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Chemical Spills: What is Coal Ash?


A plume of exhaust extends from the Mitchell Power Station, a coal-fired power plant built along the Monongahela River
Jeff Swensen/Getty Images News/Getty Images


In December 2008 in Harriman, Tennessee, a dam next to a coal plant broke, releasing about 1 billion gallons of industrial waste into the Clinch and Emory Rivers and damaging homes. It was the largest toxic spill in United States history. What was in the 84-acre holding pond? Coal ash produced at the Tennessee Valley Authority owned coal plant. Major coal ash spills had occurred before, notably in Martin County, Kentucky, where in 2000 an impoundment failed and sent 306 million gallons of coal ash in nearby rivers. More recently, in February 2014, a burst pipe leaked 82 tons of coal ash in the Dan River, in North Carolina.

What is Coal Ash?

The various combustion products resulting from burning coal are referred to collectively as coal ash. Two important components are fly ash, which is a fine combustion product that rises with the hot gases emitted when burning coal, and bottom ash, which is what’s left behind. A third component is boiler slag, a molten ash residue found in coal boilers. The composition of coal ash varies depending on the type of geological setting the coal was mined from. Silicon dioxide and calcium oxide make up a large proportion of coal ash, with smaller amounts of various elements such as arsenic, boron, cadmium, chromium, copper, lead, selenium, and zinc.

Most of the 140 million tons of coal ash produced in the United States annually comes from coal-fired power plants. About 45% of all the coal ash produced is put to commercial use. It is used most importantly in the production of concrete and other construction materials. The remaining 55% of coal ash is considered waste.

What are the Environmental Risks Associated with Coal Ash?

  • Dry storage of coal ash presents air quality degradation risks. Dry coal ash is deposited into landfills in proximity to coal plants, and winds can whip up the ash and deposit it in surrounding neighborhoods. Many elements presents in the ash, like arsenic, selenium, and manganese, are toxic at high enough concentrations. Members of the Moapa Paiute Indian Reservation in Nevada, located near the Reid Gardner Power Station and its dry storage landfill, are exposed to windblown coal ash. They have reported numerous health effects, including respiratory disease, due to this exposure.
  • Water contamination by coal ash sludge has been reported numerous times, with negative effects on aquatic life and drinking water. Again here it is the heavy metals present in the coal ash that are the problem. Elevated concentrations of arsenic have been found in coal ash contaminated water. Arsenic is a known carcinogen. In Chester, West Virginia, a 1700-acre impoundment known as the Little Blue Run has been leaking in the neighborhoods for years.  Arsenic laced water seeps in basements, saturates the soil, and contaminates ground water. FirstEnergy, the operator of Little Blue Run, announced the end of operations at the site for 2016.
  • Coal ash does often contain radioactive elements such as thorium and uranium, but the US Geological Survey considers the radioactivity risks to human health below the threshold of concern.


American Coal Ash Association. Accessed 7 February, 2014. About Coal Ash.

Fox News. Accessed 7 February, 2014. Toxic Coal Sludge Pollutes KY Town 10 Years Later.

Pittsburgh Post-Gazette. Accessed 11 February, 2014. W. Va. Residents file suit over Little Blue Run.

United States Environmental Protection Agency. Accessed 7 February, 2014. Coal Fly Ash, Bottom Ash and Boiler Slag.

United States Geological Survey. Accessed 7 February, 2014. Radioactive Elements in Coal and Fly Ash:
Abundance, Forms, and Environmental Significance

Sierra Club. Accessed 11 February, 2014. Sierra Club and Moapa Band of Paiutes Sue NV Energy for Cleanup of Contaminated Reid Gardner Site.

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