The NRA said last week that the company was 'figuring prominently' in its investigations, amid suspicions that it knew about the pollution several days before it reached taps in and around Worcester.
It took action to stop its customers drinking the water only after receiving complaints that it smelt and tasted disgusting. The disruption to supplies, which lasted two days, was the biggest incident of its kind for 10 years.
The authority, in an all- night operation, traced the pollution to a firm, Vitalscheme, on the Wem industrial estate in Shropshire, about 10 miles north of Shrewsbury. From there it passed through the Wem sewage works, which is owned by Severn Trent Water, and flowed 80 miles down the rivers Roden and Severn over several days until it reached the drinking-water supplies.
Sources say that the firm is adamant that Severn Trent Water gave it verbal permission to discharge the pollution, a cocktail of several chemicals including industrial solvents, to its sewage works.
They say that there is a growing suspicion that the water company must have known that the pollution had passed through the works long before it got to consumers' taps, if only because it would have smelt very strongly. And if Severn Trent did not know, it will have to explain why.
An NRA spokesman said: 'By the very nature of the incident, a discharge to sewer, Severn Trent is figuring prominently in our investigations.' Nobody at Vitalscheme was available for comment.
The water company reacted quickly when its customers complained about the water last weekend, advising people not to drink it. But many had already consumed it, before hearing the warning.
By late last week the pollution had worked its way right down the river to the Severn Estuary, and the NRA announced that the incident was over. But it said it would continue investigations into the cause 'with a view to possible legal action'.
A Severn Trent Water spokeswoman denied that the company was aware of the chemical cocktail being disposed of in the river. 'We did not know about the pollution until the morning of Friday, 15 April. The contamination was a result of an unauthorised discharge.'
The Government has had the power for 20 years to set up special water-protection zones where companies would be required to take particular care to avoid pollution that would contaminate drinking water. No such zones have yet been set up.
After the river Dee was polluted by a release of phenol which contaminated the drinking water of two million people in 1984, the Government admitted that some rivers were vulnerable.
The NRA made another proposal for a water protection zone last year, but ran into opposition from the Confederation of British Industry.
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