Quantifying Exceedances of Chloride Water Quality Guidelines in Niagara Escarpment Streams Around Hamilton, Ontario Using High- Frequency Data

Though stream chloride concentrations (Cl) have been shown to be increasing in cold regions worldwide, the frequency and duration of exceedances of the freshwater chronic and acute guidelines for safe exposure of aquatic life to chloride are understudied. Streamwater Cl for nine watersheds located along the Niagara Escarpment around Hamilton, Ontario were modeled using concentration-conductivity regressions with high frequency specific conductivity data between March 2020 and June 2021. Analyses of Cl dynamics at these sites suggests that stream Cl occasionally exceeds the Canadian acute guideline and regularly exceeds the chronic guideline. This study highlights the dilution of stream Cl by precipitation events in the non-salting season, which may provide temporary relief to aquatic organisms from otherwise toxic Cl. These results will help ecotoxicologists identify ecologically relevant exposure durations, ultimately serving to better inform regional understanding of the risk presented to Niagara Escarpment ecosystems from increasing Cl concentrations.