Colne Valley water quality update – August 2023

In August 2023, HS2 continue with heavy engineering works across the Source Protection Zones in the Colne Valley (near Harefield) where public tap water is intended to be drawn. HS2 have now released their monthly water quality assessment reports for the Colne Valley for May 2022 to April 2023. These can be viewed below:-

The reports contain really worrying information which shows that significant pollution is being detected.

The Blackford public water source (PWS) on Moorhall Road is temporarily closed due to HS2 works but is now clearly at risk of deterioration in chemical water quality for future use. This is water intended for public supply of tap water in Hillingdon and surrounding areas.

A summary of pollution recorded in the reports is below:

Data taken from HS2 monthly water quality reports for Colne Valley 2023

Foreseeable and avoidable pollution is now mobilised in the Drinking Water Protected Area (DWPA) where HS2 is building the Colne Valley Viaduct. The Blackford public water source (PWS) is at risk and action must be taken to prevent deterioration of this vital tap water source for London. It is illegal to deteriorate a Public Water Source (PWS) under the Water Environment Regulations UK (2017) either by introducing new pollution or by mobilising known pollution. But this appears to be happening. Affinity Water Ltd petition against HS2 (2013-14) warned that ‘… pollution of groundwater could occur due to positioning of the HS2 works and that pollutants from further afield may be able to transit more easily to the sources as a result of the piling/tunnelling processes ……such pollution could be so severe as to render a source temporarily or permanently redundant.’ These concerns still stand.

Fig 1. HS2 works in for the Colne Valley Viaduct in chalk aquifer south of Harefield

Released HS2 data for 2023 shows pollution being mobilised in the chalk aquifer in Protection zones 1 and 2 for the Ickenham and Blackford (PWS)s, this is a Drinking Water Protected Area. Objectives in this protected area must be complied with. These are to maintain good chemical quality at the Blackford water source and achieve good chemical quality at the Ickenham source by 2027. Neither appear to be happening.

Under Freedom of Information HS2 has release the water quality data from monitoring of their Colne Valley works for May 2022 to April 2023. Pollution events and exceedances are listed in Table 1 below.

One main concern is that the Blackford PWS at Moorhall Road pumping station, will not avoid deterioration in status. The Blackford source is licensed for 20 Million litres a day and had good chemical quality before it was closed in May 2021 due to the close proximity of HS2 works. HS2 say the planned closure is a precautionary measure so that pollution does not make its way to our taps. During the time Blackford pumping station is closed, replacement water is being bought and piped into the area. There is a legal duty to reopen the Blackford source and for it to produce the same quantity and quality of public drinking water as before HS2. This means the groundwater in the aquifer should be of the same quality as before, clear and not needing additional treatment. This will not be possible if pollution from HS2 works and other contamination, is mobilised in the aquifer and migrates into the Blackford SPZ. It is not possible to flush out the chalk aquifer before reopening the Blackford pumping station, pollutants may be lodged in the rock matrix which could move at any time.

Worrying examples of pollution: In March 2023 there was a worrying Turbidity spike of 1067 (NTUs) Nephelometric Turbidity Units, recorded at a borehole ML026-RO426 in Load Test Pile Site 2 Harvil Road. Align Ltd states that this may be due to mobilisation of polymers from the test piles. Polymers are a risk to drinking water as residual monomer acrylamide can be left. Acrylamide is a banned substance in groundwater as it causes cancer. We do not know whether HS2 has done any testing for acrylamide since the high turbidity readings were taken. There is no indication of the direction of travel of the polymers, or whether they are moving towards the Blackford source.

Chromium 6 is another cancer causing substance which HS2 has repeated found at their sites and is derived from cement use. Align Ltd have made various arrangements to transport waste water contaminated with chromium 6, to their Water Treatment Plant at the south portal site, to remove the chromium. Align Ltd repeatedly state that the chromium 6 being found at their sites has not entered the aquifer. On 21st March 2023 chromium 6 was measures in borehole ML026-CR032 in the chalk aquifer at 5.3 ug/l. This is a priority hazardous substance which must be prevented from entering groundwater and in particular must be prevented from entering groundwater in the Blackford SPZ.

HS2 are blaming other chemical imbalances on the Council landfill site, up hill on the eastern edge of the Colne Valley, south of Harefield. Pollution includes; TPH aromatics, Ammoniacal nitrogen, sodium, potassium, electrical conductivity, calcium. HS2 works should not be mobilising the landfill leachate or causing the pollutants to travel into the Blackford SPZ1 but this appears to be happening.

March monthly data report states. ‘TPH were previously detected on 4th January, but different carbon bands were identified (C7-C10 aromatic). Discussion with S2 contractor SCS have mentioned presence of TPH in groundwater owing to historic landfill contamination within/close to their area.’

Hazardous pollutants have been found in the Newyears Green Bourne (NYGB) which runs through the disused Contaminated Land, under Harvil Road and down to the Hillingdon Outdoor Activity Centre (HOAC) Lake which forms part of the Blackford inner source protection zone.

ML026-CRO32 is west of Harvil Road, south of Dews Lane and north of Newyears Green Bourne.

ML026-RO426 is a test borehole west of Harvil Road, south of Newyears Green Bourne, in the Load Test Pile Site 2.

Table 1: extracts from Align Ltd. C1 Monthly Water Monitoring Reports (download here)