Successful collaboration for gasworks remediation
National Grid Property Holdings knew its land, a former gasworks, was heavily contaminated by dense non-aqueous phase liquids (DNAPL), including coal tar. It wanted to sell the land but needed a specialist to investigate the extent of the contamination and devise a suitable remedial strategy. RSK was hired for its reputation and professionalism. So, a trio of companies mobilised: RSK Geosciences and RemedX to design and interpret all the investigations and Structural Soils to complete all the on-site drilling.
Sharing the load and problem-solving
The RSK–RemedX team investigated the extent and properties of the DNAPL contamination in the ground, air and water while refining remedial options through pilot trials, which they would distil further to inform the final strategy for the client. Structural Soils’ detailed drilling revealed complex ground conditions: basalt mixed with sedimentary rock at shallow depths. Boreholes revealed pervasive DNAPL contamination and accumulation throughout the site. The natural fractures enabled DNAPL to seep to depths of 10–20 m in both the basalt and sedimentary rock; DNAPL was also retained in below-ground structures associated with the former gas works. DNAPL movement was driven mainly by the fracture flow, which made planning remediation more complex. Significant differences were found in the properties of the DNAPL throughout the site, so the data mined from the laboratory and on-site testing refined the remedial strategy.
Pilot testing and clean-up
Through preliminary remedial appraisals, the team identified the best techniques for cleaning up the site, including groundwater and NAPL extraction (total fluids) at ambient temperature; dual-phase extraction at ambient temperature; thermally enhanced water flushing; total fluids extraction; steam injection; and steam-enhanced dual-phase extraction.
RemedX’s multi-technology team carried out several trials in well arrays (previously drilled and installed by Structural Soils) to determine the most effective techniques. For well arrays in the made ground, eliminating contamination was complex, whereas wholesale removal of DNAPL from the sedimentary rock and the more competent basalt was easier to complete.
After collating and interpreting the data, RSK completed its report and recommended measures for the client to meet its principal aim. The project was successful and received widespread industry acclaim: it was highly commended in the best site investigation category of the Brownfield Briefing Awards in 2018.