The city of Portland finalized a 15-year settlement with the state to clean up the Columbia Slough, the 31-mile stretch of waterways between Fairview Lake and the Willamette River.

Portland city councilors unanimously approved entering the settlement with the Oregon Department of Environmental Quality Wednesday morning.

Under the settlement, the city will pay $19.5 million to DEQ over the next three years. The agency will use the money to address storm water discharges and sediment contamination in the slough. The city also commits to building 15 facilities to treat stormwater runoff, and spending an additional $4 million on cleanup projects that it will oversee over the next 15 years.

The settlement marks a new stage of cleanup for the slough’s waterways — including its 19-mile main channel — which for the last century have collected pollution from surrounding industrial and agricultural sites. The slough had also been a local dumping ground until the 1970s.

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Some industrial chemicals remain among the sediment, including polychlorinated biphenyls (PCBs) and pesticides, which are also consumed by fish. For that reason, the state advises people to limit eating fish caught in the area.

The city owns about 200 outlets discharging storm and waste water throughout the slough. Since 2006, Portland has entered into a series of five-year agreements with the state. Those agreements, city staff explained at the council’s Jan. 15 meeting, mostly focused on investigating sources of pollution and collecting samples. The five-year agreements weren’t legally binding.

This new settlement, by contrast, is legally binding, outlines a cleanup plan, and helps protect the city from lawsuits.

“It also defines the city’s role and obligations in the long term with a work plan which will result in long-term cost savings for the city,” said Annie Von Burg, who oversees environmental remediation at the Portland Bureau of Environmental Services, at last week’s meeting.

City staff said these funds will come out of the bureau’s sewer system operating fund for this fiscal year, as well as the next two fiscal years. The bureau is working with an insurance carrier to recoup some expenses going toward the settlement.