The District of Columbia's antiquated storm water and sewer system poses a "serious public health threat" by regularly flushing raw sewage into the Anacostia and Potomac rivers and Rock Creek after heavy rains, a new study by a consultant has concluded.

Untreated sewage is diverted into the Anacostia, the most polluted of the three streams, an average of 85 times a year, according to the report prepared for the D.C. Department of Environmental Services. Sewage is pumped into the Potomac an average of 60 times a year and into Rock Creek 17 times a year.

O'Brien & Gere Engineers Inc., the firm that prepared the study, urged the city to undertake a massive $70 million, two-stage construction program to sharply reduce the episodes of water pollution.

The frequent sewage discharges have destroyed fish and other aquatic life in the Anacostia and discouraged recreational activity along the Potomac and Rock Creek, the study says.

"All three streams are aesthetically affected by the combined sewer overflows because of the release of unsightly debris," according to the 218-page report. " . . . Overflow debris is visible floating on the water surface or stranded along the river bank."

About 35 percent of the city is served by an outmoded, double-duty sewer system that carries both waste and rainwater that drains from roofs, yards and streets.

In dry weather, all of the waste in the sewers is collected by interceptors and conveyed to the city-operated Blue Plains sewage treatment plant in Southwest Washington. But in rainy periods, such as now, when the overall flow greatly exceeds the capacity of the interceptors, the city is licensed by the federal government to allow discharges of sewage at 60 locations along the Potomac and Anacostia rivers and Rock Creek.

"It's the major water-quality problem remaining in the area," said Edward Hopkins, research director for the Clean Water Action Project, a national group lobbying to improve water quality. "It's not at all unique to Washington. This is what's happening in older cities that have combined sewer systems."

William Johnson, the District's director of environmental services, said yesterday he was concerned about the problems outlined in the report, especially the pollution and resultant depletion of dissolved oxygen in the Anacostia. He described the Anacostia as a "filthy dirty" river.

However, Johnson hinted that the consultant's $70 million proposal, including the expansion in capacity of existing sewers and the construction of three satellite treatment facilities along the Anacostia near RFK Stadium, might prove to be too expensive.

"Obviously, we've got some hesitancy about these large expenditures," Johnson said. "We haven't even come to grips on that at all."

The consultant's report said that the city might be able to obtain 75 percent federal funding for the project if the U.S. Environmental Protection Agency (EPA) approves a grant application before Oct. 1, 1984.

The report recommended that the District cover its share of the cost of the 13-year construction project by increasing residential sewer user fees by an average of $3.25 a year.

The Department of Environmental Services will hold a public hearing on the consultant's proposal at 7 p.m. on July 13 in the D.C. City Council chambers, on the fifth floor of the District Building.

Federal and local authorities have waged costly efforts during the past 20 years to clean up waterways in the Washington metropolitan area, including upgrading the regional treatment facilities at Blue Plains.

The newer sections of Washington and the Maryland and Virginia suburbs have separate storm water and sewage systems. However, a significant volume of sanitary sewage continues to escape treatment because of the overflow from the outdated portion of the city in older areas of the District.

Hopkins, of the Clean Water Action Project, said that some cities have begun to experiment with low-cost flow-management systems to deal with the problem, instead of launching major construction projects like the one proposed by the District's consultant.

He noted that Congress voted in 1981 to delete funding from the Clean Water Act to finance costly projects to deal with combined-sewer overflow problems. "There are methods available that are less capital intensive that can be used to correct the problem," Hopkins said.

The consultant's report to the District was funded by the EPA as part of an overall "Potomac Strategy" for cleanup of the area's major surface water resources.