On 17 July last year, the Environment Agency received reports of oil on Stanford Brook, Surrey. Officers visited the site and found gas oil on the brook. They immediately put booms down to stop it spreading.
The spill was traced for a kilometre to Thames Water's Hockford sewage treatment works.
Investigations revealed that oil was leaking onto the floor of the works' generator room from a leaky pipe and into the brook via surface water drains.
The following day, however, the officers also discovered that oil was leaching into the brook from the riverbank. Further investigations revealed a hairline crack in the floor of the generator room had allowed oil to leach into the ground for up to five months.
Thames Water immediately cleaned up the brook and dug a hole in the bank to ensure no more oil would leach into it. In spite of that action, around 1,200 litres of oil was lost.
Appearing before Guildford magistrates on 27 January, Thames Water pleaded guilty to causing polluting matter to enter controlled waters, contrary to section 85(1) of the Water Resources Act 1991. It was fined £10,000 with £1,233 costs.