The federal government sued the Washington Suburban Sanitary Commission yesterday, charging that sewer overflows have caused millions of gallons of raw or partially treated sewage to flow into Maryland waterways.

The suit by the Environmental Protection Agency, filed in federal district court in Baltimore, alleges that the overflows pose "an imminent and substantial risk to human health," according to a summary of the suit obtained from a federal official.

The WSSC, one of the nation's largest water and sewer utilities, serves 1.6 million people in Montgomery and Prince George's counties.

The government is asking the court to issue an immediate injunction directing the WSSC to correct the problem and develop measures to prevent future overflows. An unspecified civil penalty from the WSSC is also requested in the suit, which the state of Maryland is expected to join as a co-plaintiff.

Liz Kalinowski, a WSSC spokesman, said agency officials have been expecting the suit and are in settlement talks with the federal government. Those talks have been ongoing for two years, she said.

"What they have done is expected and the next logical legal step," Kalinowski said, adding that she had not seen the lawsuit.

Officials at the U.S. Department of Justice, which is representing the EPA, declined to comment. But the summary says the suit is based on violations of federal law that have been occurring since March 1997.

A Washington Post review of WSSC records this year found that 130 million gallons of sewage had been released during the past two years.

According to the Natural Resources Defense Council, Maryland Department of the Environment records show that the WSSC had 445 overflows between January 2001 and July of this year. It is illegal under the Clean Water Act to discharge any amount of raw sewage.

The sewage, which escapes from the WSSC's network of 5,300 miles of wastewater collection pipes, pollutes the Anacostia and Patuxent rivers with human waste, putting nearby residents at increased risk of diseases such as cholera and hepatitis.

In September, several environmental groups, including the Natural Resources Defense Council, the Audubon Naturalist Society, Friends of Sligo Creek and the Anacostia Watershed Society, filed a 60-day notice of their intention to sue the WSSC for violating the Clean Water Act.

Yesterday's EPA action precludes the environmental groups from exercising their right to sue. Those groups, however, can intervene in the suit on behalf of their members.

"We are pleased that our threat to sue WSSC finally prompted the EPA to do its job to stop the sewer authority from allowing raw sewage to contaminate our streams, streets and parks," said Nancy Stoner, director of the Natural Resource Defense Council's Clean Water Project. "We will be watching closely to make sure that the government is truly serious about pursuing this case."

Kalinowski said the WSSC has begun trying to fix many of the problems that led to overflows, the biggest of which occurred last year after Hurricane Isabel.

The agency said it plans to spend $150 million over six years to upgrade its infrastructure and analyze overflow patterns. By comparison, it spent $40 million on those same programs from 1996 to 2001.

"Focusing on our wastewater system and continuing to improve it is a core of our business," Kalinowski said.