LONDON — At the 2012 London Olympics site, five 35-ton washing machines are rumbling away, removing arsenic and tar from more than 1.5 million tons of earth.

The city’s largest cleanup since World War II is part of what the head of sustainability for London’s Olympic Delivery Authority, Dan Epstein, calls the “legacy” of the games.

“The big prize is to regenerate an area of London the size of Exeter,” Mr. Epstein said, referring to a city in southwest England with 120,000 residents. “We took over an incredibly contaminated site.”

Construction of the 80,000-seat main stadium, scheduled to begin May 22, has created Europe’s biggest building site. The decision to transform industrial land, soaked with chemicals and dotted with warehouses, helped London win the games and stirred debate among residents and environmentalists.

Clearing the site will cost $708 million. The total budget, funded by local and central governments and national lottery money, has ballooned to more than $18 billion, almost triple original estimates.

Before the work started, the 246-hectare site in Stratford, east London, had more than 220 buildings, two disused landfills, and 52 pylons. After the games, the area will have 10,000 homes and the largest park built in a European capital city for 150 years, Mr. Epstein said.

His team of 16 organizers is “focused on trying to get the greenest games ever,” Mr. Epstein said. The effort includes generating wind energy, minimizing greenhouse gas emissions, and sourcing timber from sustainable forests.

More than 90% of the material from demolished buildings will be used to build the new sports facilities. Salvaged lamp-posts and manhole covers will be incorporated into sculptures in the park.

Organizers must ensure the development leaves a lasting legacy beyond 2012, a Conservative Party member of the London Assembly, Andrew Boff, said.

“The costs have wildly varied since the start of the process,” Mr. Boff said. “If all the Olympics give us is a lovely warm feeling as a memory years afterwards, then we’ve got to ask ourselves, ‘Was it worth the 9.3 billion?’ If as a result we get improved public services, improved opportunities for some of the poorest communities in the country, we can look back and say it was a job well done. It isn’t guaranteed.”

Wildlife is part of the project. The Olympic Delivery Authority, set up by the government to build the venues, moved 1,698 smooth newts and 110 common toads from the site to a new pond a mile to the north, in the Lee Valley Regional Park, the park’s biodiversity manager, Simon Wightman, said.

After the games, the 26-mile-long park will be extended two miles to the River Thames.

Still, developers have ejected from the site some users who are normally associated with so-called green lifestyles: cyclists and people who grew fruit and vegetables on allotments — which are patches of land leased to individuals for cultivation — at Manor Gardens.