The Mid Chilterns Chalk aquifer forms the chalk of much of the Chiltern Hills but it also continues under the Colne Valley. The chalk aquifer is up to 100 meters deep and the ground water held in the chalk is filtered and purified slowly. The overall direction of flow is south easterly towards the London Basin. In the Colne Valley, the ground water levels are at or near the surface, and from here 88 million litres of drinking water are drawn every day by Affinity Water Ltd for our tap water.
Blackford Water Pumping Station, in Harefield is only 25 meters from the HS2 Colne Valley Viaduct
HS2 poses three main threats to our local water supply system.
The first is the 13.5 km Chiltern tunnel that emerges near the M25 in the west of the Colne Valley. Twin tunnels are being drilled through the aquifer and risk damaging its structure. The drilling process will also require 6-10 million litres of water to be used daily to transport chalk slurry out of the tunnels, risking increased turbidity or cloudiness in the water, up-stream of the main water abstraction bore holes.
Secondly there is the 3.4 km Colne Valley Viaduct which is strangely designed to curve across five deep lakes. Over 1,000 acres of valuable waterside, aquatic and terrestrial habitats are being cleared away to make space for temporary access roads, utility diversions, work compounds and spoil dumps. The viaduct will pass only 25 meters away from one our main water pumping stations Blackford on Moorhall Road. This pumping station produces 20 million litres daily but Affinity Water Ltd has already applied for this source to be closed during HS2 construction works due to predictions of increased turbidity. Hundreds of concrete and metal piles will be driven through the bottom of the lakes and across the entire floodplain, decreasing natural water cleaning processes, increasing impenetrable and polluting surfaces, and changing the structure of the fragile water bearing rock.
Thirdly, the HS2 route is positioned to pass through an underground pollution plume at the eastern side of the Colne Valley. This pollution is coming from the New Years Green landfill site and has already been responsible for closing the Ickenham water pumping station on Breakspear Road. Two scheme elements are taking place in the area of the underground pollution plume, these are the Copthall Cutting and the deep pile driving for the start of the Colne Valley Viaduct. Both elements risk opening up new pathways for the pollution to transit further into the aquifer. If this pollution reaches the Blackford source then Blackford will need to close permanently and never again provide clean, fresh aquifer water for the public.
There is a lot of secrecy around HS2 and the future of our water supplies, with evidence of treatment plants being built and a pipeline to bring in Thames recycled water to replace aquifer water for some customers. Hillingdon Green Party is fighting for the Plans for changes to Our Water System to be released for public scrutiny.
