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Opening the gateway

For an area with an historic connection to the soap-making industry, some areas of the town of Widnes are less than squeaky clean. The waste materials and connected industries from the manufacture of soap and cleaning products has actually left a legacy of contamination, but remediation is underway at one site.

Construction of the Mersey Gateway bridge between Runcorn to the south and Widnes on the northern banks of the River Mersey is set to bring widespread regeneration to the area. Although a contractor has yet to be appointed, the remediation work already in progress in Widnes will clear the way for when the main construction starts.

The contaminated land clean up focuses on part of the site where the northern approach embankment for the new bridge will be built on the site and, although the selected contractor will be responsible for the design, it is expected that ground improvement using vibro concrete columns will be used to spread the load into the alluvial and glacial clays that underlay the made ground. 

Mersey

The clean-up of the contaminated land focuses on part of the site where the northern approach embankment for the new bridge will be built on the site

Although until recently the site was occupied by warehouses owned by St Modwen, industrial use of the site since the early 1800s presented a problem to the plan.

“The area was known as the Gaskell Deacon Works and used to process chloros, which was a by-product of the local soap production industry and yarnite,” says Ramboll senior contaminated land engineer Peter Fitch. “Chlorinated solvents were also a soap by-product and once a use for these was found in the dry cleaning industry, the site was used to manufacture the chemical.”

Route optioneering for client Halton Borough Council (HBC) by Gifford (now part of Ramboll) as far back as 2001 identified the issue but the alignment is the most viable based on traffic modelling as it keys straight into Widnes and the existing road network.

Assessment of the site and ground investigations found that products of chlorinated solvents were present in the form of dense non-aqueous phase liquids (DNAPL).

The geology at the site is formed by 4m to 5m of made ground underlain by alluvium to 11m. At around 30m to 40m the geology changes to glacial clay which overlies the Sherwood Sandstone bedrock.

The contamination problems were seen as a high risk within the Mersey Gateway project and HBC decided to advance the work and fund it separately as it felt the contractor wouldn’t have time to deal with the issue and leaving it in the main contract could result in higher costs.

HBC compulsory purchased the site in order to fast track the remediation work.

“HBC wanted the site to be cleaned up before the bridge development started as the underlying Sherwood sandstone is an aquifer and there was evidence of the contamination moving downwards,” says Fitch.

“Evidence of the DNAPL pollution was found in the alluvium. There is perched groundwater in the made ground and confined groundwater in the alluvium and glacial sands but due to the dense nature of the material it can move down and breaks down the clay to create a migration path.”

Nonetheless, free product was not actually found during the site investigation but Fitch’s team knew it was present due to other evidence.

“The aim with the MPE was to lower the water table and treat the water before re-injecting the water upstrem to create a positive gradient that would flush the area”

Trevor Bamber

But this presented a problem to the clean up – the exact volume of pollutant was unknown and the concentration was undefined. This led HBC to let the contract under the need for “betterment” to remove as much as possible rather than set a defined volume or concentration level to be achieved.

HBC and Ramboll decided to allow the contractor to design the remediation scheme in order to benefit from their expertise. According to Fitch, five tenders were received in total and these proposed three different technologies for the site.

“HBC allowed us to focus on quality when it came to assessing the tenders as the end result and the time involved was critical,” he says.

Celtic Technologies first became involved in the project early last year with pilot trials for sampling and lab testing for HBC to inform bidders on what might be achievable at the site.

Proposals included surfactant flushing, which has not been proven in the UK, and thermal technology that has never been used at full scale. But HBC wanted an approach that it could be confident would achieve its aim and Celtic’s proposal to use multiphase extraction (MPE) followed by ChemOx injection to remediate the ground insitu was successful as a result.

Celtic was awarded the contract to carry out the remediation in April 2012. Work on site started soon after and the physical elements of the clean-up were scheduled to be completed as this issue of GE went to press, but monitoring will continue at the site for another three months.

“The first part of the main contract was field trials of the technique with a break clause in place just in case it didn’t work,” says Celtic Technologies senior engineer Trevor Bamber.

The trials were successful and also located the DNAPL that had alluded previous investigations. “The trials enabled us to look at the sphere of influence of each extraction well under pumping and fine tune the borehole spacing,” adds Bamber.

With the remediation plan agreed, Celtic installed 153 boreholes to 10m to 11m below ground level. The boreholes were either 200mm in diameter for the dewatering pumps, or 50mm in diameter for the vapour extraction.

“The aim with the MPE was to lower the water table and treat the water before re-injecting the water upstream to create a positive gradient that would flush the area,” says Bamber. “Lowering the water table also allows air to drive off the chlorinated solvents. Where the DNAPL was found, the method switched to a total fluid recovery system.”

Celtic’s aim was to carry out 2.3 to 2.6 porewater volume exchanges.

The site was split into five treatment areas, which were each treated for around five weeks. “The timings were based on the concentrations,” says Bamber.

The highest concentrations encountered so far have been in the 1,000s of milligrammes per litre where there was free product but generally levels were around 150mg/l. Now ChemOx has been completed in some areas, the levels are down to 1mg/l.

“ChemOx is the polishing part of the work and oxidises contamination to benign products,” says Bamber. “It works well with more complicated chemicals such as petrol products and chlorinated solvents.

“The challenge has been that there were up to 30 different chlorinated solvents produced at the site and as they degrade they form other chlorinated solvents. This means that there can sometimes be spikes in the contamination levels as the remediation progresses and new chlorinated solvents are formed.”

Nonetheless, Bamber believes that the concentration levels are moving in the right direction and when GE visited site, Celtic had extracted 2t of DNAPL product. In total 16.5t of product, air and water has now been extracted.

While the monitoring is necessary to prove the clean-up has achieved the called for “betterment”, it is clear that both Fitch and Bamber are pleased with the results so far.

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