65% of drinking water supplied by Affinity is abstracted from the Chalk Aquifer – around 900 million litres/day. Affinity Water have a government indemnity of (at least) £70m to compensate them, should construction interfere with their operations. Such interference might result from
- Physical disturbance of the fractured chalk, diverting flows away from the water abstraction points
- Obstruction of flows by the two tunnels, and by grout (used to seal the tunnels) penetrating and sealing fractures in the surrounding chalk
- Turbidity caused by chalk finings suspended in the water. This prevents effective purification using UV light. The new treatment plant, constructed opposite Tesco (Amersham) is designed to remove turbidity (and Bentonite) from the Amersham extraction point, and also water pumped there from Chalfont St Giles
- Pollution of the aquifer by chemicals (such as Bentonite) used in the tunnel grout and when constructing the Tunnel Ventelation Shafts
Careful monitoring of the tunnelling process may avoid these impacts, but as the monitoring has been delegated to the company building the tunnel, it is very unlikely that tunnelling would be suspended for anything other than a catastrophic failure.