Cognition’s aim was to eliminate the risks from contaminated land to future users, off-site receptors, and controlled waters: the River Wey in particular. The key elements of these works included:

• Sequential demolition of all above/below ground structures including gasholders
• Dewatering and desludging of gasholders and waste oil/tar tanks
• Stabilisation of site soils to form a pile mat
• Characterisation study to delineate areas of contamination and plan ex-situ and in-situ bioremediation
• Installation of secant pile wall to intercept/contain contaminated soil and groundwater
• Bulk excavation of grossly contaminated soils during basement dig
• Bioremediation and chemical oxidation of soil
• Installation of extraction trenches and sumps
• Groundwater treatment via extraction wells, well-pointing and mobile water treatment plant

The project objectives were achieved in full. Groundwater was treated and NAPL removed using a pump and treat system to enable the construction of a watertight basement. Groundwater monitoring demonstrated the site no longer posed an environmental risk. Soils were successfully bioremediated to allow their subsequent disposal predominantly as inert or non-hazardous waste.