Terrestrial Carbon Disturbance from Mountaintop Mining Increases

Feb 8, 2010 - Within coal producing regions, increased research has been initiated .... The stable carbon isotope is given in delta notation as (1) wh...
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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. 2144

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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 VOL. 44, NO. 6, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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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 2146

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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 VOL. 44, NO. 6, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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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.

Literature Cited (1) USDOE, Energy Information Administration website. http:// www.eia.doe.gov/ and Dynamic Querying System, accessed May 27, 2009. (2) Spath P.; Mann M.; Kerr D. Life Cycle Assessment of Coal-Fired Power Production; U.S. DOE National Renewable Energy Laboratory: Golden, CO, 1999; p 172. (3) Koornneef, J.; van Keulen, T.; Faaij, A.; Turkenburg, W. Life cycle assessment of a pulverized coal power plant with postcombustion capture, transport and storage of CO2. Int. J. Greenhouse Gas Control. 2008, 2 (4), 448–467. (4) Freund, P. Making deep reductions in CO2 emissions from coalfired power plant using capture and storage of CO2. Proc. Inst. Mech. Eng., Part A 2003, 1–7. (5) Davidson, J. Performance and costs of power plants with capture and storage of CO2. Energy. 2007, 32, 1163–1176. (6) U.S. DOE. Carbon Sequestration Technology Roadmap and Program Plan; U.S. DOE Office of Fossil Energy: Washington, DC, 2007; p 48. 2148

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