The world's biggest cargo ships, some a quarter-mile long, could be docking regularly near downtown Seattle before long. After four years' study, the Army Corps of Engineers has given the okay to digging deeper shipping channels around Harbor Island at the mouth of the Duwamish River. The dredging down to a depth of 57 feet would allow the world’s longest and deepest cargo ships to carry full loads to and from the Port of Seattle. The biggest container ships can sit more than 50 feet deep in the water. Those ships can come to Seattle today, but only if they're not fully loaded: the East and West Waterways, on either side of Harbor Island at the Port of Seattle, are too shallow for them, with depths ranging from 34 to 51 feet.

“This deepening project in the Seattle harbor is critical for our future success,” said John Wolfe, the CEO Northwest Seaport Alliance, a partnership between the Ports of Seattle and Tacoma. “We would be the deepest port in the nation.” The $60 million deepening project would remove nearly 1 million cubic yards of mud from the industrial waterways at the mouth of the Duwamish River to make the shipping channels as much as 23 feet deeper than they are now. The mud is nasty stuff. “The mud of the East Waterway is contaminated with high levels of pollutants including polychlorinated biphenyls (PCBs), arsenic, carcinogenic polycyclic aromatic hydrocarbon (cPAHs), tributyltin (TBT) and mercury,” according to the Environmental Protection Agency. Such contaminants, built up over a century of heavy industry along the Duwamish, earned the East and West Waterways a place on EPA’s list of highest-priority toxic-waste sites, as part of the Harbor Island Superfund site.

Two other Superfund sites are nearby: Lockheed West, along the West Waterway, and the Lower Duwamish site, occupying the 5 miles of river immediately upstream of Harbor Island. Cleanups of the three sites are at various degrees of completion. Port officials hope to have the harbor deepened by 2024. “We will not commence any dredging work at any contaminated areas until those sites have been completely remediated and determined safe by the EPA,” said deputy district engineer Damon Lilly with the Army Corps of Engineers. James Rasmussen with the Duwamish River Cleanup Coalition said officials assured him repeatedly that the deep dredging would not stir up any long-buried pollutants.

“The only real concern we have is it’ll mean there’s more container [truck] traffic going through the communities,” Rasmussen said. “Both South Park and Georgetown — the largest air contamination we have here is diesel air particulates.” The Ports of Seattle and Tacoma aim to nearly double the amount of ship traffic they handle in the next 10 years. Last year, the ports announced they would cut their greenhouse gas emissions in line with the Paris climate agreement, a difficult task if their business nearly doubles in a decade. For a given amount of cargo, the super-size container ships are more fuel-efficient, and thus less polluting, than the smaller ships they are starting to replace. The deepening project still needs approval and funding from Congress.