HILO, Hawaii — Federal regulators have approved a radiation monitoring plan for a U.S. Army installation on Hawaii's Big Island that previously used depleted uranium.

The plan approved by the U.S. Nuclear Regulatory Commission to test sediment in the Pohakuloa Training Area could go into effect in six months, The Hawaii Tribune-Herald reported Tuesday.

The depleted uranium — a dense radioactive metal alloy — was contained in spotting rounds used in the 1960s as part of a weapons program. The rounds didn't explode on impact.

Training area spokesman Eric Hamilton said 140 kilograms of depleted uranium were used in Hawaii, but it's not known how that amount was distributed between Pohakuloa and Oahu's Schofield Barracks.

According to the Army's plan, "most, if not all, of the 140 kilograms" of depleted uranium fired at Pohakuloa and Schofield remain in radiation control areas, where the spotting rounds were fired. The control areas make up 5 percent of the impact area and are still in use, Hamilton said.

The commission, in its decision to approve the plan on Thursday, did not note any significant impacts that the depleted uranium could have on the training area.

"NRC staff concurred with the Army's position that doses associated with acute events, such as high explosive (HE) activities, were not likely to result in significant risks or necessitating air monitoring," the commission wrote in its review.

But Jim Albertini, who leads the group Malu Aina, said air monitoring is necessary for the site.

"This one sample taken every several months from sediment is totally inadequate according to any kind of scientific definition of sampling," Albertini said in a news release.

The sediment sampling to locate depleted uranium will be from a stream downslope of the impact area.

"The sampling location the Army proposed to the (NRC) was selected as the best place to look for sediment carried out of the DU area that might contain DU," said training area Commander Lt. Col. Chris Marquez in an emailed statement.

The state Department of Health has said the Army's previous use of depleted uranium, which has 40 percent the radioactivity as naturally occurring uranium, is not considered a significant health threat.

The regulatory commission is accepting requests for a hearing or petition to intervene in the monitoring plan decision by April 10.