Vapour Containment System at Dump B as seen from Coastal Footpath
As planned, the Project Team set about dealing with each of the Dumps in a sequential order, having agreed with Kerrier District Council and Cornwall County Council some time ago that it would be beneficial to begin with Dump C, to keep the Cornish Coastal Path open.
At Dump C it had been agreed with the Environment Agency that contaminated material should be excavated, sorted and disposed of to off-site landfill, and a significant amount of planning work, including the approval by planning authorities of a Notification of a Proposed Development, (under DOE Circular 18/84), had been completed. At Dumps A, B, D & E a comprehensive Assessment Study, incorporating intrusive dump site investigations, had been designed to ensure that the remediation method adopted would meet the requirements of Part IIA of the Environmental Protection Act, and ensure value for the tax-payer's money.
Defence Estates (the Agency responsible for managing MOD land and building assets) took the lead with a challenging programme of work. This was undertaken in collaboration with DEBUT Southwest (the MOD's Regional Prime Contractor in the South West), Enviros Consulting (Environmental Consultants) and Defence Science & Technology Laboratories, Porton Down, whom were supplying essential toxic operations capability.
The key objectives of the programme were met with the disposal pits at Dump C successfully excavated and some 100m3 of waste material taken to off-site landfill. Following the sucessful remediation of Dump C, the area has now been revegitated. At Dumps A & B over 60 trial pits have been dug (some just over 5m deep), with a large number of soil and water samples taken for comprehensive chemical analysis.whilst at Dumps D & E good progress continues to be made with investigations of the two former mine shafts.
To ensure the highest levels of safety, and in the unlikely event that chemical warfare agents might be present, excavations were carried out by personnel wearing respirators and protective suits. As a further precaution, work was carried out inside a Vapour Containment System, a large tent-like structure, designed to operate under negative pressure such that any potential vapour releases would result in gases being vented to the environment via activated carbon filters to remove any contaminants. The structure needed to be strong and mobile so that it could be moved regularly around the site and be capable of withstanding 80-knot wind strengths. This led to a bespoke air-supported structure being procured, for use during periods of anticipated low wind strengths, from May until early October 2006.
The results of the investigations have been very positive with no chemical warfare agents found. The vast majority of waste in Dumps A & B is non-hazardous rubble from the former buildings at Nancekuke and included: large lumps of reinforced concrete, bricks, old windows, door frames and concrete flooring. Pieces of asbestos cement sheeting were also found in both dumps, as well as small pieces of asbestos rope gaskets. A significant amount of domestic waste was found at Dump A together with a small number of items from the decontaminated chemical plant.
In addition to the building rubble at Dump B, an area of chemical waste residues was identified along with an area used to dispose of bagged fibrous asbestos wastes. The chemicals present were all substances typical of many industrial activities, including heavy metals, petroleum hydrocarbons and solvents and were all on record as being buried at Dump B.
Excavation Activities Within The Vapour Containment System
In terms of former CDE Nancekuke activities, minute traces of chemical warfare agent breakdown products: methylphosphonic acid (MPA), isopropyl methylphosphonic acid (IMPA) and thiodigycol (TDG) were all found, as well as low concentrations of the riot control agent CS. MPA and IMPA are both breakdown products of the nerve agent Sarin (GB), produced at Nancekuke during its operational life and are remnants of the decontamination process that took place when the dumps were formed. TDG is a mustard agent breakdown product and, again, was found in extremely diluted concentrations in one soil sample and in two gaskets removed from plant in Dump A. CS was found as a solid residue in one small area and is associated with research that would have taken place at Nancekuke into its effects.
In terms of toxicity these chemicals are only a minimal hazard, and no more toxic than many insecticides approved for home use so the risk they represent to the environment is considered very low. The migration of such trace amounts through the dumps would be extremely slow and the damp ground conditions would further reduce the concentrations by reaction with water. By the time they reach the groundwater, the natural hydrolysis and bio-remediation processes would eliminate any hazards.
At the two former mineshafts significant progress was made with the above and below ground investigations. The above ground work confirmed the presence of decontaminated chemical plant in one shaft and the below ground work, which has involved access into the mine from the adit at Sally’s Bottom Cove, has enabled the specialist team to reach below the intersection of the Dump D shaft. The adit entrance has now been gated to prevent unauthorized access.
Further investigations are also continuing on the ‘North Site’ and its adjacent effluent treatment plant, covering the area where Sarin was produced when Nancekuke was operational, between 1950 and 1976. Highly sophisticated laboratory analysis has detected some signs of solvents, petroleum hydrocarbons and pesticides, consistent with what would be expected to be found at any former industrial site. Some borehole samples revealed the most minute traces of two compounds - 1,4-dithiane and 1,4-thioxane - which are remnants of the decontamination process, and therefore confirm MOD’s view that the plant had been fully decontaminated. These compounds were found at such low concentrations - parts per billion - that they could only just be detected by the most sophisticated laboratory analytical techniques. To put this into context, at the very low levels detected you would need to drink several thousand litres of water to absorb a dose approaching the no observable adverse effects level.