Chemicals from the explosion at the Buncefield oil depot in the UK in 2005 may still pose a danger to the environment, a court in Hertfordshire heard.
The water supply had been severely polluted by the spillage and the firefighting chemicals used to tackle the blaze, which lasted several days.
The full extent of the damage to the environment is not yet known as the chemicals may not yet have seeped into the water supply.
Hertfordshire Oil Storage (HOSL), TAV Engineering and Motherwell Control Systems 2003 are accused of health and safety breaches.
The blast at the Buncefield depot in December 2005 caused widespread damage and injured 43 people.
The blast, which measured 2.4 on the Richter scale and could be heard 125 miles away, happened after a massive vapour cloud ignited when 250,000 litres of petrol leaked from one of its tanks, the court heard.
The court was told HOSL had "manifestly failed" in its duty to ensure the site was safe.
The jury was told Total UK, which operated the depot, had already admitted health and safety breaches in connection with the explosion but other companies were responsible too.
British Pipeline Ltd has also admitted two charges in connection with the explosion.