This site in East London had been subject to some remedial treatment for the chlorinated solvents by injection and soil mixing in the past, however this appeared to have stalled. The remediation of the chlorinated solvents needed to be completed to the satisfaction of the Environment Agency, and a LNAPL recovery and water treatment system was required on site, both to remediate the LNAPL hotspots and to be used during the wider ground remediation works.

ERS designed and installed a LNAPL recovery and treatment system flexible enough for a variety of flow rates and locations, utilising floating skimmers and pumps operated around a central treatment system.  Treatment comprised silt settlement tanks, oil water separation and management, sand filtration and activated carbon treatment.  This allowed both discharge to sewer and reuse within soil processing plant on-site when appropriate.  The system was used successfully by the main contractor on-site to reduce LNAPL to acceptable levels, with maintenance and support provided by ERS.

ERS’ solution for the chlorinated solvents involved bioremediation via enhanced reductive dechlorination (ERD) and bioaugmentation.  This required direct injection of specialist reagents and an inoculum of dechlorinating bacteria into the impacted aquifer.  A key requirement of the project was to manage upwards migration of contamination and reagents from the confined, contaminated aquifer, protecting material overlying the confining layer which was either uncontaminated or had previously been remediated.

Given these constraints, conventional direct push injection methods were considered unsuitable.  The injection of the reagents required the use of drilling casing to isolate the vulnerable overlying materials prior to forming a seal and advancing the injection rods to the target depth. Once injection of reagents was complete and the rods were being retrieved, the locations were sealed using bentonite grout to prevent upward migration of reagent and contaminated groundwater from the confined aquifer.

Groundwater was monitored for 18 months post injection to ascertains trends in contaminant concentrations and check that environmental conditions remained favourable to enhanced reductive dechlorination.  ERS also monitored the abundance of dechlorinating bacteria by qPCR analysis  as another line of evidence during the monitoring period.

The injection works were completed under ERS’s environmental permit from the Environmental Agency.