A sewer extension proposed by Fairfax County planners would help subsidize the cost of George Mason University's ambitious expansion program, benefit a member of the college's Board of Visitors and possibly open a long-protected enviromentally fragile area to suburban development.

Fairfax officials previously have wanted to keep sewer lines - which promote high-density development - out of this area because it is part of the basin that feeds the Occoquan Reservoir, the drinking water supply for 600,000 Northern Virginians.

George Mason wants to build a West Campus on presently unsewered land west of Rte. 123, west of its campus. The expansion is in an area just north 50 acres owned by John T. Hazel Jr., a developer and zoning attorney who is a member of George Mason's board of visitors.

The county planners have proposed that the land near the planned West Campus be provided with sewers to help the university pay for a required pumping station.

The proposed extension, which the Board of Supervisors is expected to consider Monday, would probably increase growth pressures, said Theodore J. Wessel, the county planning director.

"It's a gamble," Wessel said, "but we don't think it's that much of one."

For Hazel, sewer lines are critical if the 50 acres he owns are to be developed into a 99-lot subdivision called North Farm. Last year, the county sent North Farm's preliminary plan back to Hazel's engineer because the subdivision did not hav sewer service.

Hazel said if he does not get sewer service, he would lose about 10 lots by redesigning the subdivion for septic service. He said North Farm would not be developed at a density higher than permitted in the county's master plan.

"It's not a case of development or no development," he said.

With the extension of sewer lines to the property surrounding its proposed new campus, county planners estimate that the university's share of the anticipated $589,979 cost would be $340,419. Other users, such as Hazell, would pay the remainder.

George Mason did not suggest the sewer extension, the university's chief planner, Joseph I. Gurfein, said yesterday. Last November, however, university President George W. Johnson wrote the county a letter saying, "It would be most desirable if the county joined us in this project," citing the southwest quadrant of Rte. 123 and Braddock Road.

Hazel said that because his property might be affected by George Mason's expansion, he abstained from the board of visitors' vote in December when the decision to seek the expanded sewer service was made.

Supervisor Audrey Moore (D-Annandale) said yesterday that the university will not need all the sewer service it wants and suggested that George Mason might be requesting the extension to make its land more attractive to potential buyers.

"They will be doing well to complete their eastern campus in the next 20 years," she said.

But Gurfein said, "There is no way we would sell the land off. We need every bit of it."

He said the university is expected to grow from its current enrollment of 10,700 to 16,000 by 1985. The West Campus, he said, would include a fieldhouse for intramural sports, housing and academic facilities yet to be determined, possibly a medical or dental school.

Moore said she will ask the Board of Supervisors to givwe the university only enough sewer service for the fieldhouse, which, with an adjacent track, is the only element of the proposed West Campus with firm construction plans.

The George Mason property and Hazel's tract are located in the Popes Head Creek watershed, a semirural area that drains into the Occoquan Reservoir.

One of the major threats to the reservoir is pollution from runoff in developed areas. The pollution is in the form of sediment - especially during construction when large areas of land are uncovered - and a variety of chemicals - from lawn and garden fertilizers and parking lots.