LEAD deposits from old mine workings in the upper dales could mean a chemical time bomb is ticking away beneath riverside pastures, a scientist has said.

Now environment experts, local authorities and the government are poised to meet to thrash out how best to tackle the problem.

The issue, believed to affect rivers such as the Swale, Tees and Wear, is highlighted in a study conducted along the River Swale by academics from the university of Wales, funded by the national environment research council.

Preliminary findings show high levels of lead, which has been linked to brain damage in children, in grazing land alongside the river, which runs from upper Swaledale through the Vale of York.

Researchers from Aberystwyth say last autumn's floods exacerbated the problem by dislodging contaminated soil deposited during the mining heyday of the 19th century. The soil was washed on to waterside pasture where stock grazes.

Prof Mark Macklin, who heads the team, said more than 75pc of sediment samples taken along a 110km stretch of the Swale exceeded guidelines for lead levels - some by as much as ten times. Most watercourses in similar mining areas, including the Tees and Wear, were likely to be affected in the same way, he added.

"Flood plains do represent a chemical time bomb. We have known that for some time," he said. "We can assume that some of those metals brought down as a consequence of the autumn flooding will get into crops and, if animals graze on affected areas, they are likely to be affected too. It really is a long-term problem and, in the most extreme cases, the land should not be used."

Mr Bob Pailor, dales environment protection manager with the Environment Agency, stressed the findings of the study were "hot off the press" and research was not yet complete.

"We have known for some time that there is a lot of mining residue in rivers such as the Swale and Tees and it is well documented that this moves around slowly and some of the flood plain sometimes contains high levels of these metals," he said.

"This latest research has identified that, during the floods last autumn, there were some quite significant sediment movements downstream. This information is literally hot off the press; we found out about it on Thursday of last week and work has not yet been completed.

"It is a multi-agency issue and we, as guardians of the water environment, will be meeting local authorities, which are responsible for contaminated land, and the department for environment, food and rural affairs. We need to get together to assess the findings of the research, identify the issues and see what needs to be done."

It was not known when the study, which had been hampered by foot-and-mouth restrictions, would be complete.