" The entire structure used
for coal slurry waste disposal, including the embankment, basin,
beach, pool, and slurry." (National
Research Council, 2002).
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.