Sorting out CS2
Richard Thurgood of CEL International and Philip Morgan of ESI describe the investigation and remediation of a carbon disulphide contaminated site
While it is a common contaminant, it is the major risk driver at only a small proportion of sites. The site in question is a former chemical plant that manufactured CS2 and is located in a rapidly developing area of Manchester. Project team
Site investigation and remediation project was managed by CEL International on behalf of the site owner. Environmental Simulations International was the lead soil and groundwater consultant and Euro Dismantling Services the principal remediation contractor. Additional specialist technical and engineering support was provided by various organisations.
In view of the flammability of CS2 and the potential for mercaptan-like nuisance odours, special techniques were developed to undertake soil and groundwater investigations. It was surprising how many of the analytical laboratories were not able to report CS2 in their VOC analytical results nor cope with the wide range of concentrations required.
Significant contamination with CS2 was detected in a number of locations at the site; dense non-aqueous phase liquid (DNAPL) CS2 was present in localised areas, principally where storage tanks had been located. No other significant organic contamination was present.
Following a desk study and preliminary invasive site investigation, a phased programme of investigation works were undertaken to define the contaminant distribution (both on and off-site), site geology and hydrogeology.Sub-surface conditions
The former plant had been constructed on made ground underlain by a relatively thin sand aquifer, in turn underlain at about 5-6m depth by low permeability glacial till. This glacial till (30m thick) protected the underlying Triassic (Sherwood) sandstone aquifer, which locally is heavily utilised for industrial abstraction. DNAPL CS2 was detected at the top of the glacial till beneath a former tank farm and at lower concentrations in the made ground and sand aquifer across the manufacturing and former waste disposal areas of the site. This waste disposal area also contained rubble, tars and other materials that were generated in the course of site operation. No significant contamination had entered the sandstone aquifer.Risk assessments
The site investigation data demonstrated that a number of pollutant linkages existed that required quantitative assessment.
These were performed for the initial contaminant profile and were particularly complex due to the multiple contaminant sources and transport pathways, including both vertical and lateral transport of multi-phase contamination.
Human health risk assessment was undertaken using a modified risk based corrective action (RBCA) approach, which gave the best representation of the exposure pathways operating at the site. The results showed that CS2 was the only contaminant that could result in a risk to the health of future occupants.
A CS2 concentration in unsaturated made ground below 12mg/kg was shown to be acceptable and contamination in the sand aquifer would only pose a risk to health if a significant quantity of CS2 DNAPL was present.
A controlled waters risk assessment was undertaken to quantify the risks posed by the linkages associated with groundwater within the sand aquifer. These included the risk of groundwater interception by sewers located within the sand and subsequent transportation to a surface water receptor. Vertical flow of groundwater and/or DNAPL from the sand aquifer through the glacial till and into the underlying Sherwood sandstone aquifer was also assessed.
The assessment demonstrated that the only significant risk posed to any receptor, via a pathway involving groundwater transport, was for CS2 DNAPL entering the sandstone aquifer at the base of the glacial till. The model results were substantiated by site investigation data.Remediation strategy
For light industrial or equivalent land use, removal of the identified areas of CS2 DNAPL was sufficient to adequately reduce the risk to human health and controlled waters. In addition, contaminated unsaturated soils from the made ground in these areas needed to have an average CS2 concentration below 12mg/kg.
The aim of the remediation was to reduce risks to the environment and human health to acceptable levels, in a cost-effective manner, and to validate the site's suitability for use. A wide range of technologies, including novel methods, were evaluated define an appropriate remediation strategy for the site. Health and safety aspects of the technologies were key, in view of the properties of CS2. Very few remediation techniques had successfully addressed CS2 contamination and the selection of a suitable technology was constrained by a number of practical considerations.
In particular, the main DNAPL contaminated area on the site abutted an elevated canal; the chosen remediation strategy had to pose no threat to the integrity of this. Also, one of the remediation areas was partially under a building.
After a semi-quantitative evaluation, including safety, technical, economic and environmental considerations, the selected remediation strategy comprised removal of CS2 DNAPL-contaminated soils after in situ mixing with bentonite slurry to stabilise the CS2-saturated material. Stabilised material was excavated and removed to approved off-site landfill, and the areas backfilled with clean material. Reduction of residual contaminant fluxes in shallow groundwater crossing site boundaries was carried out by means of a 'funnel and gate' permeable reactive barrier system. This comprised a bentonite wall directing groundwater flow to two reactive zero-valent iron 'gates' in which dissolved-phase CS2 reacts to yield innocuous end products.Excavation for off-site disposal
The bentonite slurry stabilisation for CS2 DNAPL-containing soils was subjected to an on-site pilot-scale test undertaken by EDS, CEL and ESI to demonstrate, to the satisfaction of the regulator, that the on-site works, transportation and disposal could be performed safely, without significant volatile losses to atmosphere, and without the generation of nuisance odours.
The use of reactive iron technology for CS2 had been developed and patented by Robert Kalin's group at Queen's University Belfast and was developed by the project team for this first full-scale application.Conclusions
The remediation of this former chemical works site has been successfully achieved despite a number of technical and practical challenges associated with the safe and controlled handling of high concentration CS2 contamination.
Key factors in this successful project included effective teamwork between the project managers, technical specialists and implementers. A high level of integration across the project team, including the involvement of the site owner and future developer, led to a unified, streamlined approach, with noticeable efficiency savings and avoidance of costly errors and misunderstandings.
The team successfully fostered a constructive relationship with the regulator. By focusing on openness and transparency, including a willingness to provide practical demonstration of technologies, regulatory negotiations and allied discussions with other third parties were swiftly channelled to a successful conclusion.
Close working relationships were also formed with solution providers. The selection and involvement of key specialist and other professionals enabled the project team to provide an integrated solution to the contamination issues, with the highest level of performance.
A number of other sites with similar contamination problems have been brought to the attention of CEL and ESI. From the experience gained in the investigation and remediation of CS2 gained during this project, safe and cost-effective practices for the investigation and remediation of CS2-contaminated sites now exist.