The Fairfax County Board of Supervisors reversed itself yesterday and gave preliminary approval to a 147-acre expansion of the Lorton landfill, saying final approval would depend on an environmental study and appropriate measures to prevent leakage of pollution from the facility.

The board voted 5 to 4 to order county staff members to apply for state permits to expand the landfill, overturning a September decision not to file permit applications until an environmental impact study of the proposal could be completed.

Critics of yesterday's vote said it guaranteed that the Lorton landfill, located on 3,000 acres of federal land in southeastern Fairfax, would be expanded despite concerns that such a move could increase pollution of the Occoquan River, which eventually empties into the Chesapeake Bay.

"It's a complete sellout," said Laurie Frost, a citizen activist who has led opposition to the expansion. An environmental study of the proposal will be "a paper exercise, it's academic" if preceded by the permit process, she said.

Supervisor Joseph Alexander (D-Lee), who supported the expansion, said, "We have to take care of our refuse, otherwise it's going to be backing up into people's front yards." Democratic Supervisors Lilla Richards (Dranesville), Sharon Bulova (Annandale), Martha V. Pennino (Centreville) and Kate Hanley (Providence) also supported the expansion.

Board Chairman Audrey Moore (D), and Supervisors Gerald Hyland (D-Mount Vernon), whose district includes the landfill, Thomas M. Davis III (R-Mason) and Elaine N. McConnell (R-Springfield) voted against the expansion.

Proponents of the expansion, which was scaled back from a proposed 300 acres, said the current dump will be filled in about three years, or about the same length of time it will take to receive state permits for the expansion and build the facility to the latest environmental standards.

After burning about 2,400 tons of garbage a day, the county is left with 200 tons of unburnable trash that is much bulkier, Public Works Director John diZerga said. Without a place to dump the garbage, County Executive J. Hamilton Lambert said, the county would have to pay "astronomical" fees to haul it to another landfill in the state.

The county pays $43 a ton to dump at the Lorton landfill, diZerga said. It could cost as much as $100 a ton to dump elsewhere, not including transportation costs, he said.

In an unrelated matter, the board unanimously approved holding public hearings on a local law aimed at reducing pollution of the Chesapeake Bay from development in the Bay's watershed. The proposed ordinance, mandated by the state's 1988 Chesapeake Bay Preservation Act, would establish protected areas where construction and other "land disturbing activities" are prohibited to prevent soil erosion and runoff of pollution into the bay.

Stricter regulations on disturbing land and vegetation would be enforced across the entire 255,360-

acre county under the proposal. Some supervisors said they were concerned that the proposed ordinance could reduce property values or prevent residents from developing their land or improving their homes and yards.

The ordinance is scheduled for a Dec. 12 public hearing before the county Planning Commission and a Jan. 28 hearing before the Board of Supervisors.