DENVER — More than 1 billion gallons of contaminated water — enough to fill 1,500 Olympic-sized swimming pools — is trapped in a tunnel in the mountains above the historic town of Leadville and threatening to blow.

Lake County Commissioners have declared a local state of emergency for fear that this winter’s above-average snowpack will melt and cause a catastrophic tidal wave.

The water is backed up in abandoned mine shafts and a 2.1-mile drainage tunnel that is partially collapsed, creating the pooling of water contaminated with heavy metals.

County officials have been nervously monitoring the rising water pressure inside the mine shafts for about two years. An explosion could inundate Leadville and contaminate the Arkansas River.

“It could come out, we just don’t know where,” county Commissioner Carl Schaefer said. “We’re seeing changes and we’re very concerned. We’re not crying ‘Chicken Little’ here.”

State and federal officials agreed Thursday to conduct a risk assessment before taking any action. Critics said something should be done immediately to ease the pressure.

Peter Soeth, a spokesman for the Bureau of Reclamation, which acquired the drainage tunnel in 1959, said there was no immediate threat to Leadville’s 2,700 residents.

Officials point out that a speaker system to broadcast evacuation notices has already been installed near a mobile home park that has 300 residents near the tunnel’s portal.

The tunnel normally drains water that seeps into some of the hundreds of abandoned mine shafts and other mine workings in the mountains east and south of Leadville and deposits it into the East Fork of the Arkansas River about a mile north of town.

The Environmental Protection Agency raised concerns about the situation in letters sent to the Bureau of Reclamation, which has been assessing the concerns.

“Due to the unknown condition of the tunnel blockage and the large volume of water behind the blockages, we are concerned that an uncontrolled, potentially-catastrophic release of water to the Arkansas River from (the tunnel) is likely at some point,” said one EPA letter sent in November.

Stan Christensen, an EPA expert on the tunnel, said the likelihood that something catastrophic can happen increases the longer nothing is done.

A water treatment plant at the foot of the tunnel removes toxins and heavy metals such as zinc, cadmium and manganese before discharging the water into the Arkansas River. The mobile home park is near the treatment plant.

New springs and seepages have appeared at California Gulch, which sits below the plant. Tests have shown high levels of heavy metals typically found in mine discharge, leading officials to conclude the trapped water is finding ways out.

“No one can tell us what it means,” said Jeffrey Foley, Lake County’s emergency management director. “It’s finding fault lines and it’s pouring mine-contaminated water into the Arkansas.”

The EPA’s Christensen said the water table is rising regionwide and that his agency can’t immediately reach the same conclusion.

Leadville, which sits at 10,200 feet of elevation and some 100 miles west of Denver, rose to national prominence and attracted thousands of people after a gold rush in 1859. After the gold ran out, silver became the dominant mining industry.

Later, a mine that sits beneath 13,000-foot mountain peaks began shipping molybdenum ore in 1915. Miners have recovered 946,000 tons of molybdenum, used to harden steel, worth about $4 billion. The Climax mine closed in 1995