A groundwater sample shows the presence of tritium at 400 times the federal drinking water standard in a monitoring well 3.6 miles from the Columbia River at the Hanford nuclear reservation in Washington state.
Tritium is a radioactive isotope of hydrogen.
An internal Hanford memo, obtained by the Tri-City Herald, said it could take the underground plume as little as three years or as many as 30 years to reach the river.
The U.S. Department of Energy's (DOE) Pacific Northwest National Laboratory and the state officials will begin conducting additional test on Monday at the site.
"We're hopping on this," said Mike Thompson, DOE's official in charge of groundwater monitoring.
He hopes to know better in a couple of weeks the concentration of the tritium and how serious a threat it poses to the river.
A tritium concentration of 20,000 picocuries of radiation in one liter of water is the federal drinking water limit.
The reading at the monitoring well was 8 million picocuries.
Hanford was established as part of the secret Manhattan Project to build an atomic bomb during World War II.
Today, the mission at the 560-square-mile facility in southeast Washington state is cleaning up radioactive and hazardous waste created during 40 years of plutonium production for the nation's nuclear arsenal.