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Comparative Human Toxicity Impact of Electricity Produced from Shale Gas and Coal Lu Chen, Shelie A. Miller, and Brian R. Ellis Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03546 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 12, 2017
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Environmental Science & Technology
Shale Gas
Coal
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Comparative Human Toxicity Impact of Electricity Produced from Shale Gas and Coal
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Authors: Lu Chen1,2, Shelie A. Miller1,2*, Brian R. Ellis2
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Affiliations: 1
Center for Sustainable Systems, School for Environment and Sustainability, University of Michigan, 440 Church Street, Ann Arbor MI 48109 2
Department of Civil and Environmental Engineering, University of Michigan, 2350 Hayward Street, Ann Arbor MI 48109 *Corresponding author: Shelie A. Miller, 440 Church Street, Ann Arbor MI 48109; 734-7638645; [email protected] Keywords: hydraulic fracturing, risk assessment, life cycle assessment
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The human toxicity impact (HTI) of electricity produced from shale gas is lower than the HTI of
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electricity produced from coal, with 90% confidence using a Monte Carlo Analysis.
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different impact assessment methods estimate the HTI of shale gas electricity to be one-to-two
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orders of magnitude less than the HTI of coal electricity (0.016-0.024 DALY/GWh versus 0.69-
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1.7 DALY/GWh). Further, an implausible shale gas scenario where all fracturing fluid and
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untreated produced water is discharged directly to surface water throughout the lifetime of a well
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also has a lower HTI than coal electricity. Particulate matter dominates the HTI for both
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systems, representing a much larger contribution to the overall toxicity burden than VOCs or any
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aquatic emission. Aquatic emissions can become larger contributors to the HTI when waste
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products are inadequately disposed or there are significant infrastructure or equipment failures.
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Large uncertainty and lack of exposure data prevent a full risk assessment; however, the results
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of this analysis provide a comparison of relative toxicity, which can be used to identify target
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areas for improvement and assess potential tradeoffs with other environmental impacts.
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Introduction
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operations. Recent studies quantify water quality issues associated with shale gas,1-3 although the
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magnitude and frequency of contamination events are still not well documented. While concerns
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of water contamination and VOC emissions associated with shale gas production may be
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warranted, it is also important to contextualize the potential human toxicity impacts of shale gas
39
relative to other sources of electricity.
Potential contamination of drinking water is a major concern surrounding hydraulic fracturing
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A growing number of studies investigate the potential environmental impacts related to
41
hydraulic fracturing. A recent analysis suggests that spills of fracturing fluid could impact soil
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health due to the ionic strength of flowback water, although the study found that human health
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concerns were minimal.4 A variety of studies have highlighted concerns regarding major water
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withdrawals and contamination events due to stray gas leakage, accidental spills, and wastewater
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disposal.3,
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fracturing, although some localized analyses are being conducted.8 VOC signatures from winter
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haze events have been linked to the Bakken,9 although it is difficult to differentiate between
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activities associated with hydraulic fracturing and other oil and gas extraction activities. Other
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analyses suggest the contribution of VOCs to changes in air quality is minimal,8, 10 indicating the
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need for further investigations into VOC emissions associated with hydraulic fracturing.
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Additional studies have begun to assess the toxicity of components of fracturing fluids and
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flowback water.11
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on a variety of environmental concerns, including greenhouse gas emissions, water use, and
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criteria air emissions;1,
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analysis comparing electricity produced from shale gas and coal.
5-7
Data are scarce regarding changes to air quality associated with hydraulic
Life cycle assessments of electricity produced from shale gas have focused 12-16
however, there does not appear to be a comprehensive toxicity
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The relative human toxicity impact (HTI) of electricity produced from shale gas and coal
57
are compared to better understand how increased penetration of shale gas will affect overall toxic
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releases associated with the power sector. HTI is commonly used within life cycle assessment to
59
quantify the inherent toxicity burden associated with emissions from a product or process.17 The
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HTI is measured as disability-adjusted life years per unit of electricity generation (DALY/GWh)
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and is calculated by means of generic fate and exposure assumptions applied consistently across
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all systems. The HTI serves as a useful screening metric and an initial step toward a full risk
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assessment. Characterization factors (CFs), sometimes referred to as human toxicity potentials
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(HTP), estimate human health damage for a wide range of chemicals and are expressed as
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DALY/kg emission. The results of life cycle impact assessment are subject to a great deal of
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variability and uncertainty. Therefore, this analysis uses a variety of approaches to test the
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robustness of the results. Two scenarios, representing a baseline case and an accidental release
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scenario, are used to examine different sets of potential assumptions. Both the USEtox 2.0 and
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ReCiPe 2016 impact assessment methods are used to demonstrate how different sets of CFs may
70
influence the results.18, 19 Local sensitivity analysis in the form of one-at-a-time perturbation is
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used to assess the degree of influence associated with individual parameters. Finally, Monte
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Carlo Analysis is used as a global sensitivity method to simulate how both variability and
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uncertainty across parameter ranges affect the outcome of the analysis.
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This study conducts a comparative analysis of the toxicological human health effects of
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electricity produced from shale gas and coal. Human toxicity is a single life cycle impact
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assessment indicator and is not a comprehensive analysis of environmental factors. A complete
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life cycle assessment (LCA) should include a broader suite of impacts, including climate change,
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ecosystem quality, resource depletion, land use, and water use. The evolving nature of research
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on the environmental impacts of hydraulic fracturing is characterized by data limitations and lack
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of consensus on individual indicators such as climate change.20 Focused, in-depth analyses on
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discrete indicators are still needed to be able to compile the necessary information to complete a
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comprehensive LCA. Therefore, the results of this analysis on human toxicity must be used
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alongside compatible studies focusing on other indicators in order to understand the overall
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environmental context.
85 86 87
Methods The scope of this analysis is a human toxicity impact assessment of direct chemical
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emissions associated with shale gas and coal electricity.
The inventory includes chemical
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emissions that occur during resource extraction and electricity generation, the stages with the
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greatest associated toxicity. Baseline case and accidental release scenarios are constructed for
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both energy systems. The baseline scenario approximates normal operating conditions, while the
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accidental release scenario simulates major unintended releases of emissions for each system to 3
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calculate an upper toxicity limit. For the shale gas system, the study includes aquatic emissions
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of fracturing fluid chemicals and produced water associated with shale gas extraction, air
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emissions of NOx, PM, and VOCs during shale gas extraction, and emissions of PM, VOCs, NOx
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and SOx during electricity generation. To compensate for data scarcity associated with shale gas
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production and improve confidence in the robustness of the results, the toxicity of the shale gas
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system is overestimated whenever there is insufficient or missing data. For the coal system, the
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study estimates the toxicity associated with effluent loadings from coal mine outfalls, air
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emissions of PM, Hg, VOCs, NOx, and SOx during electricity generation, and aqueous emissions
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from coal ash impoundments. For the coal accidental release scenario, emissions of metals
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associated with a coal ash spill are also included.
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transportation, infrastructure, and cooling water are not included in the analysis, due to minor
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contributions to overall toxicity.1, 21 Figure 1 depicts the shale gas and coal systems.
Chemical emissions associated with
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.
106 107 108
a)
b)
109 110 111 112
Figure 1. a) Sketch of shale gas electricity system, including hydraulic fracturing operations and electricity generation; b) Sketch of coal electricity system, including coal mining and electricity generation.
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Pennsylvania is used as the point of origin for both shale gas and coal, given the
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abundance of both sources of energy within the region. When Pennsylvania specific data are not
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available, alternate datasets are used that can be reasonably assumed to be consistent with coal 4
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and shale gas resources originating from the region. National averages are used for efficiency
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and air emissions associated with natural gas and coal electricity generation, as the resources
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produced within the region are distributed nationally.22
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Human Toxicity Calculations
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Chemical emissions that have a direct toxicological effect on human health are included in the
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analysis. Factors that are non-toxicity related (e.g., noise, light, stress), have an indirect effect on
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human health (e.g., ozone depletion, climate change), or cause environmental damage not related
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to human health (e.g., salinity, pH, turbidity) are outside the scope of the analysis. Toxic releases
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are measured using disability-adjusted life years (DALY) per GWh of electricity produced. A
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DALY is a common metric to measure impact on human health and corresponds to the number
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of years lost due to poor health, disability, or premature death. 18, 19
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To calculate HTI of electricity produced from coal and shale gas, the USEtox 2.0 (released in
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2015) and ReCiPe 2016 characterization factors are used.18, 19 USEtox and ReCiPe are impact
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assessment methods that estimate the toxicity of a given quantity of emissions by providing CFs
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that estimate human health damage for a wide range of chemicals. Human toxicity CFs are
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averages for North America. Both USEtox and ReCiPe are long-term exposure assessment
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methods and consider the impact on the general population. The time horizon is 100 years for
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USEtox and infinite for ReCiPe’s Egalitarian assumptions. Human health impacts due to acute
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workplace exposures are not taken into account. USEtox 2.0 is a scientific consensus model
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recommended as the preferred database for calculating HTI by the United Nations Environment
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Programme and the Society of Environmental Toxicology and Chemistry’s Life Cycle Initiative.
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ReCiPe was developed by a consortium of life cycle assessment practitioners. The two methods
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were developed independently and both provide endpoint level CFs for carcinogenic and non-
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carcinogenic toxicity. Life cycle impact assessment is an accepted method to quantify the
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relative hazard and importance of pollutants when data for a full-scale risk assessment are not
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available.23
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CFs from both models are reported due to the large degree of uncertainty in developing
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appropriate CFs and differences in CFs for key constituents of this analysis. The sensitivity 5
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analysis addresses concerns regarding CF uncertainty in greater detail. Eqn. 1 is the generic form
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for calculating HTI for an individual emission.
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= ×
Eqn. 1.
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Where i represents each unique chemical, is the human toxicity impact for each emission
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(DALY/GWh), is the characterization factor for each chemical (DALY/kg emission), is
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the emissions of each chemical associated with a given amount of electricity generation
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(kg/GWh). Total HTI ( ) for life cycle electricity generation is the sum of the individual
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chemical impacts (Eqn. 2).
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= ∑
Eqn 2.
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The HTI of PM reported in this analysis is the aggregate human health damages resulting
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from both PM2.5 and PM10 formed from both primary (directly emitted to the atmosphere) and
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secondary (generated by chemical reactions associated with emissions such as NOx and SOx)
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sources.
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PM has a different mechanism for impacting human health than other chemicals in the
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inventory and the CF for PM is derived via a different approach. The ReCiPe model contains a
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CF for PM2.5, as well as for NH3, NOx, and SO2, which contribute to secondary PM aerosol
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formation. USEtox does not include a CF for PM, so the USEtox method is supplemented with a
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method described by Gronlund et al.
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and thirteen coal-fired power plants within Pennsylvania are used to calculate the HTI of PM,
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using production-weighted averages of electricity generation for each plant.25 The Supporting
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Information contains relevant assumptions and calculations for the HTI of PM.24, 25
24
Emission data collected from twenty-three natural gas
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Shale Gas
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In an effort to be conservative due to uncertainty and lack of data, assumptions are made to
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overestimate the potential toxic emissions for the shale gas baseline whenever limited data
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inhibit the analysis. A number of pathways exist for chemicals within fracturing fluid and 6
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produced water to enter the environment. Potential emission pathways include storage failures of
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fracturing fluids and produced water at the surface, produced water migration through the
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subsurface into groundwater, contamination from faulty casings, and as effluent from wastewater
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treatment plants.26
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To estimate potential releases of fracturing fluid chemicals, baseline and accidental
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release scenarios are constructed. Data from 2990 wells from FracFocus within Pennsylvania are
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used, containing a total of 368 chemicals. FracFocus 3.0 is a machine-readable database that
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includes self-reported chemicals used in fracturing fluid formulations from over 62,000 wells
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throughout the United States.27 The ID number, total fracturing fluids volume, purpose of each
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chemical (corrosion inhibitor, proppant, etc.), and the concentration of each component were
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recorded for each well. To account for differences in chemical compositions due to geographic
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and operational variations, average well information for each Pennsylvania County represented
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in FracFocus as of October 2015 was used. Not all wells use the same chemicals, so the average
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concentration was used for the 100 most frequently used chemicals from 2010 to 2015 (Table
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S1). Most of the fracturing formulations surveyed within FracFocus include less than 40
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chemicals, so using the average concentration of the 100 most frequently used chemicals
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overestimates the HTI for shale gas.
197 198
Table 1. Assumptions used for baseline and accidental release shale gas scenarios and parameter ranges found
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within the literature for comparison.
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Information.
Calculations and further justifications are found within the Supporting
201 Parameter
Unit
Average fracturing fluid volume
m3/well
Flowback water percentage share of injected fluid Produced water percentage share of injected fluid Shale gas electricity generation efficiency Shale gas well lifetime span Shale gas production volume Barium removal efficiency Fracturing fluid release rate
Baseline case 13000
Accidental release 13000
Literature Range 3500-26000
Reference 27
%
15
15
10-15
%
5
5
1-7
Mcf / kWh year MMcf %
0.0101
0.0101
28-30
15 4300 90
15 4300 0
0.009640.0134 5-30 1600-9000 0 – 99
% released
1
100
See details in text
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Currently, no database exists that systematically records spills and other releases.35 The baseline
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case is conservative to compensate for data gaps in toxicity and chemical composition. The
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baseline case assumes that 1% of fracturing fluids are emitted to surface water without any
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treatment, although the discharge could occur via any of the potential emissions release
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mechanisms. A recent study estimated the total known spill volume of fracturing fluid from 6000
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wells to be 174 m3 from 2008 to 2013,35 averaging 0.03 m3 per well during that period. The
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assessment report from the EPA lists the median spill volume as 1.6 m3 per well that reports a
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spill (range 0.19-72 m3), with 0.4-12.2 spills per 100 wells, which is similar to data reported
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elsewhere.36, 37, although it does not necessarily account for spills that go unreported. Of the
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spills that are reported, the EPA estimates that only 9% of spills reach surface water. Assuming
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13000 m3 of fracturing fluid for each well (Table 1),27 a more reasonable spill rate is 0.001% of
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injected fluids, as opposed to the 1% assumed in the baseline scenario.
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In addition to fracturing fluids injected into the wells, produced water has the potential to
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contaminate drinking water sources. The chemical composition of flowback and produced water
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are different, with larger concentrations of naturally dissolved materials in produced water than
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flowback water. Some of the chemicals in produced water, especially organic compounds,
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originate from the injected fracturing fluids. It is difficult to differentiate naturally-occurring
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organic compounds from those that derive from fracturing fluid formulations. Because of the
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difficulty distinguishing between the sources of chemicals, all chemicals within produced water
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are treated as a separate source of emissions, effectively double-counting the fracturing fluid
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chemicals contained in produced water. The volume of flowback and produced water is assumed
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to be 20% of the injected water volume and have the average chemical composition of a
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compilation of 35,000 data entries for the Marcellus shale gas region reported by Abualfaraj et
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al.38 (Table S2).
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Produced water from hydraulic fracturing operations is assumed to be sent to a dedicated
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water treatment plant, which is the most common method of disposal in Pennsylvania.39
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Alternative methods of disposal include injection into Class II disposal wells and recycling for
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additional shale gas extraction operations.
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Chemical constituents of produced water can enter surface water via the same
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mechanisms as fracturing fluids, and also as effluent from wastewater treatment due to 8
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incomplete removal. For the baseline shale gas case, 10% of the chemicals within produced
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water are assumed to be discharged to surface water. For context, a recent study estimated the
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total known spill volume of produced water from 6000 wells to be 473 m3 from 2008 to 2013,35
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averaging 0.079m3 per well. The assessment report from the EPA lists the median spill volume
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of produced water as 37.5 m3 per well that reports a spill, with 0.4-12.2 spills per 100 wells.
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Assuming 2700 m3 of produced water per well (Table 1), then the spill rate of produced water is
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likely less than 1%. In addition, some fraction of the constituents of produced water are
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discharged as effluent from the wastewater treatment facility, so the overall estimate of 10%
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discharge is assumed to be reasonable and likely an overestimate for the baseline case.
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The accidental release scenario for shale gas assumes 100% of injected fracturing fluid
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and 100% of produced water is directly discharged to surface water over the entire lifetime of the
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well. While this scenario is highly improbable, it provides a maximum threshold for possible
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toxicity loads associated with shale gas.
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For air emissions from shale gas fields, values are obtained from a comprehensive study
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by Roy et. al, which quantifies total load of NOx, PM2.5, and VOC emissions per well associated
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with hydraulic fracturing operations.40 Their study estimates total load, but does not provide
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compositional analysis of VOCs. In the absence of compositional data from this study, the CF
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for benzene was used for all VOCs in this analysis. VOC composition from oil and gas sites tend
252
to have significant proportions of alkanes and alkenes that have lower toxicity than aromatics.
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Therefore, the use of the benzene CF is considered an overestimation of VOC contribution to
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HTI.
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Coal
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An EPA study on coal mine discharge41 was used in conjunction with annual
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Pennsylvania coal production estimates42 to estimate toxicity associated with coal mining.41 The
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study calculated loading of aluminum, iron, manganese, and total suspended solids in
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Pennsylvania streams impacted by acid mine drainage.
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To estimate human toxicity of mercury air emissions from coal-fired power plants, data
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was collected from the 100 largest power producers in the United Sates in 2013.43 Combustion
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residuals, also known as coal ash or fly ash, are assumed to be stored in surface impoundments. 9
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Some of the chemicals within the coal ash leach out of the disposal site and are released to the
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environment (Table S3).44 The coal ash effluent values are assumed to be representative of coal
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originating in Pennsylvania, given the locations of the power plants where data were obtained.
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An EPA study on power plant effluent, which sampled coal-fired power plants in the
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Appalachian region45, reported similar values. It is assumed that, in addition to ash, the coal ash
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impoundment also receives discharges of flue gas desulfurization waste water and transport
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water for fly and bottom ash. These waste streams appear to be included in reported outflow
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values and are therefore not calculated as separate emissions.
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Electricity generation was calculated using nameplate capacities of each power plant,
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assuming continual operation of all plants, 365 days/year, which overestimates the generation
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capacity of each plant, underestimating the HTI associated with electricity production. Wet
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disposal of coal ash is not the only possible method of disposal. Coal ash can also be reused in
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engineering applications or landfilled in a dry state. This choice is further addressed in the
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sensitivity analysis and discussion.
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The accidental release scenario for coal uses the same assumptions for all parameters of
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the baseline case, and also includes unintentional release of coal ash into surface water, similar to
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the impoundment failures that occurred in Kingston, TN in 2008 and Eden, NC in 2014.
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Although major releases of coal ash spills are infrequent, they have historical precedent and their
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inclusion provides an analogous system to compare with the accidental release scenario for shale
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gas.
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For the coal accidental release scenario, post-remediation data from the Kingston coal ash
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spill were used.46 Remediation efforts took place in 2009 and 2010 leaving 0.18 million m3 of
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ash in the Emory River. The density of ash is assumed to be 1500 kg/m3 and the HTI is
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calculated based on the volume of the ash left in the Emory River post-remediation. Babbit et al.
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calculated the average coal ash generation from four power plants in Florida as 0.0568 kg/kWh.21
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The National Renewable Energy Laboratory estimates the average coal ash generation rate as 0.1
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kg/kWh
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more conservative HTI estimate.
47
. The smaller coal ash generation value is used in this analysis in order to provide a
293 294
Data Limitations
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The scope of this study is intended to address the full life cycles of shale gas and coal
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electricity; however, data limitations necessitated a number of choices to be made regarding the
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life cycle inventory of both shale gas and coal. Because of these limitations, systematic choices
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were made to overestimate the coal inventory while underestimating the shale gas inventory.
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Most studies that report VOCs or non-methane hydrocarbons report total load but do not
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provide compositional analysis. Benzene is used as the CF for all VOCs in this analysis, which
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likely overestimates VOC contributions to total HTI for all scenarios.
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Data relevant to calculating the HTI of coal mining is rarely reported in a format that is
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conducive to translation into inventory data.
Mining emissions tend to be reported as
304
concentrations, without sufficient data to relate concentrations to mass emission/mass coal
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extracted needed for the inventory (e.g. stream flow, time since coal extraction, overall amount
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of coal extracted from site). Although other studies have found that coal mining may have
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significant human health impacts on nearby communities,48-50 lack of appropriate data inhibit
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inclusion of several aspects of the coal system into this analysis. Potential toxicity impacts are
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associated with cadmium and selenium in mine drainage; however, limited data prevented
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quantification of loading data from either element.41 Similarly, calculation of HTI associated
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with coal mining does not include particulate matter or other air emissions associated with
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mining that are analogous for the data obtained for the shale gas baseline case. Fugitive coal
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dust emissions along transportation routes has been linked to chronic community-level exposures
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of particulate matter and metals,51 but the format of data available inhibits translation into
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inventory metrics useful for this analysis.
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In addition to limitations associated with life cycle inventory collection, ReCiPe and
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USEtox do not contain a uniform list of chemicals, so a given chemical may be associated with a
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CF in one database and not in another. Characterization factors are also highly uncertain and a
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standard operating assumption is that a CF may vary by three orders of magnitude in either
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direction. 18, 19
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Different oxidation states of metals have different toxicities; however, oxidation states
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are not available from the datasets used in this analysis. To be consistent with the systematic
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overestimation of the shale gas HTI, the more toxic oxidation state is assigned for shale gas
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emissions whenever available. Similarly, mercury in the coal system is assumed to be present in
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its inorganic form, even though it has the potential to form organic mercury, such as 11
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methylmercury and dimethylmercury, which are more toxic than inorganic mercury.52 CFs for
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radionuclides are included in ReCiPe but not in USEtox. The effects on results of these issues
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are further discussed in the Results and Discussion.
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Uncertainty and Sensitivity Analysis
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A Monte Carlo Analysis (MCA) is used to determine the range of possible results and the
332
extent of overlap between the shale gas and coal cases. Triangular distributions are assigned to
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each of the parameters within the model, using the most likely value and upper and lower range
334
boundaries (see Tables 1, S10-S12). For the CFs, which span six orders of magnitude, triangular
335
distributions are assigned to the exponent of each parameter to reduce positive bias in sampling.
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The results of 10,000 trials are reported.
337
A one-at-a-time perturbation method is used to assess the sensitivity of the results to each
338
input parameter. The effect of each parameter on the HTI is determined by changing its value to
339
the extreme ends of its range while keeping all other parameters at their initial model value (see
340
Tables 1, S10-S12). CFs from USEtox are used as the default for both the sensitivity analysis and
341
the MCA, unless a CF was available only in ReCiPe.
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In addition to the sensitivity analysis, two alternate operating scenarios are explored for
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coal. The scenarios are disposal of coal ash via a dry storage method and implementation of the
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Mercury Air Toxics Standards (MATS) regulation.53
345
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Results and Discussion
PM
VOC
As
Others
Ba
DMF
Hg (air)
Hg (coal ash)
100
HTI (DALY/GWH)
10
1
0.1
0.01 USEtox
ReCiPe
USEtox
Shale Gas
ReCiPe
Coal
USEtox
ReCiPe
Shale Gas
Baseline Scenario
USEtox
ReCiPe
Coal
Accidental Release Scenario
350 351 352 353 354 355 356
Figure 2. Baseline and accidental release scenarios for electricity produced from shale gas and coal using the USEtox and ReCiPe2016 methods to quantify HTI. Data is depicted on a log scale. Major components of HTI include particulate matter (PM), volatile organic compounds (VOC), arsenic (As), barium (Ba), N, N’-dimethylformamide (DMF), and mercury (Hg). Constituents that appear in both systems are depicted in gray scale, whereas those only found in coal system are in orange and those only found in shale gas system are in blue.
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Figure 2 shows that the baseline scenario for coal-fired electricity has a greater HTI than
358
both the baseline and accidental release scenarios for shale gas, by at least an order of magnitude.
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The HTI baseline values are 0.016 DALY/GWh (USEtox) and 0.024 DALY/GWh (ReCiPe) for
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shale gas and 0.69 DALY/GWh (USEtox) and 1.7 DALY/GWh (ReCiPe) for coal. A prior study
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on coal toxicity using CML2001, another popular impact assessment method, estimated an HTI
362
of coal electricity to be between 0.24 DALY/GWh to 2.2 DALY/GWh,54 which aligns with the
363
results of this analysis.
364
Particulate matter is the dominant toxicity contributor for both shale gas (86% USEtox,
365
93% ReCiPe) and coal (92% USEtox, 98% ReCiPe), and includes calculation of primary
366
emissions of particulate as well as secondary aerosol formation (see Tables S6-S9). Other
367
contributors to the HTI of the coal baseline are air emissions of mercury (5.6% USEtox,
Article
Comparative Human Toxicity Impact of Electricity Produced from Shale Gas and Coal Lu Chen, Shelie A. Miller, and Brian R. Ellis Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03546 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 12, 2017
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
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Environmental Science & Technology
Shale Gas
Coal
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Comparative Human Toxicity Impact of Electricity Produced from Shale Gas and Coal
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Authors: Lu Chen1,2, Shelie A. Miller1,2*, Brian R. Ellis2
4 5 6 7 8 9 10 11 12 13 14 15 16
Affiliations: 1
Center for Sustainable Systems, School for Environment and Sustainability, University of Michigan, 440 Church Street, Ann Arbor MI 48109 2
Department of Civil and Environmental Engineering, University of Michigan, 2350 Hayward Street, Ann Arbor MI 48109 *Corresponding author: Shelie A. Miller, 440 Church Street, Ann Arbor MI 48109; 734-7638645; [email protected] Keywords: hydraulic fracturing, risk assessment, life cycle assessment
17
The human toxicity impact (HTI) of electricity produced from shale gas is lower than the HTI of
18
electricity produced from coal, with 90% confidence using a Monte Carlo Analysis.
19
different impact assessment methods estimate the HTI of shale gas electricity to be one-to-two
20
orders of magnitude less than the HTI of coal electricity (0.016-0.024 DALY/GWh versus 0.69-
21
1.7 DALY/GWh). Further, an implausible shale gas scenario where all fracturing fluid and
22
untreated produced water is discharged directly to surface water throughout the lifetime of a well
23
also has a lower HTI than coal electricity. Particulate matter dominates the HTI for both
24
systems, representing a much larger contribution to the overall toxicity burden than VOCs or any
25
aquatic emission. Aquatic emissions can become larger contributors to the HTI when waste
26
products are inadequately disposed or there are significant infrastructure or equipment failures.
27
Large uncertainty and lack of exposure data prevent a full risk assessment; however, the results
28
of this analysis provide a comparison of relative toxicity, which can be used to identify target
29
areas for improvement and assess potential tradeoffs with other environmental impacts.
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Introduction
35
operations. Recent studies quantify water quality issues associated with shale gas,1-3 although the
36
magnitude and frequency of contamination events are still not well documented. While concerns
37
of water contamination and VOC emissions associated with shale gas production may be
38
warranted, it is also important to contextualize the potential human toxicity impacts of shale gas
39
relative to other sources of electricity.
Potential contamination of drinking water is a major concern surrounding hydraulic fracturing
40
A growing number of studies investigate the potential environmental impacts related to
41
hydraulic fracturing. A recent analysis suggests that spills of fracturing fluid could impact soil
42
health due to the ionic strength of flowback water, although the study found that human health
43
concerns were minimal.4 A variety of studies have highlighted concerns regarding major water
44
withdrawals and contamination events due to stray gas leakage, accidental spills, and wastewater
45
disposal.3,
46
fracturing, although some localized analyses are being conducted.8 VOC signatures from winter
47
haze events have been linked to the Bakken,9 although it is difficult to differentiate between
48
activities associated with hydraulic fracturing and other oil and gas extraction activities. Other
49
analyses suggest the contribution of VOCs to changes in air quality is minimal,8, 10 indicating the
50
need for further investigations into VOC emissions associated with hydraulic fracturing.
51
Additional studies have begun to assess the toxicity of components of fracturing fluids and
52
flowback water.11
53
on a variety of environmental concerns, including greenhouse gas emissions, water use, and
54
criteria air emissions;1,
55
analysis comparing electricity produced from shale gas and coal.
5-7
Data are scarce regarding changes to air quality associated with hydraulic
Life cycle assessments of electricity produced from shale gas have focused 12-16
however, there does not appear to be a comprehensive toxicity
56
The relative human toxicity impact (HTI) of electricity produced from shale gas and coal
57
are compared to better understand how increased penetration of shale gas will affect overall toxic
58
releases associated with the power sector. HTI is commonly used within life cycle assessment to
59
quantify the inherent toxicity burden associated with emissions from a product or process.17 The
60
HTI is measured as disability-adjusted life years per unit of electricity generation (DALY/GWh)
61
and is calculated by means of generic fate and exposure assumptions applied consistently across
62
all systems. The HTI serves as a useful screening metric and an initial step toward a full risk
2
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assessment. Characterization factors (CFs), sometimes referred to as human toxicity potentials
64
(HTP), estimate human health damage for a wide range of chemicals and are expressed as
65
DALY/kg emission. The results of life cycle impact assessment are subject to a great deal of
66
variability and uncertainty. Therefore, this analysis uses a variety of approaches to test the
67
robustness of the results. Two scenarios, representing a baseline case and an accidental release
68
scenario, are used to examine different sets of potential assumptions. Both the USEtox 2.0 and
69
ReCiPe 2016 impact assessment methods are used to demonstrate how different sets of CFs may
70
influence the results.18, 19 Local sensitivity analysis in the form of one-at-a-time perturbation is
71
used to assess the degree of influence associated with individual parameters. Finally, Monte
72
Carlo Analysis is used as a global sensitivity method to simulate how both variability and
73
uncertainty across parameter ranges affect the outcome of the analysis.
74
This study conducts a comparative analysis of the toxicological human health effects of
75
electricity produced from shale gas and coal. Human toxicity is a single life cycle impact
76
assessment indicator and is not a comprehensive analysis of environmental factors. A complete
77
life cycle assessment (LCA) should include a broader suite of impacts, including climate change,
78
ecosystem quality, resource depletion, land use, and water use. The evolving nature of research
79
on the environmental impacts of hydraulic fracturing is characterized by data limitations and lack
80
of consensus on individual indicators such as climate change.20 Focused, in-depth analyses on
81
discrete indicators are still needed to be able to compile the necessary information to complete a
82
comprehensive LCA. Therefore, the results of this analysis on human toxicity must be used
83
alongside compatible studies focusing on other indicators in order to understand the overall
84
environmental context.
85 86 87
Methods The scope of this analysis is a human toxicity impact assessment of direct chemical
88
emissions associated with shale gas and coal electricity.
The inventory includes chemical
89
emissions that occur during resource extraction and electricity generation, the stages with the
90
greatest associated toxicity. Baseline case and accidental release scenarios are constructed for
91
both energy systems. The baseline scenario approximates normal operating conditions, while the
92
accidental release scenario simulates major unintended releases of emissions for each system to 3
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calculate an upper toxicity limit. For the shale gas system, the study includes aquatic emissions
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of fracturing fluid chemicals and produced water associated with shale gas extraction, air
95
emissions of NOx, PM, and VOCs during shale gas extraction, and emissions of PM, VOCs, NOx
96
and SOx during electricity generation. To compensate for data scarcity associated with shale gas
97
production and improve confidence in the robustness of the results, the toxicity of the shale gas
98
system is overestimated whenever there is insufficient or missing data. For the coal system, the
99
study estimates the toxicity associated with effluent loadings from coal mine outfalls, air
100
emissions of PM, Hg, VOCs, NOx, and SOx during electricity generation, and aqueous emissions
101
from coal ash impoundments. For the coal accidental release scenario, emissions of metals
102
associated with a coal ash spill are also included.
103
transportation, infrastructure, and cooling water are not included in the analysis, due to minor
104
contributions to overall toxicity.1, 21 Figure 1 depicts the shale gas and coal systems.
Chemical emissions associated with
105
.
106 107 108
a)
b)
109 110 111 112
Figure 1. a) Sketch of shale gas electricity system, including hydraulic fracturing operations and electricity generation; b) Sketch of coal electricity system, including coal mining and electricity generation.
113 114 115
Pennsylvania is used as the point of origin for both shale gas and coal, given the
116
abundance of both sources of energy within the region. When Pennsylvania specific data are not
117
available, alternate datasets are used that can be reasonably assumed to be consistent with coal 4
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and shale gas resources originating from the region. National averages are used for efficiency
119
and air emissions associated with natural gas and coal electricity generation, as the resources
120
produced within the region are distributed nationally.22
121 122
Human Toxicity Calculations
123
Chemical emissions that have a direct toxicological effect on human health are included in the
124
analysis. Factors that are non-toxicity related (e.g., noise, light, stress), have an indirect effect on
125
human health (e.g., ozone depletion, climate change), or cause environmental damage not related
126
to human health (e.g., salinity, pH, turbidity) are outside the scope of the analysis. Toxic releases
127
are measured using disability-adjusted life years (DALY) per GWh of electricity produced. A
128
DALY is a common metric to measure impact on human health and corresponds to the number
129
of years lost due to poor health, disability, or premature death. 18, 19
130 131
To calculate HTI of electricity produced from coal and shale gas, the USEtox 2.0 (released in
132
2015) and ReCiPe 2016 characterization factors are used.18, 19 USEtox and ReCiPe are impact
133
assessment methods that estimate the toxicity of a given quantity of emissions by providing CFs
134
that estimate human health damage for a wide range of chemicals. Human toxicity CFs are
135
averages for North America. Both USEtox and ReCiPe are long-term exposure assessment
136
methods and consider the impact on the general population. The time horizon is 100 years for
137
USEtox and infinite for ReCiPe’s Egalitarian assumptions. Human health impacts due to acute
138
workplace exposures are not taken into account. USEtox 2.0 is a scientific consensus model
139
recommended as the preferred database for calculating HTI by the United Nations Environment
140
Programme and the Society of Environmental Toxicology and Chemistry’s Life Cycle Initiative.
141
ReCiPe was developed by a consortium of life cycle assessment practitioners. The two methods
142
were developed independently and both provide endpoint level CFs for carcinogenic and non-
143
carcinogenic toxicity. Life cycle impact assessment is an accepted method to quantify the
144
relative hazard and importance of pollutants when data for a full-scale risk assessment are not
145
available.23
146 147
CFs from both models are reported due to the large degree of uncertainty in developing
148
appropriate CFs and differences in CFs for key constituents of this analysis. The sensitivity 5
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analysis addresses concerns regarding CF uncertainty in greater detail. Eqn. 1 is the generic form
150
for calculating HTI for an individual emission.
151 152
= ×
Eqn. 1.
153 154
Where i represents each unique chemical, is the human toxicity impact for each emission
155
(DALY/GWh), is the characterization factor for each chemical (DALY/kg emission), is
156
the emissions of each chemical associated with a given amount of electricity generation
157
(kg/GWh). Total HTI ( ) for life cycle electricity generation is the sum of the individual
158
chemical impacts (Eqn. 2).
159 160
= ∑
Eqn 2.
161 162
The HTI of PM reported in this analysis is the aggregate human health damages resulting
163
from both PM2.5 and PM10 formed from both primary (directly emitted to the atmosphere) and
164
secondary (generated by chemical reactions associated with emissions such as NOx and SOx)
165
sources.
166
PM has a different mechanism for impacting human health than other chemicals in the
167
inventory and the CF for PM is derived via a different approach. The ReCiPe model contains a
168
CF for PM2.5, as well as for NH3, NOx, and SO2, which contribute to secondary PM aerosol
169
formation. USEtox does not include a CF for PM, so the USEtox method is supplemented with a
170
method described by Gronlund et al.
171
and thirteen coal-fired power plants within Pennsylvania are used to calculate the HTI of PM,
172
using production-weighted averages of electricity generation for each plant.25 The Supporting
173
Information contains relevant assumptions and calculations for the HTI of PM.24, 25
24
Emission data collected from twenty-three natural gas
174 175
Shale Gas
176 177
In an effort to be conservative due to uncertainty and lack of data, assumptions are made to
178
overestimate the potential toxic emissions for the shale gas baseline whenever limited data
179
inhibit the analysis. A number of pathways exist for chemicals within fracturing fluid and 6
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produced water to enter the environment. Potential emission pathways include storage failures of
181
fracturing fluids and produced water at the surface, produced water migration through the
182
subsurface into groundwater, contamination from faulty casings, and as effluent from wastewater
183
treatment plants.26
184
To estimate potential releases of fracturing fluid chemicals, baseline and accidental
185
release scenarios are constructed. Data from 2990 wells from FracFocus within Pennsylvania are
186
used, containing a total of 368 chemicals. FracFocus 3.0 is a machine-readable database that
187
includes self-reported chemicals used in fracturing fluid formulations from over 62,000 wells
188
throughout the United States.27 The ID number, total fracturing fluids volume, purpose of each
189
chemical (corrosion inhibitor, proppant, etc.), and the concentration of each component were
190
recorded for each well. To account for differences in chemical compositions due to geographic
191
and operational variations, average well information for each Pennsylvania County represented
192
in FracFocus as of October 2015 was used. Not all wells use the same chemicals, so the average
193
concentration was used for the 100 most frequently used chemicals from 2010 to 2015 (Table
194
S1). Most of the fracturing formulations surveyed within FracFocus include less than 40
195
chemicals, so using the average concentration of the 100 most frequently used chemicals
196
overestimates the HTI for shale gas.
197 198
Table 1. Assumptions used for baseline and accidental release shale gas scenarios and parameter ranges found
199
within the literature for comparison.
200
Information.
Calculations and further justifications are found within the Supporting
201 Parameter
Unit
Average fracturing fluid volume
m3/well
Flowback water percentage share of injected fluid Produced water percentage share of injected fluid Shale gas electricity generation efficiency Shale gas well lifetime span Shale gas production volume Barium removal efficiency Fracturing fluid release rate
Baseline case 13000
Accidental release 13000
Literature Range 3500-26000
Reference 27
%
15
15
10-15
%
5
5
1-7
Mcf / kWh year MMcf %
0.0101
0.0101
28-30
15 4300 90
15 4300 0
0.009640.0134 5-30 1600-9000 0 – 99
% released
1
100
See details in text
3
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Currently, no database exists that systematically records spills and other releases.35 The baseline
205
case is conservative to compensate for data gaps in toxicity and chemical composition. The
206
baseline case assumes that 1% of fracturing fluids are emitted to surface water without any
207
treatment, although the discharge could occur via any of the potential emissions release
208
mechanisms. A recent study estimated the total known spill volume of fracturing fluid from 6000
209
wells to be 174 m3 from 2008 to 2013,35 averaging 0.03 m3 per well during that period. The
210
assessment report from the EPA lists the median spill volume as 1.6 m3 per well that reports a
211
spill (range 0.19-72 m3), with 0.4-12.2 spills per 100 wells, which is similar to data reported
212
elsewhere.36, 37, although it does not necessarily account for spills that go unreported. Of the
213
spills that are reported, the EPA estimates that only 9% of spills reach surface water. Assuming
214
13000 m3 of fracturing fluid for each well (Table 1),27 a more reasonable spill rate is 0.001% of
215
injected fluids, as opposed to the 1% assumed in the baseline scenario.
216
In addition to fracturing fluids injected into the wells, produced water has the potential to
217
contaminate drinking water sources. The chemical composition of flowback and produced water
218
are different, with larger concentrations of naturally dissolved materials in produced water than
219
flowback water. Some of the chemicals in produced water, especially organic compounds,
220
originate from the injected fracturing fluids. It is difficult to differentiate naturally-occurring
221
organic compounds from those that derive from fracturing fluid formulations. Because of the
222
difficulty distinguishing between the sources of chemicals, all chemicals within produced water
223
are treated as a separate source of emissions, effectively double-counting the fracturing fluid
224
chemicals contained in produced water. The volume of flowback and produced water is assumed
225
to be 20% of the injected water volume and have the average chemical composition of a
226
compilation of 35,000 data entries for the Marcellus shale gas region reported by Abualfaraj et
227
al.38 (Table S2).
228
Produced water from hydraulic fracturing operations is assumed to be sent to a dedicated
229
water treatment plant, which is the most common method of disposal in Pennsylvania.39
230
Alternative methods of disposal include injection into Class II disposal wells and recycling for
231
additional shale gas extraction operations.
232
Chemical constituents of produced water can enter surface water via the same
233
mechanisms as fracturing fluids, and also as effluent from wastewater treatment due to 8
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incomplete removal. For the baseline shale gas case, 10% of the chemicals within produced
235
water are assumed to be discharged to surface water. For context, a recent study estimated the
236
total known spill volume of produced water from 6000 wells to be 473 m3 from 2008 to 2013,35
237
averaging 0.079m3 per well. The assessment report from the EPA lists the median spill volume
238
of produced water as 37.5 m3 per well that reports a spill, with 0.4-12.2 spills per 100 wells.
239
Assuming 2700 m3 of produced water per well (Table 1), then the spill rate of produced water is
240
likely less than 1%. In addition, some fraction of the constituents of produced water are
241
discharged as effluent from the wastewater treatment facility, so the overall estimate of 10%
242
discharge is assumed to be reasonable and likely an overestimate for the baseline case.
243
The accidental release scenario for shale gas assumes 100% of injected fracturing fluid
244
and 100% of produced water is directly discharged to surface water over the entire lifetime of the
245
well. While this scenario is highly improbable, it provides a maximum threshold for possible
246
toxicity loads associated with shale gas.
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For air emissions from shale gas fields, values are obtained from a comprehensive study
248
by Roy et. al, which quantifies total load of NOx, PM2.5, and VOC emissions per well associated
249
with hydraulic fracturing operations.40 Their study estimates total load, but does not provide
250
compositional analysis of VOCs. In the absence of compositional data from this study, the CF
251
for benzene was used for all VOCs in this analysis. VOC composition from oil and gas sites tend
252
to have significant proportions of alkanes and alkenes that have lower toxicity than aromatics.
253
Therefore, the use of the benzene CF is considered an overestimation of VOC contribution to
254
HTI.
255 256 257
Coal
258
An EPA study on coal mine discharge41 was used in conjunction with annual
259
Pennsylvania coal production estimates42 to estimate toxicity associated with coal mining.41 The
260
study calculated loading of aluminum, iron, manganese, and total suspended solids in
261
Pennsylvania streams impacted by acid mine drainage.
262
To estimate human toxicity of mercury air emissions from coal-fired power plants, data
263
was collected from the 100 largest power producers in the United Sates in 2013.43 Combustion
264
residuals, also known as coal ash or fly ash, are assumed to be stored in surface impoundments. 9
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Some of the chemicals within the coal ash leach out of the disposal site and are released to the
266
environment (Table S3).44 The coal ash effluent values are assumed to be representative of coal
267
originating in Pennsylvania, given the locations of the power plants where data were obtained.
268
An EPA study on power plant effluent, which sampled coal-fired power plants in the
269
Appalachian region45, reported similar values. It is assumed that, in addition to ash, the coal ash
270
impoundment also receives discharges of flue gas desulfurization waste water and transport
271
water for fly and bottom ash. These waste streams appear to be included in reported outflow
272
values and are therefore not calculated as separate emissions.
273
Electricity generation was calculated using nameplate capacities of each power plant,
274
assuming continual operation of all plants, 365 days/year, which overestimates the generation
275
capacity of each plant, underestimating the HTI associated with electricity production. Wet
276
disposal of coal ash is not the only possible method of disposal. Coal ash can also be reused in
277
engineering applications or landfilled in a dry state. This choice is further addressed in the
278
sensitivity analysis and discussion.
279
The accidental release scenario for coal uses the same assumptions for all parameters of
280
the baseline case, and also includes unintentional release of coal ash into surface water, similar to
281
the impoundment failures that occurred in Kingston, TN in 2008 and Eden, NC in 2014.
282
Although major releases of coal ash spills are infrequent, they have historical precedent and their
283
inclusion provides an analogous system to compare with the accidental release scenario for shale
284
gas.
285
For the coal accidental release scenario, post-remediation data from the Kingston coal ash
286
spill were used.46 Remediation efforts took place in 2009 and 2010 leaving 0.18 million m3 of
287
ash in the Emory River. The density of ash is assumed to be 1500 kg/m3 and the HTI is
288
calculated based on the volume of the ash left in the Emory River post-remediation. Babbit et al.
289
calculated the average coal ash generation from four power plants in Florida as 0.0568 kg/kWh.21
290
The National Renewable Energy Laboratory estimates the average coal ash generation rate as 0.1
291
kg/kWh
292
more conservative HTI estimate.
47
. The smaller coal ash generation value is used in this analysis in order to provide a
293 294
Data Limitations
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The scope of this study is intended to address the full life cycles of shale gas and coal
296
electricity; however, data limitations necessitated a number of choices to be made regarding the
297
life cycle inventory of both shale gas and coal. Because of these limitations, systematic choices
298
were made to overestimate the coal inventory while underestimating the shale gas inventory.
299
Most studies that report VOCs or non-methane hydrocarbons report total load but do not
300
provide compositional analysis. Benzene is used as the CF for all VOCs in this analysis, which
301
likely overestimates VOC contributions to total HTI for all scenarios.
302
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Data relevant to calculating the HTI of coal mining is rarely reported in a format that is
303
conducive to translation into inventory data.
Mining emissions tend to be reported as
304
concentrations, without sufficient data to relate concentrations to mass emission/mass coal
305
extracted needed for the inventory (e.g. stream flow, time since coal extraction, overall amount
306
of coal extracted from site). Although other studies have found that coal mining may have
307
significant human health impacts on nearby communities,48-50 lack of appropriate data inhibit
308
inclusion of several aspects of the coal system into this analysis. Potential toxicity impacts are
309
associated with cadmium and selenium in mine drainage; however, limited data prevented
310
quantification of loading data from either element.41 Similarly, calculation of HTI associated
311
with coal mining does not include particulate matter or other air emissions associated with
312
mining that are analogous for the data obtained for the shale gas baseline case. Fugitive coal
313
dust emissions along transportation routes has been linked to chronic community-level exposures
314
of particulate matter and metals,51 but the format of data available inhibits translation into
315
inventory metrics useful for this analysis.
316
In addition to limitations associated with life cycle inventory collection, ReCiPe and
317
USEtox do not contain a uniform list of chemicals, so a given chemical may be associated with a
318
CF in one database and not in another. Characterization factors are also highly uncertain and a
319
standard operating assumption is that a CF may vary by three orders of magnitude in either
320
direction. 18, 19
321
Different oxidation states of metals have different toxicities; however, oxidation states
322
are not available from the datasets used in this analysis. To be consistent with the systematic
323
overestimation of the shale gas HTI, the more toxic oxidation state is assigned for shale gas
324
emissions whenever available. Similarly, mercury in the coal system is assumed to be present in
325
its inorganic form, even though it has the potential to form organic mercury, such as 11
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methylmercury and dimethylmercury, which are more toxic than inorganic mercury.52 CFs for
327
radionuclides are included in ReCiPe but not in USEtox. The effects on results of these issues
328
are further discussed in the Results and Discussion.
329 330
Uncertainty and Sensitivity Analysis
331
A Monte Carlo Analysis (MCA) is used to determine the range of possible results and the
332
extent of overlap between the shale gas and coal cases. Triangular distributions are assigned to
333
each of the parameters within the model, using the most likely value and upper and lower range
334
boundaries (see Tables 1, S10-S12). For the CFs, which span six orders of magnitude, triangular
335
distributions are assigned to the exponent of each parameter to reduce positive bias in sampling.
336
The results of 10,000 trials are reported.
337
A one-at-a-time perturbation method is used to assess the sensitivity of the results to each
338
input parameter. The effect of each parameter on the HTI is determined by changing its value to
339
the extreme ends of its range while keeping all other parameters at their initial model value (see
340
Tables 1, S10-S12). CFs from USEtox are used as the default for both the sensitivity analysis and
341
the MCA, unless a CF was available only in ReCiPe.
342
In addition to the sensitivity analysis, two alternate operating scenarios are explored for
343
coal. The scenarios are disposal of coal ash via a dry storage method and implementation of the
344
Mercury Air Toxics Standards (MATS) regulation.53
345
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Results and Discussion
PM
VOC
As
Others
Ba
DMF
Hg (air)
Hg (coal ash)
100
HTI (DALY/GWH)
10
1
0.1
0.01 USEtox
ReCiPe
USEtox
Shale Gas
ReCiPe
Coal
USEtox
ReCiPe
Shale Gas
Baseline Scenario
USEtox
ReCiPe
Coal
Accidental Release Scenario
350 351 352 353 354 355 356
Figure 2. Baseline and accidental release scenarios for electricity produced from shale gas and coal using the USEtox and ReCiPe2016 methods to quantify HTI. Data is depicted on a log scale. Major components of HTI include particulate matter (PM), volatile organic compounds (VOC), arsenic (As), barium (Ba), N, N’-dimethylformamide (DMF), and mercury (Hg). Constituents that appear in both systems are depicted in gray scale, whereas those only found in coal system are in orange and those only found in shale gas system are in blue.
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Figure 2 shows that the baseline scenario for coal-fired electricity has a greater HTI than
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both the baseline and accidental release scenarios for shale gas, by at least an order of magnitude.
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The HTI baseline values are 0.016 DALY/GWh (USEtox) and 0.024 DALY/GWh (ReCiPe) for
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shale gas and 0.69 DALY/GWh (USEtox) and 1.7 DALY/GWh (ReCiPe) for coal. A prior study
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on coal toxicity using CML2001, another popular impact assessment method, estimated an HTI
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of coal electricity to be between 0.24 DALY/GWh to 2.2 DALY/GWh,54 which aligns with the
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results of this analysis.
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Particulate matter is the dominant toxicity contributor for both shale gas (86% USEtox,
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93% ReCiPe) and coal (92% USEtox, 98% ReCiPe), and includes calculation of primary
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emissions of particulate as well as secondary aerosol formation (see Tables S6-S9). Other
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contributors to the HTI of the coal baseline are air emissions of mercury (5.6% USEtox,
