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Risk strategy scores in gasworks remediation

MAY GURNEY Technical Services is carrying out remediation on the site of one of the UK's first gasworks, for a housing redevelopment at Gas Hill, in Norwich.

The gasworks was set up during the 1850s and closed in the 1970s.It left heavily polluted soil and groundwater with a cocktail of cyanides, lead, mercury and hydrocarbons.Past investigations also showed that there is a history of sand, lime and chalk workings in the surrounding area. Along with the presence of solution features in the chalk, the area is extremely prone to subsidence.

May Gurney carried out extensive investigations on the site, including trial pits and boreholes to a maximum depth of 25m, fitted with equipment for long term monitoring of groundwater levels and landfill-type gases.

The geology of the site comprises up to 5m of made ground, overlying mainly fine to course sand drift deposits, which in turn overlies chalk.Groundwater was encountered at 12m to 15m below ground.

Qualitative and quantitative risk assessment was carried out to develop a remediation strategy.All constituent materials on site, from re-deposited fill to natural soils, were classified according to material type and contaminant concentration. A key element of the assessment was the associated risk of the contaminant to people, water and the new development. May Gurney worked closely with the Environment Agency to establish suitable proposals to treat the site.

Remediation involves removal of shallow contamination across the entire site and hotspots of lead, mercury and cyanide present in the fill. Areas of hard standing are crushed and screened for re-use as fill material.

Soil testing throughout the earthworks also enables uncontaminated material to be recycled.

Cyanide is being removed from the groundwater via a chemical precipitation process as part of an on-site pump and treat system.When concentrations of cyanide are below the derived remediation target, the treated water is returned to the ground via an infiltration trench.

Further migration of polluted leachates is being controlled with a 150m low permeability in-ground containment barrier, installed along the northern perimeter of the site, to an average depth of 7m.

Treatment of the site is due to be completed in six weeks, after which May Gurney will install a series of CFA piles into the chalk for the new development.

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