ERM and QDS scooped the award for Best use of a combination of remediation techniques for their work on a former dye manufacturing facility
A former dye manufacturing facility, impacted by chlorinated benzene compounds, was remediated in nine months. The works represent a combination of solutions applied to one of the most challenging source scenarios, Dense Non-Aqueous Phase Liquid (DNAPL) at depth with associated impacted soils and waters adjacent to, and in continuity with, a sensitive surface water receptor, a river. A combination of in-situ and ex-situ technologies were applied.
Both source zone treatment and pathway intervention techniques were applied including soils excavation, biological substrate injection for reductive dechlorination, removal of river bed sediments, placement of a low permeability river bed backfill layer and injection of a microparticulate zero-valent iron slurry. It also used the UK's first application of surfactant flushing in an aquifer and the second only UK use of a Modified Waterloo Profiler (MWP). All validation criteria were met and around 9,000kg of contaminant was recovered from the site, while dissolved phase concentrations within the river decreased.
Investigation and validation
Trichlorobenzene (TCB) and its daughter products Dichlorobenzene (DCB) and Chlorobenzene (CB) were identified in soil, sediment, surface water and groundwater at the 1.9ha site. Works focused on understanding chlorinated solvent distribution in surface water, river sediment, unsaturated zone soils and underlying saturated zone drift and sandstone aquifers. Detailed saturated zone characterisation was achieved using a MWP, a direct push discrete interval groundwater sampling device, together with real-time analysis of 190 groundwater samples in an on-site laboratory, followed by installation of a number of continuous multichannel tubing discrete sampling wells.
The investigations significantly refined the conceptual site model and identified potential source zones of unsaturated soils, sediment and groundwater within the underlying heterogeneous clay, silt, sand and gravel aquifer, which was typically 10m-15m thick. DNAPL was identified within both the unsaturated and saturated zones.
The remediation works recovered over 9,000kg of contaminant mass via soil and sediment excavation and surfactant enhanced removal works - over 75% of the estimated mass. Impacted unsaturated zone soil was excavated until the site specific screening criteria was achieved. Such excavation works enabled the removal of material impacted with up to 8,000mgkg -1TCB. It is estimated that 600kg of chlorinated benzenes and nearly 5,000kg of contaminated soils from the upper margins of the saturated zone were removed.
The successful surfactant enhanced removal works removed around 2,600kg of TCB and DCB in under three months. This technique mobilised a considerable mass of TCB that had previously been sorbed onto the saturated soil into the groundwater and thus increased the potential recoverable concentration of TCB from a pre-treatment level of 48mgl-1 to 13,215mgl-1 during the peak of the flushing works.
"An entry bristling with innovative ideas and technology applications which was exciting to read." Duncan Sanders
