The Fairfax County Potomac River treatment plant, long mired in political problems and local controversy, is finally becoming a reality.

On a damp and chilly morning last week, Fred Morin, chairman of the county Water Authority, symbolically turned over a shovel of the reddish silt on the 112-acre site south of Rte. 7 where the water facility will eventually stand.

"It looks like it might rain, but the powers that be," Morin said, looking upward, "are smiling on us because we are really breaking ground on this facility.

The past three years have been difficult ones for the Water Authority, which supplies water to most of Fairfax County, all of Alexandria and half of Prince William County.

Since 1975, in the face of opposition from federal agencies and local residents, the county has pursued the development of a facility to supplement the already overtaxed Occoquan Reservoir.

Morin said that this week that such a facility was considered "absolutely critical" to assuring regional growth, particularly in the rapidly growing Burke area and in the developing western portions of the county.

The county plan called for tapping water from the Potomac River and pumping it to the treatment facility in the northwestern area of the county.

The view that Potomac River water should be shared regionally, however, was at first resisted by localities in southern Maryland and the District of Columbia.

The Army Corps of Engineers for three years held up construction of a planned Potomac intake plant, which would pump water from the river to the proposed treatment plant, while area governments worked out an agreement for sharing river water during droughts.

Meanwhile, well-to-do residents in the Great Falls area objected vigorously to the original plan to locate the facility north of Rte. 7. The residents' pressure, in the form of protests to the county Board of Supervisors, forced relocation of the plant site to south of Rte. 7. The increased distance from the Potomac raised total costs of the facility to more than $53 million.

When the Environmental Protection Agency earlier this year called the Potomac plan "environmental unacceptable," because of the impact of the intake plant on water quality downstream, and when the Interior Department raised other objections, county supervisors began casting around for alternatives. The Interior Department said it was concerned about the impact on fish life and wild life during low-flow periods at the point where water was being taken.

Federal agencies withdrew their objections when signatories to the regional sharing compact agreed to establish minimum river flows to maintain the health of the estuaries.

"As seems to be the case with . . . most major public works projects in recent times, our progress on this project has been much slower than we had hoped . . . " Morin said last week. "We are hopeful, however, that there will be no further delays."

Although steam shovels are already at work on a construction access road, the cleared site is a mass of mud and silt.The plan calls for terracing off the plant to give it as much as possible a park-like setting.

The Potomac plant will be capable of expanding to handle almost twice the capacity of the Occoquan Reservoir. However, during droughts, usually in summer and fall when demand is greatest, the authority will be able to tap only a fraction of what the plant can handle.

A potential construction delay is the recent recovery near th designated river intake site of early Indian artifacts dating back as far as 5,000 years. The Water Authority is in the process of restoring those finds for future cataloging and for study by archaeologists.