A tractor-trailer carrying paint and other chemicals rolled over on the right shoulder of Interstate 5 just south of the Nisqually River Bridge early Thursday, blocking traffic for much of the day.
The semitrailer ran off the road about 1:40 a.m.
Crews were set to work on removing it late Thursday night.
The truck had two trailers. The first contained 160 80-pound pails of water-soluble paint, and the second contained an unknown amount of organochloride pesticide.
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Small amounts of paint were leaking from the first trailer, but there was no apparent leak of the pesticide.
The truck reportedly struck the guard rail and fell onto the right shoulder embankment, according to the Washington State Patrol.
It came to rest 200 to 300 feet from the Nisqually River.
The Oregon driver was uninjured, State Patrol Trooper Brandy Kessler said.
He likely will be cited for negligent driving because he reportedly was falling asleep, she said.
Traffic tie-ups spilled onto Martin Way for much of the day.