A regional blueprint to clean up the Potomac River and keep it clean for the next 20 years has been heavily criticized at public hearings and its even losing support among public officials who approved it.

What has attracted the most opposition and misgivings is the plan's insistence that the main element of the river cleanup be construction of a big regional treatment plant at Dickerson in upper Montgomery County.

At public hearings last week in the District, suburban Maryland and Northern Virginia, there was almost unanimous criticism of Dickerson for being too costly (the estimated price is approaching $500 million), wasteful of energy (sewage would have to be pumped for 24 miles uphill) and hazardous to health (effluent would be discharged above the area's water intakes on the Potomac.

While most of the criticism came from well-organized environmentalists, who packed the hearings, one of the toughest comments was made by Thomas M. Schwarberg Jr., director of the Northern Virginia office of the State Water Control Board. In a statement studded with underlined words and exclamation points, Schwarberg said:

"I cannot overemphasize!! The Commonwealth of Virginia will not participate financially, in any way, in the Dickerson project."

Dickerson has already been vetoed - on the basis of cost - by the federal Environmental Protection Agency, although that the decision is being appealed by Maryland in federal court.

While some of Dickerson's supporters feel that long-range planning should offer an alternative to the plant, the 20-year plan, drawn up by the water resources planning board of the Metropolitan Washington Council of Government, says Dickerson is the only way to go. The water planning board's decision was a testimonial to the forcefulness of one of Dickerson's biggest supporters, Montgomery County Executive James P. Gleason. Gleason insisted that any plan to clean up the river start with the construction of Dickerson.

Though water experts hired by COG devoted a good part of their two-year, $3.5 million study to the study of alternatives, the planning board stuck by Dickerson.

However, in view of the negative reception Dickerson got at last week's hearings, members of the COG water board said they are having doubts about maintaining Dickerson as an essential element of the plan.

Charles J. Vincent, Prince William County's representative who voted for Dickerson the first time, said after the hearing in the District: "I don't think I could vote to support Dickerson."

Ben W. Gilbert, the District's planning director, said: "At this point it looks very doubtful that the majority is holding fast . . . We (the District) would not be upset at all if there was an alternative to Dickerson."

The water boar will take a second look at its 20-year plan tomorrow. A shift of two votes could wipe out the necessary two-thirds majority that Dickerson got the first time.

If the water planning board does choose to go beyond the Dickerson-or-nothing approach, the technically and financially practical alternatives would be difficult politically.

Expansion of the Pitscataway plant in Prince George's is one alternative, but that would be opposed by Prince George's officials who do not want their county to become the "sewage capital of the metropolitan area."

Another alternative is land treatment, where sewage is processed on the soil rather than in out-of-sight plants. But residents of areas that have been considered for land treatment already have risen in opposition at public hearings.

The 20-year plan is not just a paper exercise. Under the federal clean-water act, metropolitan Washington must draw up a document telling how the still-polluted Potomac will be cleaned up. The document, which must be approved by EPA, has to specify what new treatment facilities will be built to handle future needs - thus the importance of Dickerson or an alternative.

If the cleanup does not succeed, environmentalists say, the Potomac could be a dead river within 20 years - unfit for the aquatic life and unsafe for recreation.

Most of the criticism of the plan came from the citizens advisory committee. Like the decision-making water resources planning board, the committee was set up to be representative of the region. But environmental activists controlled the board, and their ideas lprevailed.

The citizens group, led by Rhea Cohen, a Sterra Club official who represented the city of Greenbelt, said the 20-year draft plan failed on every important consideration.

In place of Dickerson, the citizens group proposed that the 20-year plan find a place for land treatment, the process where sewage is returned to the soil rather than discharged into water.

The group also called for more comprehensive and stricter regulations to control agricultural and urban runoff, which is being recognized as a major contributor of water pollution.