Furious campaigners say the Environment Agency has “failed the Chilterns” and must reconsider HS2’s entire Chilterns tunnel project – as they prepare to launch a legal battle to stop what they fear will be serious harm to the landscape.

Misbourne Environmental Protection, a group of local residents including hydrologists and geologists, fear that the globally rare chalk stream the River Misbourne is at “very significant risk” from pollution and even complete destruction because of the tunnels under construction for HS2 – along with the beautiful Shardeloes Lake in Amersham and the chalk aquifer, which supplies 20 per cent of London’s fresh water.

The group is particularly concerned about HS2’s use of bentonite – a type of clay that can swell and turn into a gel when dispersed into water – during construction of the tunnels, and the effect it could have on wildlife.

Liquid bentonite is highly polluting and can damage plants and animals if it gets into watercourses, a safety document produced by Network Rail claims - although HS2 disputes this.

According to a Freedom of Information response from Align JV, HS2’s main works contractor, around 2,500 tonnes of bentonite have been “lost” at the Chalfont St Peter vent shaft site up until November 2020.

Amid their concerns, Misbourne Environment Protection have applied for a judicial review against the Environment Agency (EA), HS2 Ltd and Align.

They claim that “time and again” statutory bodies like the EA are failing to hold HS2 to account and properly scrutinise their work.

Explaining why they are “regrettably” launching a legal challenge, the group said: “Parliament passed the Act which approved the construction of High Speed 2, but they did so expecting statutory bodies, such as the EA, only to approve these works after a full and transparent assessment and review to ensure the designs comply with all of the legislation designed to protect the natural environment.

“During petitioning, it was stated that before they approve the tunnel applications, the EA would need to be satisfied that all potential risks to the river and the surrounding environment had been mitigated - but the EA approved the application in under eight weeks, and so approved the use of hazardous substances that will pollute the aquifer and cause significantly higher risks to the Misbourne than were disclosed to Parliament.”

They hope the legal challenge will force the EA to reconsider the entire tunnel project.

Andrew Cordiner, treasurer of Misbourne Environment Protection, said: “Time and again Parliament has promised to protect the Chilterns. Time and again Natural England and the Environment Agency promised to protect our wildlife, chalk streams and aquifer.

“Time and again all of these institutions have failed us and failed the Chilterns.”

A spokesman for HS2 said bentonite is of low toxicity, is not defined as a hazardous substance and is safe to use if managed appropriately. They are mainly using it in the construction of the tunnel’s ventilation shafts to reinforce the wall excavation before concrete is pumped in.

It is then removed, cleaned and reused, with an HS2 spokesman saying: “Bentonite is used in this way in building sites across the country every single day of the year without any complaints.

“It is not in any way controversial or unusual.”

Chris Wilson, Misbourne Environmental Protection secretary, has hit back at this, saying: “They may say that bentonite is a natural substance - so is asbestos.

“You wouldn’t want either in your aquifer or watercourses, particularly public drinking water supplies.”

HS2 also disputed that the EA has had eight weeks to approve the application, saying they have been working together for “almost eight years” before work started, and also had approval from Affinity Water ahead of tunnelling.

A spokesman added: “HS2 take our responsibilities incredibly seriously and we have a world-leading team of engineers, hydrogeologists, and environmental scientists in place to ensure that the tunnelling operation affords the right protection for the environment, and is completed as safely and efficiently as possible.

“Our permissions have been granted by the Environment Agency and Affinity Water following considerable engagement with those organisations and there has been no significant effect on the aquifer since the start of tunnelling five months ago.”

The EA said it could not comment on ongoing or potential legal matters.

See the crowdfunding page for the legal challenge at www.crowdjustice.com/case/savethemisbourne/