A chemical spill Wednesday on northbound Interstate 5 near downtown Seattle wasn't as serious as officials initially thought, but the mop-up of the toxic, corrosive liquid still made for a challenging commute as drivers jammed surface streets in search of alternate routes into the city.

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A chemical spill Wednesday on northbound Interstate 5 near downtown Seattle wasn’t as serious as officials initially thought, but the mop-up of the toxic, corrosive liquid still made for a challenging commute. Northbound I-5 in the area of the spill was completely closed for more than four hours, and drivers jammed surface streets in search of alternate routes into the city.

Shortly after 1 p.m., a state Department of Transportation worker alerted the State Patrol that a disabled semitruck was parked on the right shoulder of northbound I-5, just before South Holgate Street near the ramp to eastbound Interstate 90. The Seattle Fire Department’s hazardous-materials team was called to the scene at 1:30 p.m. after the tanker started leaking sodium borohydride and sodium hydroxide solution, chemicals used for the bleaching of pulp and wood products, said Kyle Moore, spokesman for the Fire Department.

Troopers started backing cars off the freeway and northbound lanes were shut down as the hazmat team — working with officials from the state Department of Ecology — came up with a game plan. While the spill was initially estimated at 50 gallons, that estimate was later downgraded to two gallons, according to Moore.

He explained that the chemicals, if mixed with water, could create a toxic, corrosive or flammable gas. A substance that Moore likened to “industrial kitty litter” was used to neutralize and soak up the chemical.

All lanes of I-5 were finally reopened just after 6:30 p.m., according to the State Patrol.

Trooper Julie Judson, a State Patrol spokeswoman, said the semi driver, who is from British Columbia, picked up his load in Elma, Grays Harbor County. He was presumably driving back to B.C. when a passing motorist pulled alongside his rig and pointed to his load, Judson said.

The driver pulled over, discovered the leaking liquid and was soon contacted by a transportation worker, who notified the Fire Department and the State Patrol, she said.

A cleaning cap had not been replaced, causing the spill, Judson said. “Whoever cleaned the rig didn’t replace the cap,” causing the liquid to slosh out, she said.