A long-standing federal goal for reducing algal pollution in the Potomac River appears impossible to reach, a new government study has concluded.

Algae, tiny nuisance plants that float in the water, are considered objectionable partly because they may deplete oxygen needed by fish and other river life, especially at night. Algae may also reduce light necessary for other river plants to grow and may make the water too murky and slimy for swimming. Excessive algae may form unsightly, smelly mats on the river's surface.

The study by the Metropolitan Washington Council of Governments found, however, that the goal set in 1971 for drastically decreasing algae in the Potomac probably cannot be achieved even if county and city governments carry out the most stringent and costly cleanup measures yet conceived.

This finding is one indication of a wide-ranging reevaluation now under way by environmental researchers and governmental officials, who are seeking to revise their strategy for the Potomac. They hope to cut multimillion-dollar costs and jettison impractical aims.

"The atmosphere has changed," said Stuart A. Freudberg, a water resources engineer for the Council of Governments who is overseeing the regional planning group's multimillion-dollar computer study of the Potomac. In the past, officials virtually disregarded costs in their drive to rejuvenate the river. Now, Freudberg noted, government leaders are seeking to spend as little as possible without causing needless environmental harm.

Stringent sewage treatment aims, once viewed as essential to revitalizing the river, are being challenged as likely to prove largely ineffective and financially wasteful. Prospects are mounting that expensive antipollution equipment may be left to stand idle, at least during some months every year, to avoid high costs of using it. Whether Washington's waterfront will ever be safe enough for swimming remains a controversial issue.

"How much is enough?" asked Deputy Arlington County Manager Anthony H. Griffin. "The water is not that much cleaner than it would be if the standards were relaxed a little bit." Added Fairfax County Executive J. Hamilton Lambert "You're speaking of megabucks."

In a controversial report early this year, the General Accounting Office, Congress' auditing arm, charged that the Potomac cleanup had "cost much more than necessary and achieved much less than desired."

In addition to questioning the algal pollution goal, the COG study also has provided tentative evidence indicating that one long-debated and extremely costly cleanup measure may be unnecessary. The technique, advocated by environmental groups, would require removal of nitrogen from the discharges of sewage treatment plants. Local governments have opted, instead, for phosphorus removal, a markedly less costly measure. The COG report supports phosphorus removal.

Estimates prepared for District of Columbia officials several years ago indicated that building a nitrogen-removal facility for the large Blue Plains sewage-treatment plant would cost $100 million and that operating expenses would amount to $25 million yearly. Today, these costs would be higher, officials note.

The COG study also raised the possibility that sewage-treatment standards might be relaxed during winter months without significantly increasing pollution. A similar loosening of waste treatment requirements may be possible in summer if the water level in the river is high, the report said. Such moves would save local governments millions of dollars.

Any easing of sewage treatment standards appears likely to draw opposition from environmental activists, who have long denounced the cleanup program as misguided. Nitrogen removal was a central issue in a research study prepared for the Environmental Defense Fund several years ago. In calling for nitrogen extraction, the group said it had established "irrefutable evidence that the phosphate-removal strategy is failing and that it will continue to fail."

Another argument offered by critics is that apparent improvements in the Potomac in recent years may merely reflect higher water levels during the 1970s than in the 1960s. "That is a fact that masks the underlying problems in the river," said Michael T. Janik, a lawyer for the Montgomery Environmental Coalition, a group currently pressing for stricter sewage treatment standards in court.

The COG review of the algae goal illustrates the complexity of the issues. The technique used to estimate the level of algae in the river is to measure the amount of chlorophyll, a green pigment found in plants. The goal set for the Potomac by the Environmental Protection Agency would allow no more than 25 micrograms, or millionths of a gram, of chlorophyll in a liter of water.

The COG study found, however, that the 25 microgram goal would amount to restoring the river to its pristine state centuries ago before Americans inhabited the Potomac River basin. The COG study showed that the most stringent chlorophyll goal that may be achieved today -- at least during key times of hot summer weather when river levels are low -- would be around 50 to 75 micrograms.

This finding, in turn, raised further questions that researchers are still exploring. Recent measurements of the Potomac found chlorophyll readings around 100 micrograms per liter near Washington. To reduce the chlorophyll content to the 75 microgram range would probably cost millions of dollars annually, researchers say. Would such huge spending yield significant benefits for fish and other aquatic life or make the river noticeably more suitable for swimming? COG water resources engineer Freudberg says the answer is not yet known.