The incident came to light on 11 October 2002 when the Agency received reports from members of the public about dead fish in the Nant yr Aber stream in Caerphilly.
Agency officers visited the site and saw over 100 dead fish in the stream. They could not find any trace of pollution.
The next day, the officers returned to the site and saw a "large discharge" of wastewater pouring out of a surface water sewer. Suspecting this discharge to be behind the incident, they traced it to Transco's Caerphilly gasholder station.
Transco - and its contractors, E Harper (York) Ltd - were decommissioning the station. They were washing it out with water mixed with a deodoriser.
Further investigations revealed that the wastewater had been accidentally discharged into the surface water sewer instead of the foul sewer. This was due to a mix-up over the site's drainage plans.
A biological survey later indicated that a 1.1 kilometre stretch of stream was affected with the majority of fish being killed.
Appearing before Blackwood magistrates on 17 January, Transco pleaded guilty to causing polluting matter to enter controlled waters, contrary to section 85(1) of the Water Resources Act 1991. It was fined £8,000 with £1,400 costs.
Transco was unwilling to comment on the case aside to say it "recognises mistakes were made" and it has "revised" its procedures to try to ensure such incidents never occur again.
The Agency is also prosecuting E Harper (York) Ltd in relation to the incident, but the case has yet to come to trial.
Transco appears to have been fined only once before for an environmental offence - in 2002 for polluting the Black Lynn burn in Oban, Scotland, with oil (ENDS Report 324, p 46 ). The Caerphilly case is its first prosecution in England or Wales.