By Laura Zuckerman
SALMON, Idaho (Reuters) - Hazardous fumes from a 10,000-gallon gasoline leak forced the evacuation and closure of the post office on Thursday in Miles City, Montana, where officials say a pool of gasoline is migrating below ground.
The post office is the second building that has been evacuated and cleared and closed because of the fuel leak, which was first detected late last month.
Authorities evacuated a 15-unit apartment complex on October 26 after instruments showed the presence of explosive gases, said Miles City Fire Chief Derrick Rodgers.
The Montana Department of Environmental Quality has traced the leak to an underground storage tank at a Conoco gas station not far from the apartment and post office buildings.
The tank is no longer leaking fuel but fumes from the pool of petrol have risen to unhealthy levels at some sites in downtown Miles City, a ranching community of about 8,500 people about 140 miles east of Billings.
Montana environmental officials are calling the leak the worst in recent history.
"We've had other releases and evacuations in the past but this is the biggest one in terms of impacts to people," Mike Trombetta, chief of the state's hazardous waste cleanup bureau, told Reuters.
Seventeen people have been forced to move out of the apartment building, at least temporarily. After several temporary stops - including a Red Cross shelter - those residents have found other lodging, Miles City officials said.
Authorities said the apartment and post office buildings are not likely to reopen any time soon, with Rodgers estimating the second week of December.
Federal officials on Thursday trucked in a mobile postal unit, which is set up elsewhere in town.
"It's been a trying situation," said Rodgers. "It's been a very dynamic incident, changing constantly. When gas is moving underground, it's difficult to anticipate what it's going to do."
Kim Brown of the First Baptist Church in Miles City said the church last month housed several people who rented apartments at the now-closed complex.
"We had to put them up in the church nursery, but that's what you're supposed to do, you're supposed to take people in," she said.
Although gasoline has contaminated groundwater near the cleared buildings, officials said the public drinking water supply, which comes from the nearby Yellowstone River, remains safe.
Clean-up crews have established a field of recovery wells to stop the pool of gas from spreading and have pumped out thousands of gallons of petrol-polluted water.
They also have installed systems to vent the two buildings and surrounding soils.
(Editing by Dan Whitcomb and Peter Bohan)