Coal ash is the toxic waste formed from the burning of coal in coal-fired power plants — the solid waste left behind when coal is burned.

Coal ash looks like gray dust.

It contains high levels of toxic compounds, such as heavy metals, that can and do cause cancer and other health problems.

Also referred to as coal combustion residuals, or CCRs, coal ash includes a number of by-products produced from burning coal, including:

Fly Ash, a very fine, powdery material composed mostly of silica made from the burning of finely ground coal in a boiler.

Fly Ash

Bottom Ash, a coarse, angular ash particle that is too large to be carried up into the smoke stacks so it forms in the bottom of the coal furnace.

Bottom Ash

According to Earthjustice, coal ash is the second largest industrial waste stream in the country.

The EPA issued a final rule on coal ash disposal on December 14, 2014 in accordance with a court order. Under the rule, coal ash will be treated as a non-hazardous waste under Subtitle D of RCRA with certain minimum requirements for storage in both new and existing sites. Coal ash is not, therefore, subject to federal protections, and state laws governing coal combustion waste disposal are usually weak or non-existent.

Far more common than a dam break is leaching of contaminants from ponds and landfills: the process by which toxic materials in coal ash dissolve in water and percolate through the earth.

The dissolved toxics, called “leachate,” can endanger public health and the environment by contaminating surface water or groundwater used for drinking supplies.

Leaching may be less spectacular than a rupture, but it happens with much greater frequency and may continue to release toxic substances into the environment for decades.

Landfills are dangerous to ground water, especially when they are not properly lined or the lining becomes damaged. Stormwater can seep through the dry ash, collecting toxins as it percolates down to the ground and into the water table.

Dry coal ash can be dispersed by wind and blown into neighboring plots of land as “fugitive dust”; this carcinogen-laden dust can be a major health concern when inhaled, affecting the lungs and bronchia.

According to the U.S. EPA, an estimated 36 percent of the coal ash waste generated by utilities in 2007 was disposed of in dry landfills.