Environ. Sci. Technol. 2010, 44, 2144–2149
Terrestrial Carbon Disturbance from
Mountaintop Mining Increases
Lifecycle Emissions for Clean Coal
J A M E S F . F O X * ,† A N D
J. ELLIOTT CAMPBELL‡
Civil Engineering Department, University of Kentucky,
Lexington, Kentucky 40506, and College of Engineering,
University of California, Merced, California 95343
Received October 30, 2009. Revised manuscript received
January 25, 2010. Accepted January 29, 2010.
The Southern Appalachian forest region of the U.S.sa
region responsible for 23% of U.S. coal productionshas 24
billion metric tons of high quality coal remaining of which
mountaintop coal mining (MCM) will be the primary extraction
method. Here we consider greenhouse gas emissions
associated with MCM terrestrial disturbance in the life-cycle
of coal energy production. We estimate disturbed forest carbon,
including terrestrial soil and nonsoil carbon using published
U.S. Environmental Protection Agency data of the forest floor
removed and U.S. Department of Agriculture-Forest Service
inventory data. We estimate the amount of previously buried
geogenic organic carbon brought to the soil surface during MCM
using published measurements of total organic carbon and
carbon isotope data for reclaimed soils, soil organic matter and
coal fragments. Contrary to conventional wisdom, the lifecycle emissions of coal production for MCM methods were
found to be quite significant when considering the potential
terrestrial source. Including terrestrial disturbance in coal lifecycle assessment indicates that indirect emissions are at
least 7 and 70% of power plant emissions for conventional
and CO2 capture and sequestration power plants, respectively.
To further constrain these estimates, we suggest that the
fate of soil carbon and geogenic carbon at MCM sites be
explored more widely.
1. Introduction
According to data from the U.S. Department of Energy, Energy
Information Administration, coal consumption for energy
production and manufacturing was responsible for 2.10
billion metric tons per year of CO2 emitted to the atmosphere
in the United States from 1997 to 2006 accounting for 36%
of the CO2 produced in the United States due to the burning
of fossil fuels, and 9.77 billion metric tons of CO2 per year
were emitted worldwide due to coal burning accounting for
38% of world’s CO2 emissions due to fossil fuel consumption
(1). Meanwhile, coal accounted for 23.2 quadrillion Btu’s per
year of the energy produced in the United States or 33% of
the total energy produced, and globally coal accounted for
102.4 quadrillion Btu’s per year of the energy produced or
25% of the world’s energy source (1). Thus, coal is both a
primary energy source and contributor to CO2 emissions.
* Corresponding author phone: (859)257-8668; fax: (859)257-4404;
e-mail: [email protected].
†
University of Kentucky.
‡
University of California, Merced.
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The interaction of coal production and its environmental
consequences is an ongoing discussion specifically with
regard to achieving sustainability when advancing clean
energy that works toward decreased pressures on the global
climate. Looking forward, the abundant reserves of coal in
industrialized as well as developing countries serve as a viable
means for inexpensive energy production while helping
countries work toward energy independence. However,
environmental mandates and carbon budgeting efforts have
placed increased importance upon reduction of greenhouse
gas emission from coal-fired plants. Recent societal focus
has placed emphasis upon production of coal via clean
technologies that employ CO2 capture and sequestration
(CCS) methods to reduce greenhouse gas emissions. CCS
research and development efforts are undoubtedly focused
upon reducing CO2 emissions during coal burning in order
to produce a more sustainable technology. Within coal
producing regions, increased research has been initiated by
state governments and industry to perform experimental and
pilot studies of CCS increasing the realization of its application. Research on the fundamental processes underlying clean
coal technology has addressed greenhouse gas emissions
associated with life-cycle components of coal production
(2, 3), such as emissions during mining, refinement, and
coal transportation in order that emissions can be accurately
budgeted.
Notwithstanding the importance of CCS efforts to improve
the imprint of coal burning on the environment, the lifecycle emissions also should be further investigated and
quantified to determine their significance under coal production scenarios. One aspect of coal production that has
received less attention is the redistribution and loss of
terrestrial carbon during surface or mountaintop coal mining
(MCM) methods. Quantification of MCM as a disturbance
agent impacting carbon storage has not been reported in the
literature presumably due to the higher, more apparent flux
associated with CO2 emissions from coal-burning power
plants. However, as “clean coal” continues to be promoted
and CCS methods are adopted (4-6), the terrestrial carbon
impacted by MCM disturbance should be quantified to
determine its significance under current coal burning
practices and under future CCS methods.
The objective of this study was to quantify the impact of
MCM disturbance upon terrestrial carbon storage in the
Southern Appalachian forest region (SAFR) of the U.S. The
SAFR, located in southern West Virginia, eastern Kentucky,
southwestern Virginia, and portions of eastern Tennessee,
has been impacted by widespread use of MCM over the past
two decades, resulting in removal of 6.8% of forests and
production of 23.3% of the coal in the United States (7, 8).
It is estimated that 23.9 billion tonnes of high quality coal
remain in the SAFR making coal from this region important
when considering application of CCS methods during coal
burning (7). Herein, estimates of terrestrial carbon impacted
by MCM as well as CO2 emissions during coal burning,
extraction, and transportation and are calculated using
published data and equations from the literature and federal
agencies, that is, U.S. Department of Energy (USDOE), U.S.
Environmental Protection Agency (USEPA), and the U.S.
Department of Agriculture-Forest Service (USDA-FS). Thereafter estimates of terrestrial carbon impacted by MCM are
discussed relative to (i) CO2 emissions due to coal extracted
using current power-plant combustion practices; (ii) greenhouse gas emissions associated with other components of
coal production; and finally (iii) the anticipated CO2 reductions from CCS methods implemented in the future.
10.1021/es903301j
 2010 American Chemical Society
Published on Web 02/08/2010
2. Materials and Methods
In the following subsections, the details are presented
regarding the methods and calculations to estimate the lifecycle emissions from MCM in the SAFR. Specifically, methods
are presented to estimate soil carbon removed, nonsoil
carbon removed, geogenic organic carbon transferred to the
soil reservoir, carbon emissions during coal burning for
energy production, and fossil fuel carbon burned during coal
extraction and coal transportation.
2.1. Soil and Nonsoil Carbon Removed During MCM.
MCM includes surface mining methods that can be classified
as steep-slope mining, mountaintop removal, contour mining
and area mining. For all MCM methods, the nonsoil carbon
consists of above ground biomass carbon and is clear-cut
and thereafter scraped from the land surface, and soil organic
carbon is removed and drastically disturbed (9). Thereafter,
remaining overburden is removed using blasting and excavation. Due to the initial removal of nonsoil and soil carbon,
MCM is a forest disturbance agent that causes a source flux
of CO2 to the atmosphere and redistributes carbon stored in
plant biomass, woody debris, and soil organic matter (10-13).
An estimate of the disturbed forest carbon, including
terrestrial nonsoil and soil carbon, impacted by MCM was
calculated for the SAFR. The forest carbon pools for all
counties in the SAFR are summarized by age class and forest
type using USDA Forest Service inventory data (14). The USDA
Forest Service Carbon On Line Estimator (COLE) was used
to provide estimates of total forest carbon including soil
carbon and nonsoil carbon (live tree, dead tree, under story,
down dead wood, forest floor) for each county in the SAFR.
COLE combines USDA Forest Service Forest Inventory and
Analysis (FIA) data with carbon conversion parameters as
described in Smith et al. (14). COLE was used to provide
average soil and nonsoil carbon pools for each forest type
in the region as well as a weighted average for the SAFR
based on geospatial analysis. Uncertainty was included in
the COLE analysis. The disturbance estimate for the region
includes the weighted averaging of many different forest sites
impacted by MCM and thus the uncertainty associated with
the variance of the sample mean was used (i.e., (2 std. dev.).
The carbon density for the forest pools was normalized by
the forest area removed and disturbed due to MCM in the
SAFR. Estimates of the disturbed forest area have been
previously reported for the region (7, 8).
The net increase in atmospheric CO2 resulting from this
forest disturbance will depend on the fate of the disturbed
nonsoil and soil carbon. The fate of nonsoil carbon will
depend on harvested wood, natural regrowth on MCM sites,
and natural sequestration foregone due to disturbance. First,
wood may be burned on site or harvested. We consider the
potential for harvest-related sequestration in our low emission estimates and for burning on site in our upper bounds.
Harvest will reduce net CO2 emissions associated with
terrestrial disturbance through sequestration from wood
products that are in use, in landfills, and burned with energy
capture. We consider the potential sequestration from harvest
using published relationships between growing stock and
harvest-related sequestration (product in use, landfill, and
energy capture) (14). Second, regrowth on MCM sites after
disturbance will reduce estimates of net CO2 emissions.
Reclaimed areas in the Appalachian coal belt show regrowth
of only 3% of nonsoil carbon after 15 years. However, the
carbon recovery may be even more limited because only 2%
of land disturbed by coal mining in the United States has
been reclaimed and bond released (9) and existing reclamation in the SAFR has focused on erosion prevention and
bankfill stability and not reclamation with trees (7, 8). We
estimate regrowth as 3% of undisturbed nonsoil carbon every
15 years (15). Third, the natural sequestration foregone due
to disturbance will tend to increase net emissions estimates.
The amount of carbon that would have been sequestered
had mining not occurred is estimated using COLE carbon
stocks and age class data. For the analysis of net emissions
we consider the accumulated emissions during the first 50
years after mining which may be conservative if timber
products continue to decay after this point while foregone
sequestration accumulates at a faster rate than natural
regrowth (see Supporting Information Table S1).
The fate of soil carbon may be emitted to the atmosphere
due to mineralization or sequestered through incorporation
into surface or buried layers. While most field studies show
diminished soil carbon on reclaimed mines, some studies
find recovery after disturbance to levels similar to undisturbed
lands in 15 years for surface soils (15). The fate of soil carbon
is highly uncertain and needs further research so we consider
the potential for complete sequestration in our conservative
estimates and for complete mineralization in our upper
bounds.
2.2. Geogenic Organic Carbon Transferred to the Soil
Reservoir. As MCM is completed, the land surface is replaced
with compacted mining spoil after mining in order to prevent
erosion and maintain backfill stability (7). Mining spoil
contains high amounts of coal fragments, termed geogenic
organic carbon (GOC). The mining operation effectively
relocates once buried GOC to the soil column and enables
potential interaction of GOC with plant biomass in the
terrestrial environment.
An estimate of the amount of GOC brought to the soil
column during MCM was calculated for SAFR using a mass
balance mixing model analysis within a Monte Carlo
uncertainty simulation based on stable carbon isotopic
measurements of coal, spoil from reclaimed soils, and
undisturbed soil samples collected from the SAFR and
following existing methodologies (16, 17). The stable carbon
isotope is given in delta notation as
δ13C )
(
)
Rsample
- 1 103
Rstd
(1)
where Rsample is the isotope ratio (13C/12C, where C is carbon)
of the sample, Rstd is the isotope ratio of the standard (Vienna
Pee Dee Belemnite, VPDB), and the equation is multiplied
by 103 in order to convert δ13C to per mil (‰). Soil samples
collection and isotopic analysis has been previously reported
(18), and methods are briefly explained here. To collect the
reclaimed mining soil samples, a 4 ha grid was established
and 10 soil pits were excavated and samples were collected
from the 0-5 cm depth, dry sieved with a 2 mm sieve, and
homogenized. The soil was representative of conditions
broadly defined as reclaimed mining soil but was a mixture
of GOC from coal fragments as well as SOC that was
accumulating at the soil surface. A GOC isotopic end member
fingerprint was obtained using published carbon isotopic
data from 22 coal samples with a range of different maceral
contents collected in the region (19). An SOC isotopic
fingerprint was obtained by collecting surface soil samples
from the region in areas where surface mining had not
disrupted the surface soils, and the samples included both
grassland and forest samples (18). Four samples were
collected and homogenized similar to the reclaimed mining
soil. All samples were analyzed using isotope ratio mass
spectrometry following the methods in Campbell et al. (18).
Information that resulted from the soil analysis is provided
in Table 1 for the grassland and forest samples as well as in
Campbell et al. (18). Mean and standard deviation of the
GOC samples were 23.57 and 0.64‰. To estimate proportion
the amount of GOC in the reclaimed mining soils, a mass
balance unmixing model analysis was performed that
included Monte Carlo sampling to assess the uncertainty
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TABLE 1. Measurements of Total Soil Samples for δ13C (‰)
and C (%) for Reclaimed Soil (RS), Forest Soils (F1, F2, and
F3) and Grassland Soil
TABLE 2. Terrestrial Carbon Reservoirs by Forest Type in the
Southern Appalachian Forest Region (kg C m-2)
analysis
RS
F1
F2
F3
GS
forest type
total forest
carbon
soil
carbon
δ13C (‰)
C (%)
-25.81
5.71
-27.97
5.40
-28.15
3.59
-28.18
4.41
-27.06
3.02
eastern white pine
black locust
eastern hemlock
nonstocked
loblolly pine/hardwood
other exotic hardwoods
black cherry
loblolly pine
eastern redcedar
sugarberry/hackberry/elm/green ash
sassafras/persimmon
pitch pine
white pine/red oak/white ash
mixed upland hardwoods
post oak/blackjack oak
black walnut
shortleaf pine/oak
willow
shortleaf pine
sweetgum/yellow-poplar
virginia pine/southern red oak
virginia pine
red maple/oak
elm/ash/locust
yellow-poplar
swamp chestnut oak/cherrybark oak
sycamore/pecan/american elm
white oak/red oak/hickory
yellow-poplar/white oak/red oak
scarlet oak
white oak
chestnut oak/black oak/scarlet oak
baldcypress/water tupelo
northern red oak
chestnut oak
eastern redcedar/hardwood
sugar maple/beech/yellow birch
cherry/ash/yellow-poplar
red maple/upland
other pine/hardwood
river birch/sycamore
hard maple/basswood
weighted average
6.1
7.1
7.2
8.5
8.8
9.5
10.5
10.8
10.9
11.5
11.8
11.9
12.1
12.4
12.4
12.5
13.5
13.8
13.9
14.3
14.4
14.4
14.6
14.7
14.8
15.1
15.4
15.5
15.7
15.9
16.1
16.1
17.3
17.4
17.4
17.9
19.0
19.7
19.8
19.9
20.0
20.8
15.8
4.6
4.1
3.8
6.6
4.2
6.1
4.2
4.5
3.8
5.0
4.2
4.2
4.5
4.2
3.9
4.5
4.2
9.6
4.2
3.9
4.5
4.2
4.5
6.9
4.3
5.3
6.4
4.3
4.4
4.0
4.2
4.4
5.3
4.8
4.5
5.2
6.1
6.8
6.5
5.8
8.1
6.4
4.7
associated with estimates of the end-member contribution.
An end member unmixing was performed using the δ13C
data and a mass balance mixing model as follows
δ13CRS ) PGOCδ13CGOC + PSOCδ13CSOC
(2)
PGOC + PSOC ) 1
(3)
and
where δ13CRS is isotopic value of the reclaimed soil sample
and δ13CGOC and δ13CSOC are the mean carbon isotopic delta
values of the GOC and SOC end-members, respectively. PGOC
and PSOC are the fraction of organic carbon derived from
each source. Equation 2 was solved for PGOC and PSOC using
eq 3, which constrains that the fractions sum to unity. The
unmixing was solved using Monte Carlo realizations that
drew samples from the δ13C distributions of the sources, and
each realization was solved independently. The δ13C distributions of the data were assumed normal, and 10 000
realizations were performed.
2.3. Carbon Dioxide Emissions During Coal Burning
for Energy Production. Estimates of the carbon flux from
coal burning as CO2 emissions under current power-plant
combustion practices were also calculated for coal extracted
using MCM in the SAFR. Coal extraction information using
MCM in the SAFR, including southern West Virginia, eastern
Kentucky, southwestern Virginia and eastern Tennessee, has
been reported by the USDOE for the time period from 1995
to 2007 (20). The mean and variance of coal extracted per yr
was calculated and carried through the analysis to provide
average estimates as well as the uncertainty associated, and
it was calculated that 118((15) million t coal yr-1 was
extracted from MCM in the SAFR. The method of Quick and
Glick (21) was used to calculate the CO2 emissions associated
with burning the coal. Quick and Glick (21) provide a
nomograph that can be used to calculate CO2 emission factors
based on the coal type and grade on a net energy basis,
which overcomes limitations of CO2 emission estimates based
on numerical factors applied to broad rank categories. Using
the method of Quick and Glick (21) (see Figure 1, pg 806 in
ref 21), a ratio of 2.56((0.3) kg CO2 emitted to kg gross coal
extracted was found for the high-volatile, bituminous coal
extracted from the SAFR region and mean and uncertainty
values were used throughout the analysis.
2.4. Fossil Fuel Carbon Burned During Coal Extraction
and Transportation. It is known that greenhouse gases are
emitted during mining, refinement and coal transportation
and thus these components should be considered in the lifecycle of coal production. Emissions of CO2, CH4, and N2O
emitted during coal extraction and transportation were
estimated on a per ton of coal extracted basis using the lifecycle emission factors presented by Spath et al. (2) and
Koornneef et al. (3) for DOE life-cycle modeling of surface
mining in the central and eastern United States. The CO2
equivalent (CO2-e) emissions rates for mining and transportation are 0.06 g CO2-e/g coal-1 and 0.04 g CO2-e/g coal-1,
respectively.
3. Results
Table 2 summarizes the forest carbon pools estimated using
COLE for the different forest types in the SAFR that were
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used to calculate the soil and nonsoil carbon removed during
MCM. The weighted average of the carbon stocks for the
SAFR is indicated in Table 2 and includes of 4.7((0.1) kg C
m-2 and 11.1((0.2) kg C m-2 for the soil and nonsoil C pools.
Using previously published information for the SAFR reported
in USEPA (7) and Wickham et al. (8), it was estimated that
15,190 ha yr-1 of forest will be removed and disturbed between
1992 and 2012 due to MCM. Based on the average C density
for the soil and nonsoil forest pools, it was calculated that
0.71((0.03) Tg C yr-1 is removed as soil and 1.69((0.03) Tg
C yr-1 is removed as the nonsoil carbon pool.
The contribution of the nonsoil carbon disturbance to
net CO2 emissions depends on the wood harvest, natural
regrowth, and foregone sequestration. Timber may be burned
on site or harvested. For the case of harvest we find that
sequestered carbon (wood products in use, wood in landfills,
wood burned with energy capture) at 50 years after disturbance is 14% of the nonsoil carbon disturbed or 1.6 kg C m-2
for the SAFR. Natural regrowth and foregone sequestration
of nonsoil carbon at 50 years after disturbance are 1.1 kg C
m-2 and 2.5 kg C m-2, respectively. The sum of these terms
suggests that the net carbon emissions from nonsoil carbon
is 2% smaller than gross disturbance if wood is harvested
and 12% larger if wood is burned at the mining site.
The end member mixing model in eqs 2 and 3 was solved
using Monte Carlo realizations in order to estimate the
TABLE 3. Estimates of MCM Disturbance in Comparison with Existing Life-Cycle Emissions for Conventional and CCS Power
Plants. All Values Are Reported in Tg CO2 yr-1
GOC transferred to soil reservoir
net forest soil emissions
net forest plants/litter emissionsa
extraction emissions
transport emissions
power plant emissions
conventional-Low
conventional-High
CCS-low
CCS-high
0.0
0.0
6.0((0.1)
9.4
5.7
302((78)
27.5((5.9)
2.6((0.1)
6.9((0.1)
9.4
5.7
302((78)
0.0
0.0
7.8((0.1)
12.2
7.4
39.3((10)
35.8((7.6)
3.4((0.1)
9.0((0.1)
12.2
7.4
39.3((10)
a
Low emission assumes wood harvest and soil C reaches undisturbed conditions and high emission assumes wood is
stock piled and burned during mining and soil C is mineralized and remains degraded. b Fossil fuel burned during
extraction and transportation are based on the aggregate of CO2, CH4 and N2O emission.
contribution of GOC (PGOC) transferred to the soil reservoir.
Results were that the soil column of the reclaimed grassland
sites have a GOC concentration of 2.73((0.59) g C per 100 g
soil. Assuming a homogeneous overburden soil with depth
and a constant bulk density of overburden equal to 1.8 × 103
kg m-3, it was estimated that the GOC in mining soils was
49.1((10.4) kg C m-2 per one meter soil column. An estimate
of GOC transferred to the soil reservoir was calculated by
multiplying the GOC density by the disturbance rate of MCM
in SAFR of 15,190 ha yr-1 of forest removed. It was estimated
that 7.5((1.6) Tg C yr-1 of GOC that was previously buried
is transferred to the soil reservoir.
Carbon flux from coal burning as CO2 emissions under
current power-plant combustion practices was estimated
using analysis of the USDOE information (20), which provided
a result of 118((15) million t coal yr-1, and the ratio of
2.56((0.3) kg CO2 emitted to kg gross coal extracted from
Quick and Glick (21) for the high-volatile, bituminous coal
of the SAFR. It was estimated that 302((78) Tg of CO2 yr-1
was emitted to the atmosphere due to burning coal extracted
from the SAFR.
Greenhouse gases emitted were calculated using published factors (23) for mining, refinement and transportation
of coal extracted from the region and the results are presented
as CO2-e. It was estimated that the additional components
of the coal life-cycle were 9.4 Tg of CO2-e emitted yr-1 during
coal extraction and refinement and 5.7 Tg of CO2-e emitted
yr-1 for transportation of coal.
Table 3 summarizes the estimates of carbon redistributed
due to coal production in the SAFR on an average per year
basis. The table provides values reported as CO2. Both lower
and upper bounds are included in the table. In Table 3,
estimates of carbon redistributed from MCM in SAFR was
also accessed for the scenario when 90% of CO2 is captured
under CCS technology, which is an overall goal of the USDOE
Carbon Sequestration Program (6). When calculating the
percentages under the CCS technology, a 30% increase in
coal production needed to power the CCS methods was
applied in order that carbon fluxes could be analyzed for the
same energy output when comparing current practices and
CCS targets (22).
4. Discussion
The fate of removed soil C during MCM needs further
quantification and management. While stockpiling of topsoil
might be performed initially, replacement of topsoil to the soil
surface after mining is not practical for soil stability due to the
steep slopes of the SAFR, and soil C is either mineralized after
the disturbance, eroded, and transported to forest streams or
siltation ponds, buried in neighboring valleys, or mixed with
mining spoil and replaced to the soil surface (15).
The fate of newly deposited GOC within the terrestrial soil
reservoir is also relatively unknown and it is suggested that a
portion of the GOC will enter the active carbon pool and interact
with the soil-plant-atmosphere system while some will be
eroded to streams (16). While oxidized biomass, or biochar,
has recently received attention for its ability to retain carbon
in agricultural systems (23), the fate of GOC including coal
fragments and different macerals is relatively unknown and
results from stable carbon isotopic analysis of lignite C have
suggested that the GOC may be incorporated into fresh plant
material becoming part of the active C cycle (16). Further,
erosion of mining spoil during mining and in the three years
following mining completion can produce sediment loads 1000
times undisturbed conditions containing GOC that are transported in mining watersheds (24); and analysis of published
data (24, 25) show that total sediment yield from post-SMCRA
surface mining operations averages 193 t ha-1. The fate of GOC
associated with transported sediment particulate organic matter
is essentially unknown and requires further research as to its
export, outgassing, and burial.
Given the uncertainty associated with soil C and GOC, a low
estimate of potential life-cycle emission that neglects these two
factors suggests that life-cycle emissions are at least 7 and 70%
of power plant emissions for conventional and CO2 capture
and sequestration power plants, respectively. The high estimate
places life-cycle emissions at 17 and 173% of power plant
emissions for conventional and CCS power plants. The significance of the terrestrial carbon impacts is apparent. Further,
the increased relative importance of the terrestrial reservoir
after CCS technologies are put in place is also apparent.
In both cases of current combustion practices and future
CCS goals, the terrestrial carbon storage impacted by the
disturbance of MCM is shown to be significant. It is argued
here that the terrestrial carbon impact be included in the
ongoing discussion of coal mining life-cycle emissions and
be considered when discussing energy production and
environmental sustainability. Further, terrestrial carbon
redistributed under CCS technology should be accounted
when setting future goals. A discussion is needed of what
incentives may be put in place in order that interactions
between terrestrial carbon disturbance and coal production
via surface mining methods can be optimized, e.g., optimal
mining surface disturbance practices, soil and biomass
storage, and reclamation practices.
In order to agree on informed decision-making, the sustainability discussion begs the need for ongoing and future
scientific research, discussion, and thereafter management to
address a sustainable trajectory for terrestrial carbon and coal
production interactions. While a number of studies have been
performed to understand the dynamics of mining and reclamation in the SAFR (26, 27), specifically, three areas are
highlighted that require further quantification for assessing
MCM impact on terrestrial carbon including (1) the management of removed soil C and nonsoil C from MCM sites; (2) the
long-term uptake of carbon from the atmosphere during
recovery of the forest terrestrial system; and (3) the fate of newly
deposited GOC within the terrestrial soil reservoir.
A number of potential indirect terrestrial carbon changes
associated with MCM can also occur that could require further
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quantification and are associated with hydrologic and geomorphologic changes impacting dissolved and particulate
organic carbon transport, including increased erosion of forest
carbon due to mining road construction, decreased time to
peak flow and increased magnitude of peak flow due to
decreased soil permeability of the soil surface after mining,
and increased transport of fine coal C particulates in the
subsurface of mined lands. In general, the carbon flux associated
with these hydrologic processes in streams and rivers is small
relative to other carbon reservoirs and fluxes (28); however,
CO2 flux associated with aquatic systems has been shown to
be significant in some systems (29, 30) and these topics would
require further research. Furthermore, the effects of MCM on
streamwater ecosystems can be large (31-33) and should be
considered as a further biproduct of the MCM practice.
Another result of the present analysis worth discussing is
the ratio of carbon disturbed relative to the annual coal carbon
produced for this region, which was found to equal 0.03.
This estimate considers that land disturbed by the mines is
immediately used to harvest coal and thus the 20 yr estimate
is reflective of finished MCM and their impact upon forest
carbon pools. For smaller surface mines, which typically have
short life expectancies lasting one or two years, the consideration is justifiable; however, larger mines can have a life
expectancy of 10-15 years (34), and some of the disturbed
areas reported here will continue to produce coal, which
potentially overestimates the ratio of disturbed forest to coal
production. Alternatively, our ratio of disturbance relative to
coal production may be an underestimate when projecting
into the future because minable coal seams are becoming
deeper and thinner in the SAFR, thus larger areas of forest
disturbance will be needed to harvest the same amount of
coal as in previous years (35). Further, the amount of
disturbed forest carbon is potentially underestimated because
the average county-level carbon densities (14), can be less
than half the carbon densities in the higher elevations mining
sites where high precipitation enhances forest growth and
low temperatures inhibit respiration and decomposition (36).
Given these uncertainties, our estimated ratio of carbon
disturbed relative to the annual coal carbon produced for
this region is considered justifiable when considering present
conditions as a baseline for improved carbon sequestration
strategies in future coal energy production practices.
Note Added after ASAP Publication
The author affiliations were incorrect in the version published
on February 8, 2010. The correct version was published March
11, 2010.
Supporting Information Available
Table S1. This material is available free of charge via the
Internet at http://pubs.acs.org.
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