Underneath parts of Fairfax County are large but generally untapped reservoirs of groundwater that could, some experts say, be used to head off the water crisis expected to hit most of Northern Virginia as early as 1980.

Utilized in the dry summer period, when supply is low but demand is high, the groundwater could supply as much as a third of the country's water needs by 1980, according to John Tilghman, a planner in the county's environmental and technical services branch.

Tilghman said that preliminary studies indicate groundwater could deliver as much as 28 million gallons daily if tapped for only part of the year - during the dry seasons and droughts. It is at those times that the Occoquan Reservoir - Fairfax's major source of water - is already hardpressed to meet needs.

Based on projected growth in its service area - most of Fairfax, the city of Alexandria and part of Prince William County - the reservoir will not be able to meet all customer demands by 1980, officials say. That year, according to estimates, the reservoir's supply, plus wholesale purchase from Falls Church, will be 108.4 million gallons daily, but demand will be 118.3 million gallons daily.

Tilghman said test wells would have to be drilled to determine more precisely the potential supply of water in the ground.

On Monday, the Fairfax Board of Supervisors endorsed unanimously a resolution by Marie B. Travesky (R-Springfield), a member of the county's water committee, asking the U.S. Geological Survey to develop a well-test program in Fairfax and extend it to Alexandria and perhaps the entire metropolitan area.

Richard Johnston, a hydrologist with the geological survey's Fairfax field station, said in an interview that there are considerable groundwater reservoirs in western Fairfax (from the Herndon area southward to Manassas), in the southeastern part of the county and in southeastern Alexandria. As recently as 1960, he said, groundwater in those areas supplied 4 million gallons daily but today, because of increased reliance on utility-supplied water, the groundwater only contributes about 1 million gallons daily.

While declining to put a firm estimate on the reservoirs, he said they could probably supply "slightly less than 20 million gallons daily" on a regular basis. The supply potential would be increased if the reservoirs were used only part of the year.

If groundwater is utilized as a supplementary but major water supply in Fairfax, Tilghman said, "I think it would lead to a rethinking" of the county's controversial plans for dealing with the impending water shortage.

To meet the shortage predicted by 1980, the Fairfax County Water Authority, which operates the Occoquan Reservoir, plans to build an intake facility on the Potomac River. The intake and a treatment plant would cost about $52 million, or more than $1 million per million gallons. A million-gallon-daily well could be built for $5,000 to $7,000, Tilghman said.

But expense is not the major problem Fairfax faces in trying to tap the Potomac. Construction of the intake is being held up by the Army Corps of Engineers. The corps - which must approve all construction on major waterways like the Potomac - said it won't permit the intake to be built until there is an agreement protecting the District's water supply.

The District gets all its water from the Potomac. The corps is concerned that if Fairfax draws water from the river, and if suburban Maryland carries out plans to increase the amount it already takes, the Potomac might virtually run dry during periods when demand is at its peak.

If that happened, the District, alone wholly dependent on the Potomac, would have to turn to the estuary near Chain Bridge for its supplies. While construction is under way on an emergency intake facility, the purity of such water, which would consist in part of sewage effluent from the Blue Plains treatment plants, has been questioned.

"If Fairfax got 50 per cent of its supply in the summer from groundwater, that would change the picture entirely," Tilghman said.

In other action Monday, the supervisors:

Received a report from the Fairfax Consumer Protection Commission saying that the Consumer Affairs Department recovered $213,435 for county consumers and tenants in 1976. Commission chairman Lawrence V. Fowler said the department handled 20,705 complaints in the year and resolved 20,619 of them. Of the total received, 23.9 per cent involved automotive sales and service, 17.4 per cent house contractors, and 11.4 per cent retail stores.

Endorseo a resolution by Alan H. Magazine (D-Mason) calling on the county personnel department to translate its rules into Spanish to aid non-English-speaking Hispanic employees, many of whom work in maintenance jobs.

Appointed J. Roger Teller of Fairfax to fill the vacancy on the county school board created by the departure of Mary Anne Lecos, who did not seek reappointment when her term expired. Teller, who is 44, is an associate professor of mathematics at Georgetown University. The supervisors endorsed a motion praising Lecos, a former chairman of the school board, for her many years of service.