HARRIMAN, Tenn - Environmentalists worry the ash-laden sludge that coated a Tennessee neighborhood when a power plant dike burst could pose a health risk, although initial tests by a public utility company have shown no threat to drinking water.

Crews were expected to work through the holiday weekend to contain the aftermath of Monday's breach at the coal-fired Kingston power plant, run by the nation's largest public utility, about 50 miles west of Knoxville.

Officials at the Tennessee Valley Authority have said preliminary tests suggest that there is no danger to millions of people who get their drinking water from the 652-mile Tennessee River.

And TVA spokesman Gil Francis said crews were cleaning up the sludge.

"The cleanup is making progress," Francis said Thursday, adding that the group was moving from the road to other areas. TVA brought in 30 pieces of equipment and more than 100 workers for the effort, which will take four to six weeks to complete, he said.

A TVA news release Wednesday said there was no threat to the environment from the breach at the plant near Harriman along the Emory River, which joins the Clinch River and flows into the Tennessee River.

But environmentalists have sharply criticized TVA for what they say was something completely avoidable. Hundreds of fish were floating dead downstream from the plant Tuesday, and state and federal agencies have yet to complete water-quality testing. Environmental Protection Agency spokeswoman Laura Niles said some toxic metals could be in the muck, including mercury and arsenic.

Lisa Evans, a Massachusetts attorney for the environmental group Earthjustice, said similar spills have happened in Pennsylvania and Georgia, and she blamed the industry and a lack of federal regulation.

"The saddest thing is this is entirely avoidable," Evans said. "These people in these communities don't have to be in harm's way. This is not some complicated problem like nuclear waste. This is something the utilities know how to do."

Lawyer Chandra Taylor of the Southern Environmental Law Center estimated the sludge could have filled 798 Olympic-size swimming pools.

"The holiday disaster shows that there really isn't such a thing as a clean coal plant," Taylor said.

Francis has said the fish may have died from the freezing cold that contributed to the breach, not pollutants.

TVA officials say six inches of rain in 10 days and overnight temperatures in the teens contributed to the rupture of the dike on a retention pond, releasing about 2 million cubic yards of ash, water and mud that covered as much as 400 acres. Three homes were destroyed and several others damaged. No one was seriously injured.

TVA officials were working with the EPA and state environmental officials to determine exactly what caused the flood.