A mound along Marshala Rokossovskogo Boulevard that for years was used by children as a sledding hill actually contained radioactive waste dumped there in the 1940s and 1950s, and after beginning an excavation to enable the building of new apartments on the site, officials have suspended the operation, the Moscow Times reported Tuesday.

That has led to concerns of radioactive water running off from the partially completed clean-up site, environmentalists say. Vladimir Chuprov, head of Greenpeace Russia's Energy Unit, told the newspaper, "The bad news is that the water has flowed in. This water might contain radioactive materials. Liquid is much more difficult to recover and keep from spreading."

But Anton Matyukha of Radon, a government-backed company charged with locating, retrieving and securing radioactive waste, said the site has been sufficiently cleaned and poses no danger to the public.

The mound is one of many sites around Moscow containing low levels of radioactive waste produced as Soviet Union pushed forward with its atomic programs at military plants, research institutes and factories, the newspaper said.