Assessing the risk for contamination in West Dunbartonshire, UK

During the redevelopment of a site in Dumbarton for its new headquarters, West Dunbartonshire Council commissioned RSK to investigate a gasholder found on the site and assess any potential land contamination, such as ground gas, associated with any residual materials located beneath the gasholder.

There was a preliminary remedial strategy for the site. However, concerns had been raised about potential additional liabilities relating to the former gasholder. RSK’s geoenvironmental team was asked to provide specialist advice to the council on any possible environmental liabilities to reach financial closure of the project between the council, hub West Scotland and the Tier 1 contractor.

To ensure that this project was a success, RSK mobilised quickly, worked on a tight turnaround and expedited the site work and reporting to meet the council’s urgent requirements. A key aspect of RSKs success was its ability to offer a suite of services: RemedX, an RSK company specialising in remediation, was commissioned to dewater the gasholder and remove residual tar from its walls and base.

RSK undertook a detailed desk study to develop a conceptual site model incorporating the gasholder. This included research into gasholder types to confirm the likely historical use of this particular feature as a tar tank. A geophysical survey of the base using ground-penetrating radar and radio-detection methods to locate any buried services, pipework or voids beneath the concrete base followed.

After RemedX completed dewatering, there was concern that drilling through the base to recover samples of residual material from underneath would enable uncontrolled groundwater ingress under pressure. An innovative approach was developed that involved working with specialist subcontractor G&I Diamond Drilling to drill 40-mm-diameter probe holes through the gasholder’s base. This enabled the drillers to seal the holes quickly enough to prevent any significant ingress of groundwater while enabling RSK to obtain soil samples from directly beneath the base for laboratory analysis. Supplementary groundwater samples were obtained from existing boreholes outside the gasholder as part of the risk assessment. The resulting combined factual and interpretive report demonstrated that the residual risks were acceptably low and that the likelihood of any residual environmental liabilities occurring was very low.

Throughout the process, the client was kept up to date with all the progress made both on-site and with office-based tasks. The project was delivered to budget and to the client’s satisfaction. RSK’s report satisfied all parties’ requirements, thus enabling quick financial closure to the rest of the project.