The new research proves for the first time that this occurred because the plutonium attached itself to microscopic mineral fragments which were then carried along by flowing water underground.

Plutonium hardly dissolves in water, which is why underground storage of nuclear waste has been seen as the most acceptable disposal option. But the new study shows that over 99% of the transported plutonium did not travel dissolved in groundwater but on particles - called colloids - carried in the flow.

No complacency

Nirex, the UK authority responsible for disposing of nuclear waste, told BBC News Online: "We agree with the scientific findings - colloids must not be neglected. But we have been aware of this transport mechanism and done our own research at a fractured slate mine in Cornwall.

"This is factored into all the calculations we make about our rock repositories - we haven't been complacent," said Dr Alan Hooper, Science Director.

The pesticide DDT could also spread by fixing on to particles in groundwater

The actual amount of plutonium moved in Nevada is very small and only a tiny fraction of the total waste present at the test site was mobilised. Nonetheless, the researchers from the Lawrence Livermore and Los Alamos National Laboratories, writing in the journal Nature, warn that ignoring this way in which contaminants can be dispersed may "significantly underestimate the extent of radionucleide migration."

Vapourised rock

The US researchers looked at the Nevada Test Sites where 828 underground nuclear tests took place between 1956 and 1992. One, Benham, was the location of a 1,150 kiloton explosion just over 30 years ago. It reached a temperature of a million degrees and vapourised the rock 100 metres around.

The scientists compared the ratio of the types of plutonium found in soil at Benham with that found in groundwater 1300 metres to the south. They matched perfectly, not only with each other but also with molten rock samples taken at the time of the blast.

Filtering the very fine grit out of the groundwater samples showed that over 99% of the plutonium had been adsorbed onto the clay mineral particles. Professor Bruce Honeyman, at the Colorado School of Mines said in Nature that the study illustrated: "the striking influence colloids may have on contaminant transport."

Plutonium - ideal candidate

He stated that low solubility in water and a high tendency to stick to rock made plutonium "an ideal candidate" for this type of transport, along with thorium, cobalt and pesticides like DDT. However he told BBC News Online it was doubtful that it would provide a significant way in which people might be exposed to nuclear waste, because there the concentration of colloids is usually low.