Tests this month at the city-run Stabler's Point Rock Quarry across from Auke Bay show that it contains asbestos.
Federal regulators are scrutinizing the site to see if it poses a health risk.
Rock from the quarry is slated for use in construction projects such as the Juneau-Douglas High School parking lot, Harris Harbor and Bartlett Regional Hospital.
The city found out about the positive lab test results for tremolite, the most dangerous form of asbestos, last Thursday, from a Juneau contractor. One sample showed 3 percent tremolite while another showed 5 percent.
The Environmental Protection Agency deems building materials hazardous if they contain more than 1 percent asbestos. But the EPA does not regulate rock quarries. That responsibility is held by the federal Mine Safety and Health Administration, which may have test results finalized in three to four weeks, said Rorie Watt, the city's chief capital improvement project engineer.
Despite the rainy weather, an Anchorage field inspector for MSHA conducted air tests at the quarry on Tuesday.
"We wanted immediate results," Watt said on Tuesday. He said the city will conduct its own testing later this week.
The airborne levels of asbestos in the quarry may not pose a health risk, Watt said.
Based on his visual inspection of the quarry on Monday, Chris DeWitt, supervisor of Juneau's John Rishel Mineral Information Center, told Watt in a memo that the total amount of asbestos in the quarry could be "far less than one percent of the rock."
A state regulator said asbestos, a human carcinogen that has caused the death of many industrial workers, is only harmful when it becomes airborne.
A regional MSHA official based in Vacaville, Calif., declined to comment about Tuesday's inspection, other than "we are testing a mine, which is pretty much what we do on a daily basis."
The Stabler's Point quarry, opened for commercial use in 2002, has become one of Juneau's major sources of rock for construction projects, producing about 50,000 to 75,000 tons per year.
That has aroused some anger from private quarry operators. The president of one of those companies, Jim Wilcox of Glacier Lands, privately commissioned the asbestos tests this summer after hearing from a friend that the quarry contained asbestos.
City engineers said they had never run tests for asbestos at the quarry, but it was easy to find the fibrous bands in the rock once they started hunting for them.
But that doesn't mean that the asbestos at the quarry is a health hazard, federal and state officials said.
Asbestos health facts
Threat: Occurs only when it is disturbed and becomes airborne.
What happens when inhaled: The tiny fibers can inflame the lining of the lungs and become permanently lodged there.
Asbestos-related diseases: Cancers of the lung and the lining of internal organs; asbestosis, a potentially fatal disease that involves breathing problems and possible heart failure.
"As long as the fibers are entrapped ... like in asphalt ... then it can't hurt you," said John Stallone, chief of enforcement for the Alaska Occupational Safety and Health Section of the state Labor department.
"If you are going to put it in a (building), the owner should know about it," Stallone added, noting that the main danger is from inhalation of loose fibers.
Asbestos can become airborne through weathering or human disturbance, such as drilling.
Stallone said workers can avoid asbestos exposure by wearing high-efficiency particulate air filters.
About five years ago, the state mined the front of Stabler's Point, facing Auke Bay, to expand its ferry terminal in the bay. The positive asbestos test results came from rock at this location, according to the city.
About 10 to 20 percent of the rock goes to private contractors and the rest is used for state and city projects.
Between half a dozen to a dozen Juneau contractors use the quarry during the summertime, according to city engineers.
On Tuesday afternoon, Miller Construction was hauling off rock from the quarry for a project at Harris Harbor.
"This is the most competent rock source in Juneau," said Timothy Miller, the company manager.
His company has been planning to use the rock from the quarry to pave the JDHS parking lot this summer.
He said he hopes the new quarry test results come back safe. Otherwise, it will harm a lot of Juneau projects, he said.
Coogan Development wanted to use the rock from Stabler's Point for a Bartlett Regional Hospital project but owner Wayne Coogan said he worries about exposing his workers.
An Anchorage field inspector for the Mine Health and Safety Administration visited the quarry on Tuesday and equipped a work crew with air monitors.
But Kyle Miller, one of the construction workers wearing an air monitor Tuesday, said the rain obviously kept the dust down.