Conceptualisation and trials for complex non-aqueous phase liquid remediation at a gas works

RemedX was contracted to undertake supplementary site investigations and pilot-scale remediation trials at a former major gasworks site in the UK. The project focused on assessing the extent and behaviour of coal tar on multiple sources of non-aqueous phase liquid (NAPL) contamination under the site. A combined RemedX and RSK team delivered the project.

The combined team divided the project’s scope according to each unit’s strengths, with engineers drilling and installing well arrays for remediation pilot trials alongside the supplementary site investigation boreholes to delineate and conceptualise the NAPL sources. RSK specialists investigated the extent and properties of the NAPL in the ground, the underlying bedrock and several structures. They recorded the detailed characterisation of the bedrock properties through rock core logging, downhole geophysical logging and geotechnical testing.

The underlying ground conditions were found to be extremely complex, as the investigations revealed significant differences in the bedrock geology. Initial modelling had predicted that the basalt would be of low productivity, but it was actually heavily fractured in places. Detailed rock core logging showed NAPL present in fractures to about 10 m depth with smearing to about 21 m in the sedimentary rock. It was therefore concluded that the NAPL movement in both types of bedrock was driven not only by NAPL mass but also by fracture flow.

Working with a specialist laboratory, analysis and subsequent data modelling were undertaken to develop a more detailed conceptual site model of the NAPL sources and behaviour in the subsurface. This enabled the risks to water environment receptors, the wider bedrock aquifers and the nearby estuary, and of off-site migration of NAPL to be evaluated. RemedX’s assessment of which remediation techniques would suit the site identified dual-phase extraction of groundwater and NAPL alongside in-situ thermally enhanced treatment as the likely most effective solutions.

RemedX’s multi-technology remediation pilot test trailer was instrumental in trialling specific techniques, including groundwater and NAPL extraction (total fluids) at ambient temperature, thermally enhanced water flush and thermally enhanced total fluids extraction. Using the trial results, which confirmed the viability of effectively extracting NAPL from selected areas only, we refined the NAPL source areas identified in previous phases and developed design considerations for a full-scale site remediation.

This extensive programme of work was highly commended in the Best Site Investigation category of the Brownfield Briefing Awards in 2018.