A WATER company has admitted it is to blame for an untreated sewage leak that left more than 1,000 fish and other wildlife dead in a north Hampshire nature reserve.

The alarm was raised last Wednesday morning after members of the public spotted dead fish, including chub, perch and trout, floating in Silchester Brook.

The Environment Agency launched an urgent investigation, and within 24 hours, Thames Water had publicly accepted “full responsibility” for the incident.

The company said the deadly leak was caused by a fault at its unmanned sewage treatment works to the south-west of Silchester.

Simon Evans, communications manager at Thames Water, said: “We deeply regret this incident and have accepted full responsibility.”

The Environment Agency is now focusing its investigation on the cause of the leak, the scale of the damage to wildlife, and how long the leak lasted, before deciding on further action. This could include a prosecution of the water company.

The stream runs from the Pamber Forest nature reserve to the east, between the villages of Little London and Silchester, and into the Foudry Brook, which is a tributary of the River Kennet.

It was at 8am last Wednesday that the Environment Agency first received an alert that sewage and dead fish had been sighted in Silchester Brook.

Richard Alexander, of Clappers Farm, in Clappers Farm Road near Silchester, said his wife Linda noticed the water in the brook, which runs through their land, was cloudy.

He said: “By 9am, it was really ponging and the first dead fish were appearing.

“We have got our own makeshift aquarium and we tried to pull out as many fish as we could. We have enjoyed having this lovely stream through there but now we don’t know what is going to happen.”

After reporting the problem to the Environment Agency, the couple moved their livestock, including ponies, away from the stream.

Cally Morris, clerk to Silchester Parish Council, told The Gazette that she noticed a strong smell of raw sewage at around 9am.

She said: “It filled the air for 10 minutes and then went. I then had phone calls from residents who had seen the pollution.”

Officers from the Environment Agency were on the scene at around 9am and located the source of the leak at the sewage treatment works. The team also carried out checks on water quality and wildlife along 6km of the watercourse, while staff from Thames Water used a pump to aerate the stream.

Joe Cuthbertson, emergency planning officer at the Environment Agency, said the pollution was a “category one” sewage leak – the most serious.

He said: “This is a very serious incident. It has had a serious impact on the river environment.

“Sewage has a very high biochemical oxygen demand. That essentially takes oxygen out of the water because the bugs breed in the water, which leaves no oxygen for the fish.”

He added that aeration and rain on Thursday dissipated the pollution as it travelled downstream, but the death toll for fish was already in the “high hundreds” by that time. Invertebrates such as leeches, damselfly larvae and small shrimps are also thought to have been affected.

By Friday, it emerged that the pollution had spread to 12km of the stream, the number of dead fish was more than 1,000, and the Environment Agency was using hydrogen peroxide to increase the amount of oxygen in the water.

The sewage treatment works serves Silchester and the local area by treating sewage and releasing harmless treated effluent into Silchester Brook.

Mr Evans said he could not comment on what caused the problem, as it was still under investigation. But he added that an engineer fixed the leak just after 9am last Wednesday, meaning that the sewage treatment works could carry on operating.

Alison Cross, head of conservation in north Hampshire for the Hampshire and Isle of Wight Wildlife Trust, which runs the Pamber Forest nature reserve, said the stream and its wildlife could take months to recover, if not longer.

She said: “We are concerned about this type of incident. It shows how easy it is to damage eco-systems.”