The Fairfax County Board of Supervisors voted this week to toughen bonding requirements for builders in the county in order to prevent them from leaving sidewalks, roads and other public improvements unfinished.

Supervisor Audrey Moore (D-Annandale) sponsored the legislation which made permanent a four-month old moratorium on the use of personal bonds by builders.

Moore thanked the Board after passage of the legislation which she said was not "a political winner." She added, "The real winners may never even know what you've done here today," referring to the county taxpayers who must foot the bill on projects that builders don't complete.

The Board action this week will require 100 per cent bonding of improvement projects from builders but that may be reduced to 50 per cent at the discretion of the county government when 50 per cent or more of the development has been completed.

This requirement, explained county executive Leonard L. Whorton, is meant to be "leverage" for the county government to make builders complete their current projects before going on to new ones "by tying up the credit ability of the builder."

Supervisor Alan H. Magazine (D-Mason) attached a proposal to the legislation that ordered an evaluation of the new policy to be presented to the Board at the end of six months.

Magazine voted for the new bonding policy but said he had reservations because of the effects it might have on the cost of housing in Fairfax County.

In other matters, the Supervisors requested a briefing from the Environmental Protection Agency about what is being done to monitor the level of chloroform in the Occoquan Reservoir and to determine a maximum safe level of chloroform in drinking water.

A preliminary report released last week by the EPA revealed it has found higher levels of chloroform in the Occoquan Reservoir, which supplies the county with its drinking water, than previously has been detected.

Meanwhile, the county's office of environmental protection told the Board that "there is no basis for public panic concerning the safety of water supplied by the Fairfax County Water Authority system, nor is there any justification for immediate action."

Finally, Supervisor James M. Scott (D-Providence) suggested that the county's staff investigate the possibility of setting up a consortium of governmental bodies in the metropolitan area to buy medical insurances for their employees.