&EPA
United Sates
EnviroiiTMdlal Protection
Technical Support Document:
Potential for Excess Local Deposition of
U.S. ECU-Attributable Mercury
in Areas near U.S. EGUs
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EPA-452/R-11-010
December 2011
Technical Support Document:
Potential for Excess Local Deposition of
U.S. EGU-Attributable
Mercury in Areas near U.S. EGUs
U.S. Environmental Protection Agency
Office of Air Quality Planning and Standards
Health and Environmental Impacts Division
Research Triangle Park, North Carolina
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DISCLAIMER
This document has been prepared by staff from the Office of Air Quality Planning and
Standards, U.S. Environmental Protection Agency. Any opinions, findings, conclusions, or
recommendations are those of the authors and do not necessarily reflect the views of the EPA.
Questions related to this document should be addressed to Dr. Bryan Hubbell, U.S.
Environmental Protection Agency, Office of Air Quality Planning and Standards, C504-02,
Research Triangle Park, North Carolina 27711 (email: hubbell.bryan@epa.gov).
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Executive Summary
Previously, the analysis of the potential for excess local mercury deposition surrounding
U.S. EGUs was located in Appendix G of the National-Scale Mercury Risk Assessment. In
response to public comments, we have moved this information to a separate TSD, re-titled the
analysis, and provided additional technical details. EPA calculated the average EGU-attributable
deposition (based on CMAQ modeling of mercury deposition) in the area 500 km around each
plant and the average EGU attributable deposition in the area 50 km around each plant. The
difference between those two values is the excess local deposition around the plant. This analysis
shows that there is excess deposition of Hg in the local areas around EGUs, especially those with
high Hg emissions. Although this is not necessarily indicative of higher risk of adverse effects
from consumption of MeHg contaminated fish from watersheds around the U.S. EGUs, it does
indicate an increased potential that Hg from U.S. EGUs will impact local watersheds.
Purpose and Scope of Analysis
Published research shows that U.S. coal-fired power plants significantly contribute to
local and regional mercury deposition (Caffrey et al., 2010; Keeler et al., 2006; White et al.,
2009). As discussed in the preamble to the proposed MATS (U.S. EPA, 201 Ib), for the purposes
of the appropriate and necessary finding, EPA determined that information on the potential for
excess deposition of mercury in areas surrounding power plants would be useful in informing the
finding. The purpose of this analysis was to evaluate at the national-scale whether there
existed excess U.S. EGU-attributable deposition of Hg in locations near EGUs, which would
indicate that U.S. EGUs are potentially contributing to mercury exposures locally as well as to
the potential exposures that result from the combined deposition from U.S. EGUs at a regional
scale. This analysis does not address total mercury deposition because global sources of mercury
deposition account for a large fraction of total mercury deposition, which would not provide
useful information regarding the comparison of local and regional mercury deposition from U.S.
EGUs.
This analysis is not intended to show "mercury hotspots" based on elevated
concentrations of methylmercury in fish tissue but rather of mercury deposition hot spots,
defined as excess local U.S. EGU-attributable mercury deposition around power plants relative
to regional U.S. EGU-attributable deposition. To reduce the confusion about the term "hotspot",
we have re-titled this analysis to "Potential for Excess Local U.S. EGU Attributable Deposition
of Mercury in Areas near U.S. EGUs".
Methods
EPA evaluated the potential for "hotspot" deposition near U.S. EGU emission sources on
a national scale, based on the CMAQ-modeled Hg deposition for the 2005 and 2016 scenarios
(U.S. EPA, 201 la). Locations of U.S. EGUs in 2005 and 2016 were mapped based on latitude
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and longitudes extracted from the Integrated Planning Model (IPM), a model of the power
system used by EPA. We calculated 50 km and 500 km buffers around each EGU location using
the ArcGIS® geographic information system software (Environmental Systems Research
Institute, 2010).
We then calculated the spatial averages of the U.S. EGU-attributable deposition (obtained
by subtracting estimated mercury deposition with U.S. EGU Hg emissions zeroed out from
baseline mercury deposition from all sources including U.S. EGU Hg emissions) across the grid
cells with centers falling inside the 50 km and 500 km buffers. The average deposition within the
500 km buffer represents the likely area in which an EGU contributes to regional deposition. The
average deposition within the 50 km buffer is used to characterize local deposition plus regional
deposition near the EGU.
The spatial surfaces were generated by applying an averaging kernel to the CMAQ
deposition estimates for U.S. EGU-attributable mercury, which have a 12 km by 12 km gridded
spatial resolution. Averaging kernels assign a mean value to each grid cell based on the averages
of all neighboring grid cells within a predefined window as a method to smooth the deposition
surface. In this case, kernel sizes were 50 km and 500 km radiuses. Then 50 km radius average
values were subtracted from 500 km radius averages to create the map of excess local deposition.
If there were only general regional mixing of U.S. EGU mercury and relatively even
deposition across broad regions, then we would expect that the average U.S. EGU attributable
deposition within 50 km of an EGU would be about the same as the average U.S. EGU
attributable deposition within 500 km of the EGU. The difference between the averages of the 50
km and 500 km buffers is thus a measure of excess local deposition.
Results
This analysis shows that there is excess deposition of Hg in the local areas around EGUs,
especially those with high Hg emissions. Although this is not necessarily indicative of higher
risk of adverse effects from consumption of MeHg contaminated fish from watersheds around
the U.S. EGUs, it indicates an increased potential that Hg from U.S. EGUs will impact local
watersheds around the EGU sources, and not just impact regional deposition.
Figure 1 shows a map of the excess local deposition based on the 2005 CMAQ modeling.
Figure 2 shows excess local deposition based on the 2016 Base Case. As shown in Figure 1,
there is heterogeneity in the amount of excess local deposition around plants. Some plants,
especially those with high mercury emissions, have local deposition that is less than the regional
average deposition, suggesting that most of the mercury from those plants is transported
regionally, or that other EGUs in the vicinity of those plants dominate the deposition of mercury
near the plants.
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Summary statistics for the excess local deposition are provided in Table 1. Table 1 shows
both the mean excess deposition around all U.S. EGUs and the mean excess deposition around
just the top 10 percent of Hg emitting U.S. EGUs. Table 1 also shows the excess Hg deposition
as a percent of the average regional deposition to provide context for the magnitude of the local
excess deposition. In 2005, for all U.S. EGU, the excess was approximately 1.2 times the
average deposition, while local deposition was approximately 3.5 times the regional average for
the top 10 percent of Hg emitting U.S. EGUs. By 2016, the absolute levels of excess deposition
decrease, but the local excess still remains approximately 3 times the regional average for the
highest 10 percent of Hg emitting U.S. EGUs.
This analysis shows that there is excess deposition of Hg in the local areas around EGUs,
especially those with high Hg emissions. Although this is not necessarily indicative of higher
risk of adverse effects from consumption of MeHg contaminated fish from waterbodies around
the U.S. EGUs, it does indicate an increased chance that Hg from U.S. EGUs will impact local
waterbodies around the EGU sources, and not just impact regional deposition.
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Figure 1. Excess Local Deposition in 2005
ECU plant locations
2005
Total Hg Emissions (tons/year)
• 0.000-0.013
• 0.014-0.037
o 0.038 - 0.068
o 0.069-0.108
o 0109-0.159
o 0.160-0.226
o 0.227 - 0.309
o 0.310-0.424
• 0.425 - 0.639
• 0640-1.096
2005 Hg Deposition Hot Spots
Excess Local Deposition Values ([jg/m2)
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Figure 2. Excess Local Deposition in 2016 (Base Case)
EGU plant locations
2016 Base case
Total Hg Emissions (tons/year)
0.000-0.013
0.014 - 0.037
0.038 - 0.068
0.069-0.108
0.109-0.159
0.160-0.226
0.227 - 0.309
0.310-0.424
0.425 - 0.639
0.640 - 1.096
2016cr Hg Deposition Hot Spots
Excess Local Deposition Values (\igln\2)
^ ^
*N 4*
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Table 1. Excess local deposition of Hg based on CMAQ modeled Hg deposition
Category of Results
All U.S. ECU sites with Hg emissions >0
(672 sites)
Top ten percent U.S. ECU in Hg
emissions
(67 sites)
SOkm-Radius-Average Excess Local EGU-Attributable
Deposition values (ug/m2)
Mean Across EGUs (percent of regional average
deposition)
2005 Scenario
1.65(119%)
4.89 (352%)
2016 Scenario
0.38 (98%)
1.18(302%)
References
Caffrey, J.M., Landing, W.M., Nolek, S.D., Gosnell, K.J., Bagui, S.S., Bagui, S.C., 2010.
"Atmospheric deposition of mercury and major ions to the Pensacola (Florida)
watershed: spatial, seasonal, and inter-annual variability." Atmospheric Chemistry and
Physics 10, 5425-5434.
Keeler, G.J., Landis, M.S., Norris, G.A., Christiansen, E.M., Dvonch, J.T., 2006. "Sources of
mercury wet deposition in Eastern Ohio, USA." Environmental Science & Technology
40,5874-5881.
White, E.M., Keeler, G.J., Landis, M.S., 2009. "Spatial Variability of Mercury Wet Deposition
in Eastern Ohio: Summertime Meteorological Case Study Analysis of Local Source
Influences." Environmental Science & Technology 43, 4946-4953.
Environmental Systems Research Institute (ESRI), 2010. ArcGIS Desktop 10 Service Pack 2 with
Arclnfo Licence [Software]. Environmental Systems Research Institute, Inc. Redlands,
CA. 92373. USA. URL: http://www.esri.com
U.S. Environmental Protection Agency (U.S. EPA). 201 la. Air Quality Modeling Technical
Support Document: EGUMercury Analysis. EPA-454/R-11-008.
U.S. Environmental Protection Agency (U.S. EPA). 201 Ib. National Emission Standards for
Hazardous Air Pollutants from Coal-and Oil-fired Electric Utility Steam Generating
Units and Standards of Performance for Fossil-Fuel-Fired Electric Utility, Industrial-
Commercial-lnstitutional, and Small Industrial-Commercial-lnstitutional Steam
Generating Units. Federal Register.
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United States Office of Air Quality Planning and Standards Publication No. EP A-452/R-11-010
Environmental Protection Health and Environmental Impacts Division December 2011
Agency Research Triangle Park, NC
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