Before coal is transported on trains, it is washed and processed. The leftover water from this is called coal sludge or coal slurry, which is stored in huge sludge ponds. The coal sludge contains water, coal dust, clay, and harmful metals including mercury, arsenic, and lead. when sludge ponds leak, the people who live nearby end up drinking toxic water. 

   Sometimes the dams holding the sludge in the toxic ponds break, resulting in millions of gallons of sludge dumped into valleys and streams. A sludge dam collapsed in Kentucky in 2000, when noxious sludge flowed into the streams that branch out from The Big Sandy River. The Environmental Protection Agency called this "The biggest environmental disaster ever in the southeastern United States." On December 22, 2008, a sludge dam broke and let loose a huge wall of muck from a power plant sludge pond in Tennessee. Twenty-six housed were destroyed, leaving families without a place to live. The Emory River was also polluted with sludge.