The application of de-icing agents to roads during winter months has been widely practised in Europe and North America since the 1960s. It reduces the risk of accidents caused by ice and snow, and helps to maintain the flow of traffic. The most widely used de-icing agent is sodium chloride (rock salt), which is applied to roads as either a liquid or solid, depending upon conditions. This may be done as just NaCl or it can be mixed with grits and sands, and possibly an anti-caking agent such as sodium hexacyanoferrate (II). De-icing salts dissolve into any precipitation, lower the freezing point and thereby melt either ice or snow. They also prevent ice from forming on roads. While de-icing salt clearly improves road safety, its application does however bring problems to the environment. Between 75% to 90% of the de-icing salt applied ends up within 10m of the road, although, elevated concentrations of Na+ and Cl– have been observed tens and even hundreds of metres away from roads. Once in the soil, salts can drain to ground waters or through surface soils, and from there enter streams and rivers. Some salts are transported directly to surface waters through drainage systems.
The detrimental impacts of elevated concentrations of Na+ and Cl– on roadside soils, vegetation, and ground- and surface-waters (as well as on cars and road surfaces), have been well-documented. High salt concentrations can increase colloid mobility and that of associated heavy metal pollutants, while at the same time causing loss of soil structure. Na+ displaces other cationic species from cation exchange sites and changes the composition of soil solution. This leads to changes in pH and ion concentrations, and disruption of natural and pollutant element biogeochemical cycling. Biological communities may change to more salt-tolerant ones. Levels of dissolved organic carbon, nitrogen and ammonium in water bodies may change, and chloride can, in extreme cases, reach concentrations that are toxic to some species. In addition, there may be direct impacts upon roadside plant communities, such as leaf and needle scorch, branch die-back, disfigurement, loss of turgidity, growth reduction and sometimes death to plant life.
Research at York is investigating the role of road salting in destabilisation of the nitrogen cycle via direct and indirect effects on mineralisation, nitrification, and ammonium leaching. Some of these changes will be induced by pH shift. These processes have the potential to enhance loadings to surface freshwaters of inorganic and dissolved organic N species, dissolved organic carbon, and heavy metals. Quantitative mechanistic understanding of the changes occurring is highly relevant in the context of the Water Framework Directive.
Britain’s uplands supply the majority of our potable waters, yet these areas are subject to heavy de-icing during winter. Since many upland trunk roads track river courses, salting these roads poses a real threat to the chemical and ecological quality of our water supply.
The true extent to which roadside soils, vegetation and associated freshwater bodies have been impacted by salting is still poorly understood. Furthermore, the complex interactive nature of the processes which follow it, make remediation very difficult. Many questions about the long-term effects of road salting currently remain unanswered. Is there a lag time between introduction of road salts and impact? Are impacts from the periodic introduction of enhanced NaCl concentrations episodic with immediate recovery? Are roadside environments shifting to a new steady state, which is resilient to the effects of road salt and if so, can these cope with the episodic changes in soil and soil solution concentrations of NaCl?