HS2 and the Chilterns Chalk Streams

HS2 and the Chilterns Chalk Streams

Since its inception there have been concerns about the impact of HS2 on the Chilterns landscape. While the destruction of ancient woodlands made headlines, there has been less media attention on the impact on local rivers and particularly on the chalk aquifer.

So why should you care?

The chalk of this landscape acts both as a giant sponge and a natural filtering system providing clean, clear water. The globally scarce chalk streams of the Chilterns rely on the aquifer for their flows. The human population relies on water pumped from the aquifer for their drinking water supply.

The construction of HS2 involves boring two 10.5 m diameter, 16km long tunnels through the Chilterns chalk. Tunnelling will cause permanent changes to groundwater movement and risks introducing pollutants to water stored in the aquifer. The tunnel will pass directly underneath the River Misbourne.

Calls for full disclosure and public consultation on this issue have not been answered. In April 2021 the Environment Agency approved tunnelling work for HS2. Work has now begun and we remain deeply concerned about potential impacts.


Pollution Risk

We are concerned that sediments and pollutants resulting from the tunnel boring process will be carried through the aquifer, impacting rivers and water supplies. HS2 have already self-reported one incident of pollution of the aquifer close to the vent shaft at Chalfont St. Giles.



Of particular concern is the huge volume of water required for tunnelling – latest estimate of up to 10 million litres of water a day (equivalent of four days water supply for the whole population of Amersham). It is still unclear where the water required will come from or how the contaminated waste water will be processed and disposed of.

A great deal of campaigning in recent years has resulted in a reduction to the amount of water abstracted by water companies in order to protect precious chalk streams. We are now extremely concerned that HS2 Ltd’s water requirements will reverse this and decrease flows in the rivers once again.


Groundwater flows

HS2 has stated that based on current estimates, cement grout used to line the tunnel could infiltrate up to 10 metres into the surrounding rock in every direction. Local campaigner Paul Jennings, chairman of the River Chess Association explains ‘With two tunnels this could mean a 60m+ cemented barrier through the chalk… So, taking two 16.7km long tunnels the volume impacted (could be) 24.4 million cubic metres.’ Changes to groundwater flows could be significant. We don’t know how this could affect existing abstraction points (used to supply us with drinking water) or river flows. We don’t know if these estimates are likely to change.



The chalk the machines will be boring through is known to be deeply fractured and poorly consolidated. The machines will tunnel a mere 20m beneath the River Misbourne at Chalfont St. Giles and again at Shardeloes Lake (Little Missenden). Disturbance to the riverbed may cause water to drain away, leading to permanent damage to this rare chalk stream habitat and loss of wildlife.

Damage to the bank of the River Misbourne has already occurred during the installation of a bridge for a temporary construction road. This directly contributed to extensive flooding.


The Chilterns Conservation Board, along with local partners organisations, continues to seek reassurances about plans to monitor and mitigate against potential impacts. For our full position statement see Chilterns Conservation Board and Buckinghamshire Council Position Statement.pdf (chilterns.org.uk)