PROBLEMS OF CURRENT PRACTICE THE ENVIRONMENT

The negative impacts that chloride based de-icers can have on areas immediately adjacent to roads has been widely reported over the last three decades by Schraunfnagel (1965), Avery B D I ri (1992). These include degradation of vegetation along the roadside environment, sodium infiltration of drinking water and corrosion to vehicles and infrastructure.

Such effects are observed because salt does not remain solely on the carriageway. It is transported onto highway verges either directly from salt spreaders or with the assistance of wind and bounce from the pavement surface. According to Burtwell (2004b) the `salt mist has an impact on the local soil and vegetation, with the greatest concentration of salt found within 3m of the pavement edge and within the top 1m or so of soil`. Surface runoff from stockpiles and the carriageway can also transport salt into the natural environment.

The most frequently reported negative impact of sodium chloride is the influence its use has on roadside vegetation and soil. Incidents of vegetation injury were first reported in Minnesota, USA during the 1950s. Further incidents were quickly reported, for example in 1957 New Hampshire, USA reported 13,997 dead trees along a 3,700 mile long highway (Avery 1973).

Further investigations by Backman et al. (1996) concluded that `The damage was by far the most pronounced in the immediate vicinity of the roads`.

Vegetation damage occurs due to the intake of salt ions from contaminated soil or water, but it can also occur via salt spray from vehicles. This damage is primarily caused by excessive exposure to chloride ions rather than sodium ions. Vegetation damage can be observed by browning and falling leaves, stem dieback, stunted or abnormal growth, or premature death.

The impact that road salting has on vegetation is complex and highly site-specific. The severity of the impact can depend on the amount of salt, type of soil, distance from the roadway, slope of the road, total precipitation, wind direction, temperature, soil texture and drainage and the plant species. This was highlighted during “

impacts generated by de-icing salt (Y.C.Jin et al. 2004) `A different set of geological characteristics could have led to an entirely different outcome`.

The rate of salt application is also positively correlated with salt concentrations in roadside soil, D I . Although damage to vegetation is mainly from chloride ions, salt damage on soil is primarily due to the accumulation of sodium ions. According to TRB (1991), long-term salt accumulation can cause high soil density and lower permeability. These factors adversely affect plant growth and erosion control.

The impact of salt on vegetation and soil are interconnected with impacts on surface and groundwater. The effects of salt on surface water are confined mainly to small streams running adjacent to heavily salted highways. However for larger water systems the effect, in general, is significantly reduced. This is because the higher concentrations are quickly diluted. For example, Schraunfnagel (1965) found chloride concentrations greater than 10,000mg/L in highway spring runoff in Wisconsin, however adjacent surface waters only demonstrated a maximum concentration of 45mg/L.

Salt has a corrosive influence on infrastructure such as bridges, parking structures and most costly, to motor vehicles. The chloride ions in the sodium chloride disrupt the natural protective films of the metal surfaces and increase the conductivity of water. This induces and accelerates corrosion, see Figure 2.7 (TRB 2007). For example, when considering bridge decks, `the chloride ions in salt penetrate concrete and cause reinforcing steel bars to rust, resulting in cracking and fragmenting of the surrounding concrete` (TRB 1991). According to TRB (1991), `During the past 30 years in the Northeast and Midwest (U.S), road salt has caused more premature bridge deck deterioration than any other factor`.

Figure 2.7 Localised corrosion of steel reinforcements within concrete (TRB 2007)

The significance of the problem of corrosion is clear when further considering the summary of TRB (1991), which states that the overall cost estimate of corrosion in the U.S is between $3.5 billion to $7 billion per year.

A summary of the estimated cost of continued salt exposure on motor vehicles and infrastructure are detailed below in Figure 2.8 (TRB 1991). UK estimates by Thornes (2000) reported that the total cost of corrosion in the United Kingdom was £150 million.

Figure 2.8 Economic impacts of salting on infrastructure (TRB 1991)

Similarly to economic assessment of the overall benefits of current practice, monetary value has been placed on the environmental impact. The overall environmental impact as a consequence of using dry salt as a precautionary treatment in the United Kingdom has been estimated to be

£160 million per winter (Thornes 2000). Vitaliano (1992) reported that the true cost of salt used for de-icing roads in the United States is estimated at more than US$800 per ton (approximately

£511 per ton at 2003 exchange rate).

Whilst it can be concluded that current practice provides an overall economic solution, if the content of chloride ions reaching the environment were to be reduced, significant environmental and associated economic benefits would be gained. Burtwell (2004b) summarised the benefits `not least a reduction in the cost of structural repairs, a reduction in the contamination of highway runoff, reduced concentrations in watercourses, and reduced effects on flora and fauna`.

The use of best practice guidelines can significantly limit the associated environmental impacts of current practice, as documented by the Highways Agency Network Management Manual Part 5 and Road Liaison Group (2005). These guidance documents are produced to ensure that the service provider considers all relevant materials available and selects the most appropriate treatment for each part of the network on each occasion, whilst also taking into account the cost of storing and spreading and the environmental impacts of the material. For example `To be most effective, salt should be spread before ice forms or snow settles on the road` (Highways Agency Management Manual Part 5).

The manual also recommends that the service provider makes full use of specialised road weather forecasting services and the Road Weather Information System. Best practice guidance supports the Salt Institute (2004) philosophy for sensible salting which is based on the principle

`just enough and no more`.