About 16,000 letters will go out this week, marking what some have portrayed as the final stage in a two-decade conflict over the Occoquan Reservoir area.

The letters will inform landowners that county supervisors may rezone much of Fairfax's sparsely developed west to allow only one house on every five acres. The plan is intended to protect the water reservoir that serves 600,000 Northern Virginians, to preserve some open space in the last expanse of woods and horse farms in the suburban county and to slow and control growth in fast-growing Fairfax before it is fully developed.

Some county officials say the massive rezoning of 38,500 acres would be unprecedented in Virginia, where property owners' legal rights rarely have been challenged successfully. "I think in this case it is pretty earthshaking," says Supervisor Marie Travesky, whose district includes most of the Occoquan Reservoir basin.

But the landowners who feel threatened are not, for the most part, the county's powerful developers and major landowners who have fought--and defeated--previous slow-growth efforts. This time the county has found ways not to offend those powerful interests, thus diffusing the opposition--and the political risk.

"The most drastic effect will be on relatively small acreage owners," shrugs John T. Hazel, a developer who has successfully fought previous slow-growth efforts. "Whether it has any real effect on the ultimate development of the county is doubtful."

Much of the Occoquan Valley has been exempted from the downzoning, often where intense development has been planned for years. The county has given ample warning to landowners with enough savvy or legal talent to allow them to file plans that may exempt them from the restrictions even if their land falls within the 38,500 acres. And, while downzoning some land, the county is simultaneously planning to permit intense development in one corner of the Occoquan basin where several of the most influential developers, including Hazel, own land.

Supervisor Audrey Moore, the developers' most outspoken critic on the board, for once agrees with Hazel, although she believes the Occoquan plan is better than no action at all.

"It's a sop to the citizens," Moore says, claiming the developers have won most of the battle before it even begins. "And it's not going to accomplish what they're predicting."

Beyond the ridge where the distant Blue Ridge Mountains first break hazily into view for travelers from the District, the 100-square-mile Occoquan watershed covers one quarter of the county. No land has been fought over more fiercely by environmentalists, developers and longtime residents.

Many of those residents still use outdoor privies--often disguised as toolsheds to avoid their neighbors' scorn--while developers have won the right to build rows and rows of townhouses. Now the reservoir, say environmentalists and county staff, is in danger of dying from runoff pollution.

The staff says that Fairfax--and eventually Prince William and Fauquier counties as well--must require builders to provide sophisticated forms of drainage control and must keep much of the watershed free of construction.

Supervisor Moore recalls making the same argument before the board as a citizen-activist in the 1960s. "You don't have to be a genius to understand you don't put your outhouse above your well," she says.

Development did take place, however, where a small group of politicians and landowners wanted it to. The pattern was set in the 1950s and early 1960s, an era in Fairfax history when Supervisor Stuart DeBell--who represented a district that now contains 76,000 people--could win election by a 316-to-146 margin. It was an era when the leading zoning lawyer kept a neat ledger of payments to local politicians, and when politicians were not shy about promoting their own interests when they thought the county would benefit as well.

DeBell, who later was indicted and acquitted of charges of accepting bribes in zoning cases, believed in growth and worked hard to bring to western Fairfax sewers that would transform raw land into valuable commercial property. Those sewers allowed William J. Levitt, the builder of Levittown, to carve Greenbriar out of the Fairfax wilderness. They permitted other developers to build trailer parks near the Loudoun County line. And they helped part of DeBell's own 500-acre family farm become the Newgate shopping center.

"These weren't bad guys," says Supervisor Martha V. Pennino, elected to the board as a reform candidate in 1968 after the indictments of DeBell, who was acquitted, and 14 other officials and developers, eight of whom eventually were convicted of bribery or related offenses. "They just didn't have any vision."

The sewer lines passed by the area's longtime residents, with their privies and failing septic fields, who could not afford the tie-on costs. The resulting leapfrog pattern of development has cost Fairfax taxpayers ever since, say many planners. "There they were, a cluster of single-family houses with no way to serve them," Pennino says of Greenbriar in its first 15 years. "It was extremely costly."

About four-fifths of the Occoquan basin remains sparsely developed, and many residents who have no speculative intentions back the proposal to restrict development.

"We gave up the convenience for the quiet and the breathing space," says Ivy Mitchell, who with her husband spent five years building a house on Wolf Run. "This will ensure that what we've chosen will stay the way we wanted it to stay."

Other landowners, who effectively buried a more stringent downzoning proposal in 1978, are expected to attack again this year. Some opponents say the water quality issue is simply the latest excuse for those who dislike growth.

"This is just a pseudo-technical report wrapped up to justify a political philosophy," says Noman Cole, the former chairman of the State Water Control Board who pushed for the modern Occoquan sewage plant that opened much of the area to development.

Other opponents, such as real estate dealer Suzanne Paciulli, say the plan would keep moderate-income families out of Fairfax by making every new home an expensive estate. And many landowners, including one represented by former county attorney F. Lee Ruck, say the county cannot legally erase their right to subdivide and build.

But the county has taken several steps to make this plan more palatable to landowners, and especially to big developers, than the 1978 proposal. That plan would have allowed only one home on every 10 acres, compared to the current five, and much of the undeveloped land, with poor soil and steep slopes, could not support more than one house on five acres in any case.

In addition, landowners, who were not told of the 1978 plan until the last possible moment, have been given enough warning this year to allow them to file subdivision plans that may legally exempt them from the new restrictions.

DeBell's son, engineer John T. DeBell, mailed flyers to about 250 landowners in the valley explaining their rights and offering to help them beat the deadline for filing plans. "I've had quite a few calls," DeBell says. "They own land that they've paid taxes on for one-acre zoning all these years, and they feel they'd be losing considerable money by having it downzoned on them."

Hazel says he accelerated development plans in March for his 600-home Fairfax Station subdivision, where only half the projected homes have been built, in order to beat the downzoning.

"It's not going to accomplish what they think it will, unfortunately," says Hazel's law partner, Grayson P. Hanes. "What they're doing is spurring development on by encouraging people to come in and get vested."

County officials declined to comment on the legal impact of the early plan filings.

Huge parcels of land in the Occoquan basin have been exempted from restrictions in the county plan. In the Little Rocky Run Valley, the boundary marking proposed restrictions dips sharply to the south to avoid land that has been planned for intense development since the days of Stuart DeBell's sewers.

Landowners in Little Rocky--including Hazel, Hazel's secretary, Charlotte Garner; builder Ralph Rocks, and Cole--may build as many as 12 houses on every acre, thanks to rezonings approved by the board in the late 1970s, hundreds of acres at a time, and to a sewer line approved by the board in 1981.

County planners say Little Rocky Run is a legitimate part of the "urban envelope" because it is near Centreville. "Our object was never intended to be no growth," says planning director Ted Wessel. "We simply want to orient growth to the most appropriate spots."

The county estimates, in fact, that its downzoning proposal--along with asssociated pro-growth recommendations--ultimately would allow a population of 171,000 in the Occoquan area, a 10 percent reduction from the 187,000 that currently would be permitted.

The most rapid growth has been targeted to a 5,000-acre area where Interstate Rte. 66 and Rte. 50 come together, and where some of the county's most high-powered developers have grand plans. Hazel and his partner, Milton Peterson, own a $3 million, 600-acre parcel there that drains into the Occoquan--and that the county is proposing for a major office park that could substantially increase the land's value.

County officials say the plans for the I-66 area complement their proposal to protect water quality in the rest of the Occoquan. In one area, they can attract businesses that will pay taxes and provide jobs, while elsewhere they can restrict growth to compensate for the environmental damage intense development at I-66 may cause.

Earl Lee, who runs the general store in Clifton, is one of the smaller-acreage landowners who would be affected by the restrictions. Lee says he has no plans to develop his 70 acres--"I'm not the polluter," he says--but he still says the proposals are unfair.

"I would support the downzoning if it applied to everybody," says Lee. "I resent their trading my rights for some guy in New York or California who's going to pick up the Wall Street Journal and see, 'Five-acre lots for sale for high-rise development.' "