Vermont's health department says a radioactive isotope was detected in ground water at the now-shuttered Vermont Yankee Nuclear Power Station in Vernon.
The strontium-90 was found in water that is unavailable for consumption and at levels well below the U.S. Environmental Protection Agency's safe drinking water threshold.
The health department says there is no immediate health risk.
The water was sampled in August and the Oak Ridge National Laboratory verified the strontium-90 on Jan. 29.
Entergy-owned Vermont Yankee stopped producing electricity in December after more than four decades online. The New Orleans-based company says it continues to comply with all monitoring programs.
The plant will sit for decades while its radioactive components cool. It's expected to cost nearly $1.25 billion to dismantle the plant, which likely won't occur before 2040.