A combination of remediation methods has helped the redevelopment of a browneld site in a prime location in Rugby.
Neil Jaques reports.
A redevelopment in Rugby has given RSK ENSR Remediation director Richard Croft a welcome chance to demonstrate best practice in dealing with contaminated land.
'The whole reason for me getting into this industry in the first place was to provide an alternative to landfill and to apply creative engineering thought to the problem of ground contamination, ' he says.
For the past 10 months, his firm has been using a combination of insitu and exsitu remediation techniques at a 3ha former ceramics and spark plug factory site on St Peter's Road, which is being re-developed for housing.
Remediation began while the factory was still operating, avoiding dig and dump (as well as an estimated 2000 lorry movements). Miller Homes was able to start building six months earlier than anticipated.
Attempts to minimise disrupting the neighbours have paid off - despite the fact that the site is boxed in by offices, Rugby College and residential properties, there have been no complaints.
In fact the site has become something of a local curiosity - all that can be seen at its southern end is an array of blue hoses, pipes and wells.
'It's known as the site with the blue worms, ' says Miller Homes' regional engineer Mike Box. 'It's a focal point for people in the area as it's all they see in terms of the remediation going on.'
In May 2005, with 80% of the site covered by buildings, the problem was how to implement a quick, cost-effective and environmentally sustainable remediation strategy that could start while the factory continued to operate.
'It's been an interesting site because we had to put an awful lot of design effort into getting it absolutely right, ' says Croft, who has been using the project as an example of best practice in lectures he has given to Ciria and First Faraday (a DTI sponsored partnership for innovative remediation science and technology).
The initial site investigation con med the ground conditions as Dunsmore Gravel (a clayey silty gravel) and Wolston Clay (a laminated clay). Chemical analysis of soils and groundwater identied the key contaminants to be a freephase oil, likely to have originated from accidental spills of cutting oils, and trichloroethylene, a chlorinated solvent.
The latter was found to have migrated into the subsurface as a liquid and, as chlorinated solvents are denser than water, passed through the groundwater about 2m down, hitting an impermeable layer at about 6m.
Groundwater containing dissolved trichloroethylene was also moving away from the source area and across the site. On the whole, the contaminants had spread to four areas of the site, particularly towards the south.
'Considering the distribution and the nature of the contamination we came up with the idea of splitting the site into two different parts, ' explains RSK ENSR Remediation's project manager Frank Westcott.
'Using two different remedial approaches meant house building could begin at the northern end of the site while remediation was nished at the southern part.'
The first step was to install a 70-well insitu multiphase vacuum extraction system at the southern end, with 25mm diameter hoses fitted with lances to be plunged 6m down the wells.
The system was designed to take advantage of tricholoethylene's volatility - high vapour pressure and high Henry's constant - and to harness the contaminant's characteristics to instigate a mass recovery method.
The low hydraulic conductivity of the saturated soils meant that by applying a vacuum (0.5 bar) to the lance and the well itself, liquids and air could be removed from the well faster than they could enter it.
The resultant vacuum would then propagate the drawdown of groundwater through the aquifer and extend the recovery reach several metres beyond the well. This established areas of unsaturated soil receptive to air being drawn through them.
As air (soil vapour) is drawn to the well, volatile contaminants enter the vapour phase and are carried to the well for recovery. Throughout the process contaminant levels are monitored. When they become sufficiently low, the wells are turned off and the recovery procedure is moved sequentially across site.
Extracted contaminants are passed to a treatment plant where the water stream passes through an oil water separator to remove liquid oils or solvents and a sandfilter to remove all particulate matter.
Finally, a carbon vessel removes all trace organics. Treated water is discharged into the foul sewer. The vapour stream is passed straight through to dedicated carbon vessels before discharge of air to atmosphere.
Croft says: 'The plant is extremely expensive - it costs £12,000 a month just to keep it onsite, and that doesn't include the energy we're putting in or the manpower, so we really have to make it work for its living.
'It's a constant round of monitoring and assessing what's going on - we don't just install the system, connect the wells up and walk away and think in four months' time that it's all going to be clean.
It takes an awful lot of continuous monitoring and assessment and data analysis to optimise the system.'
Croft stresses that the functionality of the insitu design is rooted in rigorous and exhaustive pilot testing.
'We have to come on site and get some real data. If you get a well's radius of influence wrong, perhaps put wells too far apart, you may never reach far enough out and you may miss crucial areas.
'Design is so important in an insitu remediation scheme on a project of this scale, where a signifiant amount of money is being spent on remediation. To not do pilot testing and save something like £20,000 could be a huge false economy.'
Once the factory was demolished, the problem of allowing Miller Homes quick access to the northern end of the site had to be addressed.
The initial plan of using a vacuum extraction system followed by chemical oxidation polishing of groundwater was abandoned when additional investigation suggested the contaminant was spread in thin layers, and at relatively great depth, which could have slowed insitu treatment.
It was decided that the most efficient method was to excavate about 1000m3 of contaminated soil, move it to the southern part of the site and treat it using exsitu chemical oxidation.
Bench scale trials performed with a series of oxidants on soil samples identified potassium permanganate as the most successful oxidant - results that were repeated on a larger scale when the treatment was applied to four skip-loads of contaminated soil, containing a tonne each.
Wescott explains: 'The difficult approach with this kind of treatment is to get the oxidant intimately mixed with the soil because we add it in a powder form and the soil is a solid, so it's not like a reaction in a test tube. We've got to make this happen in a real solid matrix and the key to make that work successful is the mixing.'
Croft says: 'Exsitu chemical oxidation has a great future and I think we will see an awful lot more of it. It destroys the contamination completely, not just transferring it from site to landfill.'