The government has spent £12.7m cleaning up toxic and radioactive waste on part of the Olympic site, the Olympic Delivery Authority (ODA) has confirmed.
The contamination, on the banks of the River Lea in east London, where the main Olympic stadium is being built, was discovered when the site's previous owners left the land in July 2006.
Tests by the Environment Agency detected vinyl chloride in the groundwater of the soil, which is a product of the break down of chlorinated chemicals.
A spokesman for the Environment Agency said that the clean-up had been budgeted for before the site was bought, and was "no great surprise", being only a small part of a huge sum spent on decontamination of the Olympic Park.
He added: "People forget just how bad the site was beforehand, and what it's like now - it's going to be turned into a magnificent asset for London."
It is not being suggested that the company whose storage facility was previously on the site was in any way responsible for the contamination.
The ODA confirmed that the majority of the toxic waste has now been removed and cleaned, and the clean-up of the groundwater is still ongoing.
A spokesman for the ODA said: "The clearing of the Olympic Park site has been carried out in consultation with the Environment Agency, Health & Safety Executive and industry experts, with the clean-up and disposal of material meeting all relevant legislation.
"This particular site was identified during early investigations as heavily contaminated and an extensive clean-up has been undertaken with around 60,000m3 of contaminated soil removed from the site and over 50,000m3 of groundwater under the site treated and removed.
"This work has been carried out to the highest standards in consultation with the Environment Agency and planning authority."
He added: "Health and safety of the workforce and local people is our number one priority.
"In the cleaning and clearing of the Olympic Park, much of it contaminated through decades of industrial use, we have reused over 80% of previously contaminated soil that we excavated and recycled or stored for reuse over 90% of demolition materials with minimal materials taken to registered landfills."