Alternatives for Future Coal Waste Disposal - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 142

Underground injection also requires MSHA's approval. Depending on the
state, this can be handled independently or through a joint approval process.
Finally, underground injection of slurry requires approval by the regional EPA
office, whose primary concern is groundwater quality. States may interpret the
requirements of the Safe Drinking Water Act differently— for example, West
Virginia limits the injection of any slurry from circuits that employ petroleum
products, such as those used in froth flotation.
In addition to obtaining permission from the surface owners to install
injection wells, the operator must obtain the right-of-way for the pipeline to
deliver the slurry to the boreholes (Marshall Hunt, Consol, personal
communication, 2001). Variable terrain can impose severe pumping problems if
pipelines must be laid over hills and down into valleys to access injection sites.
The accuracy of mine maps, with regard to underground workings and surface
surveys, is important if the slurry boreholes are to intercept the underground
workings in the desired location.
Filling above-drainage mine workings with slurry may increase hydraulic
head on the coal barriers and result in a blow-out, making evaluation of mine
workings above a surrounding stream valley critical. The mine maps must be
evaluated for accuracy, and the underground barriers for adequacy to contain
the slurry. Mines below the surrounding natural drainage level offer more
secure underground disposal sites.
Blind flushing is used in mines where access has been obstructed, such as
by roof falls. In this method of slurry injection, the underground cavity may be
dry, partially filled with water, or completely filled with water. The volume of
the voids is estimated from old mine maps and any other available, relevant
data. Since it is nearly impossible to determine how much slurry a borehole will
accept, a series of holes is drilled; when one borehole becomes clogged,
injection moves to the next. The slurry is pumped to the borehole and injected
into the mine at a relatively high velocity. Once the slurry leaves the turbulent
area at the bottom of the borehole, the coarser material will settle.
In flooded mines, the injected slurry will displace the water, which often
results in a new or increased discharge elsewhere (Marshall Hunt, Consol,
personal communication, 2001). Depending on the quality of the displaced
water, treatment facilities may have to be upgraded to handle the additional
volume, or the mine pool may have to be pumped to avoid discharge at an
undesired location.
Paste backfilling has been demonstrated in other types of mines. Although
it has not been used to dispose of coal waste, it may be possible to extend the
technology to this application.