So reports the state Ecology Department, following up on the sheen sighted on the river on May 29th (WSB coverage here) – read on for their followup:

The Ecology news release:

The Washington Department of Ecology (Ecology) has determined that an oil spill last month on the Duwamish Waterway in Seattle was jet fuel. The Boeing Co. (Boeing), which operates a jet fuel terminal nearby, is cooperating with Ecology’s investigation.

Ecology and the U.S. Coast Guard responded to a report of oil on the Duwamish Saturday morning, May 29, 2010. There was no known source of the oil sheen found in the vicinity of Slip 4, an inlet on the east bank between the First Avenue South and 16th Avenue South bridges. Responders took samples for laboratory analysis to determine the type of fuel involved.

During the Saturday morning response, the Coast Guard used a federal oil spill contingency fund to hire a cleanup contractor. Oil recovery continued that morning while the tidal conditions permitted recovery efforts.

On Tuesday morning, June 1, Boeing informed Ecology that the company had responded to a jet fuel spill that occurred Friday night, May 28 at the company’s facility. Boeing officials originally believed the spill was contained within the terminal. Ecology collected samples from the terminal to compare with the earlier samples.

Ecology’s preliminary laboratory results indicate that the fuel in the waterway on Saturday was jet fuel.

Additional information – still being gathered and analyzed – indicates that jet fuel at Boeing’s facility flowed from the site on the night of May 28 and through a private stormwater line that empties into the Duwamish Waterway. Witnesses who made the Saturday morning report to Ecology also reported a petroleum odor along the river Friday night.

Investigators will continue to examine the incident. Boeing continues its efforts to clean out the drain line and affected shoreline areas on the company’s property. The on-water portion of the spill dissipated with wind and tides May 29.