Coal slurry is a mixture of finely crushed coal and water, typically consisting of 60 to 75% coal, 25 to 40% water, and about 1% chemical additives. It’s created either intentionally, as a way to transport coal through pipelines, or as a waste byproduct of cleaning and processing coal at mining operations. In both cases, the result is a thick, black liquid that poses significant logistical and environmental challenges.
What Coal Slurry Contains
The coal in slurry is ground into fine particles, sometimes as small as grains of sand or smaller. Water makes up the remaining bulk, and a small fraction of chemical additives called dispersants keep the particles from clumping together. Without these additives, the solid particles would settle and aggregate, clogging pipes or forming dense sludge. Even with dispersants, the mixture is considered thermodynamically unstable, meaning it naturally wants to separate over time.
The chemical makeup of coal slurry depends on the source coal. Contaminants present in the original coal, including heavy metals like arsenic, mercury, and selenium, end up suspended in the slurry along with everything else. This is one reason disposal is such a concern.
How Coal Slurry Is Used and Transported
One primary use of coal slurry is pipeline transport. Pumping coal as a liquid through a pipeline can move large volumes over long distances without railcars or trucks. The only coal slurry pipeline that operated in the United States was the 273-mile Black Mesa pipeline, running from a mine in northeastern Arizona to a power plant in Nevada. It carried roughly 4.8 million tons of coal per year and ran continuously from 1970 until it was shut down in 2005.
Pipeline transport requires substantial water. The Black Mesa pipeline drew about 3,200 acre-feet of water annually from underground aquifers. Proposed longer pipelines from Kentucky to Florida were estimated to need 50,000 to 80,000 acre-feet per year. Pumping stations are required every 50 to 100 miles to maintain the flow velocity needed to prevent the coal from settling and blocking the pipe. Each station occupies several acres and needs additional water storage nearby for emergencies.
Compared to other coal-based energy processes, slurry pipelines actually use relatively little water. Coal gasification requires about three times as much water per unit of energy delivered, and mine-mouth power plants use roughly eight times as much. But compared to rail transport, which needs negligible water, the consumption is significant, especially in arid regions.
Where Coal Slurry Gets Stored
The far more common form of coal slurry in the United States is waste slurry, the leftover material from washing and processing coal at mines. This slurry has to go somewhere, and the main options are surface impoundments, slurry cells, and underground injection.
Surface impoundments are essentially large ponds held back by dams, often built across valleys or on hillsides using the coarse leftover rock (called refuse) from mining itself. The Mine Safety and Health Administration classifies these structures by their potential hazard: facilities where a failure could kill people receive the most conservative design requirements, including the ability to withstand a probable maximum flood. Designs vary based on geography. Cross-valley dams span a valley to create a reservoir behind them, while diked impoundments use constructed walls on flatter terrain.
Underground injection sends slurry into abandoned mine tunnels below the surface, essentially filling old mine voids with waste. This avoids the risks of surface dams but raises concerns about groundwater contamination, since the slurry sits in direct contact with underground rock formations and water sources.
Major Disasters Linked to Coal Slurry
The dangers of storing billions of gallons of coal slurry behind earthen dams have been demonstrated catastrophically more than once. In February 1972, three coal waste dams on Buffalo Creek in West Virginia failed in rapid succession. The collapse of the uppermost dam sent millions of gallons crashing into the two dams below, and all three gave way within minutes. A wall of nearly 130 million gallons of water and slurry, estimated at 300 to 400 acre-feet in total volume, tore down the valley at roughly 20 feet per second. The flood killed 125 people and injured more than 1,100.
Nearly three decades later, an even larger spill occurred. On October 11, 2000, a 72-acre impoundment operated by Martin County Coal Corporation in Inez, Kentucky, released an estimated 250 million gallons of coal slurry. The bottom of the impoundment broke through into abandoned mine tunnels below, and the slurry poured out through the old mine openings into local waterways. The spill contaminated more than 75 miles of streams and rivers, including tributaries of the Ohio River. It was roughly 25 times the volume of oil released in the Exxon Valdez disaster, though it received far less national attention.
Environmental and Health Concerns
Coal slurry contaminates water in two main ways: through catastrophic failures like the ones described above, and through slower, chronic leaching from impoundments and underground injection sites. The heavy metals and chemical compounds in slurry can seep into groundwater over years or decades, potentially reaching wells that communities depend on for drinking water.
Communities near coal slurry impoundments and injection sites in Appalachia have long raised concerns about water quality. Residents in parts of West Virginia and Kentucky have reported discolored tap water and unusual health problems in areas where slurry was injected underground. The full scope of groundwater contamination from these practices remains difficult to measure because underground water movement is complex and monitoring has historically been limited.
How Coal Slurry Is Regulated
Coal slurry impoundments fall under the jurisdiction of the Mine Safety and Health Administration, which sets structural and inspection requirements for dams at mining operations. Separately, the EPA regulates coal combustion residuals, the ash and waste left over when coal is burned at power plants, which can include slurry-like waste stored in surface impoundments.
In 2015, the EPA finalized national regulations requiring technical standards to prevent contaminants from leaking into groundwater, escaping as airborne dust, or causing dam failures. Facilities must conduct inspections, monitor conditions, and post compliance data on publicly accessible websites. In May 2024, the EPA extended similar requirements to “legacy” impoundments at inactive power plants, facilities that had previously escaped regulation. Owners of these legacy sites must complete facility evaluations by early 2027 and 2028 under the new deadlines.
Repurposing Coal Waste
Some companies are exploring ways to turn coal waste into useful materials rather than simply storing it. One approach involves processing waste coal ash from landfills into lightweight aggregate, a material used in concrete production. A startup called SusMaX, funded by the National Science Foundation, has developed technology to manufacture this aggregate from coal ash regardless of its chemical composition, which had previously been a barrier. The resulting material has a higher absorption capacity than traditional aggregate, which could reduce the amount of cement needed in concrete mixes and improve the finished product’s durability through a process called internal curing. These efforts are still relatively small-scale compared to the hundreds of millions of tons of coal waste already stored across the country.