The site comprised the main buildings of Chelmsford Gas Works dating back to the mid-19th Century, including a retort house and gas holders, that has left a legacy of soil and groundwater contamination. Due to the site’s geographical position, between the River Chelmer around 30m to the south-west and the Chelmer and Blackwater Canal to the north, a complex hydrogeological regime exists across the site that required detailed assessment.

Prior to purchase of the site from National Grid it had been suggested that remedial works may not  be necessary, but CampbellReith highlighted to the client that this  would only be the case if the site  remained undeveloped and the site was proposed for residential development.

CampbellReith reviewed all the historical information, and this identified two possible non aqueous phase layer (NAPL) plumes, one, light (LNAPL) and the other, dense (DNAPL). Understanding the extent of these plumes was one of the main priorities for the subsequent site investigation, since it had not been done previously. The risk of vapour migration from the NAPL plumes to neighbouring receptors was also to be assessed due to newly constructed residential development north of the site.

The aquifer beneath the site has been significantly impacted by the site’s historic use but detailed  quantitative risk assessment  (DQRA) showed that groundwater contamination does not present a significant, i.e. statutory, risk to the River Chelmer and this was accepted by the Environment Agency (EA).

However, following discussions with the EA, it was highlighted to the client that groundwater remediation would likely be required as part of any planning application. As such, the cost of any proposed development would be higher than originally supposed by Chelmsford City Council. CampbellReith provided a cost estimate for the remediation which enabled Chelmsford City Council to renegotiate the site purchase.

A ground investigation was then carried out to investigate data gaps, enable additional DQRA, and to undertake a remedial options appraisal as part of a Remediation Strategy. A series of phases of remediation were recommended, based on Chelmsford City Council’s development programme and diversion works associated with the active gas mains across the site.

As part of the first remedial phase, CampbellReith specified and managed NAPL removal from identified plumes, via the Non-aqueous Extraction Technique (NET), also called FabEx, supplemented by targeted chemical oxidant injections to enhance recovery of very viscous DNAPL.

These works were successfully completed prior to the proposed construction works and CampbellReith subsequently advised the client on cost effective methodologies to deal with the outstanding remedial requirements.