Résumé

Every year, roads and highways in Metropolitan Toronto receive more than 100,000 tonnes (t) of NaCl road de-icing chemicals. While much of this salt is flushed from the region every winter season by overland flow, a proportion will enter the sub-surface and eventually discharge to urban streams as baseflow. To determine annual retention rates of de-icing salts in an urban watershed, a chloride mass balance has been applied to the Highland Creek basin, a typical urban catchment in eastern Metropolitan Toronto. The catchment has an area of 104 km² and groundwater recharge is estimated to be 162 mm per year. Chloride in put to the catchment was determined from municipal records. These show that the catchment receives approximately 10,000 t of chloride annually, predominantly in the form of NaCl de-icing chemicals which are applied to road, highways and parking lots during the winter months. Chloride output was estimated from stream flow and electrical conductivity measurements recorded at 15-minute intervals over a two-year period. The balance reveals that only 45% of the salt applied to the catchment is being removed annually and that the remainder is entering temporary storage in shallow sub-surface waters. If present rates of salt application are maintained, it is predicted that average steady-state chloride concentrations in ground waters discharging as springs in the basin will reach an unacceptable 426 ± 50 mg·L^-1 possibly within a 20-year time frame. The value of 426 mg·L^-1 represents a three-fold increase over present average baseflow concentrations, and is nearly twice the drinking water quality objective of 250 mg·L^-1 maximum acceptable concentration.