A charity has slammed HS2 for its “imminent intention” to start digging under the Chilterns AONB amid fears it could damage fragile chalk aquifer under the Misbourne Valley.

The Chiltern Society has warned that the chalk aquifer – which it says provides drinking water to thousands of people in the region – is “under threat” from HS2’s planned operations for the 10-mile long Chiltern tunnels.

It said in a statement on its website: “When plans to tunnel beneath the Chiltern Hills were first announced in 2010, the Chiltern Society and others raised concerns relating to the potential for significant damage to both the chalk aquifer, which supplies drinking water to many thousands of people regionally and in London, and to the Rivers Misbourne and Chess, which are internationally rare and threatened chalk streams that rely on the aquifer to support this very special habitat, and the diverse wildlife that it supports.

“The two tunnel boring machines (TBM’s), each more than 10 metres in diameter, will both carve through 16km (10 miles) of chalk from the Chalfont Lane South Portal site, just inside the M25, before emerging at top of the Misbourne valley above Great Missenden.

“On the way, these machines will cross the Misbourne Valley twice, passing approximately just 20 metres beneath the River Misbourne at Chalfont St Giles and at the head of the historic Shardeloes Lake.”

“Our geological advisor, Dr Haydon Bailey, confirms that chalk is an extremely permeable rock in this area and is known to contain intense fracturing and jointing that allow water to pass through the aquifer.”

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The charity has asked whether it should “expect partial or total loss of flow in the River Misbourne” as a result of the tunnelling adding: “Will shallow tunnelling (at c.20m depth) beneath the river at Chalfont St. Giles cause disturbance to the riverbed and drainage of water from the river leading to permanent damage to this rare chalk stream habitat and loss of wildlife?”

It also asked what contingency plans HS2 has in place “if it becomes apparent that the tunnelling is causing serious damage to the aquifer and to the River Misbourne”, and says it has concerns over the “huge volume of water required for tunnelling”.

It added: “Precise volumes have still not been fully determined, with latest estimates of the machines requiring up to 10 million litres of water a day.

Bucks Free Press: PICTURED: The River Misbourne (image: Brenda Spargo, BFP Camera Club)PICTURED: The River Misbourne (image: Brenda Spargo, BFP Camera Club)

“Where will the water come from? Although some water will be recycled, 10 million litres is the equivalent of four days water supply for the whole of the population of Amersham.

Tom Beeston, chief officer of the Chiltern Society, said: “This project, from its very start, has been carried out in totally the wrong order.

“We believe that serious questions remain unanswered about the impact of HS2’s tunnelling operations on our precious chalk streams and the availability of drinking water for local communities.”

HS2 has hit back at some of the claims, saying it is “not true” that around 10 million litres of water a day will be needed for the tunnelling, with a spokesman saying: “The real amount, even at the peak of construction with two TBMs at work, is half that.”

They also said the majority of the water used during the construction of the tunnels will be drawn from Thames Water’s supply, which “does not come from the aquifer”.

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The spokesman added: “We are working closely with Thames Water, Affinity Water and the Environment Agency to deliver a water supply solution that does not add any significant new abstraction burden to the chalk aquifer, or take any water from vulnerable chalk streams, such as the River Misbourne or River Chess.

“As part of that work, we are looking at how we can reduce the pressure on water resources over the short to medium term and improve water supply resilience into the future.

“We have met with the Chiltern Society, River Chess Association and other community groups on multiple occasions to explain our approach to tunnelling and reassure them about the protections in place.

“Ensuring the continued supply of high quality of drinking water from the chalk aquifer is an absolute priority for HS2 and we will continue to work closely with Affinity Water and the Environment Agency throughout construction to ensure any risks are managed appropriately.”