A water company which allowed untreated sewage to enter a Woking canal, killing hundreds of fish, was today (Friday) ordered to pay a total of GBP17,403 by magistrates.

Thames Water, based in Reading, admitted a charge of allowing polluting matter to enter the Basingstoke Canal and was fined £12,000, made to pay £5,388 costs and a £15 victim surcharge by Woking Magistrates’ Court. The charge was brought under the Water Resources Act 1991.

Environment officer Ruth Clayden was called to the canal at Boundary Road, Woking, at around 8am on September 7 2009. She walked under the road bridge and noticed a discharge coming from the surface water outlet, which was entering the canal on the right hand bank looking downstream.

The water was cloudy, with a strong smell of sewage and a number of fish were seen either dead or gasping for air on the surface. Readings taken upstream of the discharge point at 9.55am showed: Dissolved Oxygen (DO) levels were 44.8% and Ammonium (NH4+) levels were 3.24 milligram’s per litre (mg/l).

A reading taken near the discharge point at 10.05am showed DO levels were 5% and Ammonium (NH4+) levels were 33.27 mg/l.

Fisheries officer Dominic Martyn arrived at 11.20am and walked further downstream with the environment officer. More readings were taken and 35 dead fish were seen over a 200-metre stretch from the discharge point. This number contained roach, bream, perch and gudgeon.

Thames Water were contacted and arrived late morning as they were working on a blocked sewer in Victoria Way, which was cleared soon after. The environment officer took another reading at the discharge point and the ammonium reading was recorded as 12.51 mg/l, a decrease since the blockage had been cleared.

Ruth Clayden walked downstream again in the afternoon and although fish seemed fine at the end of the stretch of the affected water, roughly 420 metres downstream, they were still gasping near the discharge point so it was decided that Thames Water would install aeration equipment.

The Environment officer attended the scene with a colleague the following day and more readings were taken but these showed DO levels were still low. Thames Water contractors confirmed aeration equipment did not arrive until 6.30pm the night before and had been switched off overnight.

On September 9 Thames Water gave Ruth Clayden some readings over the phone which showed DO had marginally increased. It was decided to deploy two more aeration kits and the Environment Agency gave permission for Thames Water to tanker several loads of clean water from the nearby Goldsworth Park balancing pond.

On the morning of September 10 Thames Water called and gave more readings which showed the pollution was still bad but a further reading taken on site by Ruth Clayden at 2pm showed the equipment was having a positive effect on DO levels.

Thames Water called to say 230 dead fish had been counted. The aeration equipment was removed in the afternoon of September 11 due to rising oxygen levels and the canal was monitored over the weekend.

Readings taken by Ruth Clayden on Monday 14 September showed DO readings were now at a level not to cause any harm to fish in the canal.

During an interview under caution on December 15 2009 Simon Rawles, Thames Water’s Area Network Manager, accepted the company was responsible for the sewers in the Woking area. He admitted a blockage was found in the foul sewer where two drain rods, which have since been removed, had collected fat, oil and grease.

He said once this blockage had been cleared the pollution had stopped. Although foul sewers and surface water sewers in the surrounding area were investigated, no structural defects or misconnections that could be connected to the pollution incident could be found.

Ruth Clayden, who investigated the incident for the Environment Agency, said: “This was a distressing incident and it was upsetting to see so many fish either dead or gasping for life. The area is popular with anglers due to its urban location, and is fished regularly.

“Water companies have a statutory duty to maintain their sewers, which Thames Water had not done in this case. It sends a clear message to other companies that if you fail in your environmental responsibilities you may be prosecuted.”