AFTER years of secrecy, the US government has released details of the amount and type of radioactivity lingering in the Nevada desert following underground nuclear tests. Between 1941 and 1992, the US and Britain exploded 828 nuclear bombs, hundreds of metres below ground at the Nevada test site.
Data on the radiation left behind was originally classified for fear that it might betray details of the type and size of weapons being tested. But researchers at Lawrence Livermore National Laboratory in California and Los Alamos National Laboratory in New Mexico have succeeded in persuading the US Department of Energy to release information about radioactive contamination of the desert for the first time.
The new data shows that when nuclear tests ceased in September 1992, 43 different isotopes with half-lives of more than 10 years were emitting 4.89 million terabecquerels of radiation. This is around half the total radiation released by the Chernobyl accident in Ukraine in April 1986. The levels in Nevada will decline as shorter-lived isotopes such as tritium, strontium and caesium decay. But Department of Energy figures, which are to be published in a future issue of the Journal of Environmental Radioactivity, show that in 500 years鈥� time, there will still be 8490 terabecquerels of radiation remaining, mostly from plutonium.
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Also, 258 of the tests were conducted below or within 100 metres of the water table. In an earlier study, the researchers from the Lawrence Livermore laboratory found that groundwater contaminated by traces of plutonium from a large explosion had already moved 1.3 kilometres through rock over 30 years. This was 鈥渃ompletely unexpected鈥� says investigator David Smith from Lawrence Livermore.
The find has added to fears that plutonium could end up contaminating crops and livestock in a nearby agricultural community that borders the 3560-square-kilometre test site. To the south, 30 kilometres from the nearest tests, is Amargosa Valley which is home to 1500 people 鈥� mostly farmers and their families. The valley includes Nevada鈥檚 largest dairy, supplying 30 million people on the West Coast.
Steve Frishman, the state government鈥檚 nuclear adviser, argues that the groundwater on which crops and livestock in the valley depend will, sooner or later, become contaminated. 鈥淭he question is not if, but rather when and how much,鈥� he says.
However, Smith points out that, to date, no contamination has been found outside the Nevada test site.