BAINBRIDGE ISLAND – Hundreds of thousands of gallons of oily contamination in the ground at the Wyckoff Superfund site on Eagle Harbor’s south shore will be locked in a concrete tomb.

The federal Environmental Protection Agency, which manages the cleanup of the old Wyckoff creosote treatment plant at Bainbridge Island’s Pritchard Park, announced this week it would move ahead with treating the site by injecting a cement slurry into the soil there to treat 267,000 cubic yards of contaminated soil and groundwater.

In the treatment, a large-diameter auger drills into the ground, along the way injecting the slurry in a column from the surface down deep into the soil, said Helen Bottcher, the EPA’s site manager. After about a month, the treated areas will become a solid, concrete monolith that prevents remaining contaminants from leaching out into groundwater.

Roughly 650,000 gallons of oily contamination remain in the ground at the site — evidence of the old treatment plant that once operated there — the vast majority of which will be treated with the slurry, according to the EPA.

“It’s not as hard a concrete as a street or a sidewalk, but it is very stable and the most important thing is that we’ll encapsulate and contain the contaminants,” Bottcher said.

The concrete will then be protected and could easily last hundreds of years, Bottcher said. An impermeable cap, which could be a thick layer of plastic or tightly compacted clay, will be placed over the treated areas to keep rainwater from filtering down through the soil, she said. A clean layer of soil will then be added so grasses and shrubs can be planted and the area can eventually be reopened as a park.

The state’s Department of Ecology has operated a water treatment plant that pumps and cleans groundwater at the site since 2012. Turning the soil to concrete and freezing the contamination in place would allow the state to stop its treatment operations and save about $750,000 in annual operations costs, according to the EPA.

Previously the EPA had attempted to clean the site using a steam-injection system, but a pilot project found that method didn’t work. As part of its announcement this week, the EPA said it would have equipment from that initial test and other debris removed from the site.

After a final design has been selected, the EPA estimates it will take eight to 10 years to do the work. If the work stays on schedule and there are no delays in funding, it’s estimated that construction will be finished in 2032, Bottcher said.

“This is going to take awhile,” she said.

The EPA also said it planned to have a new underground wall installed on the south side of the treatment area to completely seal it off from the surrounding environment and prevent groundwater from entering that space. A deep, U-shaped steel wall currently surrounds the west, north and east sides of that area but is rusting. The Army Corps of Engineers is designing a new wall that will reinforce the existing structure, Bottcher said.

Most immediately, the agency plans to have Creosote Place, an access driveway that runs from Eagle Harbor Drive down to the treatment facilities, rebuilt so it can better accommodate large trucks. Construction on that project is set to begin in the first months of 2020, Bottcher said.