Broad-Scale Assessment of Long-Term Stream Chloride Trends in Ontario, Canada

Abstract:

Excessive use of road salts to maintain safe winter travel conditions has resulted in increasing chloride concentrations in streams, leading to negative impacts to the structure and function of stream ecosystems. Long-term increasing stream chloride trends have been attributed to increases in urban land cover, however recent research shows that even relatively rural streams can exceed chronic water quality guidelines for chloride. Untangling the relative influences of urban growth, change in streamflow and winter maintenance practices is critical for identifying watersheds at risk of salinization, as well as future measures to mitigate chloride pollution. This study makes use of an open-source water quality dataset to assess long-term stream chloride trends across the Province of Ontario, Canada, where there has been an increased interest in road salt management plans in the last two decades. A Seasonal Mann-Kendall test on chloride concentrations recorded over 1964-1995 resulted in about 68% of the sites showing statistically significant upward trends in both salting (Nov-Apr) and non-salting (May-Oct) seasons. However, this number decreased to about 27% in the more recent time period of 2000-2018, which is after road salt was reported as a priority substance by Environment Canada (2000). To explore the relative role of changing streamflow regimes, a subset of sites with co-located streamflow measurements are compared using the weighted regressions on time, discharge and season method. For streams with an increasing chloride trend not accounted for by changes in streamflow, the relationship between the rates of change in stream chloride and increase in urban areas are examined.