The Fairfax County Board of Supervisors capped months of debate on one of the last major land-use disputes in the county by voting last night to limit growth sharply within a 100-square-mile area.

The decision, which came after a three-hour public hearing and a confused vote to exempt two subdivisions from the proposed massive downzoning, is meant to limit growth in the Occoquan River Basin, which comprises the county's western quadrant and is one of the largest watersheds in Northern Virginia.

About 50 persons addressed the board before the final decision, many protesting that a vote to downzone the Occoquan Basin from the current ceiling of one house per acre to one house per five acres would slash property values. Several speakers threatened the board with lawsuits if the county went ahead with the downzoning.

The board this spring had tied the downzoning in the Occoquan to a proposed massive development--along the lines of the huge Tysons Corner shopping center--at the intersection of Rte. 50 and Interstate 66 just west of Fairfax City. The board was scheduled to vote on that proposal yesterday as well, but at midnight decided to defer a decision until next week.

The supervisors voted to enforce sparse development in the Occoquan with an eye to protecting the watershed from runoff pollution. The basin slopes toward Occoquan Creek and Bull Run, which feed the Occoquan Reservoir, the drinking-water source for about 600,000 Northern Virginians. Development is viewed as increasing erosion and other pollution problems for the reservoir.

"The board felt very strongly that measures had to be taken to guarantee protection of the water supply," said Supervisor Marie B. Travesky. "We're not going to run the risk of having urban development along Bull Run and the Occoquan."

Several landowners contended there was no real evidence the downzoning was environmentally necessary, and said it was merely a ploy by the board to limit growth in what is often called the county's "last frontier."

The board became bogged down on the Occoquan downzoning early in the evening when it voted, 6 to 3, to exempt two subdivisions south of Dulles Airport from the downzoning and from a noise-exclusion area.

About 200 residents of the subdivisions, Pleasant Valley and Pleasant Hills, cheered the early vote, and soon left the board room. Within the hour, however, it became apparent that several board members thought they had excused the two sites only from the noise-exclusion area.

The supervisors feared that exempting the two subdivisions from the downzoning might allow other similarly affected subdivisions in the Occoquan to gain the upper hand in anticipated legal battles.

"I still don't think the Pleasant Valley thing is resolved," Supervisor Thomas M. Davis III said.

The board, realizing its confusion on the two exemptions after reading a transcript of what it had done, decided to schedule another public hearing on Pleasant Hills and Pleasant Valley for next week.

A lawyer for the two subdivisions said the board's reversal could make the county vulnerable if the issue was ever brought to court.