Coal combustion residuals are the leftover materials produced when coal is burned to generate electricity, and they represent one of the largest industrial waste streams in the world. Often called coal ash, CCRs include fly ash (the fine particles captured from flue gases), bottom ash (the heavier material that collects at the base of a boiler), boiler slag, and flue gas desulfurization materials. The United States alone generates well over 100 million tons of CCRs annually, and how this waste is stored, disposed of, and sometimes reused has become a serious environmental and public health concern. The story of coal ash is more layered than most people realize, touching everything from groundwater contamination and childhood health to rare earth element recovery and the future of concrete.
What Coal Ash Actually Contains
Coal itself carries trace amounts of dozens of metals and metalloids, and burning it concentrates those elements in the ash. Iron and titanium tend to dominate, followed by strontium, vanadium, zinc, manganese, and chromium, with smaller amounts of nickel, arsenic, copper, cobalt, lead, and mercury.1PubMed Central. Assessment of the Enrichment of Heavy Metals in Coal and Its Combustion Residues The concentrations of many of these elements jump sharply from coal to fly ash. One study of Chinese coal-fired plants measured arsenic at roughly 0.6 to 4 mg/kg in the feed coal, but 5 to 26 mg/kg in the resulting fly ash; chromium went from about 9–13 mg/kg in coal to 43–65 mg/kg in fly ash; and barium rose from 109–229 mg/kg to 777–971 mg/kg.2Fuel. Chemical speciation and leaching characteristics of hazardous trace elements in coal and fly ash from coal-fired power plants This enrichment happens because organic material in coal burns away, leaving the metals behind in a much smaller volume of mineral residue.
Not all of those metals sit locked inside the ash in stable forms. Their chemical speciation, meaning the particular mineral or chemical state they exist in, determines whether they can dissolve into water. Arsenic and manganese in fly ash, for example, can have high proportions in water-soluble and acid-soluble forms, which makes them especially prone to leaching when water flows through ash storage sites.3Fuel. Chemical speciation and leaching characteristics of hazardous trace elements in coal and fly ash from coal-fired power plants Mercury and chromium, by contrast, tend to be locked up in residual mineral phases that resist dissolving. This distinction matters enormously once the ash is sitting in a pond or landfill exposed to rain and groundwater.
How Coal Ash Contaminates Water
For decades, the most common disposal method was to mix coal ash with water and pump the slurry into unlined surface impoundments, essentially open ponds. Research across the southeastern United States has shown that these ponds routinely leak. A study examining sites across Tennessee, Kentucky, Georgia, Virginia, and North Carolina found that boron and strontium exceeded background levels at nearly every site tested. Out of 165 monitoring wells examined, 65 showed elevated boron, and 49 had concentrations of other CCR-linked contaminants above background. In surface water samples from those same areas, contamination above drinking water or ecological standards appeared in ten out of 24 samples.4PubMed. Evidence for Coal Ash Ponds Leaking in the Southeastern United States The researchers used isotopic fingerprints of boron and strontium to confirm that the contamination was specifically coming from coal ash rather than natural mineral sources.
The contamination does not stay at the pond’s edge. A study near an unlined ash pond in India found heavy metal pollution was significant within about 5 kilometers, moderate out to 7 kilometers, and low but still detectable at 15 kilometers. Lead concentrations in groundwater ranged from 0.14 to 0.41 mg/L and iron from 0.20 to 7.21 mg/L, both well above levels considered safe for drinking water in many regulatory frameworks.5Applied Water Science. Assessment of heavy metal pollution in groundwater with respect to distance from ash pond by using heavy metal evaluation index What makes unlined ponds particularly problematic is that contaminants percolate downward through soil over years or decades, creating plumes that can be expensive and slow to remediate even after the ash itself is removed or capped.
Selenium and the Damage to Fish
Among all the trace elements in coal ash, selenium has drawn the most attention for its harm to aquatic ecosystems. Selenium is an essential nutrient in tiny doses but becomes toxic at slightly higher concentrations, and it bioaccumulates through the food chain, meaning organisms at the top end up with far more than what is in the water itself. At Herrington Lake in Kentucky, where a coal-fired power plant discharged coal ash wastewater, selenium concentrations in water, sediment, and aquatic organisms reached two to nine times the level considered toxic for fish reproduction and survival. About 12% of juvenile largemouth bass examined had spinal or craniofacial malformations consistent with selenium poisoning, a rate roughly 25 times the natural background. Total selenium-induced mortality, including early life-stage losses, was estimated to exceed 25% per year.6PubMed. Selenium poisoning of fish by coal ash wastewater in Herrington Lake, Kentucky
The pattern repeats elsewhere. In the Pigeon River and Pigeon Lake system in Canada, which received seepage from a coal fly ash disposal facility, selenium concentrations in fish downstream were significantly higher than in upstream control sites. The accumulation varied by species and was not simply a function of where a fish sat in the food chain; habitat preferences mattered too, with open-water species accumulating more selenium than bottom-dwelling ones. While selenium in fish at that site did not reach levels considered dangerous for human consumption, it did exceed thresholds known to harm birds and mammals that eat those fish.7Ecotoxicology and Environmental Safety. Selenium bioaccumulation and hazards in a fish community affected by coal fly ash effluent The wildlife toll from coal ash contamination therefore extends well beyond the fish themselves.
Health Effects on Nearby Communities
Establishing a direct link between coal ash exposure and human disease is harder than documenting what happens to fish, but accumulating evidence is raising serious concerns, especially for children. A study of children living near coal ash storage facilities found that after adjusting for secondhand smoke, age, and sex, those children had roughly four times the odds of having allergies (excluding asthma) compared to unexposed children, more than three and a half times the odds of ADHD, and nearly six times the odds of gastrointestinal problems. They also had higher rates of sleep disturbances, including difficulty falling asleep and frequent night awakenings.8PubMed Central. Health of Children Living Near Coal Ash
Separate research confirmed that the exposure pathway is plausible. Nail samples from 263 children living within ten miles of coal ash facilities in Kentucky showed elevated concentrations of several neurotoxic metals, including chromium, manganese, and zinc, at levels higher than those reported in studies of both healthy and environmentally exposed children elsewhere. Iron, aluminum, and silicon, the signature metals in fly ash, clustered together in the nail samples and correlated with each other, a pattern consistent with fly ash as the common source. Proximity to power plants was associated with higher levels of fly ash metals in children’s nails.9PubMed Central. Nail Samples of Children Living near Coal Ash Storage Facilities Suggest Fly Ash Exposure and Elevated Concentrations of Metal(loid)s
There is also emerging evidence of behavioral effects. A study found that roughly 43% of children living near coal-fired power plants had detectable levels of fly ash in their homes. Those children had about 2.7 times higher odds of clinically significant externalizing behavioral problems compared to unexposed children.10PubMed Central. Externalizing behaviors in children living near coal-fired power plants These studies are observational, which means they cannot definitively prove that coal ash caused the health problems, but the combination of plausible exposure pathways, dose-response patterns, and consistent findings across multiple studies makes the association hard to dismiss.
The Radioactivity Question
Coal contains naturally occurring radioactive elements, primarily uranium-238, thorium-232, radium-226, and potassium-40. Burning coal concentrates these radionuclides in the ash just as it concentrates heavy metals. A study of a coal-fired plant in Bangladesh found that CCRs contained roughly three to three and a half times more uranium-238, radium-226, and thorium-232 than the feed coal. Fly ash was somewhat more radioactive than bottom ash, with the ratio between them ranging from about 1.4 to 1.6. Several radiological hazard indices for the CCRs exceeded thresholds recommended by international radiation safety bodies.11PubMed. Assessment of natural radioactivity in coals and coal combustion residues from a coal-based thermoelectric plant in Bangladesh
The radioactivity levels vary enormously depending on the coal source. Measurements from Greek coal-fired plants found uranium-238 in fly ash ranging from 263 to 950 Bq/kg and radium-226 from 142 to 605 Bq/kg.12PubMed. Escaping radioactivity from coal-fired power plants (CPPs) due to coal burning and the associated hazards: a review For context, typical soils worldwide contain around 30–35 Bq/kg of uranium-238 and similar levels of radium-226, so some coal ashes run well above natural background. Modern coal plants with better emission controls release far less radioactivity than older facilities. Still, the radioactivity issue becomes relevant when fly ash is reused in construction materials like concrete, where it can contribute to indoor radon levels, or when large volumes of ash sit in open impoundments where workers and neighbors may be exposed to low-level radiation over decades.
Who Lives Near Coal Ash
The question of where coal ash gets stored is not just a technical one. A nationwide analysis of census tracts found that areas in the highest quartile of poverty had more than twice the odds of hosting one or more coal ash disposal pits compared to less impoverished areas. The relationship with race turned out to be more complex than some environmental justice advocates expected: census tracts with the highest proportions of non-white residents were actually far less likely to have coal ash pits than the whitest tracts, suggesting that the environmental burden falls disproportionately on poor rural and small-town white communities rather than following the pattern seen with other industrial waste sites.13PubMed Central. Association Between Census Tract-Level Poverty and Non-White Race with Location of Coal Ash Disposal Pits in the United States The researchers noted that this pattern reflects the geography of coal-fired power generation, which historically concentrated in Appalachia, the Midwest, and parts of the Southeast. The poverty link, however, is clear and consistent, reinforcing the broader principle that communities with less political power tend to absorb more environmental risk.
Beneficial Reuse in Construction and Agriculture
Not all coal ash goes to a disposal site. A substantial fraction is recycled into other products, and in some applications the reuse genuinely solves two problems at once. Fly ash has been used as a partial replacement for Portland cement in concrete for decades. The silica and alumina in fly ash react with calcium hydroxide released during cement hydration, forming additional binding compounds that can improve concrete’s long-term strength and durability while cutting the carbon footprint of cement production. Road base, structural fill, and wallboard production are other common construction-related uses.
In agriculture, flue gas desulfurization (FGD) gypsum, the calcium sulfate material captured from smokestack scrubbers, has shown promise as a soil amendment. It can lower the pH of alkaline and sodic soils, reduce sodium levels, and improve conditions for plant growth.14PubMed Central. Influence of Flue Gas Desulfurization Gypsum Amendments on Heavy Metal Distribution in Reclaimed Sodic Soils In one study on reclaiming coal mine soil, FGD gypsum reduced heavy metal toxicity enough to support growth of camelina, a bioenergy crop, and extended the plant’s vegetative period compared to normal agricultural soil. Interestingly, when lime was used instead of FGD gypsum for the same purpose, mercury accumulated in the seeds at 0.34 mg/kg, whereas no detectable mercury appeared in seeds from the FGD gypsum-treated soil.15PubMed. Flue gas desulfurization gypsum as a sustainable amendment for coal mine soil reclamation and camelina-based bioenergy crop production
Reuse is not risk-free, though. FGD gypsum can carry arsenic and mercury, and some studies show a positive correlation between the amount of FGD gypsum applied and the concentrations of these elements in the treated soil and its leachate.16PubMed Central. Influence of Flue Gas Desulfurization Gypsum Amendments on Heavy Metal Distribution in Reclaimed Sodic Soils The practical question is always whether the benefits of reuse outweigh the risk of introducing new contaminants, and the answer depends on the specific ash chemistry, the application rate, and the receiving environment.
Mining Coal Ash for Rare Earth Elements
One of the more unexpected chapters in the coal ash story involves rare earth elements. These 17 metals are critical components of electronics, magnets, batteries, and defense technologies, and global supply chains are dominated by a small number of countries. Coal ash, it turns out, contains meaningful concentrations of rare earths, and researchers have been working to figure out whether extracting them is economically and technically feasible. Ashes from Appalachian Basin coals tend to have the highest total rare earth concentrations of any U.S. coal source, but the elements are trapped inside a glassy aluminosilicate matrix that resists simple acid leaching. Recovery from those ashes has required a two-step approach: first dissolving the glass with a strong alkali, then leaching the freed metals with dilute acid, which can recover up to about 85% of the rare earths present. Ashes from Powder River Basin coals in the western U.S. are easier to process, with near-complete recovery using straightforward acid leaching, though they start with lower total concentrations.17International Journal of Coal Geology. Aqueous acid and alkaline extraction of rare earth elements from coal combustion ash
The appeal is obvious: reprocessing an existing waste stream for valuable materials while reducing the volume that needs long-term storage. The challenge is scaling laboratory methods to industrial volumes at costs competitive with conventional rare earth mining. Work continues on newer extraction approaches, including solvent-based methods using deep eutectic solvents and ionic liquids, as well as membrane-based separation techniques.18Journal of Environmental Chemical Engineering. Extraction and separation of rare earth elements from coal and coal fly ash Several pilot projects are underway in the United States, driven partly by federal interest in reducing dependence on foreign rare earth supplies.
Impoundment Failures and Structural Risk
Coal ash impoundments are essentially earthen dams holding back millions of tons of wet ash, and when they fail, the consequences can be catastrophic. The most notorious example in the United States was the 2008 breach at the Tennessee Valley Authority’s Kingston Fossil Plant, which released over a billion gallons of ash slurry across 300 acres, damaging homes and contaminating the Emory River. But that was not a one-off event; the structural integrity of ash dikes depends on their design, the materials they are built from, and how well water seepage through the structure is managed. Analysis of different construction methods shows that safety improves substantially when dikes include adequate drainage systems, and that the method of construction (whether ash is piled upstream, along the centerline, or downstream of the original dam wall) significantly affects stability.19Advances in Transdisciplinary Engineering. Stability Analysis of Slope Induced with Coal Ash Dykes
The risk of failure is not evenly distributed. Many older impoundments were built to minimal engineering standards, and some have been accumulating ash for decades without comprehensive structural evaluations. Federal regulations adopted in 2015 established groundwater monitoring, structural stability requirements, and closure timelines for coal ash impoundments, but enforcement and compliance have varied. Many utilities are now in the process of closing surface impoundments, either by dewatering and capping the ash in place or by excavating it and moving it to lined landfills. Each approach carries trade-offs in cost, engineering complexity, and residual risk.
Cleaning Up Contaminated Water with Wetlands and Zeolites
Where contamination has already spread, remediation options exist but require patience. Constructed wetlands have proven surprisingly effective at treating coal ash leachate over long time horizons. A wetland built at a coal ash site in Alabama was studied after ten years of operation and was still removing up to 99% of iron and 91% of cadmium from contaminated water passing through its vegetated cells, dominated by cattail and soft rush. Removal rates for zinc, sulfur, boron, and manganese were lower but still meaningful, ranging from about 50% to 63%.20PubMed. Trace element removal from coal ash leachate by a 10-year-old constructed wetland Lab-scale experiments have corroborated this: emergent aquatic plants exposed to fly ash leachate reduced elevated concentrations of boron, molybdenum, and manganese in the water over time, with boron and molybdenum accumulating in plant tissue.21PubMed Central. Influences of Coal Ash Leachates and Emergent Macrophytes on Water Quality in Wetland Microcosms
Another approach repurposes fly ash itself as a remediation tool. Researchers have synthesized zeolites, highly porous aluminosilicate minerals, from coal fly ash. These synthetic zeolites can be used in permeable reactive barriers placed underground in the path of a contaminated groundwater plume, absorbing heavy metals and certain organic pollutants as the water flows through.22Applied Clay Science. Reactive barriers with fly ash zeolites for in situ groundwater remediation The same zeolite synthesis process has shown high removal efficiency for strontium and cesium from water, which could be relevant not just for coal ash contamination but for nuclear-related cleanup as well.23The University of Western Australia Research Repository. Utilization of coal fly ash for zeolite synthesis and geopolymer production There is a certain poetic logic to using the waste material to treat the contamination it created, though the economics and scalability of fly ash zeolite production are still being worked out.
Geopolymers and the Next Generation of Ash Reuse
Beyond conventional concrete and soil amendment, researchers are exploring geopolymers, a class of binding materials made by activating aluminosilicate-rich waste (like fly ash) with an alkaline solution. Geopolymers can substitute for Portland cement in certain applications and have the advantage of curing at relatively low temperatures. One research group developed a process that uses solar radiation to accelerate the curing step, potentially making geopolymer production cheaper and more energy-efficient.24The University of Western Australia Research Repository. Utilization of coal fly ash for zeolite synthesis and geopolymer production Geopolymer concrete can also encapsulate heavy metals within its matrix, reducing the leachability of contaminants that would otherwise pose risks in a disposal setting. If these materials reach commercial scale, they could absorb large volumes of fly ash that currently sit in storage, though quality control is trickier than with standard concrete because the chemical composition of fly ash varies so much from plant to plant and even load to load.
The broader trajectory for coal combustion residuals is a slow transition from seeing them purely as waste toward treating them as a resource, while simultaneously dealing with the legacy contamination from decades of less careful disposal. That transition involves engineering, chemistry, public health surveillance, environmental regulation, and community engagement all at once. For the millions of people living near existing coal ash sites, the pace of that transition matters more than the eventual destination.