If you wander down the picturesque streets of the old town of Krakow, Poland, in winter, you may encounter a heady scent. In 2021, the city authorities adopted a novel approach to managing icy roads and paths, replacing the salt usually scattered by park attendants with spent coffee grounds donated by local cafes.
The aromatic substance is an attempt to tackle a growing concern: the grit we cover our roads in every winter is increasingly wreaking havoc on the environment. In particular, it messes with freshwater ecosystems that are already among the most imperiled on Earth. 鈥淚t is quite possible in the next 50 years, given current trends, that a lot of freshwater ecosystems could be salinised to the point of pretty severe ecological impact,鈥� says , a freshwater ecologist at the University of Toledo in Ohio.
Why is there salt in road grit?
Every winter across the globe, we chuck tens of millions of metric tonnes of salt onto our roads to lower the melting point of ice and keep them navigable for drivers. They provide entertainment, too. In Scotland, the fleet of 230 gritters are given funny names by the public: things like Snowcially Distanced, Grittney Spears and Yes Sir, Ice Can Boogie.
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But what is increasingly apparent is the extent to which road salt is running off into precious freshwater ecosystems, putting biodiversity at risk and threatening drinking water supplies. We need a sustainable alternative and, as the residents of Krakow are learning, it isn鈥檛 as simple as switching out salt for coffee grounds. Tackling the grit problem is going to require innovation, technology and a few more surprising, and perhaps smelly, substances.
Putting salt, or sodium chloride, on roads all began as an experiment in the US, starting in 1938 in New Hampshire and Detroit, Michigan. By the winter of 1941, around 5000 tonnes of salt were being spread nationwide each year. Today, that figure is estimated to be around 20 million tonnes. The impact on road safety is unarguable. A study in the US estimated that gritting the roads .
What effect does it have on wildlife?
At the same time, concerns about the damage grit is causing have been growing for a while. In 2005, researchers at the University of Maryland Baltimore County found that road salt seeping into rivers in the north-east US has made .聽Then, in 2022, Hintz and his colleagues found that increased salt pollution in freshwater ecosystems . These microscopic organisms are eaten by fish and insects, making them the bedrock of aquatic food chains.
Lakes are in trouble too. In 2012, and Ren茅 G盲chter at the Swiss Federal Institute of Aquatic Science and Technology found that the salt concentration in Lake Constance, the second largest freshwater lake in central Europe, . There are other sources of contaminants, including wastewater and agriculture, but the study found that road salts were responsible for 52 per cent of the increased salinity.
On current trends, within the next 50 years, more than a dozen US lakes are on track to record chloride concentrations that would be fatal for aquatic life. At this concentration 鈥� 230 milligram of chloride per litre 鈥� a deterioration in drinking water taste also becomes perceptible. The US Environmental Protection Authority recommends limiting chloride concentrations in drinking water to a maximum level of 250mg per litre.
There have been some unexpected beneficiaries of this salty pollution. Plants like Danish scurvygrass, wild carrot and bladder campion, typically found in coastal areas of the UK due to their high salt tolerance, are now proliferating along roadside verges, says , a specialist in freshwater ecology at Swansea University. Danish scurvygrass is now one of the fastest-spreading plants in the UK.
Despite some winners among salt-loving plants, Forman says the impact of salt on freshwater species is a huge concern. In particular, he says, the wood frogs of the US, whose embryos are severely affected by salt pollution, are under threat. One 2017 study by researchers at Yale University and the Rensselaer Polytechnic Institute in New York found that when exposed to road salt.
What alternatives are there?
These concerns have got people around the world searching for alternatives, and many are turning to food products. While people in Krakow are waking up and smelling the coffee on an icy morning 鈥� the abrasive and insoluble coffee grounds are thought to help make the ice less slippy 鈥� several cities in North America are spreading tens of thousands of litres of pungent sugar beet molasses, mixed with brine, as a replacement for grit on their roads. In Wisconsin, the cheese-producing capital of the US, meanwhile, there have been trials using cheese brine, a salty waste product of the manufacturing process, while other highway departments have experimented with pickle brine and waste from the brewing process. These food-related brines are less salty but also stickier than normal brine, meaning they are less likely to wash away.
Each of these alternatives has its drawbacks. Sugar beet molasses, for example, has been found to leach sugar into groundwater, where they are broken down by microogranisms, reducing the oxygen concentration of the water. It also contains potassium, which has been found to be highly toxic to freshwater mussels in their early-life stages. Sand, a widely used alternative, has been found to silt up riverbeds, affecting invertebrates. As for coffee grounds, 聽due to our surging tea and coffee habit. Other alternatives, such as calcium chloride and magnesium chloride, can, just as sodium chloride does, mobilise toxic heavy metals like lead, mostly from human pollution in soils and pipes, further polluting water.
The food-based alternatives haven鈥檛 been entirely welcomed by people either. Pickle brine and beet molasses are pungent, to say the least. One attempt to use cheese brine in Washington state a few years ago came to an end after a mother complained that her child was lactose intolerant.
When it comes to road salt alternatives, Hintz insists there is no silver bullet. Instead, the answer lies with using salt better, for example by employing smarter methods of spreading. Several grassroots initiatives are already trialing new approaches. Among them is Wisconsin Salt Wise, a partnership that has worked to drive salt use in the state to a 30-year low over the past decade. Chloride levels in the lakes in the state have begun to tail off, too. Allison Madison, who works for the partnership, says these reductions have been achieved by refitting gritting trucks to spray salt brine to wet road surfaces before they apply the salt, helping the salt to stick more efficiently, meaning less needs to be applied. As part of the partnership, one community in Glendale, Wisconsin, has repurposed an old fire truck so that it can spray brine for anti-icing purposes. Currently, Madison is assessing the viability of cheese brine pilot schemes before considering rolling it out further across the state.
Calibrating equipment to ensure the salt is spread in the right places is at the heart of the strategy, too, Madison says. She also stresses the importance of physically removing snow before any grit is applied. This method dates back centuries: the relics of which can still be seen in some towns and cities. In my home town of Sheffield, a hilly city in northern England, for example, a few historic wooden snow gates 鈥� once used to shovel snow off the roads and into rivers 鈥� can still be spotted around the city.
On the challenge of spreading smarter, technology is helping, too. Sensors can be fitted to gritting trucks to identify where salt is required. 杏吧原创s at the Charles III University of Madrid in Spain, for example, have developed which causes existing salt on roads to fluoresce, making it visible so workers know not to add more on top. And 鈥減recision forecasting鈥� can predict exactly where and to what extent ice will form.
This winter, England has made moves towards this kind of innovative method. National Highways, which is responsible for the country鈥檚 major road network, is embarking on a 拢15.4 million upgrade of roadside weather stations, equipping them with the latest forecasting tools to better inform its fleet of 535 gritting trucks. Where gritting previously relied on the general weather conditions of the surrounding area, the new stations are being fitted with atmospheric and road sensors that instantly relay real-time weather conditions on any given road with a weather station. The hope is that this technology can lead to a far more judicious application of grit, as the scattergun approach of previous winters finally runs out of road. With any luck, the roads of the future might not have to smell of a mix of coffee, pickles and beetroot.
