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A chemical spill on northbound Interstate 5 at South Holgate Street is expected to shut down northbound lanes — and possibly even southbound lanes — for hours as the Seattle Fire Department’s hazardous-materials team figures out how to mop up an estimated two gallons of the toxic, corrosive liquid that spilled from a tanker truck. (Photo by Dean Rutz / The Seattle Times)

Some  northbound lanes of Interstate 5 at South Holgate Street have been opened to traffic, easing the impact of a chemical spill that was initially thought to be serious enough to close the freeway for hours.

However, the state Department of  Transportation says it will be at least until 7:30 p.m. before all northbound lanes are reopened. Traffic at 6 p.m. was still backed up about four miles.

The spill from a disabled tanker was initially estimated at 50 gallons, but the Seattle Fire Department’s hazardous materials team – working with officials from the state Department of Ecology – now estimate that only two gallons of the toxic, corrosive liquid oozed onto the freeway, said Fire Department spokesman Kyle Moore.

“I’ve never seen anything like this go quickly,” said Trooper Julie Judson, warning drivers that the closure is bound “to change the traffic dynamic” in downtown Seattle this afternoon.

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A state Transportation worker contacted the State Patrol and reported a disabled semi on the right shoulder at 1:09 p.m., Judson said. According to Moore, firefighters were called to the scene at 1:30 p.m. The semi, which was leaking fluid, is on the right-hand side of northbound I-5 to eastbound Interstate 90, just before South Holgate Street, he said.

The liquid is sodium borohydride and sodium hydroxide solution, chemicals used for bleaching pulp and wood products, Moore said. If mixed with water, it can create a toxic, corrosive or flammable gas, he said. It was not clear if foam can be used, or if it too could potentially cause an adverse chemical reaction.

Crews are using sand and some other material to soak up the liquid, then planned “to scoop it up” and haul it away, he said.

I-5 northbound at Spokane Street has been reopened. Two right lanes remain closed just south of I-90. The HOV lane is open to all from Columbian Way until just past I-90.

The northbound I-5 exit to I-90 is open.

Drivers should expect severe congestion on routes into Seattle, including surface streets.