Officials in Northern Virginia have formed an "Occoquan Alliance" designed to limit development -- and possible uranium mining -- in the Occoquan Reservoir watershed, which supplies water for more than 700,000 area residents.

At last week's initial alliance meeting, Fairfax County officials urged neighboring Prince William County to join the Northern Virginia Regional Park Authority. The park agency has bought and protected as parkland more than 25 miles of shoreline on the Fairfax side of the reservoir, while Prince William has allowed housing developments along the south shore.

Kathleen Seefeldt, chairman of the Prince William Board of Supervisors, told the alliance her county had been concerned about the cost of joining the park authority, but will reconsider.

Since 1977, when a new sewage treatment plant eliminated a major pollution hazard for the reservoir, the principal pollution threat has been from rain runoff from developments and from fertilizers and animal refuse on farms.

Last summer, in a move to prevent the pollution that comes with dense development, Fairfax rezoned much of its 100-square miles of Occoquan watershed from one- to five-acre minimum lots. It has urged other Occoquan basin jurisdictions to consider similar restrictions even though more than two dozen lawsuits have been filed challenging the rezoning. The alliance decided to support proposed state laws that would provide stronger support for county rezonings.

A New York firm recently bought mining rights on 16,000 acres of land in the area but last month announced it was dropping the rights. Local officials are still worried, however, about the pollution uranium mining in the watershed would cause and the alliance tentatively decided to hire a geologist to determine how much uranium there is in the area.

Occoquan is fed by a watershed of streams that wind through Prince William, Fauquier, Fairfax and Loudoun counties and through Manassas and Manassas Park, all of which have joined the alliance.