Every year, millions of tons of salt is applied to roads. Its ability to lower the freezing point of water helps to keep roads open – and economies running – throughout winter.
But there is growing evidence linking the widespread practice of road salting to a host of damaging environmental effects.
Road salt harms ecosystems
A 2018 study concluded that the salinity of more than a third of rivers in the US has increased over the past 25 years.
Rising levels of salinity can have dire consequences for freshwater ecosystems and wildlife. Studies of small waterborne organisms such as mayfly larvae have shown they have increased mortality rates and decreased reproductive capability associated with higher concentrations of salt.
Tiny creatures like these form the foundations of food chains and are an essential source of nourishment for many larger animals.
Increased salinity also affects agriculture as crops are often irrigated using freshwater sources. Estimates suggest that more than half of US rivers will contain 50% more salt by the end of the century, which is not only bad news for the environment but for human life, too.
While rising salinity can be toxic to aquatic wildlife, studies have shown that moderately increased salt concentrations in roadside plants could have a positive effect on certain butterfly species. Male monarch and cabbage whites were shown to develop stronger muscles while females had larger eyes.
The impact on infrastructure and health
As well as disturbing ecosystems, salt can also wreak havoc on road surfaces and infrastructure, and can cause vehicles to rust quickly.
Estimates put the cost associated with road salting as high as $3,000 per ton across the US.
Due to the sheer amount of salt used during winter in the cold north-eastern US states, the region is sometimes referred to as the “Salt Belt”.
Salinification of drinking water also poses a significant risk to human health. In one high-profile case, the Flint water crisis in Michigan, pipes corroded and lead leached into the water supply after the city switched its source to the Flint River, which has high levels of chloride.
This left the water supply for over 100,000 people contaminated with lead, leading to serious health problems.
Road salt naturally dissolves into chloride and sodium ions, making it a risk factor for water contamination. These salt ions can dissolve all sorts of toxic metals and other substances, introducing them to the water supply and into the food chain.
Studies of Daphnia, or water fleas, have revealed a possible link between salt and disruption to the circadian rhythm, the natural body clock which regulates various processes in living organisms.
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What are the alternatives?
Road salt is not the only source of accelerating freshwater salinization: industrial processes such as shale extraction, mining and irrigation are also to blame.
As part of a drive to use less salt, some US states have tested alternative methods to de-ice roads.
In North Dakota, transport authorities use a combination of salt and sugar beet wastewater to tackle road ice.
Along with sugar beet water, various other natural vegetable brines have been tested as more environmentally-friendly alternatives to salt.
In some circumstances though, these alternatives have been proven to be detrimental to aquatic ecosystems as they promote algal growth - in addition to local residents complaining of their foul smell.
Other proposed alternatives include solar-heated roads which remain above 0°C to stop ice forming and melt snow.
Although there is currently no replacement for road salt in widespread use, awareness of the damage it causes to the environment is increasing, which may help encourage research into viable alternatives.