The Virginia State Water Control Board has labeled the Occoquan Reservoir, which supplies 600,000 Northern Virginians with drinking water, the second most polluted Lake in the state.

Two Fairfax County Lakes - Lake Accotink and Lake Fairfax -- also were ranked among the state's most polluted Lakes.

While the designation implies no immediate danger to Northern Virginia's drinking water, state and federal officials say that unless they can stop phosphate and nitrogen from running into the lake, the continuing growth of algae will kill fish and the water will become sour and malodorous within a few years.

Even so, water taken from the Occoquan should continue to be safe for drinking for at least a decade, experts said.

"If this problem is not tended to, you couldn't even stand the smell of the water, you couldn't take a shower, regardless of how much the water treatment facility does, because the algae would be so thick," said Jerry Bowie, an official of federal Soil Conservation Service's Richmond office.

"At this time, the lake is still used for recreational purposes," said John T. Hartigan, director of the engineering-planning division of the Northern Virginia Planning District Commission. "But unless there are controls, in the future the recreational uses will be significantly reduced."

Fairfax officials said they were unaware of the rankings, which have not been widely circulated or publicized. But the thrust of rankings -- that the Occoquan basin is in danger of being significantly polluted -- is no surprise. Last March, the Fairfax Office of Comprehensive Planning released a 162-page report that said the county will "in the near future contribute major increases in . . . runoff polutants" to the reservoir.

"We are certainly concerned about this and will look into it, but I want to say that the Occoquan is a very clean lake now," said John F. Herrity, chairman of the Fairfax County Board of Supervisors. "I am assured of that by experts. We have recently taken action to 'down-zone' some property in the county to help reduce the problems of urban runoff. What else we will do, I don't know . . . We need to be careful not to alarm people into thinking there is a problem with their drinking water."

Fairfax Supervisor Audrey Moore, who has squared off with Herrity on many growth issues, is not so confident. "I'm amazed it wasn't named the first," said Moore, a consistent critic of Fairfax's handling of the Occoquan. "The pollution problem has been significantly aggravated because the county's policy has been to encourage developemnt in the watershed. And I've become known as Mrs. No-Growth."

The Occoquan is closed to swimming because it is a reservoir for drinking water, but it is open for limited boating and extensive fishing. If the water quality continues to decline, experts at the U.S. Department of Agriculture said, the type of fish remaining in the reservior probably could not be eaten.

Four years ago, the Upper Occoquan Sewage Authority's advanced waste water treatment plant near Manassas replaced 11 outdated plants with a state-of-the-art system that continues to meet federal drinking water standards. Debate has long raged in Fairfax over the environmental impact of development in the Occoquan basin and the county board recently voted stricter zoning requirements for much of that area to ensure less growth -- and less pollution -- in the future.

The zoning change should ease much of the Occoquan runoff problem, said David W. Stroh, an environ mental expert at the county planning office. But developers and home owners are expected to challenge the new restrictions in court.

Fairfax actually contributes only about 17 percent of the phosphorous pollutants to the reservoir, while Fauquier and Prince William counties contribute about 76 percent. Most of the Fauquier and Prince William pollutants come from agricultural runoff, which is not as easily controlled as urban runoff. And because pressure for further development in Fairfax is expected to remain strong, Fairfax's Occoquan study concluded that the county should play a major and early role in environmental controls.

Barron L. Weand, associate director of the Occoquan Watershet Monitoring Laboratory in Manassas, said he has seen progress in the watershed. "In terms of water quality, in spite of the . . . treatment facility, the lake is still in an eutrophic state, which means there is excessive growth of algae, but the problems are not as severe as they were 10 years ago."

State ranking of lakes, required by federal law, was meant to trigger federal assistance to clean up a state's most polluted lakes. But a recent budget cut at the Environmental Protection Agency eliminated funding for new cleanup projects:

In an attempt to fill part of that void the Department of Agriculture's Soil Conservation Service has agreed to help Fairfax, Loudoun and Prince William counties mitigate flood and water quality problems in the Bull Run watershed, which contributes about half of the pollution and half of the water that runs into the Occoquan Reservoir.

The Occoquan's contaminants come from lawn fertilizers, pet excrement, oil drippings, trash on the street, fertilizers from gardens and farms, and herbicides, said Raymond E. Bowles, director of the Water Control Board's bureau of surveillance and field studies.

The high ranking of the Occoquan enabled the agency to assist the three counties in devising an affordable way to improve water quality and flood control in the Bull Run watershed, added the Soil Conservation Service's Bowie.

"We're not sure how much we can reduce the pollution in the Occoquan yet," he said, "but we could make a significant dent in the problem. The erosion rate is four to five tons of material a year per acre, and each ton carries with it a specific amount of phosphates and nitrogen. We feel we can cut that erosion rate in half, and cut the contamination rate in half."

The state's ranking of the Occoquan and 103 other lakes was based on a 100-point system that measured recreational uses such as fishing, boating and swimming that are impaired by pollution, the amount of pollutants and the transparency of the water, each Lake's regional significance and the local committment to its cleanup, and the percentage of population served in the area of each lake,

Stumpy lake in the Tidewater region ranked as the state's most polluted lake, receiving 74 points. The Occoquan was second with 66 points. Lake Accotink tied for fourth place on the pollution list with 62 points and lake Fairfax tied for ninth with 52 points.

Fairfax has a $56 million bond issue on the November ballot to improve county parks and clean up lake Accotink.