The little-publicized sewage treatment plant the Navy is proposing to build at the National Naval Medical Center in Bethesda could pour another 70,000 gallons of treated sewage a day into already polluted Rock Creek as it flows through Rock Creek Park.

A much larger sewage treatment plant under construction in Rockville by private developers, but undergoing review by the U.S. Army Corps of Engineers, would put 3 million gallons of treated sewage a day into Rock Creek beginning next fall. This would mean that in summer months more than half the water in the Montgomery County portion of the stream would be treated sewage effluent, because average summertime stream flow now is 2.4 million gallons a day.

Navy officials said last week that they may never need to build their Bethesda plant because they are conserving water and using much less than their present county sewage allotment. But they are nonetheless seeking approval for the new plant in case Montgomery County's sewer moratorium continues and additional sewage capacity is needed for the Navy's new Uniformed Services University. The university is under construction on the medical center grounds on Wisconsin Avenue, opposite the National Institutes of Health.

Even if they build the sewage plant, Navy officials estimate that it would produce only 31,000 gallons of effluent a day, with the remaining 39,000 gallons merely reserve capacity. Both the Bethesda and the Rockville plants would give sewage so-called tertiary treatment, which removes about 98 per cent of the pollutants. They would be the most advanced sewage treatment facilities in the Washington area.

The Environmental Protection Agency is receiving public comment on the Navy's proposed sewage plant this month - until Oct. 29 - but so far has received none, according to officials in EPA's regional office in Philadelphia. No public hearing is scheduled on the proposal, but the EPA administrator could call one "if there's sufficient public outcry," an EPA spokesman said.

The Rockville plant, although approved by EPA, and state and county agencies, is being reviewed by the U.S. Army Corps of Engineers, under new federal laws that give the Corps responsibility for the tributaries of the nation's navigable waterways. A public hearing will be held on that proposal Nov. 18 at 7:30 p.m. in Rockville's Thomas Wootton High School.

The Corps of Engineers is not reviewing the Bethesda plant because it would discharge into Stoney Creek, an offshoot of Rock Creek which the Corps says has less than a 5 cubic feet per second flow. The Corps does not review construction or discharges in streams below 5 cfs. The Navy's own environmental review of its plant's effect on the stream estimates its flow at 5-10 cfs.

The District's chief water and sewer officials said they were unaware of the large Rockville plant until this summer - months after the plant had been approved by EPA and Maryland officials - and did not know of the proposed Bethesda sewage plant until a reporter called them last week.

The Rockville plant is about 10 miles above the District line. The proposed Bethesda plant is about two miles from the city limits.

John D. Brink, chief of the city's bureau of air and water quality, this week accused EPA of "making one hell of a switch on Rock Creek. The federal and District governments have been working to clean up the stream ever since I came here in 1950. It's been a sensitive issue, and EPA only a few years ago put out a big color brochure on Rock Creek and how it would soon be clean enough to swim it. Now they're approving sewage treatment plants on it."

An EPA spokeswoman said she could not remember such a brochure but in general EPA was pleased with the two plants because they will provide tertiary treatment of sewage, which is more advanced than federal law now requires and which will "reduce to a minimum the effects of the discharge into Rock Creek."

Brink and other city officials have complained that the District was neither informed nor consulted about the plants, that the plants will dump a significant quantity of sewage effluent into Washington's oldest and largest park and that the effluent, no matter how well treated, will further degrade the quality of the stream's water. Founded in 1890, Rock Creek Park is the major natural area and open space in the nation's capital.

A 1975 federal law requires states to develop anti-degradation policies, designed to keep the nation's waters from further deterioration. Such policies must prohibit any degradation of "high quality waters considered an outstanding natural resource, such as waters of natural and state parks, and waters of exceptional recreational or ecological significance."

Maryland has had such an anti-degradation law since at least 1974 but state Water Resources officials say they believe it doesn't apply to Rock Creek because it is not a high-quality water.

The National Park Service, which protects the District sections of Rock Creek Park, was unaware of the large Rockville sewage plant and has requested a thorough environmental review by the Corps of Engineers on its possible effect on Rock Creek. On the Bethesda plant, local Park Service spokesman George Berklacy said, "The information we have does not show that there will be any significant degradation of Rock Creek, but we are reviewing it."

Attorneys for the Park Service say, however, that there appears to be little they can do about the plants even if they oppose them. "The lawyers say it's up to EPA to decide whether the plants will degrade the stream," Berklacy said.

Those wishing to make written comments on the Navy's sewage plant may write to the Environmental Protection Agency, Region III, 6th and Walnut Streets, Philadelphia, 19106, ATTN: Don Knott.