On average in Britain we throw some two million tonnes of salt on to our roads every year, and during the recent bad weather the Highways Agency was spreading 25,000 tonnes a day. Yet have you ever stopped to wonder what happens to all the salt after it has been spread? After all, if the ancients used it to sterilise land, it must have some environmental effect. So where does it all go?
When salt is applied to any icy road, it dissolves into the meltwater to form brine and breaks down into sodium and chloride ions. Chlorides are resistant to natural decomposition and can move freely through the surrounding environment. They can parch vegetation and leach into waterways and contaminate ground water. Research in Canada has shown that salt run-off from roads can increase local chloride levels to between 100 and 4,000 times normal levels.
The most visible effects of road salt can be seen in plants which are prevented from absorbing water and nutrients, resulting in dehydration and death. High chloride concentrations can alter the soil’s pH chemistry and elevate levels of heavy metal pollutants, while at the same time causing a loss of soil structure and killing off micro-organisms. All of this can render soil unable to support plant growth.
The Canadian study revealed that urban waterways can see chloride levels rising to more than 1,000 milligrams per litre, and sometimes as high as 18,000 milligrams per litre. Other studies have shown that as much as ten per cent of aquatic wildlife is adversely affected by levels as low as 240 milligrams per litre.
We’ve all seen the effects salt has on creatures such as slugs but it can also seriously harm amphibians who, with their permeable skin, are particularly vulnerable. In fact, amphibians have been observed refusing to cross treated roads which can separate them from their traditional breeding areas. And even if they do make it, pools with high salt levels have been shown to kill frog and newt eggs.
Another slightly bizarre side effect is the addictive quality salt has on our wildlife. Mammals such as deer, rabbits, and badgers are attracted to roadside salt pools which have long been identified as a major factor in levels of road kill. Birds are particularly hard hit, with those that are not poisoned outright left in a sort of druggy haze that slows their reaction times and makes them apathetic to the threat of approaching vehicles.
Run-off is not the only way road salt can spread into the environment. As anyone unfortunate to be walking beside a road as a gritter truck thunders past can attest, a great deal of it ricochets off the road surface at near-ballistic speeds. Also on high-speed carriageways passing car tyres throw brine into the air where it spreads as an aerosol over hundreds of metres.
Eventually the chlorides can make it to the water table, making drinking water saltier and increasing pipe corrosion which releases lead and other toxic metals into our water.
Obviously nobody is suggesting that we stop gritting our roads, but even if you dismiss all this as scaremongering, please spare a thought for our highway’s fizzing worms and stoned birds.
Scary stuff, don’t you think?
