( 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 do n't want to drink it . It does n'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 ca n'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 ? ''
Wall breach sends sludge downhill , causing damage to 15 homes . Spokesman says 400 acres are coated by more than 500 million gallons of sludge . Area affected is bigger than Exxon Valdez oil spill , spokesman says . Footage shows dead fish , but sludge ca n't be deemed toxic until tests are done .