DON鈥橳 panic, but your dustbin could contain radioactive tritium. A government-commissioned study has found that British landfill sites contain far more tritium than most radioactive wastes.
鈥淭he numbers are still too low to be a public health hazard,鈥� says Howard Robinson of the waste consultants Aspinwall & Co, who led the study, 鈥渂ut we simply can鈥檛 explain where the tritium is coming from.鈥�
Robinson鈥檚 finding, which has set government researchers rummaging through the nation鈥檚 rubbish, emerged from a five-year investigation of the polluted water, or leachate, that seeps from the bottom of landfill sites. The study will be published by the Department of the Environment (DoE).
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Tritium is a form of hydrogen containing a proton and two neutrons in its nucleus, rather than the usual single proton. It emits beta particles and has a half-life of 12 years. Its concentration is measured in tritium units (TUs), with one unit representing a concentration of one tritium atom for every 1018 hydrogen atoms.
In 1963, when most countries banned nuclear weapons tests in the atmosphere, rainfall in the northern hemisphere contained 2000 TUs. Today鈥檚 rainwater has less than 20 TUs. And because of radioactive decay, a bucket of 1963 rainfall would contain only a few hundred TUs today.
Yet when Robinson and Jan Gronow of the DoE analysed the leachate of 30 landfill sites from Glasgow to Dorset, they found an average concentration of more than 7000 TUs, with a maximum of 39 000 TUs at two tips. This is more than government inspectors find at most dumps licensed for low-level radioactive waste. 鈥淭ritium takes the place of hydrogen inside the landfill, whether in water molecules or methane or other compounds,鈥� says Robinson.
Where the tritium in landfill comes from is unclear. 鈥淚t can鈥檛 be illegal disposal of radioactive waste or the contamination wouldn鈥檛 be so widespread,鈥� says Robinson. The source has to be some 鈥渃ommon component of domestic or commercial wastes鈥�, he says.
One possibility is the luminous paint on the dials of discarded clocks and watches. Another is the first generation of warbling Trimphones, manufactured for British Telecom between 1964 and 1976, which had luminous dials. But Robinson believes neither of these sources contains enough tritium. His best guess is that most of the tritium comes from luminous green exit signs used in cinemas and theatres and technically known as 鈥済aseous tritium light devices鈥�. The largest of these devices are supposed to be returned to the manufacturers when their useful life is over so that the tritium can be recovered. In practice they are often dumped. Robinson calculates that one large sign could contaminate the leachate from a medium-sized tip for several years.
Whatever the tritium鈥檚 source, its discovery could have a beneficial spin-off. 鈥淲e have stumbled on a very useful chemical marker for leachate from waste tips,鈥� says Robinson. 鈥淚f a tip is leaking into drinking water, now we will know.鈥�