The Natural Resources Defense Council, which compiled the report from U.S. Environmental Protection Agency data, said the likely culprit was sewage and contaminated runoff from water treatment systems.

"Aging and poorly designed sewage and storm water systems hold much of the blame for beach water pollution," the group said in a statement. "The problem was compounded by record rainfall, which added to the strain on already overloaded infrastructure."

Other factors include urban sprawl in coastal areas, which destroys wetlands and other natural buffers like dunes and beach grass that could otherwise filter out pollution, the group said.

In its 17th annual report on beach water quality, the group found the number of no-swim days at 3,500 U.S. beaches doubled from 2005 to 2006 along the oceans, bays and Great Lakes.

The beaches at highest risk are those that are most popular, close to pollution sources or both. Of those high-risk beaches, Ohio, Indiana, Illinois, Rhode Island and Minnesota ranked lowest for failing to meet national health standards.

The report is available online at (www.nrdc.org/water/oceans/ttw/titinx.asp).