Page 316 - Advances In Productive, Safe, and Responsible Coal Mining

292 Advances in Productive, Safe, and Responsible Coal Mining

that have been compacted to become impervious or placed in areas where infiltration
will not cause water-quality problems. Such drainage networks can also include
upland retention ponds, to slow runoff and capture sediments, and are often developed
at low points near the permit boundary to control waters that are leaving the mine site.
Water management may also include structures to move groundwater out of mine
backfills quickly and in a manner that reduces water-spoil contacts. Water controlled
by such structures may include groundwater entering the mining disturbance from
adjacent unmined strata and those originating from rainfall infiltration on the mine
site. Groundwater management structures may include French drains or other perme-
able channels within mine backfills to move water out of the fill and to reduce its con-
tact time with disturbed materials. Blanket or bottom drains are constructed of coarse
permeable rock to also promote transmission of water from mine backfills. Among
essential water management practices are those employed to reduce water contact
with spoil materials identified during overburden analysis as problematic due to
the potential to release elevated concentrations of acids, acid-soluble metals, TDS,
and/or Se.
On some mine sites, water management practices also involve treatment to remove
chemical contaminants so as to minimize off-site pollution and to satisfy regulatory
requirements. Ideally, effective problematic-spoil identification and management
would enable mining to occur without need for water treatment; but disturbance of
acidic strata, if present in the overburden column, can be expected to mobilize acids
and, perhaps, acid-soluble metals such as Al, Fe, and Mn. Both active and passive
water treatment methods are commonly applied to treat acidic discharges so as to mit-
igate both acidity and such metals. Active AMD treatment relies on constant addition
of industrial chemicals, and requires frequent resupply and maintenance [34]. Passive
AMD treatment relies on natural biological, chemical, and physical processes to neu-
tralize acidity, and to oxidize and precipitate metal contaminants [88]. With proper
premining overburden characterization and spoil handling, any water treatment that
proves necessary should be temporary. If a proposed mine’s overburden contains vol-
umes of acidic material that are so large that hydrologic isolation is not possible and
acidic drainages requiring treatment over long terms would be expected, it is often
prudent to avoid mining in such areas.
Active water treatment methods for TDS and Se are available; however, the pri-
mary method employed to date, reverse osmosis, is very costly and is typically not
employed by mining operations in the absence of strong legal incentives. Methods
for passive treatment of TDS are not known. Experience has demonstrated that passive
treatment systems employing sulfate reduction processes [88] can be effective in
removing Se from discharge waters. However, passive treatment for Se is an emerging
technology and, to the best of the authors’ knowledge, has not been described in pub-
lished literature.

14.4.2 Ground water management

Mining activities can impact the quantity, quality, and usability of groundwater sup-
plies. Underground mining for coal by longwall or room-and-pillar mining methods
often interrupts and depletes groundwater and can alter its quality. Surface mining can