Mobil Oil Corp., Fairfax County's biggest and most prestigious acquisition in its search for new industry, has told the county government there is "increasing concern" within the firm about Northern Virginia's current water crisis.

Mobil's reaction is the first publicized example of corporate nervousness about the crisis, which had led to mandatory water restrictions and the prospect of more stringent ones if the level of the Occoquan Reservoir continues to decline.

But Fairfax business leaders are becoming increasingly restless about the political stalemate holding up a long-term solution to the county's water needs.

"We're worried," said James M. Guiliano, executive director of the Fairfax County Chamber of Commerce. "We're worried about businesses already in the county and we're worried that some business or industry might decide not to come because of the uncertainty about water supply."

A Mobil spokesman in New York said that while the company is concerned about the water crisis, "the situation would not have any effect on our plans." The company has already decided to move its 800-employee domestic division to Fairfax, and is considering moving its entire 3,500-employee corporate headquarters to the county.

In a letter Oct. 6 to John F. Herrity, chairman of the Fairfax County Board of Supervisors, W. W. Richardson, assistant to Mobil chairman Rawleigh Warner Jr., wrote:

"We need to assure our people in the U.S. Division that the residents of Fairfax County will have an adequate water supply in the future and that action is under way for a sound long-term solution."

Richardson's letter asked three questions: 1) Is there a feasible solution that would increase the county's water supply within three years, or by the time Mobil expects to move its domestic division to a new headquarters it plans to build at Gallows Road and Rte. 50? 2) Is there public money available to finance a soltion? 3) Can all the principals agree to a solution?

"The answer to the first two questions is yes," Herrity said in an interview. "As for question three, we hope so."

The stalemate centers on the Fairfax County Water Authority's failure thus far to get a permit from the Army Corps of Engineers to pumps water from the Potomac River.

The authority, which supplies about 616,000 people in Fairfax, Prince William County and Alexandria, wants to build a Potomac pumping and treatment facility as a supplement to the Occoquan Reservoir on the Fairfax-Prince William county border. By 1981, and possibly as early as 1980, the authority estimates it will not be able to meet all water demands in its rapidly developing service area.

In addition to long-term problems, the water authority also is trying to cope with the present emergency involving the Occoquan. If the drought continues, county water planners fear, the reservoir could run out of water sometime in January.

The Corps of Engineers has balked on issuing a permit for the Potomac supply project until Virginia, along with Maryland, which already uses the river, signs an agreement guaranteeing the District of Columbia's future water supplies.

This District depends on the Potomac for all its water. Under an agreement signed last year by Virginia and Maryland - and opposed by the corps - the faster-growing suburbs would get a bigger allocation during periods of low flow of the Potomac than the District. Low flow is an infrequent problem now, but as demand increases with population, it could become a more serious issue.

Rep. Herbert E. Harris (D-Va.), who represent Alexandria, southern Fairfax and Prince William, two weeks ago proposed a compromise that would break the stalemate between Virginia and the Corps of Engineers.

He suggested that the two sides sign a temporary agreement that would permit the water authority to go ahead with the Potomac project, and at the same time continue talks on a long-term solution of low-flow allocation.

Under Harris' proposed temporary agreement, Virginia would agree not to take any water at all during low flow. During such periods, Harris said, the water authority could rely on the Occoqan, then shift back to the Potomac when the flow improves.

While Harris said his proposal had the support of the corps, Col. G. K. Withers, district engineer of the Army office, raised considerable doubts about the compromise. Furthermore, the water authority itself last week went on record refusing to make any concessions on a low-flow agreement in order to get a permit from the corps.

After discussing Mobil's reaction to the water crisis yesterday, the Board killed a motion by Supervisor Audrey Moore (D-Annandale) that would have had county staff members consider whether there should be a moratorium on water connections until the emergency was lifted.

"We should be careful about waving red flags," said Supervisor Martha V. Pennino (D-Centreville), expressing concern that such action could be interpreted as a signal to industry that its water needs cannot be met in Fairfax.

County Executive Leonard Whorton told the Board that none of the drought-stricken jurisdictions in the West have elected to hold up new water connections during their crises.

The water authority, at its meeting last week, approved a motion by Mrs. Moore's appointee, David Russell, calling for the authority's staff to look into the question of a water moratorium.

Meanwhile, the level of the Occoquan Reservoir yesterday was six inches lower than it was last Friday, despite almost an inch of rain over the weekend.