Road runoff

Did you know that the road surface collects lots of different pollutants? These come from all sorts of places – most commonly, residue from petrol and oil, as well as tiny particles created by tyre and brake wear. When it rains, these pollutants mix with water to form ‘road runoff’.

As road runoff washes away from the road surface, it can enter the environment via two mechanisms: outfalls and soakaways. Soakaways, as the name suggests, let road runoff soak into the ground. Outfalls direct road runoff to a combined sewer where it mixes with sewage. Most of this sewage-runoff cocktail goes to a wastewater treatment plant, where harmful particles such as microplastics and specific chemicals can be removed. However, not all contaminants are removed, and those that are can end up in a product called sewage sludge which is then spread on to agricultural land as a fertiliser. In addition, during wet weather, sewage may be released into rivers and seas without any treatment.

Research suggests that road runoff is one of the most significant sources of pollution that doesn’t come from a single, identifiable location, like a factory. Additives found in tyre particles can kill species including coho salmon, brook and rainbow trout. Other chemicals present in tyres can harm microalgae and mussels. Polycyclic aromatic hydrocarbons, or PAHs, have long been known to negatively impact aquatic animals, and they are found in tyres. It’s clear that road runoff isn’t good for the marine environment, but the scale of the risk is currently poorly understood.

National Highways is responsible for operating and maintaining motorways and major A roads in England. That means they’re also responsible for the soakaways and outfalls connected to these raods.

Currently, National Highways uses a risk assessment tool to determine if their outfalls and soakaways are likely to impact water quality. At the time this was written, of the 18,000 outfalls and 7,975 soakaways on their network, National Highways class 1,236 as having “a potential high risk of pollution”. Despite this, National Highways only proposed to install mitigation schemes up to 17 outfalls/soakaways in 2024-25. In the meantime, road runoff may be causing irreversible pollution of our natural environment.

The Marine Conservation Society are calling for change in three key areas.

• A national monitoring programme to assess the scale and severity of pollution from road runoff: Despite knowing that road runoff contains a cocktail of harmful contaminants like chemicals and tyre particles, there is no requirement for regular monitoring of highway outfalls. A national monitoring programme would help us develop a better understanding of the impact of these outfalls on the marine environment.
• Wider monitoring of contaminants in the environment: Currently, new and emerging pollutants are not included in water quality monitoring. What’s more, monitoring is not consistent across marine, freshwater and terrestrial environments. Sufficient monitoring is vital for recognising contaminants of emerging concern and will support a better understanding of the true impact of different pollutants on ocean health.
• Upstream solutions to reduce microplastics from tyres and the chemicals associated with them in the environment: The European Union is currently carrying out research and pursuing legislation which will reduce tyre particle pollution. The United Kingdom should aim to match, if not exceed, the ambition shown by the EU. We also believe that providing information on tyre wear rate will enable consumers to make informed choices.