Fairfax City appealed yesterday to its 8,400 water customers to curtail their water usage because problems with a faulty valve have caused a sharp drop in the community's water reserves.

The problems began last week when a defective valve on a 16-inch main forced an eight-hour shutdown of the Fairfax City water treatment plant. The plant, located on the Goose Creek in Loudoun County, ordinarily can process an estimated 10 million gallons of drinking water a day.

City water and sewer sevices director Richard R. Fruehauf said yesterday that hot-weather demands have prevented the city from replenshing its reserves even though the plant is operating at peak capacity. "We fell behind the eight ball, but we never did catch up because the ground just got dry and everyone started turning on their sprinkler," he said.

Fruehauf said consumers in Fairfax City and some parts of Loudoun and Fairfax counties would expierence a reduction in water pressure until the city's reserve tanks can be refilled later this week.

Fruehauf said that the city's water situation is "not an emergency", but he urged area residents to refrain from watering lawns and washing cars until the city's three water reserve tanks can be refilled.