FALLBROOK -- A large audience crowded into the Fallbrook Public Utility District boardroom Friday night for a discussion on the potential ill effects of the Liberty Quarry, in what officials said signaled increasing local opposition to the proposed quarry.

About 175 people heard experts detail how the 155-acre Liberty operation could impact air quality, local agriculture and the neighboring Santa Margarita Ecological Reserve.

Many of the details were familiar -- for example, Fallbrook Planning Group member Jim Oenning re-emphasized the health risks invisible, airborne particles called "crystalline silica" produced when rock is crushed.

Another panelist, Penny Newman of the Center for Community Action and Environmental Justice, said that she has seen quarries lead to widespread environmental degradation.

"Once you have a quarry, you're on the map as an industrial mess no one cares about," Newman said, recounting how former "pit mines" in other parts of the state have been used to hold garbage and toxic waste once the mining operation is finished.

The proposed quarry would sit on 311 acres in Riverside County, just north of the San Diego County line and west of Interstate 15, near Rainbow Valley Boulevard. About 5 million tons of sand and crushed rock would be mined from the site every year for 50 to 60 years, according to the developer, Granite Construction Co.

An environmental impact report on the project is due out this year, as the city of Temecula pursues an annexation agreement that would put the quarry site within city boundaries, thus giving the City Council the authority to decide on its fate.

Granite Construction did not send a representative to speak at Friday's meeting, but the company has said repeatedly that it would take steps to minimize airborne pollutants and safeguard the Santa Margarita Ecological Reserve.

Matt Rahn, director of the 4,500-acre San Diego State University field station, said the company's promises do not dismiss the threat a huge commercial mining operation would pose to the fragile research conducted on the reserve.

"If you were to throw a dart at a map of Southern California, you'd be hard pressed to find a location as ecologically sensitive," Rahn said.

He said Granite Construction has looked into how it could avoid interfering with current research, but added: "I'm not really as concerned about our current research as I am about the next 75 years."

A similar meeting last March drew 60 people and marked the beginning of the opposition effort in Fallbrook. Before then, rallies had been held only in Temecula and Rainbow, and most Fallbrook residents were uninformed about the issue.

In September, the Fallbrook Planning Group voted to form a committee to investigate the potential effects the Liberty Quarry would have on Fallbrook residents, including health risks, traffic and air quality.

Granite Construction maintains that its operation would not affect anyone living in Fallbrook, even during Santa Ana conditions.

One hurdle for quarry opponents in Rainbow and Fallbrook is that the Riverside County Board of Supervisors has the final say on the project, and that board does not have much incentive to be responsive to the residents of San Diego County.

Newman, of the environmental justice center, told members of the crowd in Fallbrook that they must find ways to influence the Riverside County politicians who hold the decision-making authority.

"You have a community that has a lot to offer; our elected officials need to start seeing that," she said.

Contact staff writer Tom Pfingsten at (760) 740-3516 or tpfingsten@nctimes.com.