Technical Support Document:

Potential for Excess Local Deposition of 

U.S. EGU-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, 2011b), 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, 2011a). 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
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Figure 2. Excess Local Deposition in 2016 (Base Case)
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Table 1. Excess local deposition of Hg based on CMAQ modeled Hg deposition

Category of Results

50km-Radius-Average Excess Local EGU-Attributable 

Deposition values (μg/m2)

Mean Across EGUs (percent of regional average 

deposition)

2005 Scenario 2016 Scenario

All U.S. EGU sites with Hg emissions >0

(672 sites) 1.65 (119%) 0.38 (98%)

Top ten percent U.S. EGU in Hg 

emissions

(67 sites)

4.89 (352%) 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., Christianson, 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 

ArcInfo Licence [Software]. Environmental Systems Research Institute, Inc. Redlands, 

CA. 92373. USA. URL: http://www.esri.com

U.S. Environmental Protection Agency (U.S. EPA). 2011a. Air Quality Modeling Technical 

Support Document: EGU Mercury Analysis. EPA-454/R-11-008.

U.S. Environmental Protection Agency (U.S. EPA). 2011b. 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, IndustrialCommercial-Institutional, and Small Industrial-Commercial-Institutional Steam 

Generating Units. Federal Register.
United States

Environmental Protection

Agency

Office of Air Quality Planning and Standards

Health and Environmental Impacts Division

Research Triangle Park, NC

Publication No. EPA-452/R-11-010

December 2011