The Army Corps of Engineers and the Environmental Protection Agency have been accused by the Washington Area's federal planning agency of approving a huge sewage treatment plant just above Rock Creek Park without an adequate study of its impact on the historic park or the city.

Although the plant near Rockville has been operating for more than six months, the National Capital Planning Commission (NCPC) unanimously called upon the Corps of Engineers to review plans to build a permanent sewage pipeline that would dump the effluent into Rock Creek. A temporary pipeline now pumps the treated sewage into the creek at Southlawn Lane, near Rockville's Red Gate Golf Course.

The review is not expected to stop the 3 million gallons a day of highly treated sewage that the plant ultimately will put into Rock Creek because the corps says it has no power to review what goes into the creek-the effluent. The corps says it can only approve or disapprove the pipeline that carries it.

The District government, the National Park Service and the NCPC previously voiced concern that sewage from treatment plant, although treated to remove 98 percent of the pollutants, would further degrade Rock Creek waters. During dry summer months, the sewage effluent will make up more than 50 percent of the water in Rock Creek.This concerns officials because the permit approved by EPA last March 19 allows the plant to dump raw sewage into the creek in emergencies, such as a plant breakdown.

A brief, four-page environmental assessment released by the corps this spring, after almost two years of study and after the sewage plant already was in operation, was criticized by the NCPC staff as coming too late, being inadequate and filled with unsupported conclusions.

More than a year ago the corps was urged by NCPC, city officials and the park service to prepare a comprehensive environmental impact statement, which would have been required if the sewage plant had been built with federal funds. The plant was financed by a consortium of private developers to provide sewage treatment facilities for about 25,000 houses the developers want to build in the area. It is to be given to the Washington Suburban Sanitary Commission when the trial operating period ends in the next few weeks.

In its short envirommental assessment, the corps' Baltimore district engineer, Col. G.K. Withers, said he reviewed the requests and federal regulations and concluded that a detailed environmental statement "is not required."

The corps "has not studied this one at its highest level . . . what it has done is dump the whole problem of effleunt into the lap of EPA," said John Parsons, the National Park Service representative on the planning commission.

A further review would be conducted at a level higher than the corps' district section.

The NCPC staff, while highly critical of both EPA and the Corps of Engineers, recommended against requesting additional review by the corps. It would be a waste of time, staff officials said, because the corps insists it will not look at the pollution question, but only at the environmental consequences of the pipeline construction.

EPA, Maryland state authorities and officials of Prince George's and Montgomery counties have insisted the sewage effluent would be cleaner than Rock Creek's waters are now, because the stream has been badly polluted for more than a decade. WSSC officials said this week that since the plant opened last fall, all aspects of its operation have received "A" grades and the effluent has exceeded federal water quality standards for treated sewage.

The study by th corps was requested, in part, at the urging of District officials, who are suing the EPA over the treatment plant and have requested the corps to review its actions. The corps declined, but under federal regulations it must initiate a review if requested to do so by a sister federal agency. The suit to block construction of the plant was dismissed last year but is being appealed.The appeal now asks that the plant be barred from operating.

A major criticism of the project has been that EPA and Maryland officials approved it before the park service, the District, NCPC and the public were aware that 3 million gallons of treated sewage would be pumped daily into Rock Creek, the federal government's oldest and largest urban park.