The Edisto River and other Lowcountry freshwater rivers have some of the worst levels of mercury contamination in fish in the country, a federal study released Wednesday shows.

Levels of mercury in largemouth bass swimming in the North Fork of the Edisto River rivaled those in fish found in polluted rivers next to Western gold mines, where mercury is a byproduct of the mining process, researchers with the U.S. Geological Survey found.

The study examined fish in 291 streams in the United States between 1998 and 2005. About a quarter of all the fish they sampled had mercury levels higher than the Environmental Protection Agency considers safe for human consumption.

The findings are no surprise to many state officials, fishermen and conservation groups who are well aware of South Carolina's long-standing mercury contamination problems.

But the study underscores the pervasive nature of mercury contamination in the nation's freshwater lakes, rivers and streams.

It also provides new ammunition for critics of coal-fired power plants, cement kilns and incinerators, the main sources of man-made mercury pollution.

Mercury, a potent neurotoxin linked to brain damage and other health problems, is one of the most serious contaminants threatening the nation's waters, the U.S. Geological Survey said in its study.

The agency's scientists examined fish in various types of rivers. The worst levels of mercury in fish were in a river polluted by a gold mine in Nevada. The next highest mercury levels were found in largemouth bass taken from the North Fork of the Edisto River near Fairview Crossroads. Researchers also found high levels in the Santee River.

Scientists said that the Carolinas, Georgia, Florida and Louisiana are particularly vulnerable to mercury problems because the states' blackwater rivers take elemental mercury and convert it to a more toxic form called methylmercury. Methylmercury is more readily absorbed by fish and other animals.

"This study shows just how widespread mercury pollution has become in our air, watersheds and many of our fish in freshwater streams," Secretary of the Interior Ken Salazar said in a statement. "This science sends a clear message that our country must continue to confront pollution, restore our nation's waterways, and protect the public from potential health dangers."

State leaders have grown increasingly concerned about mercury in South Carolina waters.

Two years ago, a Post and Courier series called "The Mercury Connection" identified mercury hotspots in South Carolina, including one around the Edisto River, and how people who frequently eat fish from these places have unusually high levels of mercury in their bodies.

The state Department of Health and Environmental Control later posted signs on some rivers warning people not to east certain fish, or to cut consumption of certain species to one or two servings per month. DHEC also is seeking public comments for new regulatory efforts to reduce mercury pollution.

Concerns about mercury contamination also have shaped the debate over Santee Cooper's plan to build a coal-fired power plant along the Great Pee Dee River. The plant site is in an area where fish have high levels of mercury.

The federal study dealt only with fish in freshwater waterways. Researchers said they found some level of mercury in every fish they sampled from 1998 through 2005.