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WILD AT HEART

Early contractor involvement on a huge remediation project in Derbyshire is designed to encourage innovation.Meanwhile the wildlife has moved in.Damon Sch³nmann reports.

A Derbyshire site described as one of the most polluted in western Europe is, ironically, a habitat for several rare species, including water voles and great crested newts.

Just south of Chesterfield, the former Avenue Coking Works, which ceased operation in 1992, is now owned by the East Midlands Development Agency (Emda) and is the subject of a £104.5M project funded by the National Coalfields Programme.

About 65% of the 99ha will be remediated as open spaces such as conservation areas and a wetland in the River Rother valley. Some of the remaining 35% will be developed as offices and housing.

But the task of cleaning up ground contaminated not only by the coking works, but by a coal gas plant, one of the biggest sulphuric acid production facilities in Europe and a landfill, will not be an easy one.

The scheme is one of the first remediation jobs in Britain to feature early contractor involvement, as pioneered on UK Highways Agency projects. The project team, which includes consultant Jacobs Babtie, believes this will encourage inventive solutions.

'We want innovation from the contractor, ' says Jacobs Bab ie divisional director Jon Smithson. 'It would have been nonsense for us to say you will use 'this' method.

'What we're asking the tenderers to do is give an indication of the combinations of processes they will use and an indication of what volumes will go through each process against an assumed volume that we will give them.' When GE visited the site in March, a shortlist of six consortia for the work was about to be released. The successful candidate should begin a four-year programme in autumn 2006.

The end classifi ation for processed soils will not be 'clean' or 'dirty', as there will be six reuse categories determined by land use, depth and distance from the river. In some cases, complete recovery may see some soil being reused elsewhere on the site.

The principal contaminated areas are two lagoons and a solid waste tip at the north of the site. The worst consists of three lagoons that have merged into one (now called lagoon two). Part of the reason for the level of pollution here is the migration of liquid waste run-off from the solid waste area. Vibrocore sampling from pontoons in the lagoon was used to confi m the degree of pollution in late 2002.

Only about 9% of the site's contamination was directly caused by the coking works, which used up to 1,400t of coal per day; the rest is from the licensed landfill.

Emda engineering manager Phil Reeve says: 'This is the clearest example of contaminated land I've ever seen. You go past a lagoon that stinks to high heaven with the river running alongside and you don't need a site investigation to tell you it's contaminated.

'The contamination is sufficiently bad that if we did nothing we would have been prosecuted so there wasn't a do nothing option. The river has been diverted at least three times and it's grossly polluted.' Yet, he says, there are water voles even in the most contaminated parts of the river. Conditions for these protected animals are about to improve.

Projects already under way at the Avenue include the reclamation and landscaping of a former railway sidings by Cheetham Hill Construction.

Its £750,000 contract includes creation of a wetlands habitat.

Reeve says one of the most persistent contaminants is benzo(a)pyrene as it binds to the soil matrix. 'It's good because it doesn't go anywhere, but it's hard to recover.' The contaminant list includes 'blue billy' (a combination of ferro and ferric cyanides, free sulphur and sulphates), naphthalene, phenols, ammoniacal nitrogen, thiocyanate and benzene. There are also lower concentrations of arsenic, lead, mercury and nickel.

Ascot Environmental carried out the most recent river diversion to allow for lagoon stabilisation work, but the course was previously altered to make room for the coke stocking area.

The lagoons are unlined and the worst one is only a few metres from the river. The historic river bed is 15m below ground level due to infi ling, but contaminants have got into the old river channels and the varying permeability of this layer has aided their movement through the alluvial beds.

All that separates the river from the edge of lagoon two is an embankment with a factor of safety of little more than one. 'It was built on its limit, ' says Smithson.

To stabilise it, the remediation team organised the building of an additional half-height bolstering berm to prevent it collapsing.

The site and depth of the lagoon has also caused problems. Jacobs Babtie senior site coordinator Brian Sims says: 'They just scraped up the alluvium to create the hole and used it to create the berm to give double the depth.' Digging out the alluvium meant an increased degree of permeability as the layers above were less permeable clay instead of sand. The deeper the excavation went, the coarser the particles became, increasing the ease with which material could escape.

Contamination has also been detected about 800m from the eastern edge of the site where 100m deep boreholes located free product 70m down.

Smithson says:'The main lagoon is 2.5-3ha and sits on permeable coal measures so we couldn't treat the contamination insitu.' It cannot be sent to landfill because the total organic carbon content is about three times higher than allowed. It was decided to completely treat material on site rather than to pre-treat and then send to landfill.

For the most grossly contaminated 200,000-300,000m 3 of material, low temperature thermal desorption will probably be used. While incineration could do the job, it would not be cost effective.

The balance of the 650,000m 3 total material to be cleaned will most likely be dealt with using bioremediation, cement soil stabilisation and soil washing. But Emda is keen to stress that other innovative techniques suggested by the remediation team will be considered.

Emda was looking at treating a much greater volume than this before the 2002 introduction of contaminated land exposure assessments (CLEA). Reeve explains that using previous rules would have meant treating 2.3M. m 3, almost four times as much as that identifi ed by using site specific assessment criteria which CLEA helped facilitate.

Remediation at the site is expected to be complete by 2012.

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