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OCR for page 60
5
Mining: Methods and Impacts
OVERVIEW OF SURFACE MINING, SPOIL
HANDLING, AND RECLAMATION
There are three major types of surface mining
procedures, which in the context of the whole
mining and reclamation process have common features
as well as significant differences. The three
mining types contour mountaintop removal and
, . . .
area mining, differ in their size of operation,
equipment use, spoil handling, and final
landscaping. The mechanics of the mining process
involve several steps, including:
1. planning and permit approval (for the
complete operation);
2. site preparation (topsoil storage, sediment
pond construction);
3. mining (blasting, spoil dumping, coal
extraction);
4. reclamation (landscape stabilization and
revegetation); and
5. bond release (operator no longer responsible
for the site).
Two reports from the National Research Council
(1981a,b) provide comprehensive summaries of
surface mining methods, impacts, and land
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restoration. One of these reports, prepared by the
Committee on Ground-Water Resources in Relation to
Coal Mining, provides a full description of each of
the main surface mining methods (NRC, 1981a). A
brief summary is provided here as a framework for
addressing the recharge focus of this report.
Contour Mining
The overburden above the approximately horizontal
coal beds in the mountainous areas (Figure 5.1) of
the Appalachian Coal Basin is removed in a stepwise
procedure that includes topsoil removal and
overburden blasting and removal to expose the coal
seam. The spoil from the first cut in a contour
operation may be stored for replacement in the
reclamation of the last cut (generally not favored
due to double handling), reclamation of an adjacent
abandoned surface mine (encouraged), or by
head-of-hollow fill (most frequent). Contour
mining is often accompanied by lateral angering to
extract the unexposed coal seam. Horizontal holes
up to 60 m long at spacings from 15 to 60 cm can
become subsurface reservoirs influencing the
occurrence and movement of ground water in the
reclaimed site. In general the landscape is
returned to approximate original contour unless a
variance has been granted in the mine plan.
Previously stored topsoil or a substitute material
is placed over the spoil and is stabilized by
mechanical compaction on sloping terrain, and the
area is revegetated with herbaceous, shrub, and/or
tree species. The main features of this mining
method that influence recharge include initial
vegetation removal, compaction of the reclaimed
soil profile and the mine floor (e.g., clay
material underlying the coal seam), spoil
generation with an increase in porous material
volume, and change in vegetation type.
OCR for page 62
- 6 2 -
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~.C3'
,~,j~,_.~ ~
o
Ed
En
o
o
Cal
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En
.
Us
C)
OCR for page 63
-63-
Mountaintop Removal
In this procedure complete extraction of upper
coal seams in a mountain (Figure 5.2) is achieved
by sequential removal of all overburden above the
seams. Spoil material is placed and graded to
favor surface runoff toward the center of the
mountain in the restructuring of the landscape.
The volume increase in spoil due to the porosity
being greater than that in the original overburden
rock is usually handled by deposition in a
head-of-hollow fill. The main features of this
mining method that influence recharge are similar
to those described for the contour method.
However, the greater size of the operation and the
longer duration of the mining process may result in
greater impacts on recharge in the vicinity of the
mine site.
Area Mining
This method uses the largest equipment. It
well established in the western United States
(e.g., the Northern Great Plains Region), southern
Illinois, Indiana, and in western Kentucky, and is
also being practiced in mountain areas of
Appalachia where several mountaintop removal
operations are combined. These operations have
common features with the other mining methods in
terms of vegetation removal, topsoil storage,
blasting, overburden removal, coal extraction,
landscape restructuring, and revegetation.
Sequential mining of the area is usually practiced
whereby current overburden is used to fill the
previous extraction pit (Figure 5.31. Sometimes
the last pit is landscaped as a water body, thus
eliminating the cost of hauling large amounts of
stored overburden. Recharge and ground water
systems can be subject to the greatest disturbance
when the area mining method is used, due to the
large mine size, heavy equipment use, and long
duration of the mining operations.
.
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,
FIGURE 5.2 Surface mining mountaintop removal and
valley fill method
OCR for page 65
- 6 5 -
~~
14~:'.
.
~ W~ ~i.
GRADED AR:|
HDISTURBED
LAND
~.~
R ~ ~ ~ ~—~ Of —~ ~:——_
~ '_·_~
FIGURE 5.3 Typical area mining method with
s tr ipp ing shove 1 .
OCR for page 66
-66-
Common Features of Three Mining Methods
The common features of these three mining methods
that can influence recharge of ground water in the
reclaimed landscapes are
1. initial vegetation removal (eliminates
transpiration);
2. blasting (increases volume of overburden,
fracturing of adjacent and underlying bed rock);
3. mine floor compaction (reduces recharge to
lower aquifers);
4. disruption of aquifers) (dewatering,
destruction of storage zone);
5. water storage in spoil (greater porosity,
extra fill areas);
6. surface compaction (greater surface runoff);
7. unfavorable reclaimed soil (poor water
storage in root zone); and
8. change in vegetation type (change in rooting
depth and growing season).
MINESOIL PROPERTIES
Surface coal mining and other land disturbances
often significantly change soil properties.
Minesoils, which are the materials on the restored
land after mining, have properties that reflect the
character of the coal overburden that becomes the
parent material of the soil. Proper placement of
soils and overburden after mining produces
minesoils suitable for plant growth, but haphazard
placement of these earth materials may result in
minesoils that are difficult to vegetate even with
lime and fertilizer amendments. Haphazard
placement of overburden materials generally results
in extreme variability of minesoil properties.
Some minesoil properties change rapidly over time,
especially for the first few years after
revegetation. These changes are caused by the
weathering of fresh, unweathered, or partially
weathered overburden materials, a process that may
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-67-
be accelerated or decelerated by applying
amendments such as lime, fertilizer, or sewage
sludge to the minesoil.
Most minesoil textures are loamy, but some are
clayey or sandy. Rock-fragment content varies, but
most minesoils in areas without an original loess
cover of 60 cm or more have greater than 35 percent
by volume rock fragments in subsoil horizons (some
as high as 80 to 90 percent) and fewer than 35
percent fragments in the surface horizons (Ciolkosz
et al., 1985; Thurman et al., 1985~. Minesoils, in
general, have more rock fragments than do
contiguous native soils (Bussler et al., 1984;
Pedersen et al., 1978; Thurman and Sencindiver,
1986).
Soil structure develops very quickly in some
minesoils, but it is generally more strongly
developed in older minesoils. New minesoils
constructed with scrapers tend to have massive,
compacted layers, but minesoils constructed with a
mining wheel excavator in combination with belt
transportation tend to have a fritted structure,
which is a porous structure with rounded aggregates
loosely compressed together (McSweeney and Jansen,
1984).
Surface horizons of minesoils generally have
higher bulk density, lower porosity, and lower
water-holding capacity than do those of contiguous
native soils (Bussler et al., 1984; Potter et al.,
1988; Smith et al., 1971; Thomas, 1987; Thurman and
Sencindiver, 1986; Younos and Shanholtz, 1980~. As
minesoils age, however, these properties become
more like those of the native soils. The subsoil
horizons of minesoils also may have properties that
differ from those of the subsoil horizons of native
soils.
Infiltration rates and saturated hydraulic
conductivity of minesoils are highly variable and
may be lower or higher than are those of contiguous
native soils (Hnottavange, 1987; Pedersen et al.,
1978~. Large macropores in some minesoils cause
water to move rapidly through the profile, but
compaction of clayey-textured material may cause
water to move very slowly through the soil.
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Other factors a~ffecting physical and hydraulic
properties of minesoils are the presence or absence
of topsoil and land use. Topsoil layers on
minesoils typically have higher water-holding
capacities than do the surface layers of
non-topsoiled minesoils, because topsoil generally
has fewer rock fragments (Thurman et al., 19859.
However, water may infiltrate more rapidly and move
through the profile more quickly in non-topsoiled
. . ~ . ~ . · . ~
._ r_ __ Layers
(Rogowski and Jacoby, 1979~. Grazing of minesoils
by livestock may significantly lower the
infiltration rate (Hnottavange, 1987~.
m~nesoils than in minesoils with topsoil
ACTIVE SURFACE MINING EFFECTS ON RECHARGE
In the site-preparation phase of mining,
vegetation is removed, topsoil is scraped off and
stored, and overburden is blasted and excavated to
expose the coal. Significant changes in the
landscape's water-budget components
(evapotranspiration, drainage, storage) result from
these activities; however, the effect on recharge
depends on the season of the year and the coal
field's location. Differences in mining effects on
water-budget components for an eastern and a
western coal region are given as examples.
Appalachian Coal Basin
Summer Operations
Site preparation that is initiated in May,
followed by mining, site reconstruction, and
completion of revegetation by September may have a
small impact on long-term recharge. Recharge is
v Removal of
A —-r -
usually negligible cur: no the .~'mm~r
vegetation eliminates
~ . ~
transpiration at the site 2
and summer ralntall can result in increased
recharge if infiltration is not restricted.
However, surface compaction usually limits
OCR for page 69
-69-
infiltration and causes surface runoff to dominate
the hydrologic processes during mining. In
undisturbed forest, surface runoff is essentially
nonexistent due to the high infiltration rates of
forest soils. Ephemeral stream flow (apparent
surface runoff) is generated by storm events from
subsurface water exfiltrating to the surface by
convergent flow induced by topography. Surface
water diversion into sedimentation ponds and pond
discharge into stream waters are the dominant
impacts of summertime mining operations
, replacing
the normal evapotranspiration water-loss component
of the water budget
Winter Mining
.
Most recharge in eastern coal basins occurs
during the winter and the spring into a natural
fracture system within the outer rock zones of
mountains. These recharge zones have formed as a
result of stress-relief fracturing during landscape
erosion (Wyrick and Borchers, 19819. Mining
operations destroy some of the natural fracture
system, and during the winter period mining can
significantly reduce recharge through surface
compaction effects caused by mining equipment.
Reduced infiltration leads to enhanced surface
runoff, which is routed to sedimentation ponds if
the mine operation is conducted according to
approved procedures. Overflow from the ponds is
channeled into local stream waters. The net effect
is a bypassing of the natural ground water
recharge-discharge process by overland routing to
streamflow.
Summer and winter mining represent two extremes
of direct mining effects on recharge, with patterns
of impact expected to be intermediate for
operations conducted during the spring or autumn.
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Western Coal Basins
The large mining operations of western coal
basins have a long period of coal extraction often
lasting for a decade or more, and they occupy large
areas relative to most eastern mines. ~ ~
The aquifer
systems in western coal basins can be extensive,
and several different aquifers, with differing
recharge areas, may exist at a mine site. Mine
operations often require dewatering of the intended
mining area since the coal seams are usually a
significant component of the aquifer system.
Ground water pumping causes a drawdown of the
aquifer, forming a cone of depression in the water
table (Figure 5.4~. The extent of drawdown may be
small in relation to the whole aquifer, but the
local effect is necessarily large (Woessner et al.,
19791. Van Voast and Reiten (1988) estimated that
the aquifer drawdown at the Decker mine in
southeastern Montana extended over several
kilometers (Figure 5.51. The surficial and deep
aspects of recharge are noted for the western coal
basins.
v
Surficial Recharge Processes
There is a definite seasonality to recharge in
the western coal basins. The occurrence of frozen
soil and the dynamics of snowmelt result in runoff
to nearby alluvial depressions and valleys during
spring. Snowmelt can be a very dramatic pulse
event resulting in water accumulation in the lower
landscape positions. Recharge to the upper
aquifers in the landscape takes Place largely
during the snowmelt period.
~ ~ ,
Rainfall during winter
and early spring can also ce effective in
recharging the upper aquifers in the landscape.
The operations at a mine site disrupt recharge
within the mine area. However, this local effect
may be offset to some extent by the collection and
discharge of mine site precipitation through
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-71-
COMBINED
, .
MINE ONLY
C_
-
~1 D~ ~
W~ ~
NON—MINE
__
_I
it_
'! ~
'~1
''~1
'2~
my.
_
C-A W~
SCENARIO D YEAR 2032
.,:
-
/~\~N'
/' ~'~"
/ ~
ma\
~ Ky3&r_ -~ ~ /
\~ K~ ~ ~ R r
:~.~7< t
FIGURE 5.4 Three-dimensional representation of
drawdown and percent of that drawdown resulting
from mining for Scenario D in 2032.
SOURCE: Office of Surface Mining Reclamation and
Enforcement, 1988.
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~ / (a)
~ ~ ~ j —~ w r O
~ Drawdown (~)
c
`,, ,,~ land surface
~ _ i_
2
~ Km
2 1 Mi
'< Fault C1 Min. pit
:: ~ WHIP ='
\D-1 and D-2 coal bedim— ~
_ _ potent~om~ric profile
FIGURE 5.5 Aquifer drawdown at the Decker mine in
southeastern Montana. (a) An area of
potentiometric decline more than 15 miles long and
5 miles wide has developed for the D-2 coal bed;
(b) lowered potentiometric levels pass unaffected
beneath valley bottoms and perennial streams.
SOURCE: Van Voast and Reiten, 1988.
(b)
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-73-
sedimentation ponds to surface waters, which may
contribute to recharge of surficial aquifers
outside the area of active mining.
Deep Recharge Processes
Mine site operations may have little effect on
the recharge of deep aquifers since in many cases
the recharge occurs in permeable upland areas
remote from the mine site. These permeable areas
are formed in the approximate location of an
outcrop of a former coal seam. Coal at or near the
surface usually had been ignited by natural causes
in earlier geological times, and the heat from the
combustion had fractured the surrounding rock,
forming the highly permeable material called
clinker or scoria. Part of the precipitation
received on clinker percolates through this
material into adjoining coal seams, recharging deep
aquifer systems. Recharge to such deep aquifers
proceeds unimpaired during mining operations.
RECLAMATION EFFECTS ON RECHARGE AND
ON WATER QUALITY
Water Quantity
Restoration of the mine site to approximately the
pre-mining landscape by spoil placement, surface
application of topsoil or other approved material,
and revegetation can lead to an increase or
decrease of recharge to aquifer systems relative to
that of the original landscape.
The factors contributing to an increase in
post-mining recharge are
Poor vegetation establishment. Reduced
evapotranspiration increases soil water drainage.
· Revegetation with species having more shallow
root systems. Smaller available soil water storage
favors reduced transpiration.
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· Reduced soil water storage in root zone--leads
to increased drainage below the root zone and to
lower transpiration.
~ Increased porosity of the vadose
zone--provides greater water storage for potential
recharge to aquifers.
· Increased permeability of the vadose
zone--allows more rapid movement of water to the
aquifer.
· Increased fracture porosity in the aquifer
(from blasting--results in greater aquifer water
storage to receive and transmit recharge.
o Water impoundments. ~
nine plans allowing lakes
and ponds may increase recharge through reduced
runoff and increased seepage from the impoundments
into aquifers.
· Enhanced zones of stress-relief fracturing in
the rocks above buried head walls, especially in
sites that received extensive angering.
Post-mining recharge may be decreased by:
· Reduced infiltration of the reclaimed
surface. Greater surface runoff bypasses aquifer
recharge.
· Slope instability and erosion--expected to
increase surface runoff in channel flow bypassing
recharge to the aquifer.
o Enhanced evapotranspiration. Prolific
vegetation may have greater canopy interception and
transpiration than did the original vegetation.
ED Reduced effective corositv in the acuifer
(from blasting--less storage to receive and
transmit recharge.
· Reduced permeability in vadose zone due to
compaction during spoil placement and reshaping.
· Mine floor compaction--limits recharge to
deeper aquifers.
o Elimination of surficial scoria or other
high-recharge areas from within the mine site
boundaries.
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One of the dominant effects of the mining
operations on recharge is the conversion of rock to
rubble. In the pre-mining landscape, deep
percolation may often be restricted by layers with
low hydraulic conductivity, leading to perched
water table development and lateral outflow as
seeps or springs. In the post-mining landscape,
deep percolation is often enhanced by the high
hydraulic conductivity and high porosity of the
spoil material.
One critical zone in the restoration of recharge
occurs at the surface. Restoration of the
infiltration rate of the reclaimed site is a
prerequisite for restoration of recharge.
Reduction in infiltration rates below pre-mining
rates can be caused by compaction by earth-moving
equipment during the landscape reconstruction.
Compaction is deliberately used on sloping terrain
to stabilize the spoil material and minimize
erosion during revegetation. Some spoil materials
from eastern Kentucky can withstand compaction
without lowering infiltration to unfavorable levels
(Wells et al., 19829. Not all spoil materials
exhibit favorable infiltration properties. In
particular, spoils from a western Kentucky surface
mine were shown to have extremely low infiltration
rates that were associated with high bulk density
and a well-graded particle size (Wells et al.,
1982~. Deep ripping of the restored profile prior
to revegetation can have a helpful influence on
infiltration in some cases. Materials that form
surface seals during wetting and crusts during
drying will also reduce recharge by enhancing
surface runoff rather than infiltration.
Vegetative cover is one of the better means of
overcoming surface sealing and crusting problems.
Several non-mining changes in land use due to
natural and human-induced causes can alter
landscape water budgets, and these provide some
comparisons with the effects of surface mining on
recharge. A discussion of these other land use
effects on recharge
given in Appendix C.
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In low-rainfall areas,
recharge may not be
limited by recharge capability, and restoration of
recharge capability could be viewed as an excessive
requirement. In high-rainfall areas such as the
coal provinces east of 100° W longitude, recharge
capability can limit recharge rates, as evidenced
by lateral subsurface flow and wet weather seeps.
Such seeps have been observed, for example, in
vertical faces of rock formations with vertical
fractures that terminate on horizontal strata of
low hydraulic conductivity. Seeps then occur above
the outcrops of the less-permeable strata.
Restoration of recharge capability to pre-mining
conditions at a surface mine permit area requires
two main component steps: (1) the infiltration
characteristics of the reclaimed site need to
favor/produce recharge rates that equal or exceed
the pre-mining rates, or exceed the highest
rainfall intensity of the area, whichever is
smaller, and (2) the hydraulic-conductivity values
of the root zone and the vadose zone need to equal
or exceed the equivalent pre-mining values. Also,
revegetation should be carried out in such a way
that the evapotranspiration rates after mining are
not greater than those that occurred before
mining. Due to soil and subsurface heterogeneity,
it is not practical to undertake major field
hydraulic-conductivity measurements (Klute, 1986),
and indirect methods of evaluation should be used.
In most cases, the hydraulic-conductivity
characteristics of the restored vadose zone are
much higher than the corresponding pre-mining
values due to the greater porosity of the reclaimed
spoil materials relative to the original rocks and
other geologic deposits and due to the breakup of
layers of low hydraulic conductivity.
Restoration of infiltration rates is needed to
initiate the recharge process. Compaction of
spoils is the major factor that may inhibit the
recharge capability at a mine site, and such
effects can usually be identified by the occurrence
of surface erosion and poor establishment of
vegetation. The reclaimed permit area should not
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have any visible signs of particle erosion rates
exceeding pre-mining values. This requirement
provides evidence that the surface hydrologic
regime has been restored and that surface runoff is
not exceeding pre-mining levels. Additionally, the
requirements for revegetation specified in SMCRA
favor the restoration of recharge capability by
maintaining the integrity of the soil surface for
infiltration as well as the permeability of the
root zone for soil water drainage. Special
attention needs to be given to evaluating
compaction in reclaimed spoils as part of the
post-mining assessment of recharge capability,
particularly in areas with loess deposits, such as
in western Kentucky.
Water Quality
From the standpoint of water quality, coal
surface mining has its greatest impact on the
shallower aquifers. The most notable effect is
increased total dissolved solids and increased
sulfate, calcium, and magnesium concentrations. In
some cases there have been increases in the
concentration of trace metals, including lead,
manganese, nickel, chromium, cadmium, zinc,
arsenic, and selenium (NRC, 1981a; Appendix D of
this resort).
-
~ ,
Increased selenium content in ground water is a
particularly onerous problem. The selenium content
of some coals is 10 to 200 times the crustal
abundance of this element Waken, 1973~. Selenium
is mobilized as selenate (SeO4 ~) in alkaline
coal spoils and is readily transported by ground
water. Selenate is the form most available for
accumulation by plants (Presser and Barnes, 1984~.
Some species of Astragalus accumulate up to several
thousand ppm selenium (Walter et al., 1972), and
selenium is concentrated by animals. Fish in the
selenium-contaminated waters of the Kesterson
National Wildlife Refuge were found to have
selenium concentrations 100 times that of fish
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found in selenium-free waters (Presser and Barnes,
1984~. Above levels required for good health (0.04
to O.1 mg/liter) selenium is toxic, causing
mutations in waterfowl hatchlings (Presser and
Barnes, 1984) and acute selenosis in other animals
(Fishbein, 1977~.
Ground water quality data that have been
collected to date are obtained from samples
collected on or very near coal surface mine sites.
Data are collected from probable hydrologic
consequence (PHC) reports and compiled by the
regulatory authority into cumulative hydrologic
impact assessments (CHIAs) to evaluate cumulative
effects of multiple coal surface mines in a given
area. Although no long-term consistent trend
(i.e., several decades to centuries) has been
confirmed through evaluation of analytical data,
(Groenewold et al., 1983; Van Voast and Reiten,
1988), it is anticipated that there may be
long-term water quality impacts.
CONTROL OF ADVERSE EFFECTS OF COAL SURFACE
MINING ON GROUND WATER QUALITY
All three phases of coal surface
mining--pre-mining exploration, active mining, and
post-mining reclamation--can potentially have
negative impacts on ground water quality.
Exploration boreholes, drilled to determine the
extent and quality of the coal seams, are, after
data collection, now plugged as mandated by the
Federal Surface Mining Control and Reclamation Act
of 1977. This action is an important step in
preventing contamination of deeper aquifers that
often have better water quality than do shallower
aquifers. Many coal companies now have a routine
program of sealing all exploration drillholes that
they find even though they may have existed from
exploration periods predating the 1977 Surface
Mining Control and Reclamation Act (W. A. Van
Voast, Montana Bureau of Mines and Geology,
personal communication, 1989~.
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During mining, blasting can create vertical
fractures or widen existing fractures, allowing
percolation of contaminated water into deeper
aquifers of higher quality. Little can be done
during the actual mining phase to prevent this from
occurring other than continual dewatering to
minimize the amount of water flowing to or from the
deeper aquifer.
During post-mining reclamation, considerable
effort is devoted to minimizing the short- and
long-term impacts of coal surface mining on ground
water quality. Blending of spoils and selective
placement of spoils are techniques now being
developed rurcner, and in some cases practiced, to
minimize the deterioration of water percolating
through the backfill of reclaimed coal surface
mines (Phelps and Saperstein, 1982; Groenewold et
al., 1983; Caruccio and Geidel, 19891. One example
of spoil blending is the mixing of acid-producing
spoils with alkaline spoils to "neutralize" acid
produced by iron sulfide oxidation.
Selective placement of weathered overburden may
play an important role in controlling ground water
quality in the western United States. The
uppermost zone of the alkaline overburden of the
West is highly weathered, and soluble salts have
migrated only a few meters into the underlying zone
over geologic time, because of low precipitation.
It is this zone of very soluble salts that has
often been placed on the mine floor as a result of
overburden handling. Lateral and vertical ground
water recharge into the spoils forms a new aquifer
with the mine floor as the aquitard. Material high
in water-soluble salts is readily leached by the
new mine floor aquifer, resulting in serious
_
deterioration of the new aquifer (Pagenkopf et al.,
-- ~ 1979 W A N7an Voast
1977; Woessner et al., _ . , ~ a_ ,
Montana Bureau of Mines and Geology, personal
communication, 19899. To avoid increasing the
salinity of the new aquifer, it has been proposed
(Pagenkopf et al., 1977) that the zone containing
the water-soluble salts be perched below the root
zone and above the mine floor aquifer by selective
spoils handling.
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Toxic and acid-producing spoils, identified
during the pre-mining evaluation of the overburden,
are often isolated with clay barrier materials and
placed well below the root zone and above the mine
floor aquifer. This technique establishes
differential permeabilities, reducing percolation
through the undesirable spoils, which could lead to
ground water deterioration. However, this
technique may only provide a temporary solution,
since clay barriers may eventually leak.
The reduction of recharge by decreasing
permeability may at first blush appear inconsistent
with the mandate of the SMCRA to restore ground
water recharge capacity. However, if local
reduction of recharge preserves water quality, then
isolation of toxic and acid-producing spoils should
be implemented. Because such isolation practices
are localized and percolation is diverted, not
prevented, the overall recharge may be preserved.
Should recharge, however, be substantially reduced
over a given area because of isolation practices,
then artificial recharge through zones, engineered
to assure good water quality, should be
implemented. If isolation practices are required
to protect water quality and the isolation
technique reduces recharge and this reduction in
recharge cannot be overcome by engineered recharge
zones, then the area should be evaluated under the
"unsuitable-for-mining" provision of SMCRA (Sect.
522).
Representative terms from entire chapter:
surface mining
