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HS2 has admitted losing a huge quantity of a potentially highly polluting substance into a chalk aquifer during the construction of a ventilation shaft for a tunnel on the high-speed rail link. The railway gave reassurances earlier this year that leaks would be prevented.

Europe鈥檚 largest infrastructure project lost 1,600m3 of bentonite slurry - enough to fill 48 shipping containers - into an 鈥渦nexpected fissure鈥� at the Chalfont St Peter shaft site late last year. Align, a joint venture comprising Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick, which is the main contractor on the site, has produced a report on the incident.

HS2 said the slurry was 97% water and 3% bentonite, and that the Environment Agency was 鈥渘otified on a daily basis鈥� of the total quantity lost.

Network Rail describes bentonite, a type of clay formed from volcanic glass which can swell to up to eight times its volume when it absorbs water, as 鈥渉ighly polluting鈥�. In an on the substance, Network Rail warns that bentonite can cause damage to plants and animals if it enters watercourses or drains.

The guide adds that any spillages should be immediately reported and 鈥減romptly cleared up鈥�.

HS2 said that the bentonite had 鈥渟ettled in fractures and fissures in the area around the shaft, where it will remain unless disrupted by direct excavation into these areas鈥�.

An HS2 spokesperson said: 鈥淏entonite is a safe, naturally occurring form of clay that is widely used across the construction industry as well as in agriculture and cosmetics. 

鈥淲hile we regret the loss at this location, monitoring has not identified any adverse effects on the aquifer or on water supplies, and the Environment Agency and water company have raised no concerns.鈥�

The Environment Agency is now facing a judicial review over its approval of the works brought by local campaign group Misbourne Environmental Protection.

The group intends to force the government agency to reconsider the tunneling project on the railway, which includes the construction of the 16km Chiltern tunnels, to ensure that any tunnels run entirely through solid chalk.

Paul Jennings, of the , which monitors the environmental impact of works such as HS2 on the area, told the Guardian: 鈥淭his incident casts doubt on the competence of HS2/Align as project operators.

鈥漈hey seem surprised to have lost all this bentonite slurry at the Chalfont St Peter vent shaft. The lost bentonite sits in the aquifer waiting to be moved by migrating ground water as it flows through a matrix of cracks and fissures.鈥�

The Environment Agency said it takes the protection of groundwater and watercourses, as well as the wildlife and ecosystems that rely on them, 鈥渧ery seriously鈥�. However, it declined to say whether it agreed with Network Rail鈥檚 assessment of bentonite as a highly polluting substance.

The Environment Agency spokesperson added: 鈥淭he agency is working closely with HS2, their contractors, local community groups, the water industry, and local authorities to ensure the environment is protected throughout the project.鈥�

In June, HS2 insisted that it had the 鈥渞ight tools for the job鈥� to stop water supplies being contaminated amid concern from local groups.

Mark Clapp, the railway鈥檚 senior project manager, said that 鈥渁 lot of science鈥� had gone into preventing water supplies being impacted by the risk of chalk dust leaking into underground aquifers during the construction of the Chiltern tunnels.

Both organisations signed off the tunnelling works after HS2 agreed to fund 拢100m worth of added capacity to local water treatment plants to maintain the quality of local supplies.

HS2 said it takes its responsibilities 鈥渋ncredibly seriously鈥�. It added: 鈥渨e have a world-leading team of engineers, hydrogeologists and environmental scientists in place to ensure that the tunnelling operation affords the right protections for the environment, and is completed as safely and efficiently as possible.鈥�

Bentonite slurry is used to fill and support the sides of excavations during the construction of certain types of walls.