Virginia Gov. John N. Dalton yesterday declared a state of emergency in the kerosene-polluted Occoquan watershed of southern Fairfax County and in Fredericksburg, where tight water conservation measures are still in effect.

Fredericksburg officials yesterday shut down the city's water treatment plant for the second time in as many days after a leak was discovered in a temporary dam built to keep heating oil out of a massive fresh water storage pond.

The plant was restarted 12 hours later after experts determined the pond was uncontaminated.

Nevertheless, city officals strongly urged the Fredericksburg area's 55,000 residents to curtail water use to preserve emergency reserves estimated at 10 days' worth. Federal environmental experts predicted yesterday that cleanup efforts could take up to 15 days to complete.

"We've still got a critical situation here," said Assistant City Manager Peter Kolakowski. "We don't want people to forget that because it's going to be a long haul."

Federal and state health officials said they expected about 260,000 gallons of heating oil and kerosene, spilled when a pipeline ruptured last Thursday, would casue "massive" fish kills in the Rappahannock and Occoquan rivers that could leave fishermen disappointed for years to come.

Water officials in Fairfax County, which draws most of its drinking water from the Occoquan, were optimistic about prospects for a complete cleanup of the Northern Virginia spill without danger to water supplies for the system's 660,000 customers.

They estimated that 80 to 90 percent of the 200,000 gallons of kerosene spilled into a tributray of the Occoquan had already been cleaned up, and said the remaining material was being effectively held away from Fairfax water intakes.

Several thousand dead fish have already been sighted in the headwaters of the Occoquan. Workers used cannon and shotgun blasts to frighten wild fowl, including endangered bald eagles, away from the gummy pollutant still floating on parts of the river.

Dalton toured the Fredericksburg site for more than an hour after declaring the state of emergency at the request of the city council, which wanted to put muscle behind its efforts to end the city's continuing water crisis.

Paul Edwards, a spokesman for the governor, said the declaration would free virtually unlimited state funding and resources for the cleanup project, if necessary, and lend state help in coordinating a crew of more than 100 state, federal and local workers.

The latest treatment plant shutdown came only half a day after one-third of the city's residents had endured a two-hour water-shutoff while they awaited a favorable state Health Department evaluation of the 10-million-gallon holding pond. Today's shutdown did not cause any break in water service because the plant had built up a reserve of treated water while city residents slept Monday night.

Arthur Cossey, superintendent of the plant, said the shutdown was ordered when city workers discovered a leak of potentially tainted water into the holding pond.

He said the continuing water crisis has left Fredericksburg water officials "holding their breath" in hopes that the city can continue paring its water intake by one-third. According to current estimates, the city is using less than 2 million gallons of water daily, down from its usual intake of almost three million.

Meanwhile, water crews used tanker trucks to transport an estimated half-million gallons of fresh water froma nearby gravel pit, and prepared to suck 12 million gallons more from two city ponds. Another million gallons of water is being shipped in daily from neighboring Spotsylvania County.

City schools, closed when the crisis began, were reopened yesterday with sharp limitations on water use. School prepared meals were eliminated and students were forbidden to take showers after physical education classes. Many restaurant and other businesses that consume large amounts of water remained closed.

"It's become almost a status symbol to have a dirty car and dirty hair here," said Kolakowski, adding that the lack of bathing resources was having a distinct effect on the city's residents. "We talk to each other from four feet away," he said.

James A. Warfield Jr. of the Fairfax County Water Authority reported that precautions are being taken to protect the county's intakes from the floating kerosene, even though the pollutant is believed to be effectively corralled at Bull Run Marina, almost 10 miles upstream.

Of the three water intakes, the county is using only the one farthest below the water's surface, Warfield said, virtually eliminating the possibility that the lighter-than-water kerosene can make its way into county supplies.

In addition, Fairfax officials are gearing up a carbon filtration system to be used in case of an emergency, and are building a dam across the small, unnamed tributary of the Occoquan into which the kerosene was dumped last week.

Continuous testing of drinking water supplies in Manassas and Fredericksburg has failed to turn up appreciable amounts of the pollutants, federal environmental officials said.