The UK's Environment Agency (EA) is looking to solve a pollution problem that has killed a stretch of the river Rother in Derbyshire.
Water contaminated with tar is leaking from a lagoon at a derelict coking works into the river at Wingerworth and, according to the EA, has killed all river life up to Chesterfield.
The culprit is the derelict Avenue Coking Works at Wingerworth. The former works took coal and removed tar and other contaminants to create clean coke for fuel. Once extracted, this waste was pumped to a lagoon where it was left to drain into groundwater. The operators of the site had permission to discharge to the river.
'The site closed in 1992 but the lagoon still exists,' says Jill Credland, one of the heads of the EA's environmental protection team that is responsible for water and waste regulation in the river Rother and Chesterfield area. 'This has been leaking and is largely responsible for devastating the river Rother, which is now dead from Wingerworth to the centre of Chesterfield.'
A massive clean-up operation was launched on the site last year by the East Midlands Development Agency. The project will stabilise the walls of the 80,000m3 lagoon by building a half-height towbund made from crushed bricks. This will prevent catastrophic failure but it will not stop the slow seepage of contaminants that are reaching the river through the layers of gravel and soil below the lagoon.
'Some of the seepage is being intercepted at certain points and pumped back into the lagoon,' says environmental protection officer Mark Hutchinson. 'This has been quite effective: after heavy rain there used to be a layer of effervescence clearly visible on the surface of the river, you don't get that anymore.' But this is not a long-term solution to the problem.
Dr A Penman, chairman of icold's Mine and Industry Tailing Dams committee, says that polluted lagoons are frequently contained by the construction of a deep trench around the perimeter, which is backfilled with a slurry comprising bentonite, sand and cement. 'For extra impermeability, this can be complemented with a PVC barrier,' he adds, 'although this is a tricky business as the PVC is mounted on large frames which have to be lowered separately into the trench without any gaps between them.'
'A trench will protect the river but it won't stop the vertical migration of contaminants,' says John Smithson, condition manager at Allot and Lomax, the main contractor involved in the cleanup project. Smithson says that the most effective solution will be to remove the lagoon entirely. 'We need to investigate the lagoon thoroughly before deciding what to do with it. It's a mixture of solids and liquids, and we need to find out what the proportions are before we decide how to remove it.'
If the lagoon contains mostly liquid material, it will be pumped to a treatment area on site. The technology available for this process is being assessed in laboratory trials at the moment. The operation is likely to last four to five years.