Why HS2’s groundwater impacts are under scrutiny

Until now, concerns over HS2’s impacts on wildlife and the broader environment have been largely focused on what will happen to sites of special scientific interest and ancient woodlands.

Construction around what will be a service road for HS2 build traffic, along the proposed HS2 route in Great Missenden. Photograph: Dan Kitwood / Getty Images

But in mid-October, a new threat emerged, and that is the risk posed to water supplies and chalk streams as a result of the 13.5km tunnel that will be bored through chalk in the Chilterns Area of Outstanding Natural Beauty (AONB), from the M25 in the south to Mantle’s Wood, near the village of Hyde Heath, in the north.

Conservationists say they have three main concerns. The first is that the estimated 6-10m litres of water – needed every day to facilitate the drilling – will bleed the already depleted aquifers dry, and result in further damage to the region’s chalk streams. 

“This whole issue has evolved considerably over time,” said Alan Beechey of the Chilterns Chalk Streams Project Team. “Previously, they were saying they wouldn’t need extra water for tunnelling.”

Over-abstraction is already a major problem in the Chilterns – a report published by The Rivers Trust earlier this year said taking water from aquifers was responsible for “chronic low flows” in seven out of nine chalk streams. More than half of the River Misbourne is dry, Beechey said.

The water is needed to transport slurry out of the tunnels during the drilling phase, and the second concern is what happens to this water afterwards. Beechey said the plan is to pass the slurry through various treatment processes such as holding lagoons, but that the water will hold such tiny particles in suspension it will be impossible to remove them. Instead, he feared, it will end up going back into streams turbid and cloudy, with this reduced water quality harming the wildlife that lives in them.

Finally, Beechey fears the actual tunnelling process will mobilise pollutants long held in the chalk – including chemical agricultural run-off – that will eventually find itself into the aquifers, thereby contaminating significant water sources.

Asked to comment on these three separate potential impacts, HS2 told ENDS in a statement that it was working with both Thames and Affinity Water and the Environment Agency to find a solution that did not “add any significant new burden to the chalk aquifer or take any water from vulnerable chalk streams”.

Affinity Water only responded in its statement on efforts to find a solution to where the water needed for tunnelling would come from. “We have informed HS2 that we are unable to meet their current water requirements for tunnelling operations,” it said. “However, we will continue to advise HS2 in developing solutions that minimise the risk to the chalk aquifer and a water supply solution that does not impact the public water supply.”

Leaving aside whether or not the water is taken from the Chilterns or elsewhere, are the other concerns for contamination of groundwater justified? According to an interview given to New Civil Engineer earlier this year by Align, modern new boring machines means they can manipulate the thickness of the slurry to make sure "thinner slurry" isn’t getting into water abstraction sites and polluting them.

“In these areas, with our variable density tunnel boring machines (VaDTBMs), we can increase the density of slurry at the face and prevent pollution occurring,” Align’s HS2 project director Daniel Altier told the journal.

ENDS contacted Align for further details but was referred to the HS2 press office.

Because of the uniqueness of the Chilterns’ geology, there are almost no infrastructure projects that have encountered similar problems. The London Crossrail project, though mostly tunnelling through clay, did have to bore through a short 2.6km tunnel from Plumstead to North Woolwich under the Thames which involved going through chalk, requiring the use of slurry tunnel boring machines.

In the Environmental Statement (ES), the project estimated that up to 30 million cubic metres of groundwater would need to be abstracted in total over three years, with a peak flow rate of 50,000m3 a day, equivalent to 50 million litres – considerably more than HS2 is planning to use in the Chilterns.

But the ES also pointed out that impacts on groundwater would be minimal, because “this is an area where infiltration from the Thames has been occurring in the long-term and the aquifer has non-potable water quality as a result”. The impact of the dewatering effluent discharged to the River Thames would also be minimal, the ES said, because chloride concentrations would not be any higher than those already in the river.

Another much smaller infrastructure project involved constructing a tunnel in London to carry high voltage electricity cables between two electrical substations. This was – like HS2 – mostly boring through chalk and close to a number of sensitive public water supply wells. To minimise impacts, purpose-drilled monitoring wells were fitted with sensors to monitor groundwater levels and turbidity, and some of this information could be relayed directly back to the consultant in charge of this aspect of the operation.

But neither this nor Crossrail appear to bear a significant resemblance to the problems that could be encountered in the Chilterns, and it is still far from clear whether HS2 is developing plans to mitigate for these impacts.