A four-dimensional contaminated land mapping technology is being used for the first time to develop a remediation strategy for a former gas works in Stamford in Lincolnshire. The CLARET project (contaminated land: assessment of remediation by electrical tomography) allows real-time monitoring of sites long after remediation has been completed and complements existing practices, such as intrusive sampling and contaminant transport modelling, to give a more accurate assessment of the success of remediation methods.

CLARET is being industry-led by brownfield regeneration specialists VHE Group and Interkonsult, an engineering practice specialising in sub-surface environments. The British Geological Survey, a world leader in the development and application of electrical imaging, provides the research and development expertise. South Kesteven District Council, as the fourth partner of the consortium, provides a regulator’s perspective. The project is part funded by the DTI technology programme, which supports innovative projects in priority areas.

Assessing the technique

The project is assessing the use of a geophysical imaging technique, known as timelapse electrical resistivity tomography (ERT), for the in-situ monitoring of contaminated land remediation. The same technology has many other potential applications such as monitoring groundwater resources, seawater intrusion, landfills, geohazards, earthing impedance and pollution plumes.

CLARET comprises multiple work packages, which include instrumentation, laboratory experiments, site trials, cost benefit analysis and market research.

CLARET involves the insertion of probes into the ground to feed geoelectrical data back to an automated remote control monitoring system, which provides monitoring throughout the remedial works and long-after the decontamination work has been completed. South Kesteven District Council determined the site, now the Wharf Road car park, as contaminated under Part 2A of the Environmental Protection Act following the discovery of eight classes of pollutants contaminating groundwater and the River Welland.

South Kesteven’s environmental protection officer Peter Rogers says: “We became aware of CLARET through Interkonsult, with whom we had worked previously. Interkonsult was seeking a trial site and as the only severely contaminated site within the authority, the Wharf Road car park was ideal.”

Sensitive site work

The former gasworks is bounded by the River Welland, causing the scheme to be highly environmentally sensitive. Furthermore, the site’s location in Stamford’s town centre has generated a high degree of public interest, necessitating measures including odour control and monitoring, careful traffic routing of lorries to minimise disruption, and letter drops to address points of public concern. In addition, all information about the project, including the remediation strategy and methodology, is in the public domain.

During the remediation scheme undertaken by VHE, the car park was temporarily closed and all street furniture was ripped out. Around 5,000m2 of tarmac overlying cyanide, benzene and hydrocarbon-contaminated hotspots was then broken out. This was followed by the excavation of 2,400m3 of contaminated material which was taken to a hazardous waste landfill site.

On-site bioremediation treatment of up to 400m3 of material was undertaken, and 4,000m3 of material was crushed, screened and processed so it could be reused on-site. Grading of the car park’s sub base and re-surfacing followed before geoeletrical probes and a monitoring system was installed. Pay stations, lighting columns and barriers were also put in, as well as drainage including new gullies and a petrol interceptor.

“The scheme has gone very well,” reports Rogers. “Whereas the car park was previously on two levels, it is now a single level with the added benefit of an increase in parking spaces. Monitoring contaminated sites has previously been expensive, involving taking samples on site, sending them to the laboratory and waiting for the results. CLARET will provide real-time ‘on-demand’ subsurface imaging of the site, enabling us to monitor the success of the remediation works at low cost and without any interruption to the users of the car-park.”

Assessment continues

The remediation was completed this summer and the car park reopened to the public in August. The site will continue to be monitored using CLARET until at least next summer. The scheme follows an earlier successful project undertaken by VHE on a site which was also determined under Part 2A of the Environmental Protection Act at Yarm, Stockton-on-Tees.

The Yarm scheme required VHE to carry out remedial works to the gardens of six terraced properties following the discovery of cyanides, benzene and benzo(a)pyrene as a result of the site’s former use as a gas works. The site was developed in 1970s without undertaking any remediation. The Environment Agency took over responsibility for the remediation works from Stockton-on-Tees Borough Council due to the potential for contamination of the aquifer underneath the site.

The scheme involved highly sensitive work requiring constant daily liaison with occupiers of properties, and a close working relationship with the Environment Agency. VHE worked carefully to minimise disruption to residents, who all occupied their properties throughout the duration of the works. This project won the EA’s national Customer Focus Award.

VHE Group


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