The Fairfax County Board of Supervisors approved a sewer extension yesterday that would allow George Mason University to expand its campus. It also would benefit a developer who is a member of the university's governing board.

The action was defended by some members of the board as the best way to meet the university's needs.

It was called a "terrible mistake" by Supervisor Audrey Moore (D-Annandale) who aruged that it could lead to high-density development in a long-protected environmentally fragile area that surrounds the university's campus near Fairfax City.

When the supervisors initially voted on the sewer project last year, they restricted the sewer expansion to the university's land without further extension to nearby property partly owned by John T. Hazel Jr., a developer and zoning lawyer who belongs to the George Mason board of visitors.

The sewer approved last July was designed to serve only a new 200-acre west campus, starting with an athletic field house, on land west of Rte. 124 west of the main George Mason campus. The west campus is just north of 50 acres being developed by Hazel.

When the Virginia General Assembly failed to include all of the more than $400,000 needed for the sewer expansion in this year's proposed state budget, Hazel offered to contribute nearly $300,000 for the sewer work if the supervisors would permit a sewer extension to his property.

Now that the Fairfax, supervisors have given their approval, the sewer project must also be endorsed by the Upper Occoquan Sewage Authority and the governments of Manassas, Manassas Park and Prince William County. a

University President George W. Johnson, arguing for the expansion at yesterday's board meeting, said that without Hazel's contribution plans to begin constructing the field house laterthis year might be jeopardized. He said it now appears the university will get only about $100,000 from the state.

Supervisors Martha V. Pennino (D-Centreville) and Nancy Falck (R-Dranesville) said Hazel's proposal was the best way to meet the university's needs under the circumstances.

"It would be terribly unfortunate not to let the expansion go forward," Pennino said.

But Moore said the university's expansion was being used by Hazel as a subterfuge to faciliate high-density development in the Popes Head Creek watershed, a semirural area that drains into the Occoquan Reservoir.

"The number one priority for development interests is to get into the Popes Head watershed," one of the few remaining areas in the county without sewer lines. Moore charged. Sewers usually result in higher-density development than is possible on land served only by septic tanks.

Hazel, in an interview, termed Moore's claims "absolute nonsense." Noting that the county master plan calls for low-density development in the area, he said he knows of "no reason or pressure" to change that.

Hazel, who already has the zoning to develop about a 100-lot subdivision called North Farms on the land near the university, said his contribution will be paid for by the larger number of lots he will have with sewer lines.

The board approved the sewer expansion by a 6-to-3 vote, with Supervisors Moore, Marie B. Trvesky (R-Springfield) and Chairman John F. Herrity (R) voting against it. Voting for it were Supervisors Joseph Alexander (D-Lee), Pennino, Falck, James M. Scott (D-Providence), Sandra L. Duckworth (D-Mount Vernon) and Thomas M. Davis III R-Mason).

In other action yesterday the board voted 7 to 2 to endorse a compromise attempt by Fairfax States Sen. Adelard L. Brault (D) to get a regional sales tax passed by the General Assembly to help fund Metro.

Yesterday's action reversed the board's decision last week to oppose Brault's plan and came after the senator sharply criticized Chairman Herrity when Herrity stated the board's opposition at a Senate subcommittee hearing last Thursday.