The U.S. Army Corps of Engineers has decided to review the environmental impact of the sewage treatment plant now under construction by private developers near Rockville, which daily will dump 3 million gallons of treated sewage into Rock Creek and Rock Creek Park. The $9 million plant is scheduled to be completed next fall.

The National Park Service and the National Capital planning Commission have asked the Corps to hold public hearings and do a comprehensive environmental review of the sewage plant because of the effect it may have on Rock Creek Park, one of the first federal parks in the United States. Rock Creek itself already is badly polluted and is subject to frequent flooding, having been heavily damaged in Tropical Storm Agnes in 1972 and Hurricane Eloise in 1975.

Construction of the plant will continue during the Corps of Engineers review, although one Corps official said last week "the developers are taking a $9 million gamble we will approve it."

Neither the Park Service nor NCPC, the planning agency which safeguards the federal interest in Washington, was informed of the sewage plant's approval by Maryland and Montgomery County officials and knew nothing of it until after construction was begun last spring, according to spokesmen for the two federal agencies. They say they learned of the plant from the Corps of Engineers, which only last fall acquired jurisdiction over Rock Creek under legislation that makes it responsible for the tributaries of navigable waterways like the Potomac.

District officials also said they were unaware of the Rock Creek Interim Advance Sewage Treatment Plant, as it is officially called, apparently for much the same reason: it is a private project although once completed it will be given to the county to operate and was not required to go through the same environmental review process required whenever federal funds are used. Nor, apparently, was there any requirement to inform or seek the opinion of those downstream.

John D. Brink, the chief of the city's bureau of air and water quality, said "we were never informed and we have a concern about it. We have just made a preliminary review and it looks like the plant might not contravene water quality standards in Rock Creek, but there is no question that it will degrade the water quality, though maybe not below the minimum standards."

Brink said any lessening of water quality in Rock Creek would appear to be a violation of federal law, which prohibits degredation of waters "considered an outstanding natural resource, such as waters of national and state parks, and waters of exceptional recreational or ecological significance."

The major natural park area in the nation's capital, and one of the country's first federal parks, created in 1890. Rock Creek would "almost certainly fall into that category," according to local park Service spokesman George Berklacy. The Park Service is asking for a comprehensive review of the effect 3 million gallons a day of effluent will have on Rock Creek, particularly in the summer when treated sewage would constitute a major portion of the stream's water.

Rock Creek has run as low as only 300,000 gallons a day and averages a summertime flow of only 1.3 million gallons a day once every 10 years, at the city's guaging station near Sherrill Drive, according to Brink.

In the Rockville area near Southlawn Lane and Avery Road, where the plant's pipes would empty into Rock Creek - the exact location has not been determined - the 3 million gallons a day would more than double even the average summer stream flow of 2.4 million gallons a day.

No Park Service, NCPC or District officials have said so far that they oppose the plant, only that it could have a significant effect on the quantity and quality of water in Rock Creek and that such a major project should not be built without full publicity, public hearings and a comprehensive study of its effects. They claim this has not been done. The developers disagree.

The quantity of water will have a negligible effect on flooding, says Howard Wilson, assistant chief of the Maryland Water Resources Administration municipal and agricultural waste section, which approved the project, since the 3 million gallons a day will represent less than 1 per cent of the stream's flood level as it passes through Rock Creek Park in the District.

As for the quality of Rock Creek's water, that may actually improve, says Wilson, because the plant will be one of the most advanced designs and its effluent "will be of a somewhat higher quality than the background water" already in the stream."

"If we learn this effluent will have any detrimental effect on the District's waters - or that we made any omissions or errors in reviewing it then we can revoke or revise our permit," Wilson said.

Charles Dalrymple, attorney for the constorium of private developers that is constructing the $9 million treatment plant, said this week "there may have been some problem in who got notified . . . and in the degree of notice . . . but that is a state water resource's administration problem. As far as the developers are concerned we're not fighting the Corps of Engineers review because we feel we having nothing to hide."

David G. Sobers, director of Montgomery County's office of environmental planning called the Rock Creek plant "a vital part of our interim sewer program and there is no question we are concerned with a challenge that could result in its not being built.

"I feel bad that the District and federal agencies didn't know about this . . . and in our bureaucratic relationship this sometimes happens. But the plant will be 9-10 miles above the D.C. line and its greatest impact will be on county residents. Nonetheless I think the District and federal agencies deserve a review and hope and feel confident they will find it will not be deleterious to the waters of Rock Creek," Sobers said.

Dairymple said the plant itself will use "the most advanced treatment there is, and produce effluent a heck of a lot purer than any effluent dumped in any water around the Washington area" by existing sewage treatment plants.

"This kind of effluent is so pure that some public officials have drunk it to demonstrate how good it is," said Dairymple. However, the District's chief sewage expert Levesque says "I'd like to know what public official has been doing that. I wouldn't advise anybody to drink it. The tertiary treatment that this plant and Blue Plains will use will chlorinate the effluent and remove 98 per cent of the suspended particles . . . but not 100 per cent. It isn't pure."

The extent of the Corps of Engineers review has not yet been determined, said James Durkey, chief of the river basin unit for this area. In addition to the hearing, expected to be scheduled sometime in Novemebr, the Corps will do a comprehensive environmental impact statement or a shorter impact assessment.