Assessing Water and Chloride Transport Dynamics of an Urban Stream Following Seasonal De-Icing With Event 18O-Discharge and Chloride-Discharge Relationships

In Southern Ontario, Canada, cool winter temperatures with frequent freeze-thaw cycles pose transport hazards that are mitigated with applications of chloride-rich rock salt to roads, sidewalks, and parking lots. De-icing activities are associated with in-stream chloride concentrations nearing or exceeding Canadian water-quality guidelines and that exacerbate degraded river quality and threaten aquatic organisms. In heavily salted, densely urbanized areas, like the Black Creek Watershed of Toronto, there is considerable uncertainty about the pathways of water and chloride transfers due to a mosaic of impervious and pervious surfaces and heterogeneity in both river morphology and land-to-stream transitions (i.e., natural rivers, engineered concrete channels, and other infrastructure for water transmission). This knowledge gap complicates our understanding of water and chloride transport, the quantification of legacy chloride stored in the watershed, and the potential for groundwater contamination. To better understand these factors in the watershed and other similar environments, we aim to synthesize information from temporal dynamics of stable isotopes in subsurface, precipitation, and stream water with chloride concentrations. In particular, assessing event 18O-Q (18Oxygen - Discharge) and Cl-Q (Chloride - Discharge) relationships across hydrometeorological and seasonal conditions may help reveal details about chloride transport during individual events and how residual chloride is flushed from soils following the suspension of de-icing during warmer weather. Preliminary results show complex and highly variable event 18O -Q and Cl-Q patterns that hint at a combination of distinct pathways contributing to water and chloride transport to streams during events. There is also evidence of seasonality in dominant chloride transport pathways to streams and a long lag in elevated concentrations following snowmelt. We observed temporally variable chloride releases during warm weather that are difficult to predict and are likely from soils that were affected by the variable spatial distribution of historic salt applications throughout the watershed.