Page 318 - Advances In Productive, Safe, and Responsible Coal Mining

294 Advances in Productive, Safe, and Responsible Coal Mining

groundwater quality and quantity due to intercepting and changing underground water
flow paths [93]. In areas of northern Appalachia where high sulfur coal exists and no
limestone layers are present for neutralization, the greatest environmental impact from
underground mines has been on surface water quality from AMD [94]. Treatment by
chemicals reduces the acidity and removes the metals, but active treatment is expen-
sive and must be continued for decades [95]. Passive methods for treating AMD are
also available and work well when appropriately designed for specific water
conditions [88].
Water quality changes over time in underground mines [96,97]. Demchak et al.
[98] observed that changes in water chemistry over time differ between below-
drainage (flooded) and above-drainage (not flooded) underground mines, with
flooded mines rebounding to much better water quality within a decade and unflooded
mines remaining acid for much longer. Lambert and Dzombak [99] found that flooded
underground mines in Pennsylvania change from very acid water to neutral or
net alkaline water shortly after complete flooding [100,101]. Borch [102] found sim-
ilar results in the flooded Meigs mine in Ohio and suggested the following reasons for
the dramatic water-quality improvement within a few years after flooding at Meigs:
(i) Pyrite oxidation ceased in flooded sections; (ii) after the initial flush, there was less
readily available iron sulfate salts to dissolve; (iii) alkaline strata in the roof rock of the
mine pool provided some neutralization; (iv) dilution and influx of alkalinity occurred
from groundwater inflows; (v) the groundwater flow path exhibited some short
circuiting, so areas of rapid transport or flow exhibited better water quality than areas
of restricted water movement; and (vi) geochemical reactions, such as sulfate reduc-
tion and cation exchange, occurred along the underground water flow path, thus
improving the quality before discharge.
Above-drainage mines did not show the same dramatic improvement as below-
drainage mines; they tended to improve slightly in water quality but remained acidic
[98,99,103]. Some sections or voids of abandoned above-drainage mines are flooded
or partially flooded, which virtually removes those pyrite reaction surfaces from con-
tributing acid products. Many other areas within the mine remain open to oxygen and
water exchange and are susceptible to reaction. These exposed pyrite surfaces produce
less acidity over time due to: (i) weathering products forming an iron hydroxy sulfate
coating, which reduces air and water contact and release of acid products [104], and
(ii) the more morphologically reactive pyrite (framboidal) is depleted first, thereby
leaving the less reactive pyrite (massive) for subsequent oxidation. Therefore, changes
in pyrite reaction rate and availability of surfaces in these areas can result in drainage
quality improvement. Only during roof or pillar collapse are fresh pyrite surfaces
exposed to the mine atmosphere and water. Once mines are closed, ventilation systems
cease, which greatly reduces the availability of oxygen for pyrite oxidation. Land sur-
faces over underground mines can be compacted or altered to reduce the amount of
infiltration, or surface cracks can be clogged, thereby inhibiting direct inflow of sur-
face water into the mine. Roof or pillar collapses within the mine can change flow
paths or create pools of water in the mine. Although all of these factors presumably
decrease acidity with time, most are difficult or impossible to validate. Researchers
are therefore left with empirical predictions of decline based on long-term data sets.