INDEPENDENCE -- Independence officials shut off the flow of water at its well field off Corvallis Road late last month after tests revealed trace amounts of tetrachloroethylene (PCE).
PCE is a man-made chemical, widely used in in the dry cleaning industry and as a metal degreaser in manufacturing, according to the health division of the Oregon Department of Human Services (DHS).
Public Works officials detected PCE during routine testing Oct. 27. City Manager Greg Ellis said those affected wells were taken off line that same day.
The Environmental Protection Agency's Maximum Contaminant Level (MCL) -- or allowable limit -- for PCE in water is .005 milligrams per liter (mg/L). Officials found .007 mg/L in some of its wells on the city's south well field.
The chemical was not found in the town's other well field off Polk Street.
Ellis said the chemical has never before been found in the Independence's water supply. Department of Environmental Quality representatives are trying to determine where the PCE originated from and how it got into the wells, he said.
"We're testing different areas around town, but don't want to make any assumptions," he said.
A city-wide notice was mailed to water customers Nov. 7. Ellis said there is no risk in using Independence water, and that the wells off Corvallis Road would be treated with a carbon filtration system to eliminate the chemical.
They won't be activated until at least January, Ellis said. "We want to make sure they are clean and that we find the source," he said.
Independence is required to test its wells for drinking water quality by the EPA every three years, said Ed Butts, the city's water engineer.
He said there's no way to pinpoint how long the contaminant may have been in the aquifer between tests in 2002 and 2005 or what the level may have been when it reached customers.
He also said he couldn't quantify the risk from drinking it if it was affected during that period.
The contaminant level of the water as it entered the city's water system may have been at .0071 mg/L, but was pumped out at MCL or below, Butts said.
"The city was testing in compliance with state and federal standards; they were testing when they were supposed to," he said.
Long-term exposure to PCE in water over the MCL increases the risk of cancer. Under the EPA's guidelines for carcinogenic agents, however, this assumes that a person injesting 2 liters of water with an MCL of .005 mg/L daily for 70 years.
Dave Leland, drinking water manager for the state, said the health threat from consuming Independence water from the affected wells was small, even if the water had been contaminated for the entire three-year period between tests.
It would take seven years of exposure at an MCL of .070 mg/L before the EPA would consider it "a small risk," Leland said.
Leland said because of the time it takes for volatile chemicals such as PCE to move underground, it's likely that the city "deteced the problem at the beginning of the contamination rather than in the middle of it."
Being limited to water production from one of its two well fields shouldn't disrupt service to Independence's 2,139 residential, commericial and industrial customers.
"We're lucky, though, that this has come up at this time of year, when water use is down," Ellis said. "We're not in danger of having a water shortage right now ... of course we want to make sure we get those wells back on line."
Most PCE enters the environment by evaporating into the air from factories, storage tanks, or other contaminated areas. It can also enter the soil and groundwater, where it takes much longer to break down than in the atmosphere, ODHS said.
Short-term effects from inhaling PCE include dizziness, headaches and nausea.