Since April 1, when a 300-foot section of concrete wall collapsed at the huge Blue Plains treatment plant, the District of Columbia has been dumping 300 million gallons of partially treated sewage into the Potomac River each day.

The flow, while far exceeding federal standards for discharge into the river and potentially dangerous to those who come into contact with the bacteria it carries, has not caused a crisis.

Stopgap repairs are under way that should permit the city to resume bacteria-killing chlorination tomorrow, but repairs to the wall will delay a resumption of full treatment operations for an uncertain period.

The dumping of unchlorinated sewage into the river began when the 300-foot section of wall that formed a dike for the fluid collapsed, knocking out a vital section of the plant's electric power system.

John R. (Russ) Thomas, chief of wastewater treatment for the D.C. Department of Environmental Services, said levels of disease-causing fecal coliform bacteria going into the Potomac are averaging 20,000 parts in each 100 milliliters of fluid -- 100 times the amount permitted by Environmental Protection Agency requirements at Blue Plains. At the peak of the problem, Thomas said, the coliform level reached 170,000 parts in each 100 milliliters. A milliliter is slightly smaller than two-thirds of a cubic inch.

Meanwhile, EPA technicians will begin taking samples of water from the Potomac today to determine the extent to which the sewage is being diluted and how much harm has been done to the river.

Federal officials said last week that the main potential danger could come from body contact with the water, and that people can still eat fish caught in the river. Swimming is prohibited in that part of the river.

Blue Plains, located at the far southern tip of the District below Anacostia, is operated by the District government, processing sewage from the city and from parts of the Maryland and Virginia suburbs. A $500 million expansion of the plant, begun in 1971, is nearing completion.

The section of wall that collapsed April 1 is part of the expansion program. It formed a dike that channeled the fluid into a large tank for its fourth and last stage of treatment, which normally includes a final filtration of remaining solids.

Although it was in operation before the collapse, that final stage is not actually required by EPA until June, Thomas said. As a result of the collapse and the time needed for repairs, he said the District will be unable to meet the deadline.

William B. Johnson, D.C. director of environmental services, said repairs to the damaged section would cost an estimated $360,000 and replacement of the entire wall could cost $2 million.

Johnson said an effort was being made to fix blame for the collapse in order to decide who should pay for the repairs.