Monitoring transport and fate of de-icing salt in the roadside environment Modelling and field measurements

University dissertation from Stockholm : KTH

Abstract: Roads and traffic are a major non-point source of pollutants and may have severe impacts on surface water, groundwater, soil and vegetation. In cold climates, de-icing salt is one such pollutant that may cause increased chloride concentrations and induce other effects on the environment. Monitoring and quantifying environmental effects are crucial for governing decisions towards more suitable use of de-icing salt in order to achieve and maintain good environmental status around roads. This thesis presents an operational modelling tool for monitoring the transport and fate of de-icing salt in the roadside environment in order to quantify changes in the environment at various spatial and temporal scales, using salt application data, meteorological data, geology and generic descriptions of hydrogeological environments as main inputs. A combination of modelling and various independent field measurements provided an efficient means for evaluating and describing the spread of de-icing salt from the road to the surroundings, the deposition of salt and ploughed snow in the roadside, and the corresponding increase in chloride concentration in soil and groundwater. Both the spatial and seasonal variation in soil chloride concentration were significantly affected by de-icing salt application. The importance of type of soil, vegetation type, groundwater conditions and distance from the road was clearly demonstrated for modelling the transport and fate of de-icing salt in the roadside environment. Salt emissions from the road by surface runoff were estimated at 50-80% of applied salt and transport by snow ploughing and air emissions at 20-50%. The uncertainty in the spatial distribution of snow and salt deposition close to the road was high and a previous proposed exponential decline in salt deposition with distance from the road could not be justified within a couple of metres from the road. Future monitoring should include both modelling and systematic data collection in order to reduce the uncertainty in predictions of the environmental impact of de-icing salt. Modelling of chloride concentration, soil water content and soil temperature and measurements of electrical resistivity may be a cost-effective solution for quantifying changes in the roadside environment.