The development of the Olympic site in east London after the Games have finished could be in jeopardy because of radioactive waste buried beneath the site, experts have warned.

According to a Guardian investigation, any development of the site risks unearthing a hundred tonnes of radioactive waste dumped at the former landfill site decades ago. Documents obtained under Freedom of Information (FOI) rules reveal that, contrary to government guidelines, waste from thorium and radium has been mixed with very low-level waste and buried in a so-called disposal cell close to the Olympic stadium - about 250m to the north.

After the Games, the demolition of the Olympic stadium in Stratford to make way for housing is a possibility because government and sporting authorities so far have been unable to agree on its future use. Despite a possible bidding war between AEG, which runs the O2, and Live Nation to possibly turn the stadium into a music venue, bookmaker William Hill recently made demolition of the Olympic stadium 5/1 third favourite behind its continued use for athletics or conversion into a home ground for West Ham United. "There seems to be no obvious usage for the stadium after the games," a William Hill spokesman said.

But while officials insist there is no risk from the waste to athletes or spectators during the event, further development of the site could expose the waste, which some experts claim should have been moved to a safe site.

John Large, an independent nuclear analyst, said: "The Olympic site's hurried and unplanned development may have resulted in a great deal of public harm to the local communities remaining around the site. Overall, there is some doubt about the applicability and validity of the radiological risk analysis undertaken for the future legacy use."

His sentiment is shared by Andrew Boff, a member of the London Assembly and Conservative spokesman on the Olympics. "I thought the £9.3bn cost would provide a remediation level sufficient for future development. But what we are left with is remediation which is just enough for us to hold the Games. The ODA is very proud that it came in under budget on remediation. I wish it had spent the whole amount and made the site fit for the future."

Assessment



Boff is tabling questions for London mayor Boris Johnson on the Olympics site. He wants to know if the mayor will commission an independent review of the way in which radioactive material has been disposed of and he also wants the mayor to ask the Olympic Park Legacy Company, responsible for transforming the site into a lasting metropolitan area once the Games are over, to investigate the additional costs of remediation that may be required for redevelopment.

Developers are concerned that a complete reassessment of the site would have to be carried out for any future work to go ahead. Steve Wielebski, divisional development director at Miller Homes and a leading authority on contaminated land, said any housebuilder will want to examine any contaminated land carefully before constructing new homes.

"There are many safeguards put in place by the planning and regulatory regime, which ensure that a finished development is fit for purpose. However, if a subsequent development represented a change of use to a more sensitive end-use, for instance the construction of houses with gardens, then the developers would need to go through the whole process again of assessing the potential risk to end users," Wielebski said. "No house builder will compromise on safety."

The Olympic Park Legacy Company said: "Our assurance from the Olympic Delivery Authority (ODA) is that the land has been remediated to the highest possible standard."

The site where the world's greatest athletes will compete in two years' time was once home to several dirty industries working with thorium and radium and also home to a number of landfill sites, where illegal dumping of toxic waste was commonplace in the 1950s and early 1960s. Although the Radioactive Substances Act of 1960 tightened the law, dumping was allowed to continue for another three years.

In July 2008, the ODA told the Environment Agency that it had found 40 cubic metres, about 50 tonnes, of waste that showed radioactive readings up to three times higher than the levels at which waste is treated as exempt. But it argued that when put together with 1,500 cubic metres of material that was "definitely exempt" this would bring the whole waste into the exempt category.

The Agency accepted this argument in July 2008. By the time the waste was buried on the site three months later the total waste had doubled to about a hundred tonnes and the total exempt waste had risen to 7,500 tonnes.

"Overall contamination levels in the waste at the Olympic site is within exempt levels. The waste has been correctly disposed of and no rules, or laws, have been broken," the Environment Agency said.

Much of this information is contained in a dossier of hundreds of documents obtained under the Freedom of Information Act by campaigner Mike Wells and intelligence analyst Paul Charman, who have spent more than three years trying to establish what has been going on at the Olympic site where 28,000 documents have been submitted for planning approval.

Disposal



The dossier has been reviewed by John Large, who has been engaged to analyse the data. He said: "As part of the on-site assaying, the ODA contractors were clearly separating out exempt waste from non-exempt batches of low-level waste with the intention, then, of sending this higher [radio]activity non-exempt waste to a radioactive waste disposal site, such as Drigg [in Cumbria]."

He continued: "At the best, this might be interpreted as a misplaced interpretation of the radioactive waste regulations or, at the worse, some might view it as blatantly cooking the books to save on the high cost of off-site radioactive waste disposal."

The ODA denied that it had misinterpreted or manipulated data and Allan Ashworth, the principal consultant who oversaw the remediation work on the Olympic Park, said it was the Environment Agency that had encouraged the elevated radioactive waste to be kept on site in a disposal cell. "EA guidance says the waste should stay on site and Drigg should only be used where strictly necessary," Ashworth said.

Asked about Ashworth's claim, the EA said: "The waste did not go to Drigg because it is so low risk."

The discovery of so much waste has prompted John Large to call for an independent review to remove any doubts about risks that workers and former site residents may have been exposed to.

But the ODA director of infrastructure and utilities, Simon Wright, said: "The Olympic Park and all venue sites are completely safe with no radiation measurements above the normal background levels that people would experience in their everyday lives.

"We have been open and transparent about the contamination found on the Olympic Park site, a former industrial landfill site. As previously announced, a small amount of soil containing traces of very low-level radioactive material, classed as 'exempt' under current environmental law, has been safely buried in a cell on site … using a proven, safe and approved method of disposing of such material."

• This article was amended on 21 June and 12 October 2010. The original said that the waste in question had been buried "under, or close, to the Olympic stadium", and a sub-heading said that this could render the Olympic site useless after the 2012 Games. This was initially corrected to give a distance of 500m from the stadium. The ODA subsequently confirmed that the distance was 250m. The text above has again been corrected.