(CNN) -- A wall holding back 80 acres of sludge from a coal plant in central Tennessee broke this week, spilling more than 500 million gallons of waste into the surrounding area.

Environmental Protection Agency officials are on the scene and expect the cleanup to to take four to six weeks.

The sludge, a byproduct of ash from coal combustion, was contained at a retention site at the Tennessee Valley Authority's power plant in Kingston, about 40 miles east of Knoxville, agency officials said.

The retention wall breached early Monday, sending the sludge downhill and damaging 15 homes. All the residents were evacuated, and three homes were deemed uninhabitable, a TVA spokesman told CNN.

The plant sits on a tributary of the Tennessee River called the Clinch River.

"We deeply regret that a retention wall for ash containment at our Kingston Fossil Plant failed, resulting in an ash slide and damage to nearby homes," TVA said in a statement released Tuesday.

TVA spokesman Gil Francis told CNN that up to 400 acres of land had been coated by the sludge, a bigger area than the 1989 Exxon Valdez oil spill.

Video footage showed sludge as high as 6 feet, burying porches and garage doors. The slide also downed nearby power lines, though the TVA said power had been restored to the area.

Francis said Environmental Protection Agency officials were on the scene and estimated the cleanup could take four to six weeks.

Some of the goop spilled into the tributary, but preliminary water quality tests show that the drinking water at a nearby treatment plant meets standards.

"I don't want to drink it. It doesn't look healthy to me," Jody Miles, who fishes in the Clinch River, told CNN affiliate WBIR. "Do you reckon they can bring all this life back that's going to die from all this mess?"

Still, there is the potential for more sludge to enter the water supply through waste runoff.

"We're taking steps to stabilize runoff from this incident," Francis said.

Although video from the scene shows dead fish on the banks of the tributary, he said that "in terms of toxicity, until an analysis comes in, you can't call it toxic."

One environmental attorney called that statement "irresponsible." The ash that gives sludge its thick, pudding-like consistency in this case is known as fly ash, which results from the combustion of coal.

Fly ash contains concentrated amounts of mercury, arsenic and benzine, said Chandra Taylor, staff attorney for the Southern Environmental Law Center.

"These things are naturally occurring, but they concentrate in the burning process and the residual is more toxic than it starts," she told CNN.

Appalachian environmentalists compared the mess with another spill eight years ago in eastern Kentucky, where the bottom of a coal sludge impoundment owned by Massey Energy broke into an abandoned underground mine, oozing more than 300 million gallons of coal waste into tributaries.

The water supply for more than 25,000 residents was contaminated, and aquatic life in the area perished. It took months to clean up the spill.

"If the estimates are correct, this spill is one and a half times bigger," said Dave Cooper, an environmental advocate with the Mountaintop Removal Road Show, a traveling program that explains the effect of an extreme form of mining.

While the full scope of the TVA spill is being determined, coal critics are already concerned about its long-term effects.

Cleaning up the mess, which could fill nearly 800 Olympic-size swimming pools, could take months or years, Taylor said.

"We're very concerned about how long it's going to take" to clean the spill, she told CNN.

Cooper agreed, saying, "It's 4, 5 feet deep. How are you going to scoop it up? Where are you going to put it?"

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