To save the Occoquan Reservoir, Northern Virginia's main source of drinking water, nearly $80 million has been spent on an ultra-sophisticated, fail-safe sewage treatment plant. Now that investment is being questioned.

"Knowing what I know now," engineer Clifford W. Randall told the Fairfax County Board of Supervisors this week, "I would have gone to a lesser degree of treatment, and taken some money and put it in storm-water runoff control."

Randall was alluding to the findings of the Occoquan monitoring program that he has headed since its creation 4 1/2 years ago. According to those findings, most of the pollution in the reservoir is caused not by poorly treated sewage - the problem the new plant is supposed to start correcting when it opens next year - but by runoff from farmland and urbanized areas within the watershed. Few measures have been undertaken to control runoff, partly because of the focus on sewage cleanup.

Runoff, Randall told the Fairfax supervisors Monday night, accounts for 93 per cent of the phosphorus, 85 per cent of the nitrogen and 98 per cent of the sediment found in the reservoir.

Besides generally fouling the reservoir, these pollutants have been linked, directly and indirectly, to the high levels of chloroform - a suspected human carcinogen - found in the treated water that is drunk by 600,000 Northern Virginians.

Chloroform is created when chlorine, used to disinfect raw water, reacts with organic matter in the water.

Scientists also think - though they have not yet proved conclusively - that chloroform is created when chlorine reacts with decaying algae. The algae are created by organic matter primarily nitrogen that enters the reservoir.

While Randall said hindsight indicates less emphasis on sewage treatment and more on runoff control, that view is disputed by engineer and environmentalist Noman M. Cole Jr. As head of the State Water Control Board in the early 1970s, Cole was instrumental in securing the regional agreement for construction of an advanced sewage treatment plant on Bull Runn that is designed to purify most of the sewage entering the reservoir.

"I haven't heard anything that changes my mind," Coles said yesterday. "I can call on other scientists who will say just the opposite (of what Randall said)."

Cole said that nitrogen entering the reservoir from runoff does not trigger growth of algae blooms with the same explosive force as nitrogen from sewage effluent.

The new treatment plant, when it is fully operational in the fall of 1978, is expected to remove almost all sewage-borne nitrogen.

At their meeting Monday, some of the Fairfax supervisors were critical of Prince William County for what they claimed was an ineffective attempt to control urban runoff.

"We're trying to protect our own area near the reservoir by keeping development out, but Prince William has not done this," said Supervisor Joseph Alexander (D-Lee).

Again, after critics of the chloroform concentrations in the Occoquan made statements before the board about health hazards, Alexander said, "I strongly suggest you take your message to Prince William and Loudoun. It's they who need the coming of the messiah."

In Prince William, heavy development is taking place virtually up to the shoreline of the Occoquan. The Lake Ridge subdivision of 10-to-the acre town houses and quarter-acre detached houses already has 2,500 units, with about 6,500 more to be built.

Joseph Colgan, director of the Prince William office of comprehensive planning, says the count can't legally halt or modify Lake Ridge. Controls elsewhere - such as limiting density - will be difficult, he said, because of Virginia's legal tradition of emphasizing the rights of the property owner.

However, he said, the county attorney's office is trying to develop legally acceptable controls that would implement some of the recommendations made by Prince William task force on the runoff problem.

Adam L. Crist, a U.S. Department of Agriculture conservationist, said Fairfax's strip of parkland buffer on its side of the Occoquan is not going to protect the reservoir from most runoff on the Fairfax sides Pollutants, he said, can be carried 20 to 30 miles down streams and then deposited in the reservoir.

After hearing a number of witnesses on the chloroform problem at the Occoquan, the Fairfax supervisors passed a motion saying "urban runoff must be reduced substantially." But they took most of the teeth out of the resolution, offered by Supervisor Audrey Moore (D-Annandale), by deleting her language requiring that new policies to control runoff be enacted before the advanced treatment plant accepts sewage from more sewer hokup.

Mrs. Moore had put in the proviso because new hookups will mean more development, which, in turn, will create more urban runoff.