The U.S. Environmental Protection Agency has agreed to advance most of the $1.5 million needed to rebuild a concrete wall at the District of Columbia's Blue Plains sewage treatment plant that collapsed last month, causing partially treated sewage to pour into the Potomac River, the head of the city's sanitary engineering agency said yesterday.

William B. Johnson, director of the D.C. Department of Environmental Services, told the City Council's environmental affairs committee he is studying reports from two consulting firms hired to help fix the blame for the collapse. But he refused to disclose who -- the wall's designer or its builder, or a combination of both -- may have been responsible.

Johnson said current plans call for replacing the entire 1,900-foot wall, which formed a dike channeling the flow of partially treated sewage between two treatment basins, and not jut repairing the 300-foot section that collapsed.

The job will take about six months, and it has not been decided when work will begin, Johnson said.

EPA will advance 75 percent, about $1.2 million, of the funds needed for the reconstruction, Johnson said. The District, which owns and operates Blue Plains, and the Maryland and Virginia suburbs, which pipe some of their sewage there for treatment, will pay the balance. EPA would expect to recover the funds after the District fixes the blame and collects damages, Johnson said.

The collapse caused the District to dump 300 million gallons of unchlorinated sewage into the river each day between April 1 and last Wednesday, when stopgap repairs were completed. Johnson said treated sewage now going into the river meets EPA standards for bacteria content.

Johnson said the collapse and the time needed for repairs will force a postponment of EPA's June deadline for more intensive treatment of Blue Plains sewage. "They're being very understanding -- they know it's not our fault," Johnson said.