristine rural vs. potentially contaminated urban watersheds indicate the influence of varying degrees of infrastructure failure within municipal systems. Although significant losses of treated water from infrastructure is common to most cities, little is known about the evolution of such water once it enters the natural hydrologic system.

Austin draws municipal water from the Colorado River, which drains a terrain comprising multiple rock types/ages having relatively high ⁸⁷Sr/⁸⁶Sr, compared with those of the Cretaceous carbonate bedrock of Austin’s watersheds. This creates a compositional distinction between municipal water/wastewater and local natural stream water. Stream water acquires dissolved constituents via water-rock interaction (WRI) processes such as dissolution, precipitation and recrystallization. We use elemental concentrations and ⁸⁷Sr/⁸⁶Sr values in seven watersheds to model municipal water inputs and WRI processes to assess the evolution of municipal water as it is transmitted to the natural hydrologic system.

Stream water in Waller and Shoal Creek, Austin’s most extensively urbanized watersheds, have high ⁸⁷Sr/⁸⁶Sr values, closer to those for municipal water than rural streams. These stream waters are less chemically evolved, with lower Sr/Ca values, than stream waters from five other watersheds Waller and Shoal stream water compositions can be accounted for by limited WRI via dissolution of this watershed’s Austin Chalk limestone bedrock. Stream water from the other watersheds can be modeled by more extensive WRI via dissolution and reprecipitation of the Glen Rose and Edwards limestone bedrock. The limited WRI reflected in Waller Creek may be a consequence of fracture flow in the chalk, whereas more extensive WRI may be facilitated by higher matrix permeability in the Glen Rose. These results indicate that the geochemical evolution of municipal water, once transmitted into the natural system, is influenced by multiple WRI processes that reflect subtle differences in watershed geology.