A company owned by oil giants Total and Chevron last week pleaded guilty to causing groundwater pollution following the massive explosion at the Buncefield oil depot in December 2005.
After the explosion, escaping fuel and firefighting foam, contaminated soil and groundwater.
Perfluorooctane sulphonate from the 750,000l of foam, and benzene, toluene, ethyl benzene and xylenes- a group of volatile hydrocarbons and methyl tertiary-butyl ether - both found in petrol, have been found in groundwater underlying the site.
Hertfordshire Oil Storage (HOS), which is 60% owned by Total and 40% owned by Chevron, brought an eight week trial at St Albans Crown Court to an end last week, when it admitted polluting ground water under Buncefield in breach of the Water Resources Act 1991.
Two days earlier HOS was found guilty of failing to take necessary measures to prevent major accidents and limit their consequences to persons and the environment, contrary to the Control of Major Accident Hazards Regulations and Health and Safety at Work Act 1974.
Flames tore through the oil storage depot near Hemel Hempsted on 11 December 2005 in what was the largest peacetime explosion in Europe.
A Health & Safety Executive report said that safety systems which shut off the petrol supplies to the storage tanks preventing overfilling failed. As a result 300t of petrol escaped. Bunds designed to contain escaping liquid also failed significantly. The petrol formed a vapour cloud which exploded, injuring 43 people.
Sentencing will take place on 16 July when unlimited fines could be imposed.