M25 runoff ponds possibly full of hazardous waste due to National Highways ‘avoiding liabilities’

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Runoff ponds are situated around main roads to collect surface water that exits the carriageway via drainage systems. This helps with water management and prevents flooding but due to the presence of pollutants in the runoff, they must be cleaned regularly or there is a risk that the harmful chemicals will enter the wider catchment.

In its response to a Freedom of Information (FOI) request from NCE, National Highways identified 94 runoff ponds it is responsible for around the M25, as well as 54 interceptors which are used to catch oil or sediment on its way from the highway to the ponds.

However, National Highways could only provide the most recent maintenance dates for seven of its 94 runoff ponds on the M25, with the remaining 87 having their maintenance described as “not yet required”.

In its FOI response, National Highways said: “A number of ponds have been added to the system recently and are due their first visit in 2024. These are shown on the list.

“One pond is shown as ‘Unknown’ and we are working to understand why. It is likely to be because a change in the asset reference number has de-linked the inspection record.

“For all ponds, the de-silting is based upon highway standards and best practice guidelines but is also aligned with condition records from inspections.

“We employ a contractor to undertake all of the inspections and maintenance of these assets.

“Therefore, those ponds marked as ‘Not yet required’ were either maintained prior to the start of the current contract and do not require a de-silt based on their inspection score or were installed recently and/or do not require a de-silt based upon their inspection score.

“Where assets were de-silted, the last record year is stated.”

Road runoff

When vehicles are driven, they undergo wear and tear. Solid parts like tyres and brake pads shed tiny particles into the environment. In addition, liquids like fuel, oil and screen wash fall onto highways and into drainage systems.

A significant proportion of these pollutants will exit highways via less-managed pathways like gulleys, and some will enter fully managed drainage systems which could feature hard engineering solutions such as oil and water separators and runoff ponds. From these drainage routes, the road runoff pollutants end up in the wider water catchment.

This process has significant failings that contribute to waterway contamination, a major issue in the UK.

Only 14% of rivers meet ‘good’ ecological status in the UK according to the Environment Agency. The government says a river has good ecological status when there is only a ‘slight change from natural state as a result of human impact’.

There are in excess of 1M outfalls across the whole country. The Strategic Road Network (SRN) alone has 18,000 deemed to be at risk of discharging potentially contaminated runoff, and the SRN represents just 3% of the total road network.

National Highways and the M25 runoff ponds

National Highways is the public body responsible for the SRN including associated pollution.

Various types of ponds have been built from the late 20^th century on England’s SRN as environmentalism became a mainstream concern, including during the construction of the M25 which started in 1975. The orbital motorway was opened in 1986.

Stormwater Shepherds is a not-for-profit working to improve the health of waterways and its director of operations UK Jo Bradley managed a joint Environment Agency/Highways Agency project to prioritise the highway outfalls on the SRN in terms of their pollution risk. She has also worked with design engineers to create effective treatment schemes for some of those priority outfalls.

Explaining the history and importance of the ponds, Bradley said: “The runoff ponds that were built at the time that the M25 was built will be flood storage ponds, built to attenuate flows off the motorway, to manage the peak flow of the runoff during rain events and to manage the volume of runoff entering local streams and rivers.

“However, their role in pollution control was recognised and well understood. The guidance published in 1993 contained design details relating to pollution capture and retention, and described the function and design of oil separators.

“They are important because the construction of a motorway changes the hydraulic behaviour of rainfall across the catchment by introducing a large impermeable area, increasing flood risk, and often draining all the rainfall to one discharge point.

“This would lead to increased flood risk, increased pollution and increased risk of erosion and physical harm to the river unless these ponds/reservoirs were installed.”

Increased flood risk and pollution

As National Highways is unable to identify when the majority of M25 runoff ponds were cleaned, Bradley argued that it is failing in its responsibilities as a public body. This is increasing both flooding and pollution risk.

“[The data shows that] pond condition inspections are being carried out, but the sediment is not being removed from the ponds,” Bradley said.

“The guidance says that the accumulated sediment from ponds like these should be removed every 10-15 years to retain the functional capacity of the device and to prevent pollutants from being flushed out.

“This list shows no evidence that these ponds (apart from four of them) have ever been emptied since they were built which, for some of them, was over 30 years ago.

“This failure to remove sediment means that the devices no longer provide the flood risk management for which they were designed, nor prevent pollution effectively.”

National Highways accused of avoiding liabilities

National Highways’ reasoning for the lack of data on cleaning raises further questions. It is unclear why the roads body is not sharing the last maintenance dates on the ponds which it says were “maintained prior to the start of the current contract”.

This obfuscation makes National Highways appear to either not know about its historic maintenance records or it is choosing not to share the data.

Thames21 describes itself as “the voice for London’s waterways, working with communities to improve rivers and canals for people and wildlife”.

In response to the sediment removal “not yet required” classification from National Highways, Thames21 head of improving rivers John Bryden questioned: “How are they coming up with that criteria? How are they making that decision?”

He continued: “I think there are people within National Highways who are aware of their liabilities [regarding pollution from roads runoff] but that it’s just been de-prioritised.”

He said he thought the reason for the de-prioritisation at National Highways was due to a lack of funding to maintain the ponds and interceptors. Additionally, Bryden said, admitting liability to the Environment Agency via applications for pollution permits would make National Highways “liable for huge costs”.

Critical interceptors sporadically installed

Of the 94 ponds, only 54 have interceptors associated with them. Of those 54, a third (18) have their last cleanse listed as “in forward programme” by National Highways.

Interceptors are devices that are designed to capture and retain oil in the highway runoff and to prevent it from washing into rivers.

“For highway runoff, the inclusion of an interceptor is advisable, if not essential, because there will always be a risk of hydrocarbon spills on the carriageway,” Bradley explained.

“The design of the interceptors was well understood when these ponds were built and design guidance was available.

“Their function in capturing sediment was also recognised. The need to maintain them regularly to allow them to function properly was also recognised and the ones that have been included on some of the sites are very large devices.”

Bradley was uncertain why there are no interceptors included at some sites and suggested that it could be due to National Highways anticipating lower levels of runoff from these parts of the M25 or because there was not space to install them.

Many of the attenuation ponds are located in cramped spaces near rivers, junctions or low-lying parts of the road.

Bradley suggested that some of the ponds may have had a forebay rather than an interceptor. Forebays are devices near the inlet of a pond or basin that can capture a proportion of the sediment before it can pass into the pond or basin. They are designed to be easy to empty, with vehicular access and they often have a concrete base.

“However, if these forebays are not emptied often enough, they simply fill with sediment and then the sediment starts to fill the rest of the device,” Bradley added.

“The capture of sediment in [interceptors and forebays] extends the function of the pond, allows the sediment to be removed for treatment cheaply and easily, and isolates the pollution from the local wildlife.

“Their ability to capture sediments was recognised at the time that these structures were built, although our understanding of that has improved since.”

Reacting to the “in forward programme” classification for when some interceptors will be cleaned, Bryden said: “It’s highly uncertain what this is, but it’s an indication that these interceptors need maintenance.”

Comparing the interceptors’ last inspection against the date of their last cleanses, Bryden said “it means [National Highways] knows it needs to do maintenance, but it hasn’t done it for nearly a year”.

Of the 18 interceptors listed as having their maintenance scheduled “in forward programme”, the latest inspection was on 1 August 2023.

Reflecting on the fact that National Highways’ data shows that many of the M25 runoff ponds don’t have interceptors, Middlesex University professor of environmental science Lian Lundy said that this “opens up more questions”.

“Why do some of them not need [interceptors installed] and some of them do?” she queried.

“Give that motorway runoff is generally heavily contaminated, you’d expect them all to have them.”

She added that the interceptor inspection dates are “suspiciously recent”.

“From a research perspective, this doesn’t tell me anything about how [the ponds and interceptors] are performing or if the process is robust,” Lundy said. “What is their inspection programme and is it fit for purpose?”

As part of its response to NCE’s FOI request, National Highways said: “Interceptors have been matched where possible using GIS [Geographic Information System], and the last date of inspection and/or cleansing added.

“Similarly, the ponds are cleansed on a condition requirement following the inspection results.”

The Department for Transport (DfT) said that after the general election on 4 July 2024, funding for the 2030 Water Quality Plan is expected to be confirmed in the third roads investment strategy (RIS3) for 2025 to 2030.

The DfT noted National Highways has a statutory duty to ensure that discharges from its network into the receiving water environment do not cause pollution and highlighted that permits are a matter for the Environment Agency.

National Highways did not deny that the majority of its M25 runoff ponds may be full of hazardous waste.

National Highways director of environmental sustainability Stephen Elderkin said: “We are committed to addressing the risk of pollution from road runoff. Our Water Quality Plan 2030 sets out a high-level programme of work to achieve this.

“We’ve also invested in a programme of research to understand the risk of pollution from microplastics in road runoff.

“High quality road infrastructure can support positive environmental outcomes, and that is at the heart of our work in this area.”

The Environment Agency was approached for comment.

National Highways has no permits for road runoff pollution

In May 2024, Stormwater Shepherds and CIWEM (the Chartered Institution of Water and Environmental Management) published a report which found National Highways and other highways authorities in England have no permits in place for managing the pollutants which are washed off their roads.

The report described road runoff as “hazardous”. It said: “Research by the Environment Agency and Highways England (predecessor to National Highways) has identified a number of pollutants commonly found in highway runoff: copper, zinc, cadmium, fluoranthene, pyrene and polyaromatic hydrocarbons (PAHs).

“These are identified as specific pollutants, priority substances or priority hazardous substances (substances of major concern for European waters due to their toxicity, bio-accumulating properties and/or persistence in the environment that under the Water Framework Directive and its regulations should be progressively reduced or eliminated from surface water).”

Stormwater Shepherds and CIWEM’s report also said: “The Environmental Permitting Regulations 2016 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.”

It went on to say: “National Highways and other highway authorities in England have no permits in place for managing pollutants in any of their outfalls. Water company surface water sewers that convey highway runoff into the water environment do not ordinarily have water discharge activity permits either.”

The normal process for the regulation of polluting activities is not being followed by National Highways, according to Stormwater Shepherds and the CIWEM, because it “has a voluntary Memorandum of Understanding (MoU) with the Environment Agency”.

In the Environment Agency’s response to a request for comment from NCE at the time of the May 2024 report, it said a change in the regulations and funding would be required to enable the agency to undertake a permitting-led approach and such an approach would be disproportionately resource-intensive to establish and manage.

M25 runoff pond dataset

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