Reclaiming the land

University of Greenwich expert Hilary Jones says there are two methods of solidification and stabilisation.

"You can use the in situ method which is drilling into the soil and injecting the ground with cement, so you have a column of treated ground, or dig up the ground and mix it with the cement. It depends on how much soil you have to treat."

Jones says that this type of remediation is most commonly used for treating land that is badly contaminated, especially with heavy metals. "Anywhere that has been used for metal production, firework factories, any type of old industry really. It has also proved to be successful where other remediation techniques have failed."

Although this technique has been used in the US for 30 years, it is relatively new to the UK. "Solidification and stabilisation has been used in the UK for about a decade and there are only a few instances of its use," says Jones. "Landfill tax has been so cheap, there was no reason to turn to land reclamation, but that's changing."

Jones went to the US to see the technique in action. One example was the remediation of an oil recycling plant in Tampa, Florida. The 15.5 acre-site was polluted with waste oil products and a previous attempt at remediation had failed, leaving a pile of contaminated ash. 27,000 tonnes of material were treated over eight months and the soil was extracted before being treated.

Blue Circle Land Remediation Systems is developing solidification and stabilisation in the UK. General manager of land remediation Keith Bradshaw says that the cost can vary greatly. "It can range from £12-15 per tonne if the contamination is light, to up to £50-60 per tonne if there is a high concentration of pollution and depending on what kind of contaminates there are, timescale and soil type."

Soil washing
Soil washing has been in use in the UK for about 10 years and is especially useful for cleaning up gas works and coalmines.

It works by separating large particles of soil from small particles. Soil washing doesn't get rid of all the pollutants, because it only eliminates the small, highly contaminated particles. Some larger, less contaminated, ones remain.

Soil washing can be conducted in situ or by pumping the soil through a machine, but the in situ method has only been trialled in the US and is still experimental.

Lattice, the property arm of British Gas, used soil washing to clean up a 19 acre site in Basford, Nottinghamshire that had been in use for almost 120 years.

Barnsley-based groundworks contractor VHE Construction washed 152,000 tonnes of soil over 18 months for Lattice — at a cost of £21 per tonne. Lattice estimated this was 50p per tonne less than using dig and dump.

Soil vapour extraction
This uses a vacuum to draw contaminants through the ground. It is a popular technique and has been used for about 15 years in the UK.

"You drill a series of bore holes in the contaminated land," says James Baldock, a hydrogeologist for chemical engineers Fluor Daniel GTI, "and then apply a vacuum to the holes which draws the contaminates from the ground."

Baldock adds that this technique is most suitable where the ground is polluted with volatile contaminants such as benzine. Soil vapour extraction is typically used to clean up former gas works and petrol stations. It is not suitable for extracting metal pollution.

"Vapour extraction is a cheaper option than many other remediation techniques, as it avoids the expense of digging up the soil," says Baldock. "It costs about £20 to £30 per m3 on average."

Fluor Daniel used this technique to clean up a section of the site that became the Millennium Dome in Greenwich, formerly a British Gas site.

The 4.5 ha site of an old gas works was cleaned up in six months, though Baldock states this was a particularly quick and aggressive project. The value of the work to Fluor Daniel was just under £1m and 130,000 m3 of soil was remediated.

Bioremediation
Bioremediation is a new technique that is favoured by the Environment Agency. British Gas has used it for five years, and last year several government-funded companies were set up to look into its use.

There are two methods of bioremediation, using plants or bacteria. Plants pull pollutants in the soil into their root zone and break down the contamination. Successful trials have been carried out on sites contaminated with metal and radioactive waste.

"Bacterial remediation works by converting organic pollutants into water and carbon dioxide," says Pat Harvey, a principle lecturer in biochemistry at the University of Greenwich.

WSP Remediation is using bacteria to reclaim an 8 ha site in St Peter Port, Guernsey which is polluted with oil and tar. The site is marked out for commercial, retail and housing use, and remediation is being carried out alongside construction. The project is still in progress, but is phased so a batch of soil is released every six to eight weeks. So far 6000 m3 has been reclaimed at a cost of £20 per m3.