INSITU BIOREMEDIATION is solving the problem of deep hydrocarbon contamination on the redevelopment of a former GlaxoSmithKline site in Welwyn Garden City.
Demolition and removal of contaminants to landfill was nearing completion when an underground fuel storage tank was found.
The tank was removed but there was not enough time to dispose of 1,500m 3 of fluvio-glacial sands and gravels contaminated with petroleum hydrocarbons before the introduction of the EU Landfill Directive in July last year.
The reduction in the number of licensed hazardous waste landfills under the new regime and the consequent rise in disposal costs meant dig and dump was no longer a viable option.
Exsitu bioremediation was ruled out because the depth of contamination, between 6.5-7m, would have meant costly and time-consuming excavation. Extensive disturbance of the ground would also have complicated foundation design, preventing the use of strip foundations for the new houses being built.
Remediation contractor Vertase, working for developer Bloor Homes, opted for insitu bioremediation.
A bioremediation liquid from Soil Water and Environment was injected through 135, 6.5m deep wells on a 1.5m grid across the contamination plume.
The liquid consists of naturally occurring microbes mixed with nutrients. The micro-organisms break down the contaminants, converting them into mainly oxygen and water.
Wells were formed using sonic drilling, which uses sonic vibrations to drive a hollow steel tube with a sacrifi cial tip into the ground. As the tube is withdrawn, the bioremediation liquid is injected.
Injection was carried out at the beginning of this year and cleanup was expected to take only six weeks.
Final validation sampling to confi rm the remediation target of 500mg/ kg of hydrocarbons has been met was expected this month, and main construction will start soon after.