The low level of radioiodine-131 detected in precipitation at a sample location in Massachusetts is comparable to findings in California, Washington state and Pennsylvania and poses no impact to drinking supplies, public health officials said.
Air samples from the same location in Massachusetts have shown no detectable radiation.
The samples are being collected from more than 100 sites around the country that are part of the U.S. Environmental Protection Agency's Radiation Network monitoring system.
"The drinking water supply in Massachusetts is unaffected by this short-term, slight elevation in radiation," said Massachusetts Public Health Commissioner John Auerbach. "We will carefully monitor the drinking water as we exercise an abundance of caution."
Officials said that at the concentrations found, the radioiodine-131 would likely become undetectable in a "relative short time."
Trace amounts of radiation believed to have originated from damaged Fukushima Daiichi reactors in the aftermath of Japan's devastating 9.0 earthquake on March 11 also have been detected in air samples in several western U.S. states, but at levels so small they posed no risk to human health.