THE CLEANUP OF the Wynn Resorts casino site in Everett is a case of addition through subtraction.

Before the Wynn tower could start going up, all of the contaminated soil on the property had to be removed.

An estimated 500,000 tons of dirt containing PCBs, arsenic, lead, ash, petroleum products, and asbestos were carted off to special landfills in Massachusetts, New Hampshire, Maine, Rhode Island, Quebec, Michigan, Ohio, and Georgia. Most of the dirt was shipped out on two trains, each with 28 cars, that pulled into the property on a daily basis. About 100 trucks also did pickups every day.

Chris Gordon, president of Wynn Design and Development, says the cleanup so far has gone pretty much as expected. The company drilled about 2,000 bore holes on the site and then examined the dirt samples to see what was below ground. The top layer, about 6- to 8-feet deep, was relatively clean crushed stone, much of it brought in from the drilling of the nine-mile Deer Island sewage outflow tunnel under Boston Harbor during the 1990s. Below that was a layer of dirt full of contaminants left over from the Monsanto chemical plant that once occupied the site. Both layers were removed.

There was one surprise. Gordon says excavators found parts of a boiler room and pipes that were wrapped in asbestos. There were also wood timbers and bricks, presumably remnants of buildings on the site that had been bulldozed and buried.

Wynn budgeted $30 million for the site cleanup, but the tab may rise higher than that, in part because of the unexpected discovery of asbestos. Still, Gordon says, the transformation of the property is amazing to watch. “It took about three months to clean up a site that had been polluted for just about forever,” he says.

Monsanto acquired the site in 1929 and continued to operate there until 1992. A Boston Globe story from 2014 quoted local residents who remembered smoke-belching plants prone to fires, explosions, and acid leaks. Many residents complained of a strong and persistent sulfur smell.

In the 1800s, Hawes Atwood, a founder of Boston’s Union Oyster House, owned the property and used to harvest shellfish there from the Mystic River. But those days are long gone. Environmental activists in the 1980s accused Monsanto of dumping toxic waste into the river and state officials accused the company of severely underestimating the size of a spill there.

Gordon deployed a robot-like machine along the bottom of the Mystic River to take pictures and obtain soil samples. The pictures and samples didn’t offer much good news.

“The report came back and said it was a biological desert,” Gordon says. “We didn’t find any living organisms in the top layer of the sediment. It was pretty bleak.”

Wynn is now preparing to dredge the river channel to remove contaminated sediments there.  The dredging is expected to start in September and end in February 2018. Wynn will also remove five barges abandoned in the river, some of which only emerge at low tide. Gordon says the barges will be sold for scrap.

Wynn is hoping to deliver a lot of patrons to its casino site via boat, so the dredging and the restoration of coastal vegetation is a way to make that option attractive to customers. Gordon says the water in the Mystic currently isn’t dangerous to humans, but it is hazardous to most plant life. He hedges on whether people will be water-skiing on the Mystic once the dredging work is done.

“Let me put it this way,” says Gordon. “By cleaning up our site and cleaning up that inlet, it’s going to be dramatically better. What that means for the overall river, I don’t know. But we have a pretty rough site that’s going to get dramatically better.”

Ed Coletta, a spokesman for the state Department of Environmental Protection, says few people thought swimming would ever be possible in the Charles River, but now some environmental advocates are pushing for a designated swimming area. Coletta says the Wynn  cleanup may have a similar catalytic effect on the Mystic River. “You have to start somewhere,” he says.