WATER company United Utilities and a partnership of construction companies are facing the possibility of hefty fines after admitting polluting Bradshaw Brook and killing hundreds of fish.

A diluted toxic chemical flowed through broken pipework into the water course. A 1.7km stretch of the brook leading towards Jumbles Reservoir was so badly polluted that virtually all aquatic organisms, including fish, shrimp and earthworms were killed.

Bolton Crown Court heard how KMI Plus were contracted to carry out improvement works at Wayoh water treatment works at Turton Bottoms.

As part of the work, in December 2013, the contractors were due to empty and remove a tank which contained the bleach, sodium hypochlorite.

The Honorary Recorder of Bolton, Judge Timothy Clayson heard that the majority of the tank was drained, but up to 300 litres of the highly toxic chemical was left in the bottom.

Richard Bradley, prosecuting on behalf of the Environment Agency, said that instead of pumping or siphoning the remaining liquid out, a decision was taken to put a hosepipe into the tank and dilute the sodium hypochlorite with water, letting it run out into a drain.

From the evening of December 4 the hose ran for an estimated 16.5 hours and hundreds of litres of the solution ran out towards a drain hole.

The contractors and United Utilities assumed the liquid would flow through drainage pipes to another tank, but a risk assessment had not been carried out and the pipework was broken, leading to the bleach solution spilling into Bradshaw Brook.

"Although it was diluted, once in the water course it was highly toxic," said Mr Bradley.

On December 6, 2013 the Environment Agency was alerted by a member of the public who had found dead fish floating in the water.

Subsequently up to 900 dead fish were recovered, including Brown Trout, Loaches and Bullheads, but Mr Bradley said the number killed is likely to be much greater.

"Some would have been swept away, some would have sunk to the bottom where they remained and some would have been taken away by scavengers," said Mr Bradley.

It was several days after the dead fish were first discovered before the cause of the pollution was identified.

The brook was a spawning ground for Brown Trout, but a survey carried out six months after the spillage showed there were still very few live fish.

Mr Bradley said that since then it has been restocked with fish from elsewhere in the brook.

KMI Plus, a partnership of J Murphy and Sons Ltd, Kier Infrastructure and Overseas Ltd, Mouchel Ltd and Interserve Construction Ltd, pleaded guilty to causing polluting matter to enter a water course, as did United Utilities Ltd.

Mr Bradley stressed that KMI Plus had not carried out a proper risk assessment prior to emptying the tank or produced a method statement and United Utilities had allowed the work to ahead without them.

Judge Clayson commented that it would not have been difficult to establish the integrity of the drains by flushing a harmless dye through them before starting the work.

Timothy Horlock QC, for United Utilities said he was instructed to express the company's "deep regret" over the incident.

"I am instructed to apologise to the court and the community in general for this offence," he said.

He stressed that it had arisen out of a misunderstanding about the method used to dispose of the chemical, but United Utilities would not have agreed to the procedure KMI Plus chose to use.

Both KMI Plus and United Utilities admitted being negligent, but KMI Plus claimed it had been the water company's responsibility to check the pipework.

The court heard that since 1990 United Utilities has had 202 convictions for environmental offences and been before the courts on 135 occasions.

They included an incident three years ago at Buckton Castle water treatment works at Mossley, which was similar to the pollution at Wayoh.

KMI Plus has had no previous convictions.

Judge Clayson adjourned giving his final judgement until later this month to give him time to consider the prosecution and defence arguments.