Prince William County became the first large Northern Virginia jurisdiction last night to adopt a Chesapeake Bay protection ordinance intended to prohibit wetland disturbance and reduce pollution from development.

The Board of County Supervisors' unanimous vote followed a 90-minute public hearing at which environmentalists praised the measure and developers complained that the requirements would cost them too much.

In 1988, the Virginia General Assembly directed 89 jurisdictions in the eastern part of the state to adopt such water quality ordinances by this September, but only 31 have done so, state officials said.

Falls Church and Fairfax City are the only other Northern Virginia localities that have approved the required laws. Fairfax County will hold a Planning Commission hearing on its ordinance on Dec. 12.

Prince William's law prohibits development within 100 feet of wetlands and streambeds, as mandated by state law.

The county and other jurisdictions have struggled to identify other vulnerable areas, where developers can build but must take special precautions to prevent erosion and preserve the existing trees and landscape.

County planners in both Prince William and Fairfax recommended requiring special measures for their entire counties, but the development community argued that the precautions -- such as requiring storm water ponds and reserve drainfields -- should apply only to clearly vulnerable areas, such as steep slopes.

But the board rejected the developers' argument and opted for countywide protection of water quality.

The board agreed with developers' demands that projects already zoned for development be exempt from the new water quality rules.

Prince William's ordinance may have to be revised after state officials review it for compliance with the original state legislation. In addition, several business and development groups are trying to get the General Assembly to change -- environmentalists say weaken -- the regulations.

The supervisors also unanimously approved $110,000 in emergency funding to stem a rising tide of child abuse.

The Department of Social Services plans to hire five social workers to work with parents who have abused their children but still have custody of them. The department also plans to add five more people, using state funding and money within its own budget.

Reports of child abuse and neglect rose 76 percent from 1983 to 1989 -- nearly twice as fast as the county's population -- and Social Services workers cannot keep up, said director Ricardo Perez.

"Our workload is exploding . . . . We will investigate {a report of abuse} but we will not be able to transfer that case to a treatment team," Perez said.

Without help, many parents continue to abuse their children, and the children end up in foster care, Perez said.

County Executive James H. Mullen had opposed the $110,000 supplement. Prince William is expecting a $17 million revenue shortfall in the next fiscal year, which begins in July.