A new report by CIWEM (Chartered Institution of Water and Environmental Management).and Stormwater Shepherds on highway runoff and the water environment is warning that a “toxic cocktail of pollutants” is running into rivers and water sources every time it rains and is calling for urgent action to tackle the issue.

The report Highway runoff and the water environment is warning that across England’s road network, surface water runoff polluted with microplastics, trace metals, hydrocarbons and other organic pollutants is washing into the environment and entering rivers through outfalls from the road network.

However, the highly critical report is highlighting that highway outfalls are not currently subject to regulation under the permitting regime, due to a voluntary agreement between the Environment Agency and National Highways and a lack of routine environmental monitoring of outfalls by the two organisations and local highway authorities.

Polluting discharges to water must be controlled under the Environmental Permitting Regulations 2016, subject to a sequence of tests being met. The Regulations dictate that any discharge of poisonous materials into a watercourse is a criminal offence, unless the discharge is made in compliance with a permit to discharge.

Nationally, there are more than 18,000 known outfalls associated with the motorway and trunk road network operated by National Highways. According to the report, there could more than a million local highway drains discharging directly to watercourses, although it cautions that there is currently no reliable data on their number, location and condition..

The report is based on samples taken by Stormwater Shepherds in the North West of England between 2021 and 2024, as well as older research – the samples provide a fuller picture than has previously existed of the extent and effects of pollution runoff from different types of roads.

The findings show that the discharges fail the Environmental Quality Standards (EQS) for several polyaromatic hydrocarbon (PAH) pollutants and that the pollutants cause harm to the waters into which they are discharged.

Samples analysed by the research from nine locations (three motorway outfalls and six local highway outfalls) show that the discharges concerned fail the EQS for several PAHs. These consistently exceeded their maximum allowable concentrations – and in one case were 730 times higher. The EQS levels were exceeded in samples taken at outfalls from both motorways and local roads.

The report’s authors say the findings raise the question of whether road runoff discharges should require permitting (as other polluting discharges would) as a mechanism to control highway pollution.

Image credit: Highway runoff and the water environment

Highway runoff contains multiple polluting poisonous and harmful substances from multiple sources which are deposited on the road surface as well as passing into the atmosphere around the road, including:

• Road surface and markings worn down through use
• Microplastics particularly from vehicle tyres
• Dust containing heavy metals from brakes, clutches and other components
• Fluids from windscreens, engines, brakes and hydraulics and fuel additives
• Combustion emissions and unburned fuel
• Spillages from accidents and leaks on vehicles
• Road salt
• Herbicides used on road verges
• Metals from road furniture
• Soils, sand and sediment from tyres or from washed or blown sediment onto the road surface from surrounding land

Currently, the Environment Agency does not have a specific monitoring programme in place to assess the impact of highway runoff on the water environment. Likewise, National Highways and highway authorities do not routinely monitor the discharges of highway runoff entering the water environment from their network. Instead they rely on their own risk assessment tool.

Highway runoff discharges are not currently covered under a permitted regime - Section 100 of the Highways Act 1980 gives a highway authority the power to discharge surface water from highway drains into any inland waters, or into any tidal waters.

Ordinarily a discharge of polluting matter into a watercourse would require a permit under the Environmental Permitting (England and Wales) Regulations 2016. The report says the Environment Agency currently does not require highway authorities to apply for permits for discharges from urban surfaces - despite having the powers to do so.

National Highways and other highway authorities in England have no permits in place for managing pollutants in any of their outfalls. The report points out that water company surface water sewers that convey highway runoff into the water environment do not ordinarily have water discharge activity permits either.

According to CIWEM and Stormwater Shepherds, the research illustrates that the levels of pollution are sufficiently high that the discharges should – according to regulations – be controlled using the environmental permitting regime and that the voluntary agreement with National Highways is not effectively protecting rivers from pollution.

The report suggests that putting a permitting regime in place for high-risk outfalls from the road network would deliver a number of important benefits, including:

• it would enable the Environment Agency to control them more effectively.
• it would generate an income to allow the Environment Agency to resource the control of the outfalls
• it would provide an incentive for highway authorities to reduce pollution at source by working with vehicle manufacturers and to develop more effective treatment devices

In addition, treatment technologies and devices for managing highway runoff are “readily available” in the UK. The report’s authors are also calling for far greater emphasis on the control of pollutants at their source.

Sustainable and equitable funding mechanism needed to control highway pollution

CIWEM and Stormwater Shepherds say a sustainable and equitable funding mechanism must be found to control highway pollution. However, they warn:

“Achieving pollution control across many hundreds or thousands of outfalls is going to cost billions of pounds over several decades. The design and construction of a treatment scheme for an existing, polluting motorway outfall can cost millions of pounds.”

Potential funding options include:

Introduction of a Stormwater Utility Levy

Similar to that in Germany, where each household pays a monthly fee into a central or regional fund to pay for better management of surface water. This can include the design, construction and operation of retrofit schemes, as well as the operation and maintenance of new SuDS schemes serving new roads and developments. In Germany and parts of the USA the fee is levied according the impermeable surface area of a property.

Introduction of product levies

Product levies could be introduced to create a fund specifically to pay for the retrofitting of treatment devices at polluting highway outfalls. Products that could be targeted include tyres, fuel oils and brake pads.

Department for Transport should provide funding

The outfalls that are owned and operated by National Highways should be remediated by National Highways with funds provided by the Department for Transport.

The control of pollution from the National Highways network is a statutory duty – CIWEM and Stormwater Shepherds are recommending that this should be paid for from core budgets by the Department for Transport and an ambitious settlement for environmental protection should be included within National Highways’ next road budget.

However, the report acknowledges:

“These changes and new funding mechanisms will be unwelcome and difficult to negotiate, but it is essential that a funding mechanism is found to deliver pollution control for highway runoff across the UK if the health of our rivers is to be restored.”

Click here to download the full report