By Environment Correspondent Alex Kirby

UK scientists have found that some highly toxic contaminants are not dispersing in water as rapidly as they had thought.

The discovery was made by researchers at the Natural Environment Research Council's (NERC) Centre for Ecology and Hydrology, and the University of Aberdeen.

Searching for improved ways of predicting and managing pollution, the scientists have been conducting detailed studies of river dynamics. This has involved them in looking at sediments, which have not been so intensively studied before.

A NERC scientist said: "It is here, in the pores between the particles on the river beds, that contaminants still lurk.

Notorious pesticides

"These are only slowly broken down or moved on down the channel and out to sea. We may still find polychlorinated biphenyls (PCBs) in the North Sea 20 years on.

The legacy of some notorious pesticides and industrial chemicals is going to be with us for much longer than we would like."

PCBs, developed originally for use in electrical equipment, are now banned, but large quantities remain in existence. They are carcinogenic, and capable of damaging the liver and nervous system, and the reproductive system in adults.

Another chemical the researchers have found in sediments is dieldrin, a carcinogenic pesticide.

Sewage works

They have found persistent levels of these pollutants in river catchments in west Yorkshire, which had a long history of contamination by chemicals used in wool processing.

NERC says it is not simply industrial areas that have problems, as run-off in rural areas can be equally suspect.

"Water coming from sewage treatment works, particularly during summer when this is a bigger part of the overall flow, may remain a source of chemicals for some time to come."

It says the chemicals probably come from industrial towns and the rural areas that feed into the sewage works.

But NERC says "the overall improvement in river water quality is the biggest UK environmental improvement story of the century.

"Better management and the identification of the worst culprits in industrial processes have done wonders for many rivers that were once no better than open sewers."