The nation's beachgoers lost nearly 20,000 opportunities to go in the water last year because the beaches they frequent were too polluted for swimming, according to an analysis of EPA data released yesterday. Maryland, Virginia and Delaware beaches in 2004 were closed at a rate that was four times as high as in the previous year, the report's authors said.

The study, produced annually by the New York-based Natural Resources Defense Council, said more than one-third of U.S. coastal and Great Lakes beaches were closed or had an unsafe swimming advisory posted for at least one day last year. Most of the actions were prompted by tests that found fecal bacteria, a problem often associated with storm-water runoff or sewage overflows.

"The overall trends are negative," said Nancy Stoner, director of the council's Clean Water Project, who presented the report yesterday in the District. "Americans deserve better than to have to swim in human and animal waste."

The group's interpretation of the data is more pessimistic than the Environmental Protection Agency's assessment of the numbers, which it released Wednesday. The federal agency stressed that far more jurisdictions are monitoring their beaches than in previous years and that the number of lost days is relatively small.

"Only 4 percent of beach days were lost in 2004 due to advisories and closures triggered by monitoring for bacteria," said Benjamin H. Grumbles, assistant administrator for the EPA's Office of Water.

A closing or posted advisory on any day at any reporting beach counts toward a state's tally of lost beach days.

In Maryland, mostly along the Chesapeake Bay, there were 197 lost beach days last year due to closures or safety advisories, more than five times as many as the 2003 total of 39 days. The total for Virginia beaches was 186 days, at least twice as many as in the year before, study author Mark Dorfman said. Delaware beaches, a popular destination for Washington area vacationers, showed improvement: They tallied 19 days lost beach days last year, compared with 60 in 2003.

None of the region's largest beaches, Ocean City in Maryland or Rehoboth and Dewey in Delaware, had to shut down or post warnings last year because of contamination.

In 1991, the first year of the Natural Resources Defense Council study, the organization found that 2,500 beach days were lost to contamination. This year, the number is about 20,000. While reporting and monitoring have increased greatly over that period, the study's authors said, so have the sources of pollution.

"The more we look for pollution, the more we find," Stoner said. "Some beaches violate public health standards 20, 25, up to 50 percent of the time."

Pollution from storm runoff caused the majority of closing and advisory days last year, the council's study found, followed by sewage spills and overflows. Heavy rain, which overwhelms storm water and sewage systems, is blamed for most incidents. High levels of fecal bacteria can lead to a variety of symptoms, most commonly vomiting, diarrhea and fever.

In the EPA's news release, titled "Better Health at the Nation's Beaches," the federal agency said about one-quarter of reporting beaches had problems last year, a figure lower than what the Natural Resources Defense Council found.

In the case of Virginia, "we found out from [the state] that the EPA had incorrect information," said Dorfman, one of the study's authors. The advocacy group's study said the EPA had recorded no closings or advisories for the state's beaches in 2003.

In checking with the state, Dorfman said, he found that although Virginia did not record lost beach days as such in 2003, the number of "events" of contamination increased from four in 2003 to 35 in 2004, totaling 186 lost beach days reported to the EPA last year.

The EPA did not reply to a request for a response yesterday. But in a conference call Wednesday to discuss beach data, Grumbles said: "We look forward to reviewing their information . . . but we feel confident the data we've gathered from the states represents the most scientifically sound data to date."

The EPA said the number of beaches monitored has more than tripled since 1997 to nearly 3,600 in 2004.