The Vermont Department of Health said it has found detectable traces of radioactive tritium from the Vermont Yankee nuclear power plant in the Connecticut River.
"We have been tracking the plume of tritium-contaminated groundwater as it moves slowly toward the river, and this new finding confirms that tritium has traveled from the Yankee site to the Connecticut River," Harry Chen, the state's health commissioner said in a statement issued on Wednesday.
The state's laboratory measured 534 to 611 picocuries of the radioactive substance per liter, a level well below the 20,000 picocuries per liter the Environmental Protection Agency suggests as the maximum threshold for drinking water, said Bill Irwin, the health department's chief of radiological and toxicological sciences.
Plant owner Entergy Corp said in a separate statement it had tested the same river water samples as the state did and found lower levels of tritium. Entergy would only specify that it registered below the official "minimum detectable" limit.
"We are very interested in working with the state to understand the discrepancy in the test results," it said.
Vermont Governor Peter Shumlin said the health department's findings were evidence of the need for the plant to dig additional wells to remove tritium from ground water and called for increased monitoring of the nuclear facility.
The announcement comes two weeks after the state announced it found traces of another radioactive isotope, strontium-90, in a fish caught in the river near the Vermont Yankee plant.
Entergy denied that its plant was the source of that radiation find.
Vermont's Senate voted in 2010 to shut down the plant in 2012. But Entergy filed suit against the state of Vermont earlier this year in an effort to keep the 39-year-old nuclear plant open. The case goes to trial next month.
The Nuclear Regulatory Commission says that tritium emits a weak form of radiation, a low-energy beta particle similar to an electron. Tritium radiation does not travel far in air and cannot penetrate the skin, the agency says on its web site.
Most tritium produced in nuclear power plants stems from boron used for absorbing neutrons from the plant's chain reaction. Boron either is added directly to the coolant water or used in the control rods for the chain reaction.
As a form of hydrogen, tritium can bond with oxygen to form water. When that happens, the resulting "tritiated water" is radioactive. Nuclear plants routinely release dilute and monitored concentrations of tritiated water, NRC said.
