Crews hired by the U.S. Army Corps of Engineers to clean up the site have uncovered 18 "toxic gas" cylinders containing traces of mustard agent, chloropicrin, phosgene and lewisite, Corps spokeswoman Andrea Takash said.
"That sounds bad, but it's not," Takash said. "What was there was very low-level."
The work site is covered with a vapor containment tent, and none of the chemicals escaped into the air, Takash said.
Steve Gobin, deputy general manager of Quil Ceda Village, the tribes' economic arm that controls that land, could not be reached for comment.
The cleanup is scheduled to be finished by mid-April, Takash said.
That's 10 months late, according to estimates provided by the Corps when the project began last May.
The project stalled in August when workers noticed a strange smell and were taken to a local hospital.
That was the same day that the crews uncovered broken glass vials that may have once contained a liquid form of mustard gas, Corps Project Manager Rodney Taie said late last year.
Work stopped at the site, and the Corps asked the U.S. Department of Defense for more money for the project. The cost of the project doubled to about $4.5 million because of the incident and the items the crew uncovered.
So far, the Corps has spent $4.3 million on the project, Takash said.
The federal government used a nearly 700-acre section of the Tulalip Reservation for storage and testing of ammunition, including chemical weapons, during World War II. The site was decommissioned in 1947 but was used in the Korean War era again for storage and training.
When the military closed the site for good, it buried leftover weapons and storage containers. When crews began investigating the site last year, they used ground-penetrating radar and detected dozens of unusual deposits buried throughout about 10 acres of the site.