DELINEATING CONTROLS ON WATER-QUALITY OF URBAN STREAMS IN AUSTIN, TEXAS, USING GEOCHEMICAL AND ISOTOPIC APPROACHES (Invited Presentation)

The City of Austin, Texas is rapidly growing, and water quality in urban streams often degrades as urbanization increases. Identifying the sources and processes that affect water compositions can be challenging due to complexity inherent to urban environments. To address this, geochemical, isotopic, and mass-balance studies of urban stream water-quality in the Austin area were conducted. Increasing solute concentrations over time (salinization) was documented in the urban streams despite Austin being an area with mild winters where deicers are rarely applied. Geochemical modeling of south Austin stream-water compositions and geochemical characterization of pipe outflows along a 1-km stretch of an urban stream in central Austin indicate that anthropogenic inputs (e.g., tap water, swimming pool discharge, and treated and untreated wastewater) are among the multiple controls on stream-water quality. Evapo-concentration of source inputs (presumably in the soil and vadose zone), however, makes quantitative estimation of source-water contributions challenging. Furthermore, mass balance accounting yields greater solute (Cl and Na) exports than inputs, indicating possible contributions from an unaccounted source. Spatiotemporal variability in δ³⁷Cl values in urban streams indicates multiple Cl sources that vary with changing hydrologic conditions. The ability, however, to delineate Cl sources using isotopic fingerprinting is limited because of variability in and overlap between isotopic signatures of potential sources. Principal components analysis of stream and source-water compositions indicates that urban stream-water compositions reflect competing effects of i) dilution from storms, and ii) salinization from anthropogenic inputs and alkalization that may result from water interaction with concrete infrastructure. Assessment of concentration-discharge relations indicates that the extent of solute dilution with increasing runoff varies among watersheds, likely reflecting differences in flow routing, solute availability, and (or) variability in source-water inputs with hydrologic condition. Results indicate multiple urban sources and processes that contribute to water-quality degradation, and highlight the challenge that cities face in ensuring water-quality resiliency.