Coal ash is the waste left over after coal is burned to generate electricity. It contains a mix ofite particles, heavy metals, and other residues that don’t combust during the process. Power plants in the United States have produced roughly 3.1 billion tons of it since the mid-1970s, and about 65% of that total has been dumped in landfills or stored in large ponds rather than recycled.
The Four Types of Coal Ash
Not all coal ash is the same. The combustion process creates four distinct byproducts, each collected at a different point in the power plant.
- Fly ash is the finest type, a powdery material made mostly of silica. It’s light enough to rise with exhaust gases and gets captured by filters in the smokestack before it can escape into the atmosphere. Fly ash makes up the largest share of coal combustion waste.
- Bottom ash consists of coarser, angular particles too heavy to travel up the smokestack. These collect at the bottom of the furnace.
- Boiler slag forms when bottom ash melts inside certain furnace types and is then cooled with water. The rapid cooling produces smooth, glassy pellets.
- Flue gas desulfurization material is a byproduct of scrubbing sulfur dioxide out of smokestack emissions. It can be a wet sludge or a dry powder, composed mainly of calcium-based compounds.
What’s Actually in Coal Ash
The concern with coal ash isn’t theite particles themselves. It’s the heavy metals concentrated inside them. When coal burns, organic material combusts away, but metals and minerals don’t. They become more concentrated in the leftover ash than they were in the original coal.
Fly ash tends to concentrate metals more than other types. In one analysis comparing raw coal to its fly ash, arsenic levels jumped from about 39 mg/kg in coal to 119 mg/kg in fly ash, roughly a threefold increase. Lead went from about 11 mg/kg to 40 mg/kg. Mercury more than doubled, from 1.3 mg/kg to 3.0 mg/kg. Other metals present in significant concentrations include chromium, nickel, zinc, vanadium, and manganese. All of these become potential contaminants when coal ash is stored improperly or comes into contact with water.
How Coal Ash Is Stored
Coal ash ends up in one of two places: surface impoundments (essentially large ponds) or landfills. The United States has more than 735 active surface impoundments, averaging over 50 acres each with depths around 20 feet. These hold ash mixed with water in a slurry. There are also more than 310 active landfills for dry disposal, averaging over 120 acres and about 40 feet deep.
Surface impoundments carry the greater risk. When their earthen walls aren’t properly constructed or maintained, they can leak contaminants into groundwater, release dust into the air, or fail catastrophically. The structural integrity of these ponds depends on factors like soil stability, flood capacity, and regular safety assessments. If a pond can’t meet minimum safety thresholds, federal rules now require it to close.
The Kingston Disaster
The risks of coal ash storage became impossible to ignore on December 22, 2008, when a 40-acre containment pond at the Tennessee Valley Authority’s Kingston Fossil Plant in eastern Tennessee collapsed. The failure released more than 5.4 million cubic yards of fly ash into the surrounding valley and tributaries of the Tennessee River. It was the largest industrial spill of its kind in U.S. history.
Between 2009 and 2015, the TVA spent approximately $1.1 billion on cleanup and invested additional funds in community recovery, economic development, and long-term environmental monitoring. The spill also had lasting consequences for the workers hired to clean it up, many of whom later reported serious health problems. Kingston became a turning point in how the federal government regulates coal ash.
Health Risks for Nearby Communities
Coal ash stored in open-air landfills and ponds can release fine particles into the surrounding air and leach metals into groundwater. People living near these sites face exposure through breathing contaminated dust, drinking contaminated water, or contact with soil where ash has settled.
A cross-sectional study comparing adults living near a coal ash storage facility to those living farther away found significantly higher rates of respiratory problems in the exposed group. Residents near the facility were more than five times as likely to report chronic cough, about four times as likely to experience hoarseness, and roughly two and a half times as likely to report shortness of breath. Respiratory infections were also nearly twice as common. Overall self-reported health was significantly worse among people living closer to the site.
The particles in coal ash are small enough to penetrate deep into the lungs, and the metals they carry, particularly arsenic, chromium, and lead, are associated with a range of chronic health effects beyond the respiratory system when exposure continues over time.
How Coal Ash Gets Recycled
About 35% of all coal ash produced in the U.S. between 1974 and 2021 was put to some beneficial use rather than dumped. The most common application is in concrete manufacturing. Fly ash contains reactive silica that acts as a binding agent, and many concrete plants already mix it into their products as a partial substitute for traditional Portland cement. When coal ash is locked inside concrete, the heavy metals are encapsulated and largely prevented from leaching into the environment.
Newer approaches are pushing this further. One process pulls carbon dioxide from the air and combines it with coal ash through a chemical reaction, producing a cement substitute that both sequesters carbon and repurposes waste. The resulting product reportedly contains more reactive silica than raw fly ash alone, giving it stronger binding properties in concrete.
Other recycled uses include road base material, structural fill, and wallboard manufacturing. The key distinction is between encapsulated uses (like concrete, where metals are sealed in) and unencapsulated uses (like spreading ash as fill material, where metals can still leach into soil and water). Encapsulated applications are generally considered far safer.
Federal Regulation of Coal Ash
For decades, coal ash fell into a regulatory gap. It wasn’t classified as hazardous waste, and there were no federal standards for how it had to be stored or disposed of. That changed in 2015, when the EPA finalized the first national regulations for coal ash disposal. The rule set requirements for groundwater monitoring, structural safety at impoundments, dust control, and closure of sites that failed to meet standards.
A significant loophole remained, however. The 2015 rule didn’t cover inactive ponds at facilities that had already stopped generating electricity. A federal court struck down that exemption in 2018, and on May 8, 2024, the EPA finalized a rule extending protections to these “legacy” sites. Owners and operators of legacy coal ash ponds now face deadlines to comply with groundwater monitoring, corrective action, closure, and post-closure care requirements.
The 2024 rule also created a new category called CCR management units, applying groundwater monitoring and cleanup obligations to any location where coal ash was placed at a regulated facility, regardless of when it was deposited. This closed another gap that had left older, informal dump sites unregulated even at active power plants.