A Montgomery County mining company whose principal product contains cancer-causing asbestos fibers, has been granted a permit to discharge excess rainwater near the Potomac River intake valve supplying much of the area's drinking water.

Several government studies indicate no detectable levels of asbestos fibers present in the accumulated rainwater, but the permit, which becomes effective today, requires monthly monitoring of the rainwater discharge.

The Rockville Crushed Stone Co., located near Rte. 28 two miles west of Rockville, will be permitted under the agreement to discharge approximately 65 millions gallons of rainwater each year.

The water will be pumped across a field located neat the 133-acre quary, and then will flow into the Sandy Branch and Piney Branch tributaries of the Potomac River, according to William E. Chicca, chief of the Industrial and Hazardous Substances section of the Maryland Water Resources Administration.

The two tributaries will carry the rainwater to the Potomac near the Washington Suburban Sanitary Commission's main water intake, he said.

The WSSC supplies drinking water to Washington's Maryland suburbs. Additionally, the city of Rockville has its own water intake valve located near the WSSC facility.

"There are no standards regulating the amount of asbestos fibers permitted in water," Chicca said. "Therefore, there was no reason to deny the permit. But if standards ever are developed, then we want to have a constant supply of information available so we can modify the permit," he said.

The information will be produced in monthly reports made by subjecting rainwater samples to visual analysis under a microscope, and by annually subjecting water samples to far more precise microscope analysis, according to the permit.

Asbestos fibers at the Rockville Crushed Stone Co. quarry are released when serpentinite, its principal product, is crushed, milled or ground up for industrial use. The quarry supplies most of the paving stone for driveways and roads, and playgrounds in the Washington area.

In October 1976 Dr. Irving J. Selikoff, regarded as the nation's leading expert on the health hazards of asbestos, reported that the level of asbestos fibers in the air near the Rockville quarry were 1,500 times greater than the highest previously reported level found anywhere in the United States.

As a result of Selikoff's finding local governments including Montgomery Country removed gravel made from serpentinite from country parks and schoolyards, surfaced roads on which the gravel previously had lain exposed, and resurfaced roads on which the macadam covering had worn thin.

Asbestos fibers, which can be so tiny they can be seen only under high-powered microscopes, can cause mesothelioma, an always-fatal form of cancer.

The only applicable standards regarding asbestos fibers are those protecting miners and quarry workers, and the Rockville Crushed Stone Co. is in complete compliance with those regulations, according to U.S., state, and local officials.

"The water we're sending off meets drinking water standards," said James G. Topper, general manager of the quarry. "The water that eventually will reach the Potomac will not include any of the water used for industrial purposes, such as the mining process itself, or for hosing down layers of loose gravel, Topper added.

Frederick K. Erickson, deputy director of the Montgomery Country Department of Environmental Protection, said that while there were no "detectable levels of asbestos fibers present in the rainwater" from the quarry, the country would have had qualms about granting the permit without the monitoring requirements.