During the process of mining and cleaning coal, waste is created and must be permanently disposed of in an impoundment. Preparation of coal, also called washing, is how non-combustible materials are removed from the mine. As the coal is washed, waste is created and classified as either course refuse or fine refuse. Larger materials such as rocks and pieces of coal are defined as course refuse. Slurry, a combination of silt, dust, water, bits of coal and clay particles is considered fine refuse, and is the most commonly disposed of material held in an impoundment. Between 20 to 50 percent of the material received at a coal preparation plant may be rejected and housed in impoundments (National Research Council, 2002). The coarse refuse is used to construct the impoundment dam, which then holds the fine refuse or slurry, along with any chemicals used to wash and treat the coal at the coal preparation plant.

How is an impoundment constructed?
Whenever possible, impoundments are constructed using naturally occurring basins, but are often built up on an embankment at the mouth of a watershed. They are reinforced with course refuse and are characteristic of a typical dam. After the waste is spilled into the basin, the coal particles are allowed to settle, leaving the leftover water on top. This water is often recycled and used once again by the preparation plant. Settling ponds are constructed nearby to catch the runoff of excess water through a pumping system, and excess water from these ponds is discharged into a local waterway (Earth Science Applied to Coal Impoundment Monitoring).

Why are some impoundments considered hazardous?
An impoundment is a system of multiple parts, and thus any weakness in one of these parts affects the others. Therefore, an impoundment can fail in many ways. Embankment failure and dam construction are two major concerns. During the past decade, malfunctions of this nature have fueled better engineering and design of impoundments, but those built before then are more at risk for failure. Seepage, weakness in the walls, and undermining (in which an impoundment has been built a few feet above a mine, weakening the ground beneath it and causing it to fall through) are also major risks for failure. Breakthroughs into underground mineworkings have been the cause of more recent catastrophic failures.