As much as 3,000 gallons of a total of 5,585 gallons of aircraft fuel that spilled from an overturned tanker truck last week have yet to be accounted for, state ecology officials say.

The tanker truck, carrying 11,400 gallons of fuel, overturned the night of April 10 just south of the North Lake Samish exit on Interstate 5. The accident closed the southbound lanes for nearly 20 hours and caused the evacuation of about 25 people living along the lake.

Some of the fuel leaked into the soil near Lake Samish, and it could take years to clean up, said Ron Langley, a spokesman for the state Department of Ecology.

“Some of it is in the soil and some of it evaporated,” he said. “But how much of each we just don’t know.”

Workers from Welch Enterprises of Mount Vernon spent Monday using backhoes to remove tainted dirt near the spill. Crews have already excavated more than 300 cubic yards of tainted soil, Langley said.

A small dam will be built under a culvert to prevent fuel residue from leaching into creeks or streams feeding into Lake Samish.