Road salt impacts on ground-water quality - the worst is still to come

The Metropolitan Toronto area of southern Ontario, Canada, receives well over 100 000 t (tonnes) (Mg) of NaCl annually. Only 45% of the salt applied is being removed annually from the catchment in surface-water flow, the remainder entering temporary storage in shallow subsurface waters. The data from this study indicate that if present rates of salt application are maintained, chloride concentrations in ground waters discharging as stream base flow will eventually reach a steady-state concentration of >400 mg/L. This value represents a threefold increase over present average base-flow concentrations and well exceeds guidelines for drinking-water quality. The impact of sodium will be delayed by ion exchange, but similar calculations suggest that steady-state sodium concentrations will reach in equally unacceptable 250 mg/L. Models indicate that rates of ground-water degradation are highly dependent on the local hydrogeology, including catchment size. -from Authors