YAKIMA, Wash. — If Yakima wants to redevelop the former Boise Cascade mill site, it will likely have to clean up an old city landfill, which could add $20 million to the project price tag.

City officials had hoped to avoid disturbing the landfill and perhaps instead cover it with a layer of soil and use it as a park. But the site, located just north of the existing Fair Avenue exit off Interstate 82, is now the likely spot for an interchange being proposed by the city and Yakima County.

The interchange is critical to redeveloping the mill site, which is hard for motorists to reach, and to the county’s plans to strengthen the regional transportation network with the so-called East-West Corridor. It also would relieve traffic on Yakima Avenue and Terrace Heights Drive. The east-west street would run from Terrace Heights over Yakima River and into the city at Fruitvale Boulevard.

An earlier proposal to build the interchange to the north was nixed in March by federal transportation officials because it would have required the East-West Corridor to cross railroad tracks near I-82, which they termed too dangerous.

The former landfill, which served as a city dump from the mid-1960s to the mid-1970s, probably isn’t heavily contaminated, said Joan Davenport, who is overseeing the mill site redevelopment project for the city.

The city will know for sure after an environmental consulting group, SLR International Corp., finishes its study of the landfill later this year.

Yakima could go after state grants from the Department of Ecology to pay for some of the cleanup.

To qualify for the grants, the city will have to buy the landfill property, which is located on the southern portion of the mill site. An appraiser hired last year by the city put the fair market value of that land and a few other parcels at about $1 million.

The project could get a big boost from the state if the Legislature includes money for the I-82 interchange work in its budget.

The state Senate and House transportation budgets both include $59 million for the project. But there isn’t agreement on whether to fund this year or wait for the state revenue to improve.

The city has lobbied hard for the project in Olympia, and it’s paid off, City Manager Tony O’Rourke said.

It also helped that the co-chairman of the Senate’s Transportation Committee, State Sen. Curtis King, R-Yakima, has championed the project.

He did not immediately respond to request for comment Tuesday.

County officials have long wanted to put in the east-west street to ease congestion on Terrace Heights Drive and improve the regional transportation network.

The city has already secured a state matching grant, which could provide more than $20 million, to help pay for the project, estimated as high at $100 million. Some of the money has been used to pay for design work.

To get the rest of the money, Yakima has to start construction on the project by June 2016.

A committee made up of city, county, state and federal technical experts plans to recommend an option for I-82 that includes expanding it from four to six lanes and adding new on- and off-ramps east of the mill site. The plan would also remove the existing off-ramp by the Ledgestone Hotel and ramps connecting Fair Avenue to the highway interchange off Yakima Avenue.

Yakima would build three roundabouts: one where Lincoln Avenue and Martin Luther King Jr. Boulevard connect, a second at the south end of the mill site, and a third in the middle of the site where a new east-west city street would cross a railroad line.

• Dan Catchpole can be reached at 509-759-7850 or dcatchpole@yakimaherald.com. Follow him on Twitter at twitter.com/dcatchpole.