Wandle River Oil Spill 'Worst in 20 Years'

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May take years to recover from diesel that leaked from bus depot

Dr Jack Hogan on the Wandle

February 21, 2025

The ‘devastating’ diesel spill that polluted sections of the River Wandle in this week could damage water and wildlife for years, according to an expert. Despite an extensive clean-up taking place, many fear the work could not be enough to protect wildlife on the river against the ‘highly toxic’ diesel spill which came from a bus depot.

Politicians are now calling for those responsible to pay, fearing vital conservation work has been undone. River Wandle expert Dr Jack Hogan has been working on the river for most of his life and has been active in conservation work as a Catchment Officer for the South East Rivers Trust (SERT).

He told the Local Democracy Reporting Service (LDRS) how the 4,000 litres of diesel that polluted the river on Tuesday (February 18) could have long-lasting consequences. He said: “It’s devastating, but we won’t know the extent of it yet. This damage is not just measured in days or weeks but potentially years. Judging by what we have seen, this is potentially the worst incident since 2007.”

4,000 litres of diesel reportedly leaked from a bus depot storage tank in Thornton Heath on Tuesday 18 February. Picture: Liberal Democrats

Contractors Aller and Allan are currently cleaning up four sites along the river. They have installed ‘booms’ and electrostatically charged pads designed to extract the diesel from the water source. The clear-up sites are located at:

Water Meads near Morden
The Paper Mill in Carshalton
Morden Hall Park
Dean City Farm

However, while the contractor has removed some of the spill’s most visible effects, Dr Hogan says the work is ‘mitigation rather than clean up’. He added: “What people see on the surface is very visual, and one part of it is visible, but as that disappears and the diesel breaks down into small particles, it goes down into the water column.

“That can affect fish gills, their reproduction, and their behaviour, and it can get into invertebrates and the soil. It is highly toxic. Diesel is the worst of all the oils for the river.

“I saw a scientific article about a diesel spill in Wiltshire. When they looked at the invertebrates, they found a 50 to 90 per cent loss immediately after the event, and the recovery process took several years. If you can’t trap the diesel at the start before it breaks down, then there is no way to clear it out, so it will persist for a long time. I believe it will eventually be degraded and consumed by bacteria in time, but this story hasn’t ended then.”

Immediate clean up efforts have taken place on the river since the spill. Picture: Dr Jack Hogan

Soon after the spill, residents reported seeing nesting birds struggling to clean themselves of the diesel. So far, Dr Hogan and his colleagues have only confirmed the death of one eel on the river and are hopeful that most mature fish have survived.

The spill reportedly leaked from a storage tank at the Arriva bus depot in Thornton Heath earlier this week. Experts believe the spillage entered nearby storm drains before entering the river downstream from the Beddington Water Treatment Works.

Merton Council has warned residents and their pets to ‘keep away from the contamination’ and that the Environment Agency (EA) is investigating the spill. However, a number of local politicians have been demanding answers following the incident.

Carshalton and Wallington’s Lib Dem MP, Bobby Dean, has called upon the Secretary of State for Environment, Food and Rural Affairs, Steve Reed, to visit the river and ‘witness the extent of the damage first-hand.’

Mr Dean told the LDRS: “There is a huge environmental disaster unfolding on the Minister’s doorstep, yet he is nowhere to be seen. I have written to him asking that he join me and local conservation groups in visiting the affected areas to see the scale of the devastation.

“This spill has undone decades of work by hundreds of volunteers who have helped restore the Wandle from an ecologically dead river to a thriving ecosystem. While we await the findings of the EA investigation, one thing is clear: this should never have happened. Those responsible must pay.”

In statements published earlier this week, the Labour MPs for Mitcham and Morden and Croydon West, Siobhain McDonagh, and Sarah Jones, also called for accountability.

While the public is being asked to stay clear of the clean-up effort, Dr Hogan and SERT have said this incident should act as a ‘galvanising moment’ to encourage people to act to protect the river’s future. Dr Hogan, who grew up near the river, also recalled the last devastating spill on the river and its impact.

He said: “The 2007 incident was a big trauma incident in my life. I remember seeing the river die in front of me. In that incident, there was a bleach leak from Beddington Treatment Works. There was an accident on site that bleached about 7km of the river, and thousands of fish were killed.

“That was a big dramatic event, and a lot of the community came out to rescue fish. To not know the extent of the damage but fear there is creeping awfulness to come is horrible. I was shocked, mingled with a sense of disgust and outrage. Of all the rivers in our patch, from Basingstoke down to the Kent Coast, it is the Wandle that gets hit the most.”

Those interested in supporting the river are encouraged to contact SERT for future conservation work and visit the Wandle oil spill crowdfunder page. They are also pushing for more water monitoring volunteers, who can help assess the river’s health and gauge the current crisis’s impact.

Speaking on the need to help the river, Dr Hogan said: “After all of this, it’s not just a case of us trying to get the river back to how it was on the weekend, but also about how we make this river more resilient, how we can restore natural processes and make it more like a chalk stream again. It’s such a precious river.”

An EA spokesperson told the LDRS: “On receiving reports of an oil spill on the River Wandle, officers were immediately deployed to limit damage to the local environment and working with the London Fire Brigade we have put in place measures that stopped the leak.

“We are currently onsite remediating the spill, gathering evidence, and assessing the environmental impact for our investigation, with the pollution source suspected to be a bus depot storage tank in Thornton Heath, Croydon. We’re likely to be onsite over the weekend.”

Harrison Galliven - Local Democracy Reporter