High-Frequency Data Provides Insight into Chloride Transport Pathways and Exceedances of Chronic Chloride Guidelines for the Protection of Aquatic Life in Streams Impacted by Deicers

Assessments of elevated stream chloride (Cl) concentrations ([Cl]), predominantly sourced from winter application of road deicers across snow belt regions, are starting to use high-frequency data, more so in the United States (U.S.) than in Canada. Here, [Cl] was derived from high-frequency specific conductance (SC) measurements from nine streams draining urbanized subwatersheds around Hamilton, Ontario, Canada, between May 2020 and April 2021. We assess [Cl] dynamics to understand dominant transport pathways and characterize water quality guideline exceedances to assess ecological risk while comparing Canadian and U.S. methodologies. These streams exhibited an alarming extent of high [Cl] as six streams exceeded the Canadian short-term guideline >90% of both the salting and non-salting seasons. High-frequency stream [Cl] revealed Cl-impacted groundwater maintaining baseflow [Cl], while fast pathways (e.g., sewers) drive [Cl] pulses in the salting season and episodic dilutions in the non-salting season. Application of the higher U.S. guideline gave consistently lower exceedances. Its application of rolling averages to high-frequency data also obscures episodic dilutions that reduce [Cl] below guideline thresholds and may provide brief intervals of refuge to organisms. High-frequency data provided insight into Cl pathways and ecological risk, though exceedance results are sensitive to the guideline methodology.