Groundwater chloride response in the Highland Creek watershed due to road salt application: A re-assessment after 20 years

SummaryChloride from road salt enters streams primarily through surface runoff and groundwater discharge. Monitoring of dry-weather flow chloride concentrations in the Highland Creek watershed of the eastern Greater Toronto Area indicates the presence of a previously unrecognised, dual porosity aquifer system whereby preferential flow associated with “urban karst” exerts a significant influence on baseflow chloride concentrations early in the year. A chloride mass balance undertaken annually over four successive salting seasons suggests that as much as 40% of the chloride applied as road salt enters the shallow aquifer resulting in a net accumulation of chloride and a gradual increase in mean baseflow chloride concentrations. Assuming current road salt application rates are continued, late summer baseflow chloride concentrations will reach around 505 mg/L, almost double present levels. Elevated chloride concentrations can affect the potability of water (the Canadian aesthetic drinking water quality guideline for chloride is 250 mg/L) and can also be toxic to aquatic organisms (CCME aquatic chronic toxicity guideline is 208 mg/L). Meeting these guidelines would require that the release of salt-laden runoff to the subsurface be reduced by over 50%.

► Monitored chloride concentrations in the Highland Creek watershed of Toronto for 4 years. ► Preferential flow via “urban karst” highly influenced chloride concentrations in baseflow. ► The rate of increase in chloride concentration is consistent with the rate of urban development. ► Mass balance showed about 40% of the applied road salt enters the shallow aquifer system. ► To meet drinking guidelines would require 50% reduction in mass of salt entering the aquifer.