Duncan Howie and George Baggott of E&RS explain the site investigation and remediation process across a 200-home residential site formerly used for viscose manufacture, the
largest such investigation undertaken in Northern Ireland.
The site is a former Courtaulds site located in Carrickfergus and used for the manufacture of viscose fibre. After closure of the works the site was sold off by Courtaulds and later split up. The west of the site now comprises a residential estate on previously unutilised land but adjacent to the area of the former carbon disulphide plant.
Following Akzo Nobel's acquisition of Courtaulds in 1998, Akzo Nobel voluntarily undertook a full investigation, in line with its Corporate Social Responsibility policy, to ensure protection of all site users, residents, and the local environment. Environmental & Remediation Services (E&RS) were appointed to investigate the site.
Carbon disulphide is a difficult contaminant with properties that require careful management. It is a hazardous chemical with low ignition energy, a DNAPL which in freephase form is very volatile. When in contact with the air it burns to produce sulphur dioxide and is a very powerful solvent.
When impure, it has a distinctive, unpleasant odour. However it has close affinity with many soils and is relatively stable while it remains under water. All of these issues were addressed during the investigation, planning and remediation process.
Site investigations
In each of the rear gardens of around 200 properties, at least five holes were drilled using a geoprobe, and typically three to four soil samples, as well as groundwater, taken from each hole. Geophysics was used effectively on the effluent plant and waste tip to determine the shallow subsurface structure thereby minimising the disturbance of wastes and allowing the investigation to be more focused.
All areas of the site were investigated, including manufacturing, waste tips, chemical production plants, service facilities and tank farms, where a range of sampling technologies were utilised. E&RS, in conjunction with Physalia, a consultancy that specialises in forensic ecology, also undertook an ecological assessment of the Belfast Lough to ascertain the potential impact of the contamination.
This was one of the most diverse investigations undertaken for carbon disulphide in Northern Ireland. The complexity was added to by the number of receptors that required investigation as well as the different land uses across the large site; the residential estate in particular required a sensitive approach. Representatives from the project team and Carrickfergus Borough Council (CBC) visited each household to give a copy of the report and fully explain the findings and potential implications.
Residential focus
For the residential area, the results indicated that the concentrations of carbon disulphide present in the sub-soils at two of the 200 properties could, if present beneath the houses, lead to unacceptable concentrations of carbon disulphide vapour in indoor air.
One of the properties revealed carbon disulphide to be present across a significant proportion of the front and back gardens, with concentrations at 40 times above the site-specific assessment criterion (SSAC) for human health.
An air monitoring exercise undertaken by CBC did not substantiate an immediate risk to the occupiers, however a potential future risk remained. The second house only displayed elevated concentrations of carbon disulphide in a small and well-defined area of the back garden at a depth of 3.5m below the lawn. The risks to human health were low in this instance but could not be eliminated as a potential future risk.
Remediation works
E&RS was instructed to remediate the contaminated soils in both properties. The local geology and the made ground of the estate made the works complex, but also ensured a complete solution to the contamination. The contamination lay below 3-4m of made ground on top of the dense clay that is common in Northern Ireland, and which ensured that the contamination was localised and immobile.
Excavation was considered to be the most effective solution as it would be quick, cost effective and certain and would create least disturbance to the residents and neighbours. In situ bioremediation or oxidation technologies could not achieve the same timescales or levels of certainty.
The remediation works involved the purchase of the first property and, after further geoprobe sampling confirmed the presence of unacceptable levels of carbon disulphide under the house, its subsequent demolition to facilitate the removal of the contaminated soils. The second property required less extensive works as the contamination was confined to the rear garden.
In both cases soil was excavated to a depth of approximately 4m and disposed of at an accredited landfill in England. Validation samples were taken from all sides and the bottom of the excavations, which were then backfilled and compacted at risk (of re-excavation being necessary in the unlikely event that residual contamination was detected in the samples) with imported clean material.
When analysis confirmed no detection of carbon disulphide the garden of the second property was reinstated to the satisfaction of the resident. Re-building of the first property is currently in progress.
The remediation works were undertaken with the cooperation and understanding of the residents and neighbours. There was excellent cooperation and transparency with the Northern Ireland Environment Agency (NIEA) and CBC. The project received consistently good feedback for the standard of its work and general communications, and no complaints were received from local residents or occupants of the industrial estate.
Duncan Howie, principal consultant
