The pollution potential of road salt on aquatic environments in Ireland

Abstract

Road salt or rock salt as it is sometimes known, is a commonly used de-icing material and is used throughout Ireland by local authorities (i.e. county councils), private road operators and members of the public during times of freezing weather. County councils, road authorities and private road operators apply road salt as required as part of their winter road maintenance programs. The road salt used is predominantly sodium chloride (NaCl) as it is relatively inexpensive when compared to other de-icing agents, easy to manufacture and readily available. A number of negative environmental effects and a loss in water quality have been associated with stormwater (melt water) containing dissolved road salt entering fresh waterbodies in the vicinity of roads which are de-iced with sodium chloride. This study uses the M3 motorway in County Meath as a case study. International findings from other studies on road salt are used to illustrate the potential pollution threat posed by spreading road salt. Recognised best practice for the mitigation of this pollution threat is reviewed and stormwater treatment methods along the M3 are evaluated with regard to reducing sodium chloride levels in stormwater run-off. This study has found that treated stormwater from the M3 does not cause a significant elevation in chloride levels. The treatment methods applied to the stormwater may remove the chloride from road salting and the receiving waterbodies dilute the chloride in the stormwater to concentrations that are below recognised toxic limits. A ‘Salt Management Plan’ is the key mitigation measure. Awareness of maintenance companies and staff to the pollution potential of road salt, training of employees, well maintained salt spreading equipment, pre-wetting of salt and active management of road maintenance particularly in areas where roads run in close proximity to waterbodies are essential to limit the threat of road salt to water quality and the environment.