(CNN) -- U.S. inspectors are investigating a possible leak at the Hanford nuclear site after an elevated contamination reading was found in a leak detection pit, Washington Gov. Jay Inslee said Friday.
The elevated reading, however, doesn't pose an immediate public health threat, Inslee said.
"This is most disturbing news for Washington," the governor said in a statement. "The discovery was made during a routine pumping outside the tank when pumps are also surveyed for radioactivity."
The leak detection pit is located outside and adjacent to a double-shell tank identified as AY-102, the governor said.
"It is not clear yet whether that contamination is coming directly from the outer shell of the AY-102 but it must be treated with the utmost seriousness," Inslee said.
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The U.S. Department of Energy has assigned engineers to analyze the source of contamination through sampling and video inspection, a process that could take several days, the governor said.
The Hanford site, which once produced plutonium for atomic weapons, borders the Columbia River.
"Given the relatively early detection of this potential leak, the river is not at immediate risk of contamination should it be determined that a leak has occurred outside the tank," Inslee said.
Secretary of Energy Ernest Moniz just paid his first visit to the Hanford site on Wednesday.
Even before learning of a possible leak, Inslee told Moniz he has "serious concerns regarding the pace of addressing the leaking tanks," Inslee said.
"We will be insisting on an acceleration of remediation of all the tanks, not just AY-102. USDOE has a legal obligation to clean up Hanford and remove or treat that waste, and we ensure that legal obligation is fulfilled," the governor said.