Greenwich opted to leave, cap and contain some of its contaminated land in the remediation for a new inner-city village.
A giant remediation programme was needed to transform Greenwich peninsula, the Government's flagship brownfield site, from contaminated wasteland into a desirable London district.
Mixed-land use, including areas of high density housing called for the clean-up to be executed to exacting standards. Location of the controversial Millennium Dome on the former British Gas depot meanwhile guaranteed media attention. The remediation needed to be a model project.
Two thirds of the 120ha former British Gas depot required decontamination. Over 100 years use and abuse by industry had left coal tars, mineral oils, benzene and polyaromatic hydrocarbons. Phenols and heavy metals were found. There were underground tanks and a large tar well amongst other structures to be broken out.
Around 2,000 bore holes were sunk to assess the degree and distribution of contamination.
Contractor Nuttall started work on the £22M clean up in 1996. Cost and pressing timescales meant the most contaminated material - in all about 700,000t - was removed from site to designated dumps. This included contaminated fines from soil washing carried out to treat other material. Client for the site remediation and company responsible for land formerly owned by British Gas, BG Property says technology was geared to cost and time.
Washing was employed to tackle organic contaminants that adhere to the surface of finer particles. Coarser grade fractions were retained and used in site preparation.
Subcontractor Fluor Daniel was brought in to carry out soil vapour extraction, a method whereby benzene and petrol that had leaked from storage tanks on the site were literally sucked from the ground as vapour. 300 wells were installed in the ground to facilitate extraction. Oxygen drawn into the ground subsequently promoted aerobic biodegradation of other organic compounds and hydrocarbons.
Where low-levels of contamination could be reconciled with the masterplan for the site, ground was covered with a marker layer of orange plastic mesh to warn of underlying toxicity. It was then capped with a capillary layer, allowing gaseous exchange between ground and air but preventing contaminants rising to the surface hydraulically.
In conjunction with impermeable membranes put in place beneath buildings, vents have been installed to guard against methane and other gases rising from the site. A barrier wall has been built to prevent remaining contaminants leaching into the river Thames, bounding Greenwich peninsula on three sides. Barriers have also been erected to stop any contamination migrating from the site via service trenches.
All work had to be agreed with and was rigorously monitored by Greenwich Borough Council's environmental health officer and the Environment Agency. AM