Army Corps officials have identified the source of leaks in Ice Harbor Dam equipment after an oily sheen was seen on water below the Snake River dam.
As much as 1,680 gallons of transformer oil contaminated with small amounts of polychlorinated biphenyls, or PCBs, are estimated to have leaked slowly from three of six cooling units at Ice Harbor, according to the Corps.
Pinhole leaks apparently developed in metal tubing in the cooling units and grew over several months, it said.
Based on oil inventory records, the Corps believes the leaks started after June.
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"As good stewards of the environment, we always seek to prevent pollutants from entering the river," said District Commander Lt. Col. David Caldwell in a statement. "The Corps regrets this inadvertent incident. We're taking appropriate actions to prevent this from happening in the future."
The Corps first spotted light silver sheens on the water just below the dam Dec. 5, when seasonal river flows were reduced and water below the dam became calmer. It immediately told the Washington State Department of Ecology, according to the Corps.
On Jan. 9 the appearance of a "rainbow" oil sheen on the water required an official spill notification to the Department of Ecology, Washington Emergency Management and the National Response Center.
The Environmental Protection Agency and the Department of Ecology continue to investigate, said Curt Hart, Ecology spokesman.
Ecology worked with the Corps to troubleshoot the search for the source after the sheen first appeared on the water.
But finding the sources of the oil proved difficult because of the complexity of powerhouse systems at the dam and because some equipment operates intermittently, according to the Corps. In addition, the bundles of metal tubes where the leaks were discovered are immersed in oil contained inside a metal sleeve, making them difficult to inspect.
Ice Harbor has three water-cooled transformer systems, each with a pair of heat exchangers, or cooling units. Each pair circulates 8,000 gallons of transformer oil to cool a transformer during hydropower generation.
When the source of the leaks was discovered, the Corps isolated the failed cooling units to stop leaks and began repairs. The first transformer, which is believed to have the largest leak of as much as 800 gallons of oil, was shut down Jan. 12. A leak in a second transformer was found Jan. 25 and a leak in the third transformer was discovered Monday.
The oil that eventually became visible on the water spread quickly to a very thin layer on the surface, and the Corps was not required to attempt to clean it up.
"It was just not recoverable," Hart said.
The Corps is replacing oil in Ice Harbor's cooling units with new oil, which does not contain PCBs. It also is testing for PCBs in oil systems at other dams in the district.
In addition, all six cooling unit tube bundles at Ice Harbor are being replaced while the Corps continues to research the causes of the metal tubing failures.