Severn Trent Water admitted leaking ferric sulphate - used to treat drinking water - into one of the best stretches of salmon river in Wales, killing all but 2 per cent of the stock.
It was the company's 34th conviction since privatisation seven years ago, Cardiff Crown Court heard. Judge John Prosser told company executives that the leak was due to a combination of design defects, gross mismanagement and inferior maintenance. ''To be convicted so many times shows that the management of the company is very slack indeed,'' he said.
Prosecutor Mark Bailey, acting for the Government's new Environment Agency, said the pollution from Severn Trent's Elan Valley water treatment works at Rhayader, Powys, flowed down the small river Elan and into the Wye, where it killed 33,000 young salmon in June last year.
''The sheer number of fish killed is higher than [in] any other incident,'' he said. The chemical had turned the river water acid, causing large quantities of aluminium to be released from sediments. This metal is highly toxic to fish.
The company, which pleaded guilty to polluting the river, was also ordered to pay costs and compensation of almost pounds 44,000, including pounds 8,500 towards restocking with fish.
Severn Trent's barrister, Benjamin Nicholls, told the court that the chemicals leaked through a hairline crack in a pipe, which was repaired as soon as it was spotted.
After the verdict, Peter Gough, of the Environment Agency, said it showed that "thorough investigations into incidents such as this pay off. Companies must realise the seriousness of their actions.''
Severn Trent said it was distressed by the size of the fine,but had no plans to appeal.