Inter-annual variability in stream chloride concentration and load dynamics 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 deicers) is flushed from the landscape. In some cases, chronic Cl conditions persist into the summer growing season; however, with surface sources depleted, rain events lead to dilution of Cl concentrations. The timing of the transition from flushing to diluting behaviour indicates a shift in Cl sources (from surface to subsurface) and represents the conceptual shift from the salting to non-salting season. Estimating the proportion of Cl loads transported in these two 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 make use of a 5.5-year record of high-frequency stream Cl concentrations (estimated from 15-minute electrical conductivity sensor measurements) from an urbanizing watershed in southern Ontario, Canada. We examine the inter-annual variability in the frequency and duration of exceedances of the chronic and acute water quality guidelines for Cl and the inter-annual variability in the timing of the transition from salting to non-salting season. We also compare salting versus non-salting season Cl loads and baseflow versus event flow loads in these two seasons 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.