URBAN GEOLOGY .3. GROUNDWATER CONTAMINATION DUE TO ROAD DEICING CHEMICALS - SALT BALANCE IMPLICATIONS

Every year, roads and highways in Metropolitan Toronto receive more th an 100,000 tonnes (t) of NaCl road de-icing chemicals. While much of t his salt is flushed from the region every winter season by overland fl ow, a proportion will enter the sub-surface and eventually discharge t o urban streams as baseflow To determine annual retention rates of de- icing salts in an urban watershed, a chloride mass balance has been ap plied to the Highland Creek basin, a typical urban catchment in easter n Metropolitan Toronto. The catchment has an area of 104 km2 and groun dwater recharge is estimated to be 162 mm per year. Chloride input to the catchment was determined from municipal records. These show that t he catchment receives approximately 10,000 t of chloride annually, pre dominantly in the form of NaCl de-icing chemicals which are applied to roads, highways and parking lots during the winter months. Chloride o utput was estimated from stream flow and electrical conductivity measu rements recorded at 15-minute intervals over a two-year period. The ba lance reveals that only 45% of the salt applied to the catchment is be ing removed annually and that the remainder is entering temporary stor age in shallow sub-surface waters. If present rates of salt applicatio n are maintained, it is predicted that average steady-state chloride c oncentrations in ground waters discharging as springs in the basin wil l reach an unacceptable 426 +/- 50 mg.L-1 possibly within a 20-year ti me frame. The value of 426 mg-L-1 represents a three-fold increase ove r present average baseflow concentrations, and is nearly twice the dri nking water quality objective of 250 mg.L-1 maximum acceptable concent ration.