The Misbourne Environmental Protection (MEP) have appeared in High Court as they ask for judicial review of the consent given by the Environment Agency for HS2’s tunnel twin bore under the Chiltern hills.

A group of campaigners, local geologist, conservationist are the River Misbourne, one of a number of rare chalk streams, sitting in an Area of Outstanding National Beauty near Shardeloes Lake.

MEP also believe the groundwater in the underlying aquifer is also at risk.

Represented by law firm Leigh Day, the campaigners argue that a pollution incident at Chalfont St Peter earlier this year has laid bare the uncertainty of the methods, risks and impacts of the HS2 tunnelling project.

The ventilation shaft incident at Chalfont St Peter released approximately 2500m3 of Bentonite slurry passed through chalk and into the groundwater.

MEP believes the incident displays "huge flaws" in the HS2 engineers’ understanding of the geology and the EA should have updated its assessment of the all-round impact of the project.

MEP spokesperson Dr James Conboy said: “The water bodies affected by this vast tunnelling project are some of the most important in South East England.

"The Aquifer supplies 20 per cent of London’s fresh water.

"Shardeloes Lake carries an abundance of unique wildlife and is a visually stunning body of water popular throughout the Chilterns.

"The River Misbourne is a globally rare chalk stream and one of only 210 left in the world.

“We want to force the EA to reconsider the entire tunnel project, take proper account of the Water Framework Directive, and consult interested parties before reaching a decision.

"This should result in a deeper tunnel running entirely through solid chalk when passing under the Misbourne, and external monitoring of the tunnelling operation, to ensure an immediate response to any incident which might endanger the Aquifer, Shardeloes Lake or the Chalk Stream.”

Leigh Day solicitor Rowan Smith said: “When the HS2 project was approved in outline by Parliament, major assumptions were made, and assurances given, about the plans.

"It is now apparent that the EA did not, despite promises, ensure that all potential risks to the river and the surrounding environment had been mitigated, before it licensed the tunnel construction.It is for that reason that our client believes that there is an arguable case that the EA’s consent was unlawful and so should be quashed.”

An HS2 spokesperson said: “HS2 take our responsibilities extremely 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.”