For the first time in years, Northern Virginia residents will not have to hope for 40 days of rain to see them through a summer dry spell. A new Potomac River water plant will do the job just fine, according to water authority officials.

The plant, opened in April and dedicated last month, will end water shortages that have plagued 700,000 residents served by the Fairfax County Water Authority in the county, Alexandria and eastern Prince William County, authority Chairman Fred C. Morin said.

Last year, a drought brought a deluge of water restrictions when the Occoquan Reservoir, which provided almost all the water for the area, fell below 25 percent capacity. But now, with a flood of water coming from the Potomac River, the restrictions probably will become a thing of the past, Morin said.

"No one, not even the authority, has much control over drought, but with the new Potomac plant operating there is practically no chance of a situation arising that we can't handle," he said. "I don't see the problems we've had in previous years reoccurring for many years to come."

The $81 million plant, which was 10 years in the making, has the capacity to draw 50 million gallons of water a day from the Potomac River. Morin said this amount, in addition to the 112 million gallon-a-day capacity of the Occoquan Reservoir, should satisfy the needs of the area until 1995. At that time, the Potomac plant could be expanded to a 200 million gallon-a-day facility. Authority customers currently use about 67 million gallons of water each day.

Morin said the plant was funded through a series of bonds and notes that will be paid off over 40 years and said the authority does not need to raise water rates at this time. Rates have remained stable for the last four years.

"We are doing pretty well financially with our current revenues," he said. The authority will take in $27.2 million this year from water sales and service, according to authority reports.

Morin said construction of the Potomac plant was the largest single, locally financed public works project ever undertaken in Northern Virginia.

Virginia officials since World War II had been discussing the possibility of using the Potomac as a water source but, for various reasons of financing and feasibility, plans for the new plant were not set into motion until 1972.

Before work could begin, the Fairfax authority had to ask Maryland's permission to tap the Potomac because that state legally owns the waterway. Morin said Maryland officials were "extremely cooperative". It took five years and four months of hearings, negotiations and reports to obtain nine separate local, state and federal permits.

Construction started in September 1978 and was finished in early April.

The facilities include a network of pipes, stations and pumps spread underneath much of Northern Virginia.

Water from the Potomac is drawn into a 110-foot concrete intake on Lowes Island in Loudoun County, then pumped five miles to a water treatment plant on 112 acres off Stuart Road in Fairfax County. There it is cleaned, chemically treated, fluoridated and piped to customers.

Morin said that during the spring and early summer, when the Potomac is high, the new facility will operate at full capacity, conserving the Occoquan Reservoir water supply as much as possible. When the river is low during the dry months of September to February, the authority will turn to the Occoquan.

This plan is the result of months of negotiations with Maryland and District water authorities, which draw almost all their water out of the Potomac.

"When the river is low, we will give Maryland and the District a chance to have sole use of the water," Morin said. "The whole plan is that those who need the water the most use it at low periods, and those who have other sources use it less at low periods."

Morin said the Potomac is in no danger of being depleted, however.

The average daily flow of the river is 7 billion gallons of water, and the District, Maryland and now Virginia draw about 350 million gallons from it a day--or about 5 percent of the flow. "So we're talking of having a very small impact normally," he said.

During a drought, the Potomac may shrink to a flow of 1 billion gallons of water a day or less, which then would make it necessary for the water authorities to turn to their reservoirs.

Morin said that although the Potomac facility was not built in time to prevent water shortages over the past few years, the delay has been beneficial in some respects. "The customers have really learned how to conserve water these past years," he said.