The issue was a legislative Hobson's choice. Whichever way the Fairfax County Board of Supervisors voted, it would be accused of threatening the future of the Occoquan Reservoir, the drinking-water supply for more than 600,000 Northern Virginians. And in an election year, at that.

Faced with what may rank as one of the most important land-use decisions they will make before their term ends in January, the supervisors voted to defer action for a month to give themselves more time for thought.

The issue centered around the attempt by the owners of 45 acres upstream from the reservoir to get rezoning that would permit them to build 271 single-family homes and town houses on the parcel, or six per acre.

The present zoning permits one house per acre, but Fairfax's master plan, adopted in 1975, recommends a density of from five to eight houses an acre, well within the range the owners propose.

The most formidable opponent of the higher density was a man none of the supervisors would like to overrule -- Carlton C. Massey, the esteemed former county executive after whom the Massey Building, where the supervisors meet and county offices are located, was named. Massey spoke as a member of the board of the Upper Occoquan Sewage Authority, which operates the $82 million regional sewage plant near the 45-acre site and has voted to oppose the rezoning.

On the other side, supporting the rezoning, were both the county Planning Commission and the county planning staff. The supervisors seldom overrule the commission and their staff.

In supporting a higher density, the planning commission and the staff were deferring to the master plan drawn up with considerable citizen involvement in 1974 and 1975, and then approved by the supervisors now holding office.

County environmental planner John H. Thillmann had, for him, and even more compelling reason to support the rezoning. Without it, he said, "the county would lose its first opportunity to get a developer to control stormwater runoff as a way of preserving the water quality of the Occoquan. If we can't do it here, where can we do it?"

The owners have agreed to spend $80,000 on what water experts call "best management practices" to control runoff, which has been identified as perhaps the worst form of pollution affecting the Occoquan.

Thillmannhs and the county staffs argument is approve the rezoning and in exchange win the developer's agreement to employ antirunoff practices that could become a model for other development in the Occuquan basin.

Under the plan proposed by county staff, the developer would have to employ practices that would reduce the impact of runoff to what it would be if there were only one house -- instead of six -- per acre

Opponents said no one knows for sure whether best management practices would actually do what computer models cited by Thillmann say they would do. "United the technology is proven, we would be playing Russian roulette with our reservoir," said Millard H. Robbing Jr., executive director of the sewage authority.

When Robbins talks about "reservoir" he does not mean the drinking-water reservoir but a 45-acre "polishing pond" where the sewage treatment plant's effluent gets a final cleaning (virtually to drinking-water standards) before being released into Bull Run, which flows into the larger body of water.

Although they deferred action, several of the supervisors indicated they were leading toward rejection of the higher density.