High-frequency observations reveal acute chloride pulses and chloride legacy effects in an urbanizing watershed impacted by road salting

In watersheds impacted by urban growth and road salt usage, increasing stream chloride (Cl^-) concentrations are well-documented. Peaks in stream Cl^- concentrations that exceed chronic and/or acute water quality guidelines are typical in the winter salting season when Cl^- (from Cl^--based de-icers) is flushed from the landscape but are not easily measured with grab samples. In some cases, chronic Cl^- conditions persist into the summer growing season due to a build-up of Cl^- in the subsurface. Estimating the proportion of Cl^- loads transported in the salting and non-salting seasons is of interest for tracking the relative role of subsurface Cl^- pools to the annual load, as well as the influence of runoff events on loads across the two periods. In this study, we made use of a 6-year record of high-frequency stream Cl^- concentrations from an urbanizing watershed in southern Ontario, Canada. High-frequency measurements revealed that the acute and chronic water quality guidelines for Cl^- were exceeded for 7 and 97 % of the study period, respectively. Salting season Cl^- loads were 2 to 5 times higher than in the non-salting season, but surprisingly, inter-event periods contributed 21 to 56 % of the annual load across years. The results of this study illustrate the utility of high-frequency sensors for identifying water quality extremes that negatively impact aquatic ecosystems, identifying Cl^- transport pathways, and tracking the build-up of legacy Cl^- in the subsurface.