Testing of surface air near an underground nuclear waste site in New Mexico’s desert showed elevated levels of radiation but did not pose a threat to humans or the environment, a U.S. Department of Energy official said on Thursday.

Trace amounts of man-made radioactive elements such as plutonium were found at an air-monitoring site half a mile from the Waste Isolation Pilot Plant and are tied to a radiation leak in the underground salt formation where waste from defense research and nuclear weapons production is stored, said Joe Franco, manager of an Energy Department field office that oversees the plant.

Energy officials said over the weekend that there was no apparent surface air contamination from the accidental release of radiation that caused an air-monitoring alarm below ground to go off about 11:30 p.m. local time on Friday. That was the first such mishap since the facility opened in 1999.

The plant, located in southeastern New Mexico near Carlsbad, is a repository for so-called transuranic waste shipped from other federal nuclear laboratories and weapons sites. The waste includes discarded machinery, clothing and other materials contaminated with plutonium or other radioisotopes heavier than uranium.

No workers were underground when high levels of radioactive particles were detected […]