The Fairfax County Water Authority reported yesterday that chloroform levels in its treated water are only half as high as those found in an earlier two-year study of the Occoquan watershed.

The water authority study, which lasted from Aug. 15 to Sept. 13, found that at the Occoquan water treatment plant the level of chloroform, a suspected human carcinogen, was almost within the limit that will be recommended shortly by the federal Environmental Protection Agency.

The average reading at the treatment plant during the 30-day period was 107 parts of chloroform per billion parts of water. The EPA's soon-to-be released proposed regulations will set a total limit of 100 parts per billion for chloroform, and other suspected carcinogens in the same chemical family.

The new study was done for the authority under the supervision of Dr. J. Carrell Morris, professor of sanitary chemistry at Harvard University. The older, two-year study was done by the Occoquan Watershed Monitoring Program of Virginia Polytechnic Institute and State University.

In an other development, the water supply committee appointed by the Fairfax County Board of Supervisors has released a study showing that through "stringent, permanent conservation." Northern Virginia's water needs could be met through the year 2010 without building any news reservoirs or enlarging the Occoquan Reservoir, currently the region's chief water supply.

The study, which rejects recent state projections of heavy demand in the years to come, says needs could be met by connecting the Occoquan to the water intake the water authority has proposed to build on the Potomac River.

Though the water authority's chloroform study reports levels only half as great as those of the earlier one, the chairman of the Occoquan monitoring program said he saw no actual contradiction.

Dr. Clifford W. Randall, chairman of the VPI monitoring program, said the study this summer was done during a period of low runoff into the reservoir. Chloroform, levels are high, he said, when runoff is greater.

Runoff this year has been so low that, in the absence of significant rainfall the reservoir could be unusable in about 30 days, based on current usage and normal evaporation.

Randall said runoff carries into the reservoir the humic acids - from leaves and other decaying vegetation - that scientists believe react with chlorine - a disinfectant - to produce chloroform. The EPA plans to set a limit on chloroform and related chemicals because of their possible connection in causing human cancers over a long period, even in minute concentrations of parts per billion.

Another likely producer of chloroform, according to Randall, is algae, vegetation that grows and then decays in the reservoir. The scientist said that because of the low runoff this summer, algae growth was not as great as it was last year and in 1975, when the VPI study was conducted.

The water authority itself, in commenting on the much lower chloroform concentrations in its study, said, "The reason for this difference is as yet undertermined."

The agency also said it "fully expects to be in compliance with the proposed EPA limit" on choloroform and related chemicals that are produced in the chlorine disinfection process. The would be "easily accomplished," it said, by applying chlorine to raw water at a later stage of treatment.

Dr. Robert Harris, director of the Environmental Defense Fund and an early critic of chloroform levels in the Occoquan, said, "I have a great deal of respect for Morris' abilities as a chemist but little or no respect for his abilities to asses the health implications of carcinogens in drinking water."

Harris said Morris has in the past discounted the role of chlorine in the creation of chloroform, as well as the importance of minute amounts of carcinogens in triggering cancers.

Morris could not be reached for comment.

With Morris' findings now available, VPI's Randall said "it would be wise to have a look at chloroform levels in the Occoquan under a variety of conditions" before drawing any conclusions about the apparent differences in the results of the new study and the one made by his lab.

The new findings will be presented by the authority's board when they hold a special meeting with the Fairfax supervisors Monday night.

The supervisors will have a heavy agenda Monday on water subjects. That same day they will also receive the study completed by the water supply committee they had appointed earlier this year.

If approved by the supervisors, it could become the major element of Northern Virginia's presentation at the "water summit" that will be held Sept. 30 at the Metropolitan Washington Council of Governments. The meeting has been called to see if the jurisdictions of Northern Virginia, the Maryland suburbs and the District can develop a regional approach to their now fragmented water-supply programs.

The Fairfax supply committee's projection, in stressing the need for stringent conservation year around, differ markedly from those recently made by the Northern Virginia office oft the State Water Control Board.

For example, the Fairfax committee says future per-capita demand can be held to the present 102 gallons per day, even anticipating new industrial and business needs that are included in that figure. But the state study saw per-capita demand continuing to rise - up to 163 gallons per day by the year 2020 - in future years.

Consequently, the state study saw the need for costly new reservoirs or other additional supply sources besides the Potomac to meet future needs. The Fairfax group's conclusions, if correct, would require less drastic solutions, both financially and environmentally.

The Fairfax group says its proposals fit in a regional approach to providing more water.