The Weight of Evidence

Road salts are released into the ecosystem primarily through their application onto roadways. They also enter the environment through inappropriate storage methods and housekeeping techniques, as well as in the disposal of huge quantities of salt-laden snow. All road salts eventually dissolve into surface water and groundwater, unless they become attached to soils and plant material.

Road salts are made of chloride ions (Cl-) and an associated cation – the predominant sodium (Na+), or calcium (Ca2+), magnesium (Mg2+) or potassium (K+). Chloride ions are relatively stable, meaning that they can move through the environment in solution without being lost or broken down through natural processes. As such, almost all chloride ions that enter the environment reach surface water.

Chloride is naturally present in the environment. Background levels of chloride concentrations in Ontario’s surface water ranges between 1 and 30 mg/L. However, runoff from roadways, salt storage yards and snow disposal sites result in greatly elevated chloride levels in surface water, soil and groundwater as shown in Table 1. The Ontario Provincial Water Quality Monitoring Network has compared water quality data collected at sites across the Lake Ontario drainage basin in 1980-82 with data collected in 1996-98. Over this 16-year period, 71 percent of sites showed increasing trends for chloride concentrations. The study related this increasing trend with the increasing urbanization and development that has occurred in these watersheds since the 1980’s.

 

Summary of Ecosystem Impacts

Various studies have documented that both aquatic and terrestrial ecosystems can be adversely affected by exposure to chloride concentrations associated with the typical use of road salts. In water bodies, elevated chloride levels have been demonstrated to be lethal to many aquatic species. It is also estimated that 10 percent of aquatic species will be adversely affected by prolonged exposure to chloride concentrations greater than 240 mg/L and 5 percent would be adversely affected by exposure to concentrations of about 210 mg/L. Increasing salt concentrations can also lead to dramatic changes in the vertical mixing and oxygen levels.

Damage to vegetation is perhaps the most visible sign of road salts’ effect on terrestrial ecosystems. Salts disrupt the inhibition of water and absorption of nutrients by plants and can be directly toxic to plant cells. High salt concentrations also lead to degradation of the physical and chemical properties of soil and can adversely affect micro-organisms and other soil organisms.

Road salts have been found to have direct toxicological effects on terrestrial wildlife, mainly mammals and birds. Large mammals, such as moose, deer and elk, are attracted to roadside salt pools, which have long been identified as a major factor contributing to collisions with motor vehicles. Bird kills, some involving hundreds of birds, have been associated with road salts poisoning. In addition, ingestion of road salts has been found to diminish the ability of birds to avoid moving vehicles.

Economic and Ecological Costs

Road salts continue to be used intensively because it is perceived as the cheapest and most effective solution. The benefits of road salts use are considerable; they facilitate safer and accessible roads and the ability to maintain efficient transport of people and goods in winter conditions. However, these benefits diminish when compared with the full ecological and economic costs of road salt use, including the degradation of water bodies and ecosystem biodiversity, and the corrosion of built infrastructure, such as roads and bridges, and motor vehicles.

It is estimated that in the US, over five billion dollars are spent each year by state and local agencies to repair infrastructure damage caused by snow and ice control operations, or $333 (US) per ton of road salts. Parking garages, pavement and roadside hardware and other objects near salt-treated roads are also exposed to the corrosive effects of road salts. Indirect costs include traffic delays and lost productivity and are estimated at more than ten times the direct cost of corrosion maintenance, repair, and rehabilitation.

Health Implications

One of the major health effects of road salts is the threat posed to drinking water sources. Although road salts were not recognized as having a significant direct impact on human health in the Assessment Report for Road Salts, the loss of access to clean drinking water is a significant health impact. The drinking water chemical and physical objectives and guidelines for chloride (250 mg/L) and sodium (200 mg/L) are considered aesthetic objectives. However, the Ontario Drinking Water Technical Guidelines also suggest that a much lower standard (20 mg/L) should trigger notification of the local Medical Officer of Health so that physicians can be informed in order to deal on sodium restricted diets. The US Public Health service recommends much lower limits generally 25 mg/L of chloride.