OFFICE OF INSPECTOR GENERAL 

Catalyst for Improving the Environment 

Evaluation Report 

Monitoring Needed to Assess 

Impact of EPA’s Clean Air 

Mercury Rule on Potential Hotspots 

 Report No. 2006-P-00025 

 May 15, 2006
Report Contributors: Rick Beusse 

Carolyn Blair 

 Hilda Canes

 Susan Charen 

Sarah Fabirkiewicz 

 James Hatfield

 Erica Hauck

 James Van Orden 

Abbreviations 

CAMR Clean Air Mercury Rule 

CMAQ Community Multiscale Air Quality 

EPA Environmental Protection Agency 

Hg Mercury 

IPM Integrated Planning Model 

km kilometer 

MACT Maximum Achievable Control Technology 

MDN Mercury Deposition Network 

mg/kg milligram per kilogram 

OIG Office of Inspector General 

Cover photo: A fisherman holding a walleye, a predator fish for which mercury 

contamination is a concern (photo courtesy EPA). 
 2006-P-00025 

May 15, 2006 U.S. Environmental Protection Agency 

Office of Inspector General 

At a Glance

Catalyst for Improving the Environment

Why We Did This Review

In support of its Clean Air 

Mercury Rule (CAMR), the 

Environmental Protection 

Agency (EPA) conducted a 

detailed analysis of mercury 

emissions and deposition. 

EPA concluded that “utilityattributable” hotspots would 

not occur after implementation 

of CAMR’s mercury trading 

program. This evaluation 

assesses the basis for EPA’s 

conclusion.

Background

About 40 percent of U.S. 

man-made airborne mercury is 

emitted from coal-fired 

utilities. EPA revised a 

previous finding that mercury 

emissions from coal-fired 

utilities be regulated with a 

Maximum Achievable Control 

Technology standard. Instead, 

EPA adopted a cap-and-trade 

program to reduce mercury 

emissions. Several State 

agencies and environmental 

groups objected to these 

actions. One concern was that 

a cap-and-trade program could 

result in localized areas with 

unacceptably high levels of 

mercury, or “hotspots.” 

For further information, 

contact our Office of 

Congressional and Public 

Liaison at (202) 566-2391. 

To view the full report, 

click on the following link: 

www.epa.gov/oig/reports/2006/

20060515-2006-P-00025.pdf

Monitoring Needed to Assess Impact of EPA’s 

Clean Air Mercury Rule on Potential Hotspots

 What We Found

EPA brought significant scientific, technical, and modeling expertise to bear in 

developing a specific methodology to consider the potential for mercury hotspots. 

Several uncertainties associated with key variables in the analysis could affect the 

accuracy of the Agency’s conclusion that the Clean Air Mercury Rule (CAMR) 

will not result in “utility-attributable” hotspots. We noted:

x gaps in available data and science for mercury emissions estimates, 

x limitations with the model used for predicting mercury deposition, 

x uncertainty over how mercury reacts in the atmosphere, and 

x uncertainty over how mercury changes to a more toxic form in waterbodies. 

Two recent studies support the need for additional monitoring to ensure that EPA’s 

analysis has properly estimated the contribution of local, regional, and global 

sources on U.S. deposition. These studies are “Mechanisms of Mercury Removal 

by O3 and OH in the Atmosphere,” published in Atmospheric Environment in June 

2005; and “Sources of Mercury Wet Deposition in Eastern Ohio, USA,” submitted 

for publication in a scientific journal in February 2006. Results of both studies 

were not available until after EPA issued CAMR in March 2005, and thus could 

not have been considered in EPA's deliberations on CAMR. Although EPA 

indicated in CAMR that it would monitor the impact of the cap-and-trade rule on 

mercury deposition, the Agency has not yet developed a monitoring plan for this 

purpose. Without field data from an improved monitoring network, EPA’s ability 

to advance mercury science will be limited and “utility-attributable” hotspots that 

pose health risks may occur and go undetected.

Based on our interpretation of CAMR, EPA could not take action to mitigate a 

mercury hotspot unless the Agency first determined that the hotspot was solely 

“utility-attributable.” Therefore, EPA could not require additional utility emission 

reductions if utilities contributed significantly, but not solely, to a mercury 

hotspot. This could limit EPA’s ability to mitigate human health hazards by 

reducing potentially harmful levels of mercury in waterbodies and fish tissue. 

This could also limit EPA’s ability to reduce the number of waterbodies with fish 

consumption advisories. 

 What We Recommend

We recommend that EPA develop and implement a mercury monitoring plan to 

(1) assess the impact of CAMR, if adopted, on mercury deposition and fish tissue; 

and (2) evaluate and refine mercury estimation tools and models. Further, if 

CAMR is adopted after the rule reconsideration process is complete, we 

recommend that EPA clarify in the final rule that the “utility-attributable” hotspot 

definition does not establish a prerequisite for making future revisions to CAMR.

In response to the draft report, the Agency agreed that additional mercury 

monitoring is needed and explained that CAMR does not establish the “utilityattributable” hotspot definition as a prerequisite for future changes to CAMR.
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON, D.C. 20460

OFFICE OF 

INSPECTOR GENERAL

May 15, 2006 

MEMORANDUM

SUBJECT: Monitoring Needed to Assess Impact of EPA’s 

 Clean Air Mercury Rule on Potential Hotspots 

 Report No. 2006-P-00025

TO: William L. Wehrum 

 Acting Assistant Administrator for Air and Radiation 

This is our report on the subject evaluation conducted by the Office of Inspector General (OIG) 

of the U.S. Environmental Protection Agency (EPA). This report contains findings that should 

help EPA to better monitor the impact of the Clean Air Mercury Rule and refine performance 

standards under the rule, if necessary. This report represents the opinion of the OIG and the 

findings in this report do not necessarily represent the final EPA position. Final determinations 

on matters in the report will be made by EPA managers in accordance with established 

procedures.

Action Required 

In accordance with EPA Manual 2750, as the action official, you are required to provide a 

written response within 90 days of the final report date. The response should address all 

recommendations. For the corrective actions planned but not completed by the response date, 

please describe the actions that are ongoing and provide a timetable for completion. Where you 

disagree with a recommendation, please provide alternative actions for addressing the findings 

reported.

We appreciate the efforts of EPA managers and staff in working with us to develop this report.

If you or your staff have any questions regarding this report, please contact me at 202-566-0847, 

or Rick Linthurst at 919-541-4909.

 Sincerely, 

 Bill A. Roderick 

 Acting Inspector General 

 
Table of Contents 

At a Glance 

Chapters 

1 Introduction ........................................................................................................... 1 

Purpose .......................................................................................................... 1 

Background .................................................................................................... 1 

Scope and Methodology................................................................................. 5 

2 Monitoring Plan Needed to Address Uncertainties 

 In EPA’s Hotspots Analysis ................................................................................. 6

EPA Analyzed Potential for “Utility-Attributable” Hotspots.............................. 6 

Data and Science Gaps Exist for Mercury Emissions Estimates................. 8 

CMAQ Model Uncertainties and Limitations................................................... 9 

Study Finds Different Rates of Atmospheric Chemical Reactions.................. 10 

Uncertainties Noted with Methylation and Bioaccumulation........................... 10 

Study Shows Significant Deposition from Local Sources............................... 12 

Uncertainties Underscore Need for Mercury Monitoring Plan ........................ 13 

Conclusion...................................................................................................... 15 

Recommendation ........................................................................................... 15 

 Agency Comments and OIG Evaluation......................................................... 15 

3 EPA Needs to Clarify Conditions Under Which CAMR 

Performance Standards Can be Tightened......................................................... 17 

EPA Provides “Utility-Attributable” Hotspot Definition .................................... 17 

“Utility-Attributable” Definition Could be Interpreted to Limit EPA’s 

 Ability to Mitigate Hotspots......... ........................................................... 18 

CAMR and Revision Must be Read Together ................................................ 18 

Conclusion...................................................................................................... 20 

 Recommendation ........................................................................................... 20 

 Agency Comments and OIG Evaluation......................................................... 20 

Appendices 

A Details on Scope and Methodology..................................................................... 21 

B Models Used in CAMR Analysis .......................................................................... 24 

C Agency Response to Draft Report....................................................................... 25 

D Distribution ............................................................................................................ 28 

i 
Chapter 1

Introduction 

Purpose 

A prior Environmental Protection Agency (EPA) Office of Inspector General 

(OIG) report cited concerns about EPA’s limited assessment of the potential for 

mercury hotspots resulting from a cap-and-trade program under the Clean Air 

Mercury Rule (CAMR). We issued this prior report, Additional Analyses of 

Mercury Emissions Needed Before EPA Finalizes Rules for Coal-Fired Electric 

Utilities (Report No. 2005-P-00003), on February 3, 2005. In support of CAMR, 

EPA conducted a detailed analysis of mercury emissions and deposition and 

concluded that “utility-attributable” hotspots would not occur after 

implementation of the mercury emissions trading program. 

EPA’s Water Quality Trading Assessment Handbook defines hotspots as 

“localized areas with unacceptably high levels of pollutants.” In this evaluation 

report, however, a hotspot is a waterbody containing consumable fish with 

elevated levels of methylmercury in their tissues. 

We conducted this evaluation to assess the basis for the Agency’s determination 

that CAMR would not result in “utility-attributable” hotspots. 

Background 

Mercury (Hg) is released into the atmosphere through natural processes and 

through human activities, such as combustion processes. Once emitted, 

atmospheric mercury undergoes several chemical and physical processes and can 

then be deposited to the ground or waterbodies via wet or dry processes. In wet 

deposition, mercury is deposited by precipitation, such as rain or snow. In dry 

deposition, mercury settles to the earth's surface and sticks to or is absorbed by 

trees, soil, water, or other surfaces. The largest source of airborne mercury 

emissions in the United States is the coal-fired electric utilities industry, 

representing an estimated 40 percent of total U.S. man-made airborne mercury. 

Although airborne mercury is generally not considered to be a serious health 

concern, once mercury enters freshwater and salt-water bodies, it can 

bioaccumulate in fish and other animal tissues in its more toxic form, 

methylmercury. As methylmercury bioaccumulates in the food chain, its 

concentration becomes increasingly higher in animals at the top of the food chain 

(such as larger predatory fish) that consume smaller, contaminated organisms. 

Figure 1-1 illustrates the exposure pathway of mercury. 

1 
Figure 1-1: How Mercury Enters the Environment 

Source: EPA 

Fish consumption is the main route by which methylmercury harms human health. 

Excessive human exposure to methylmercury has been associated with severe 

detrimental neurological and developmental health effects. Research has shown 

that the developing fetus is at risk for impaired motor and cognitive skills. Thus, 

exposure to mercury by women of child-bearing age is of particular concern. 

Most U.S. Fish Advisories Due to Mercury Contamination 

When levels of chemical contamination in fish are considered unsafe, States, 

tribes, and territories can issue consumption advisories that may recommend that 

people limit or avoid eating certain species of fish caught in certain places. Each 

State sets its own criteria and decides which bodies of water to monitor. 

Monitored waterbodies may vary from year to year. Fish advisories are voluntary 

State recommendations not governed by Federal regulations. In 2004, 44 States 

issued fish advisories for mercury. The number of mercury-related fish advisories 

continues to rise as States increase fish tissue testing. 

EPA recently reported in its 2005 Performance and Accountability Report that the 

Agency did not meet its goal of reducing the number of overall fish advisories by 

at least 1 percent from 2002 levels. From 2003 to 2004, the number of mercury 

advisories rose from 2,362 to 2,436, or 3.1 percent. According to the 2004 

National Listing of Fish Advisories, the vast majority (68 percent) of fish 

advisories in the United States are due to mercury contamination, as illustrated in 

Figure 1-2. 

2 
Figure 1-2: Percent of Fish Advisories for Each of the Top Five 

Bioaccumulative Contaminants in 2004 

2% 3% 2% 

25% 

68% 

Mercury 

Mercury PCBs Chlordane Dioxins DDT & Metabolites 

Source: 2004 National Listing of Fish Advisories 

1 Regulatory Finding on the Emissions of Hazardous Air Pollutants From Electric Utility Steam Generating Units, 

December 20, 2000; Vol. 65, No. 245. 

3 

CAMR First Rule for Mercury Emissions from Coal-Fired Utilities 

On March 15, 2005, EPA issued CAMR, which established the country’s first 

regulation of mercury emissions from coal-fired power plants. CAMR uses a 

declining cap-and-trade approach to regulating coal-fired utilities under 

Section 111 of the Clean Air Act by setting a fixed national cap. Utilities can buy 

and sell credits among one another in a national emissions market. Utilities that 

cannot cost-effectively reduce emissions may buy allowances from units that 

reduced emissions below established allowance limits. Under CAMR, an interim 

national cap of 38 tons per year becomes effective in 2010 and a final annual cap 

of 15 tons becomes effective in 2018. EPA’s first cap is based on mercury 

reductions expected to be achieved as a co-benefit of implementing the Clean Air 

Interstate Rule, issued in March 2005. That rule requires utilities to take actions 

to reduce emissions of sulfur dioxide and nitrogen oxides, and those actions are 

also projected to reduce mercury emissions. 

EPA Revised its Prior Regulatory Finding Regarding Utilities 

To use a cap-and-trade program to regulate coal-fired utilities, EPA first had to 

revise a December 2000 regulatory finding1

 that indicated it was appropriate 

and necessary to regulate coal-fired utilities under Section 112 of the Clean Air 

Act. This finding required EPA to regulate utilities using a Maximum 

Achievable Control Technology (MACT) standard. MACT standards are 
industry-specific, technology-based standards designed to reduce hazardous air 

pollutant emissions. These standards can require facility owners/operators to 

meet emission limits, install emission control technologies, monitor emissions 

and/or operating parameters, and use specified work practices. In March 2005, 

EPA issued a Revision of December 2000 Regulatory Finding,

2

 stating that the 

Agency no longer found it appropriate or necessary to regulate utilities under 

Section 112. This released the Agency from the requirement to regulate 

utilities using a MACT standard. EPA issued the finding the same day it 

issued CAMR, which established a mercury cap-and-trade program under 

Section 111. 

For its Revision of December 2000 Regulatory Finding, EPA interpreted 

Section 112(n) to mean that utilities alone had to be the sole cause of a health 

hazard in order to be regulated under Section 112 and subject to MACT 

standards. Specifically, EPA developed the following “utility-attributable” 

hotspot definition for its revision: “. . . a waterbody that is a source of 

consumable fish with Methylmercury tissue concentrations, attributable solely 

to utilities, greater than EPA’s Methylmercury water quality criterion of 

0.3 milligrams per kilogram (mg/kg).” 

EPA Response to Petitions for Reconsideration 

Several State agencies and other organizations oppose EPA’s adoption of a 

cap-and-trade program for mercury. These groups separately petitioned for 

reconsideration of the Revision of December 2000 Regulatory Finding. Among 

other things, they asserted that, in its analysis, EPA underestimated the impact of 

deposition resulting from local and regional sources and overestimated the impact 

of emissions from global sources. Thus, they argue, some mercury hotspots 

already exist, and requiring sources to comply with MACT standards would 

immediately reduce deposition in those areas. Further, these opponents to the 

cap-and-trade program believe the program could result in new mercury hotspots 

if some utilities bought excess emission credits instead of reducing emissions. 

On October 21, 2005, EPA reopened for public comment certain aspects of its 

CAMR and, in a separate action, reopened for public comment certain aspects of 

its Revision of December 2000 Regulatory Finding. The action to reopen 

comment on CAMR was taken in response to petitions filed by 14 States, 

5 environmental groups, a public utility, and a waste services association. The 

Agency stated that it agreed to reconsider several aspects regarding CAMR. The 

action to reopen comment on EPA’s Revision of December 2000 Regulatory 

Finding was based on two petitions, one from 14 States and a second from 5 

environmental groups and 4 Indian tribes. The Agency agreed to reconsider the 

legal issues underlying the decision as well as the methodology used to assess the 

2 Revision of December 2000 Regulatory Finding on the Emissions of Hazardous Air Pollutants from Electric Utility 

Steam Generating Units and the Removal of Coal- and Oil-fired Electric Utility Steam Generating Units from the 

Section 112(c) List; Final Rule, March 15, 2005. 

4 
amount of “utility-attributable” mercury levels in fish tissue and the public health 

implications of those levels. The Agency also agreed to reconsider how it defined 

a utility hotspot for the purposes of its finding concerning regulation of Utility 

Units under Clean Air Act Section 112. Comments regarding this reconsideration 

were accepted until December 19, 2005. The Agency was still evaluating 

comments at the time our field work ended. 

Scope and Methodology 

We conducted our review from September through December 2005, in 

accordance with Government Auditing Standards issued by the Comptroller 

General of the United States. We performed field work at EPA’s Office of Air 

and Radiation in Washington, DC; the Office of Air and Radiation’s Office of 

Air Quality Planning and Standards in Research Triangle Park, North Carolina; 

the Office of Research and Development in Research Triangle Park; and the 

Office of Water in Washington. 

To answer our evaluation’s objective, we examined: (1) the basis for the 

Agency’s “utility-attributable” hotspot definition and the consistency of this 

definition with any prior Agency decisions regarding hotspots; (2) the key 

attributes, assumptions, and limitations of the models used to assess the impact 

of mercury emissions from coal-fired electric utility units under CAMR; and 

(3) the key variables used as inputs to the models as well as the basis for 

selecting these variables. 

To gain an understanding of the definition of “utility-attributable” hotspots, the 

modeling and analyses EPA used to determine the potential for “utilityattributable” mercury hotspots after CAMR, and the inputs and assumptions 

associated with the Agency’s analyses, we interviewed EPA staff involved in the 

development of CAMR or knowledgeable about the processes and models used 

in EPA’s analyses. We also interviewed officials from State agencies and 

external organizations familiar with CAMR’s development and EPA’s hotspots 

analysis. We reviewed data and analyses developed in support of the rule, and 

selected public comments included in the rulemaking docket. We also reviewed 

related information provided by both EPA and non-EPA officials. 

Our analysis focused on the key assumptions and limitations of the Community 

Multiscale Air Quality model, which was used to estimate mercury transport and 

deposition. We did not review in detail the assumptions, limitations, and 

uncertainties associated with the other models used in the Agency’s analyses. 

Appendix A provides additional details on scope and methodology. 

5 
Chapter 2

Monitoring Plan Needed to Address Uncertainties in 

EPA’s Hotspots Analysis 

As with any modeling assessment, uncertainties may exist. Uncertainties 

regarding EPA’s analysis and conclusion that CAMR will not result in “utilityattributable” hotspots include: 

• gaps in available data and science for mercury emissions estimates, 

• limitations with the model used for predicting mercury deposition, 

• uncertainty over how mercury reacts in the atmosphere, and 

• uncertainty over how mercury changes to a more toxic form in 

waterbodies (i.e., methylation) and accumulates in fish tissue. 

Two recent studies support the need for additional monitoring to ensure that 

EPA’s hotspots analysis has properly estimated the contribution of local, regional, 

and global sources to U.S. deposition. These studies are: 

(1) “Mechanisms of Mercury Removal by O3 and OH in the Atmosphere,” 

Calvert, J.G., Lindberg, S.E., (published in Atmospheric Environment, 

Volume: 39, Number: 18, Page: 3355-3367), June 5, 2005, referred to in 

this report as the “Mechanisms of Mercury Removal Study;” and 

(2) “Sources of Mercury Wet Deposition in Eastern Ohio, USA,” Keeler, G.J., 

et al., referred to in this report as the “Steubenville Study” (a peer review 

of the Steubenville Study was completed in December 2005 and the study 

was submitted for publication in a scientific journal in February 2006). 

Results of both studies were not available until after EPA issued CAMR in March 

2005, and thus could not have been considered in EPA's deliberations on CAMR. 

We believe the uncertainties associated with its CAMR analysis underscore the 

need for EPA to develop and implement a plan for monitoring the impact of 

CAMR on mercury deposition and mercury concentrations in fish tissue. Without 

implementation of a monitoring plan and/or improvements to current models, 

“utility-attributable” hotspots that can pose health risks may occur and go 

undetected. 

EPA Analyzed Potential for “Utility-Attributable” Hotspots 

In its Revision of December 2000 Regulatory Finding, EPA states it “does not 

believe that there will be any [utility-attributable] hot spots after implementation 

of CAIR [Clean Air Interstate Rule] and CAMR.” EPA’s analyses of mercury 

hotspots considered many factors that influence the way mercury is deposited to 

land and waterbodies. For its CAMR analysis, EPA used the Community 

6 
Multiscale Air Quality (CMAQ) model as the principal tool to predict patterns of 

mercury deposition and as an important part of assessing the potential for “utilityattributable” mercury hotspots under CAMR. 

EPA considers the CMAQ to be the most capable model available for assessing 

the impacts of CAMR on mercury deposition within the United States. The 

model is designed to estimate pollutant concentrations and depositions over large 

areas, such as the continental United States. The model accounts for variations in 

mercury emissions, differences in the atmospheric reactions of mercury, and the 

impact of those factors on deposition. 

However, there are important limitations associated with some of the inputs EPA 

used in CMAQ for its CAMR analysis. The July 2005 Final Report: Second Peer 

Review of the CMAQ Model, conducted by an independent panel that included 

State, academic, and private organizations, notes the following limitations: 

CMAQ is a modeling system that simulates a wide range of physical, 

chemical and biological processes . . . Some of these processes are well 

understood, some reasonably well understood, and some only poorly 

understood. This wide range in the level of knowledge about the 

processes being modeled, and the fact that uncertainties in characterizing 

some of the processes correspond to areas of active research worldwide, 

means that some parts of the model code are sufficiently well established 

as to be considered fixed, while other parts of the code are under 

continuing development. 

Other models also played a role in EPA’s analysis of the potential for hotspots 

under CAMR by contributing input data to CMAQ (see Appendix B for details on 

some of these other models). For example, a separate model was used to estimate 

the amount of mercury emissions from utilities based on certain economic 

assumptions, and another was used to predict weather patterns. Both the 

emissions and weather data were fed into CMAQ, and CMAQ predictions on 

deposition were fed into another model to estimate the effects of deposition on 

future mercury fish tissue concentrations. 

In its hotspots analysis, the Agency discussed instances where conservative 

assumptions were used to avoid underestimating the impact of utilities. For 

example, in its hotspots analysis EPA did not screen out watersheds in which 

sources of mercury other than air deposition were significant. According to EPA, 

this may result in higher concentrations of methylmercury in fish being attributed 

to power plants than would be the case had EPA been able to account for non-air 

sources. In addition, EPA’s hotspots analysis discusses the conservative estimates 

used in determining the oral reference dose for mercury (i.e., an estimate of the 

daily exposure to the human population, including sensitive subgroups, that is 

without an appreciable risk of deleterious effects during a lifetime). The 

reference dose and human exposure information were used to establish the water 

7 
quality criterion for methylmercury in fish tissue, the criterion used by EPA to 

represent a mercury hotspot. 

Data and Science Gaps Exist for Mercury Emissions Estimates 

While EPA has conducted activities to greatly increase its knowledge of mercury 

emissions from coal-fired utility plants, the Agency acknowledges that some 

uncertainty still exists when estimating total and speciated3 mercury emissions 

and in projecting these emissions after implementation of various control 

technologies. 

CMAQ requires the input of emissions inventory data to predict how emissions 

will transport and deposit. CMAQ was first run with a full emissions inventory to 

establish a base case scenario assuming the presence of all emissions. Next, 

CMAQ was run with emissions from coal-fired utilities removed, in what is called 

a “zero-out” run, to determine the impact of the variable that was zeroed out. 

EPA used this zero-out method to determine that no “utility-attributable” hotspots 

would occur after accounting for emissions reductions expected to be achieved 

from the Clean Air Interstate Rule and CAMR. 

The utility emissions input into CMAQ were developed from the Integrated 

Planning Model (IPM). The IPM is a model of the U.S. electric power sector that 

can be used to evaluate the cost and emissions impacts of proposed policies to 

limit emissions of pollutants, including mercury. EPA has used the IPM in 

rulemakings since the mid 1990's. As part of that process, EPA takes comments 

on the underlying assumptions of the model and makes changes as a result. For 

its Clean Air Interstate Rule and CAMR analyses, EPA used IPM to estimate base 

case and future year national inventories of unit-specific mercury emissions under 

different control scenarios. 

IPM uses equations (emission modification factors) to estimate utility emissions 

given the chemical composition of the coal being burned as well as various 

operating characteristics of the utility unit (e.g., type of control technology 

installed). These equations were based on various coal composition and 

emissions testing data collected during a 1999 Information Collection Request 

and more recent testing conducted by EPA, the Department of Energy, and 

industry participants. 

While extensive data have been collected on mercury emissions from coal-fired 

utilities, some data and science gaps still exist with respect to understanding the 

effectiveness of specific controls in reducing mercury emissions from coal. As 

noted in the EPA Office of Research and Development’s February 18, 2005, 

3

 Mercury speciates into three basic forms: elemental, ionic, and particulate. Estimating the amount of speciated 

mercury emissions is important since the type of mercury emitted impacts how effectively it is captured by control 

technologies, and how it will react when emitted into the atmosphere. Differences in atmospheric reactions impact 

the amount and location of the mercury’s deposition. 

8 
update of its study on control of mercury emissions, data and science gaps exist 

with respect to existing controls that are intended to reduce emissions of other 

pollutants with the co-benefit of reducing mercury, as well as emerging 

technologies specifically designed to reduce mercury emissions. The impact of 

these uncertainties on EPA’s estimates of mercury emissions in base case and 

future years is qualitatively discussed in Agency documents but has not been 

quantified. The uncertainties could impact the accuracy of the estimated utility 

emissions input into CMAQ and CMAQ’s resulting deposition estimates. 

CMAQ Model Uncertainties and Limitations 

CMAQ is useful for predicting regional and national patterns of deposition, but it 

has limitations that need to be carefully considered when used for modeling small 

areas of localized deposition and, thus, identifying hotspots. When emissions data 

are fed into CMAQ, the model averages the data over an area known as a “grid 

cell.” CMAQ can predict deposition results over grid cells of various sizes (or 

resolutions) as specified by the modeler. 

For CMAQ, EPA used a 36 kilometer (km) grid resolution (36 km x 36 km) for 

its Clean Air Interstate Rule and CAMR modeling, which equates to a surface 

area approximately 22 miles wide by 22 miles long, or approximately 484 square 

miles. The model provides one average concentration for the entire area. For 

example, if there is only one power plant in the corner of a grid square, that 

plant’s emissions are averaged over the entire 36 km x 36 km area. Averaging 

over grid cells may result in a smoothing out of areas of high and low deposition. 

EPA acknowledges this limitation in its Effectiveness Technical Support 

Document:4 

CMAQ immediately dilute[s] simulated emissions into the entire grid 

volume in which they are released. This causes an artificially fast dilution 

and under-represents direct deposition from air to surfaces near emission 

sources . . . 

When looking for hotspots, the ability to identify areas of localized deposition is 

important. Using the CMAQ model at 36 km x 36 km, in the opinion of some 

EPA officials we interviewed, was too coarse a resolution to be able to pinpoint 

small areas of localized deposition. Some EPA officials stated that use of a finer 

resolution, such as 12 km grid size, is possible in CMAQ. However, at very fine 

resolutions – for instance, a 4 km grid size – the meteorological components of 

the model probably fall apart and may introduce greater uncertainties in model 

results. EPA outlines three reasons for using a 36 km grid square size in its 

Effectiveness Technical Support Document. First, the larger grid size would 

account for mercury deposition that enters a watershed through groundwater 

4 Methodology to Generate Deposition, Fish Tissue Methylmercury Concentrations, and Exposures for Determining 

Effectiveness of Utility Emission Controls (Effectiveness Technical Support Document). U.S. EPA, March 15, 2005. 

9 
inflow and runoff, as opposed to a smaller grid size that may only account for 

direct inputs to surface water. Second, in larger waterbodies where there is 

substantial fishing activity, the fish species consumed by humans are likely 

migratory and the accumulation of mercury in these fish will come from 

deposition over a larger area. Third, many anglers may catch fish from a variety 

of waterbodies in a watershed, thus a larger grid size would account for this 

fishing pattern. 

Study Finds Different Rates of Atmospheric Chemical Reactions 

The Mechanisms of Mercury Removal Study developed information on the rates 

of atmospheric chemical reactions involving mercury that is different than rates 

used by EPA in its CAMR hotspots analysis. The study was published in June 

2005 after EPA issued CAMR. Rate constants, which quantify the speed or rate 

of chemical reactions, are the most important inputs affecting modeling results. 

The accuracy of rate constants can affect the accuracy of modeling results. 

Oxidation is an atmospheric process that makes mercury more reactive and is the 

most important reaction associated with mercury deposition. The mercury 

oxidation rate affects how quickly mercury is deposited and influences its 

properties and behavior. For example, oxidation makes elemental mercury more 

water soluble and more quickly deposited; if mercury emitted from a source 

comes out already oxidized, it can be immediately deposited near the source 

(depending on meteorological conditions and other factors). 

Results of the Mechanisms of Mercury Removal Study regarding mercury 

reactions and associated rates suggest that emissions from global sources 

potentially account for less mercury deposition in the United States than 

previously believed. This means that the contribution of global sources to U.S. 

deposition may have been overestimated in EPA’s analysis and the impact from 

domestic sources underestimated. According to the Agency scientist responsible 

for developing mercury capabilities in CMAQ, if the study’s results about rate 

constants are accurate, then chemical formulations currently used in all other 

atmospheric simulation models, including CMAQ, could be incorrect (when 

modeling mercury deposition). 

Uncertainties Noted with Methylation and Bioaccumulation 

Assumptions about methylation and bioaccumulation directly impact the resulting 

predictions about mercury fish tissue concentrations after implementation of 

CAMR. Mercury methylation is a complex process that occurs in the 

environment when oxidized mercury is transformed into highly toxic 

methylmercury, which bioaccumulates (builds up) in fish tissue. Some of the 

important factors affecting methylation rates and bioaccumulation were not fully 

accounted for in EPA’s analysis. Also, a lack of knowledge about some factors 

used in EPA’s analysis is a source of uncertainty in EPA’s conclusions about 

mercury fish tissue concentrations. 

10 
Methylation. Transformation of mercury to methylmercury occurs at varying 

speeds in different waterbodies, and EPA’s analysis did not fully account for this 

variation. Methylation occurs when mercury enters waterbodies and bacteria 

transform it to methylmercury, a highly toxic and bioaccumulative form of 

mercury. Methylation of mercury occurs in waterbodies at highly variable speeds 

depending on various ecosystem-specific factors, including: the bacteria in the 

waterbody, the type of land surrounding the waterbody, the quantity of certain 

substances such as sulfate and carbon in the waterbody, and the pH (chemistry) of 

the waterbody. Thus, two adjacent waterbodies with equal mercury deposition 

can have different concentrations of mercury in fish. 

EPA’s analysis did not address individual differences between waterbodies, or the 

time it takes for different waterbodies to adjust to changes in atmospheric 

deposition. The modeling assumed that the environmental factors affecting the 

formation of methylmercury remain constant. EPA acknowledges that a lack of 

knowledge about methylation is “a major contributor to overall uncertainty” in its 

analysis; however, the effect of this uncertainty on the Agency’s ability to inform 

mercury control policies is highly variable. An EPA official stated that variance 

in methylation rates was taken into account because actual methylmercury fish 

tissue measurements, which reflect varying methylation rates among different 

waterbodies where measurements were obtained, were used in the “utilityattributable” hotspot analysis. As explained in the next section, we found that 

concerns remain about these fish tissue measurements, which call into question 

how well they address methylation uncertainties. 

Bioaccumulation. EPA’s analysis did not fully account for the highly variable 

ways that mercury bioaccumulates in fish. When mercury deposition to a 

waterbody changes because of reductions in emissions, it can take time for those 

changes to be reflected in fish tissue methylmercury concentrations. Fish absorb 

methylmercury from their food and directly from water as it passes over their 

gills. To predict levels of methylmercury in fish tissue, CMAQ deposition results 

for a given area were input into a model that assumed a proportional relationship 

between declines in atmospheric mercury deposition and declines in mercury fish 

tissue concentrations. For example, a 50-percent decrease in mercury deposition 

rates was projected to lead to a 50-percent decrease in mercury concentrations in 

fish. However, drawing conclusions and making comparisons between different 

fish types is limited in that mercury bioaccumulates in highly variable ways 

among fish, both between species and within individual fish of a species. To 

establish a 2001 baseline estimate of methylmercury fish tissue concentrations, 

EPA used data from the National Listing of Fish Advisories and the National 

Lake Fish Tissue Survey: 

For included locations, samples for the same species are averaged across all 

available years (post 1998), and then the highest averaged per species 

concentration is used to represent the methylmercury concentration for that 

11 
sample location. For example, if there are two species at a location, walleye 

and pike, with three sampling dates for each species, we would first average 

over the three sample dates for each species, and then select walleye if the 

average for walleye is highest, or select pike if the average for pike is highest. 

. . . Assignment of the maximum average species concentration recognizes the 

greater risk to an individual consuming species with higher accumulation of 

mercury while respecting the fact that each sample for an individual species is 

only an estimate of the true mean concentration in that species. 

According to EPA staff, the adequacy of current fish tissue data is sparse – it is 

patchy, non-standardized from State to State, and only identifies potential 

problems where data were actually collected. Regarding EPA’s fish tissue data, 

an Agency official said, “The data does not support the conclusion that CAMR 

will not cause hotspots.” In its Effectiveness Technical Support Document, EPA 

states that, among other limitations, the model it used to estimate changes in 

methylmercury fish tissue concentrations does not account for the time lag 

between a reduction in mercury deposition and a reduction in methylmercury 

concentrations in fish tissue. However, the document stated that EPA is unaware 

of any other tool for performing a national-scale assessment of the change in fish 

methylmercury concentrations resulting from reductions in atmospheric 

deposition of mercury. 

Study Shows Significant Deposition from Local Sources 

Results from the Steubenville Study,5 a multiyear study in the Ohio River Valley, 

found that approximately 70 percent of mercury wet deposition at Steubenville, 

Ohio in 2003 and 2004 was attributable to local/regional coal combustion sources, 

predominantly from utility boilers.6 The results of the Steubenville Study suggest 

that additional monitoring is necessary to ensure that EPA’s CAMR analysis has 

properly estimated the contribution of local and regional mercury deposition. 

For example, while CMAQ results do not provide an estimate of mercury wet 

deposition for Steubenville specifically (due to its 36 km x 36 km grid cell area), 

it estimated for 2001 that 44 percent of the wet deposition in the grid cell 

containing Steubenville was from coal-fired utilities. Spatial and temporal 

differences7 between the Steubenville Study and EPA’s CAMR analysis do not 

allow for their results to be fully comparable; however, data from other 

monitoring sites further suggest that monitoring is needed to ensure that CMAQ 

5

 A peer review of the Steubenville Study was completed in late December 2005 and it was submitted for 

publication in a scientific journal in February 2006. 6

 The Steubenville Study results have an uncertainty bound of approximately 15 percent. This uncertainty bound 

does not follow a normal distribution pattern but is positively skewed, i.e., the upper bound of the 95 percent 

confidence interval extends further from the estimate than the lower bound. 7

 The Steubenville Study wet deposition results are for 2003 and 2004 and (1) represent the wet deposition for a 

specific monitoring location; (2) include wet deposition for all coal-combustion sources; and (3) have quantified 

estimates of uncertainty. Conversely, the CMAQ results are for the year 2001 and (1) represent an estimate for a 

much larger area (i.e., a 36 km x 36 km grid cell); (2) represent deposition from coal-fired utilities only; and (3) do 

not quantify uncertainty. 

12 
has not underestimated wet deposition in some locations. An Agency scientist 

noted that: 

. . .CMAQ runs conducted using 2001 emissions data for CAMR modeling 

showed that there are areas in the U.S. where domestic sources create large 

areas of enhanced deposition (e.g., up to 60% of wet mercury deposition in 

some areas originated from domestic coal combustion sources). The 

Steubenville measurements are consistent with these projections. As an 

example of uncertainties related to CMAQ . . . the University of Michigan has 

run a network of event-based mercury monitoring sites in the Midwest and 

Vermont and the 2001 CMAQ model runs systematically underestimate the 

deposition observed at these sites (in some cases by over a factor of 2). 

Senior Office of Air and Radiation officials told us that the Steubenville area is 

known to have higher-than-average deposition from coal-fired utilities, and that 

the preliminary monitoring results were not unexpected. OAQPS noted that for 

grid cells neighboring the Steubenville grid cell, the CMAQ model predicted that 

a higher percentage of mercury deposition was attributable to utility coal 

combustion (i.e., 57 to 71 percent). Preliminary results of the Steubenville Study 

were made available to Agency officials shortly after EPA’s promulgation of 

CAMR and the Revision of December 2000 Regulatory Finding in March 2005, 

but were not available for consideration by the Agency during its promulgation of 

these rules. The Agency noted that they analyzed a number of scientific studies in 

developing CMAQ, but our evaluation did not consider all of the scientific 

evidence EPA used in developing CMAQ. As noted in Appendix A, we did not 

evaluate all the inputs and assumptions associated with EPA’s mercury hotspots 

analysis. Additional limitations of our evaluation are listed in Appendix A. 

Uncertainties Underscore Need for Mercury Monitoring Plan 

In the preamble to the Revision of December 2000 Regulatory Finding, EPA 

stated that although it believed the likelihood of a “utility-attributable” hotspot 

occurring to be “remote,” it intended to closely monitor the potential for hotspots, 

continue to advance the state of the science of mercury fate and transport, and 

take appropriate action if the possibility of a “utility-attributable” hotspot arose 

after implementation of CAMR. However, at the time we completed our field 

work, EPA had not yet developed a plan for monitoring hotspots. Given the 

uncertainties associated with the inputs to the CMAQ model and the results of 

recent studies as noted, it is important for EPA to have a plan to monitor mercury 

deposition. Mercury monitoring data could assist the Agency in determining 

“utility-attributable” hotspots, and in evaluating and improving the accuracy of its 

mercury fate and transport models. Without a mercury monitoring plan, “utilityattributable” hotspots could potentially occur after implementation of CAMR but 

be less likely to be identified due to a lack of deposition data or reliable modeling 

techniques to identify mercury sources. 

13 
Field measurement of mercury deposition could improve EPA’s ability to conduct 

source apportionment studies to help to determine whether a hotspot was “utilityattributable.” To assess whether CAMR results in “utility-attributable” hotspots, 

EPA must have mercury deposition data that enable it to identify the mercury 

source. Source-apportionment studies, such as that conducted by EPA in 

Steubenville, are designed to accomplish this task. Such studies estimate a 

source’s contribution to mercury deposition and require the collection of 

deposition samples and measurements of trace elements in addition to mercury. 

Trace elements are elements that are co-emitted with mercury from particular 

sources, and help identify from which source(s) the deposited mercury originally 

came. For example, sulfur and selenium are trace elements associated with coal 

combustion. When these elements are in samples of deposited mercury, they 

indicate the mercury came from coal combustion sources. By employing a 

monitoring plan that incorporates more studies of this nature, EPA can better 

assess the impact that utilities have on mercury deposition and resulting fish tissue 

concentrations. 

Mercury deposition data would also help EPA improve its current understanding 

of mercury fate and transport, and allow the Agency to validate and improve 

mercury deposition estimation models and techniques. Model performance can 

be assessed by comparing model predictions to actual field data. While mercury 

deposition data are available through the Mercury Deposition Network (MDN), 

these data have important limitations for model evaluation, particularly modeling 

designed to identify mercury hotspots: 

• The MDN measures only wet deposition because there is no adequate field 

methodology currently available for dry deposition. 

• The MDN does not generally provide deposition monitoring data for areas 

expected to be of greatest concern for deposition from local emissions 

sources. This is because MDN monitoring sites are generally located in 

rural locations that do not have local sources of emissions. 

• There are large areas of the nation with few or no MDN monitoring sites. 

• The MDN collects deposition samples on a weekly basis, so it does not 

accurately measure the impacts of individual events, such as rain or 

snowfall. 

• The MDN sites do not collect trace element data, such as sulfur and 

selenium data for coal combustion, which is needed to conduct source 

apportionment modeling. 

Due to the limitations associated with available data from the current mercury 

deposition monitoring network, EPA is currently unable to fully assess the 

accuracy of CMAQ’s mercury deposition predictions against actual field 

14 
measurements. Agency officials told us that the EPA Office of Research and 

Development’s National Exposure Research Laboratory was already 

implementing a research plan for mercury monitoring, but recent budget 

reductions have halted the program. 

Conclusion 

EPA has acknowledged uncertainties and limitations in its analysis of the 

potential for “utility-attributable” hotspots. The results from two studies – the 

Mechanisms of Mercury Removal Study and the Steubenville Study – illustrate 

uncertainties about some of the key assumptions used in CMAQ and the 

deposition results projected by the model. Further consideration of uncertainties 

could alter EPA’s conclusions about the potential for “utility-attributable” 

mercury hotspots. EPA indicated it will closely monitor hotspots, continue to 

advance mercury science, and take appropriate actions if hotspots arose. To 

accomplish this, the Agency needs to establish a monitoring plan to conduct 

source-apportionment studies to measure the impact of CAMR and to assist in 

evaluating the accuracy of its model predictions against actual field data. 

Recommendation 

We recommend that the Acting Assistant Administrator for Air and Radiation: 

2-1 Work with the Assistant Administrator for the Office of Research and 

Development to develop and implement a mercury monitoring plan, 

including milestones and responsible program offices for implementing 

each component of the plan, to: (1) assess the impact of CAMR, if 

adopted, on mercury deposition and fish tissue; and (2) evaluate and 

refine, as necessary, mercury estimation tools and models. This effort 

should consider the suitability of the Office of Research and 

Development’s mercury research plan for addressing these objectives. 

Agency Comments and OIG Evaluation 

The Agency generally agreed with the recommendation in Chapter 2 of the report. 

However, the Agency expressed concern with our characterization of some 

scientific issues in the report and offered clarification on three specific issues. We 

accepted the Agency’s technical clarifications and have made changes to the final 

report as appropriate. The Agency also provided us with additional concerns not 

specifically addressed in its written response to our draft report. We met with the 

Agency to discuss these concerns, and made changes to the final report as 

appropriate. In response to our recommendation, the Agency stated that the 

Office of Air and Radiation and Office of Research and Development will 

continue to work together to ensure that they are using the best possible 

information to assess the transport, transformation, deposition, and fate of 

mercury emissions in the United States. We support the Agency’s commitment to 

15 
using the best possible information to assess the impact of mercury emissions in 

the United States, and continue to recommend the Agency develop a monitoring 

plan to better ensure that this happens. The Agency’s formal written response is 

in Appendix C. 

16 
Chapter 3

EPA Needs to Clarify Conditions Under Which CAMR 

Performance Standards Can be Tightened 

EPA does not clearly explain how the “utility-attributable” hotspot definition 

affects the Agency’s ability to revise performance standards under CAMR. When 

CAMR is read in conjunction with the Revision of December 2000 Regulatory 

Finding, we believe the CAMR could be interpreted to preclude EPA from taking 

action to mitigate a mercury hotspot (such as tightening the cap or utilities’ 

performance standards) unless it first determined that the hotspot was solely 

“utility-attributable.” If this were the case, such a prerequisite could limit EPA’s 

ability to reduce methylmercury fish tissue concentrations below acceptable 

levels, and thus address public health hazards that are being caused predominantly 

(but not “solely”) by utilities. This could also impact EPA’s ability to reduce the 

number of waterbodies with fish consumption advisories. 

EPA Provides “Utility-Attributable” Hotspot Definition 

In its Revision of December 2000 Regulatory Finding, EPA defined a “utilityattributable” hotspot8

 as: 

“. . . a waterbody that is a source of consumable fish with Methylmercury 

tissue concentrations, attributable solely to utilities, greater than the 

EPA’s Methylmercury water quality criterion of 0.3 mg/kg.” 

This definition only considers the contribution of one source (utilities) on 

environmental problems that could threaten human health, and would only 

consider a hotspot to be “utility-attributable” if the utility emissions alone caused 

methylmercury in fish tissue to exceed 0.3 mg/kg. 

According to the preamble of the Revision of December 2000 Regulatory Finding, 

EPA adopted this definition based on its interpretation of Clean Air Act Section 

112(n), which directed the Agency to “. . . study hazards to public health 

reasonably anticipated to occur as a result of emissions by electric utility steam 

generating units.” EPA interpreted the language “as a result of” to mean that 

utility emissions must be the sole cause of a health hazard, and not just contribute 

to causing a hazard, to be regulated under Section 112. Using EPA’s 

methylmercury water quality criterion of 0.3 mg/kg in fish tissue as the one 

 EPA uses the terms “hotspot,” “hot spot,” “utility hot spot,” and “utility-attributable hotspot” interchangeably 

throughout the Revision of December 2000 Regulatory Finding when referring to a waterbody that is a source of 

consumable fish with methylmercury tissue concentrations, attributable solely to utilities, greater than the EPA’s 

methylmercury water quality criterion of 0.3 mg/kg. 

17 

8
measure for “hazards to public health,” EPA adopted the “utility-attributable” 

hotspots definition to determine whether such utility hotspots would remain after 

implementation of the Clean Air Interstate Rule and CAMR. Based on the 

analysis described in Chapter 2 of this report, EPA stated that it did not believe 

that “utility-attributable” mercury hotspots would exist after implementing these 

rules, therefore supporting the Agency’s decision that utilities did not need to be 

regulated under Section 112 of the Clean Air Act. 

“Utility-Attributable” Definition Could be Interpreted to Limit 

EPA’s Ability to Mitigate Hotspots 

The “utility-attributable” definition could be interpreted to limit EPA’s ability to 

address waterbodies with elevated levels of mercury unless utility emissions were 

the sole cause of the problem. This could in turn limit EPA’s ability to reduce the 

number of waterbodies with fish consumption advisories where there is a health 

risk due to the combined impact of mercury from all sources, including air 

emissions. 

As discussed in its December 1997 Mercury Study Report to Congress, EPA 

stated that there is “clearly” a need to address the combined impacts of mercury 

originating from all sources, including air emissions, wherever the combination of 

sources have been related to unacceptably high mercury levels in fish. Further, in 

its December 2000 Finding, EPA recognized concerns about the potential local 

impact of mercury trading programs and acknowledged that: 

. . . approaches that involve economic incentives must be constructed in a 

way that assures that communities near the sources of emissions are 

adequately protected. 

Within CAMR and the Revision of December 2000 Regulatory Finding, EPA 

specifies several actions it might take to mitigate the effects of a hotspot in the 

event one should be identified. However, our analysis of the revision and CAMR 

suggests that the Agency may be precluded from taking any of those actions 

unless the hotspot first meets the criteria of a “utility-attributable” hotspot. EPA 

officials told us that this was not the intent of the rule, but agreed that the rule 

could be clearer. 

CAMR and Revision Must be Read Together 

Based on our reading of CAMR and the Revision of December 2000 Regulatory 

Finding, we conclude that the definition of a hotspot presented in the revision is 

intended to apply to CAMR. CAMR and the revision were issued on the same 

day and address the same subject matter. In addition, the preamble to the CAMR 

restates EPA’s conclusion from the revision, but refers to it as part of “this 

action”: 

18 
As stated elsewhere in this action EPA does not believe that utilityattributable hot spots will be an issue after implementation of CAIR 

[Clean Air Interstate Rule] and CAMR. 

Because “utility-attributable” hotspots are not discussed “elsewhere” within 

CAMR, we conclude that “this action” refers to the other, closely related action 

published by EPA on the same day. This action, the Revision of December 2000 

Regulatory Finding, defines “utility-attributable” hotspots and also explains that 

EPA may address hotspots under “other authorities under the CAA [Clean Air 

Act],” should they occur. However, the only mechanism to which EPA refers in 

order to addresses potential future hotspots – and the only mechanism presently 

promulgated – is CAMR. The revision cites the following ways it could address 

“utility-attributable” hotspots: 

. . . if in the future we determine that utility-attributable hotspots exist and 

that those hotspots occur as the result of Hg emissions from coal-fired 

Utility Units, we may promulgate a tighter section 111 standard of 

performance, provided we determine the technology can achieve the 

contemplated reductions. We could revise the standard of performance by 

adjusting the cap-and-trade program to limit trading by high-emitting 

Utility Units. . . . Thus, although we cannot conclude today which 

statutory authority we would implement to address utility attributable 

hotspots because that determination necessarily hinges on the facts 

associated with the identified hotspots, we do conclude that were such a 

situation to occur, we believe that EPA has adequate authority to address 

any such situation that may arise in the future. 

When read together, these regulatory actions suggest that a finding of a solely 

“utility-attributable” mercury hotspot is necessary to initiate Agency action to 

mitigate hotspots under CAMR. If this were the case, EPA would be precluded 

from requiring additional mercury reductions from the utility industry, even if it 

were determined that utilities were significantly contributing to a hotspot, if the 

utilities were not the sole cause of the hotspot. For example, if methylmercury 

fish tissue concentrations for a waterbody were at 0.32 mg/kg, EPA’s water 

quality criterion of 0.3 mg/kg would be exceeded. If, in this hypothetical case, 

utility mercury emissions were causing 0.3 mg/kg or less of the total 

methylmercury, under the requirement as written, utilities would be excluded 

from any additional reductions to help mitigate the problem. 

We discussed our interpretation with Office of Air and Radiation officials. These 

officials confirmed that the “utility-attributable” hotspot definition in the revision 

applies to the CAMR. However, these officials told us that this definition does 

not establish a criterion for when the Agency can adjust the performance 

standards under CAMR. They noted that under Section 111, performance 

standards are to be reviewed every 8 years, and can be adjusted for various 

reasons. 

19 
Conclusion 

The two rules related to controlling mercury emissions from coal-fired utilities 

were issued on the same day and refer to and are consistent with each other. 

Thus, it appears that they are intended to be read together. Further, the “utilityattributable” definition in the Revision of December 2000 Regulatory Finding 

applies to the discussion of hotspots in CAMR, and this definition establishes a 

criterion for when the CAMR can be adjusted to address a potential health hazard. 

If this were the case, tighter performance standards for utilities contributing to a 

hotspot could not be promulgated unless it was first determined that the hotspot 

was solely “utility-attributable.” Although not the intent of the rulemaking, EPA 

officials agreed that the rule could be clearer. We believe CAMR, if adopted, 

should be clarified to avoid any possible misinterpretation of how the “utilityattributable” definition affects EPA's ability to modify utility performance 

standards. 

Recommendation 

If EPA decides to adopt CAMR after the rule reconsideration process, to better 

ensure protection of public health and the environment, we recommend that the 

Acting Assistant Administrator for Air and Radiation: 

3-1 Explain in CAMR that the “utility-attributable” hotspot definition found in 

the revision does not establish a prerequisite for making future changes to 

the performance standards under CAMR. 

Agency Comments and OIG Evaluation 

The Agency's response did not specifically address our analysis and conclusion 

that CAMR could be interpreted to use the “utility-attributable” hotspot definition 

as a prerequisite for future changes to CAMR. The Agency commented that, 

while information regarding “utility-attributable” hotspots would be relevant to 

future possible revisions to CAMR, such hotspots are not a prerequisite to the 

Agency making changes to performance standards under CAMR. We believe the 

Agency's intent should be made clear in the final rule. Accordingly, we revised 

our final report to recommend that EPA, to better ensure protection of public 

health and the environment, explain in the CAMR that the “utility-attributable” 

hotspot definition set forth in the revision is not a prerequisite for making changes 

to the CAMR. After submitting its formal written response to the draft report the 

Agency also suggested clarifying language to parts of Chapter 3. We accepted 

some of the suggestions and incorporated them into the final report. The 

Agency’s formal written response is in Appendix C. 

20 
Appendix A 

Details on Scope and Methodology 

We conducted interviews with staff from the following EPA offices: 

• Office of Air and Radiation, including its Office of Air Quality Planning and Standards 

and Office of Atmospheric Programs. 

• Office of Research and Development, including its National Exposure Research 

Laboratory and National Center for Environmental Research. 

• Office of Policy, Economics, and Innovation. 

• Office of Water. 

We also interviewed officials from the following external organizations: the National Oceanic 

and Atmospheric Administration, including its Air Research Laboratory; the Northeast States for 

Coordinated Air Use Management; and the Clean Air Task Force. 

To understand the variables associated with mercury fate and transport modeling and, 

specifically, CMAQ, we reviewed and/or discussed with the above officials selected reports and 

studies, including: 

• Mechanisms of Mercury Removal by O3 and OH in the Atmosphere. Calvert Jack G.; 

Lindberg Steve E. Atmospheric Environment, Volume: 39, Number: 18, Page: 3355-

3367, June 5, 2005. 

• The National Oceanic and Atmospheric Administration’s September 2005 (draft) report 

by Cohen, et al, Report to Congress: Mercury Contamination in the Great Lakes. 

• EPA’s Regulatory Impact Analysis of the Final Clean Air Mercury Rule, March 2005. 

• EPA’s Mercury Study Report to Congress, December 1997. 

• A slide presentation on EPA’s Steubenville, Ohio, study, Preliminary Results from 

Steubenville Hg Deposition Source Apportionment Study, April 27, 2005. 

• The most recent peer review of CMAQ, Final Report: Second Review of the CMAQ 

Model. 

• Technical Support Document: Methodology Used to Generate Deposition, Fish Tissue 

Methylmercury Concentrations, and Exposure for Determining Effectiveness of Utility 

Emission Controls. Analysis of Mercury from Electricity Generating Units, March 17, 

2005 (revised). 

• Technical Support Document for the Final Clean Air Mercury Rule: Air Quality 

Modeling, March 2005. 

• Emissions Inventory and Emissions Processing for the Clean Air Mercury Rule, March 

2005. 

21 
To gain an understanding of State and environmental groups’ concerns related to EPA’s analysis of 

potential “utility-attributable” hotspots under CAMR, we reviewed the following selected comments: 

• The December 19, 2005 comments submitted In Reconsideration of: Revision of 

December 2000 Regulatory Finding on the Emissions of Hazardous Air Pollutants From 

Electric Utility Steam Generating Units and the Removal of Coal- and Oil-Fired Electric 

Utility Steam Generating Units from the Section 112(c) List 70 Fed. Reg. 62200 (Oct. 

28, 2005); and Standards of Performance for New and Existing Stationary Sources: 

Electric Utility Steam Generating Units 70 Fed. Reg. 62213 (Oct. 28, 2005). Comments 

Submitted by: The States of New Jersey, California, Connecticut, Delaware, Illinois, 

Maine, Massachusetts, Minnesota, New Hampshire, New Mexico, New York, 

Pennsylvania, Rhode Island, Vermont, and Wisconsin, Docket No. OAR-2002-0056. 

• The December 19, 2005 comments submitted regarding the “Revision of December 2000 

Regulatory Finding on the Emissions of Hazardous Air Pollutants From Electric Utility 

Steam Generating Units and the Removal of Coal- and Oil-Fired Electric Utility Steam 

Generating Units From the Section 112(c) List: Reconsideration,” 70 Fed. Reg. 62,200 

(October 28, 2005). Comments of: Clean Air Task Force, Izaak Walton League of 

America, Natural Resources Council of Maine, Ohio Environmental Council, U.S. Public 

Interest Research Group, Natural Resources Defense Council, Chesapeake Bay 

Foundation, Waterkeeper, Aroostook Band of Micmac Indians, Houlton Band of Maliseet 

Indians, Penobscot Indian Nation, The Passamaquoddy Tribe at Indian Township. 

To gain an understanding of EPA’s definition of “utility-attributable” hotspots and the basis for 

that definition, we reviewed the following regulatory actions: 

• Regulatory Finding on the Emissions of Hazardous Air Pollutants From Electric Utility 

Steam Generating Units, December 20, 2000. 

• Proposed National Emission Standards for Hazardous Air Pollutants; and, in the 

Alternative, Proposed Standards of Performance for New and Existing Stationary 

Sources: Electric Utility Stream Generating Units; Proposed Rule, January 30, 2004. 

• Final Rule – Preamble - Standards of Performance for New and Existing Stationary 

Sources: Electric Utility Steam Generating Units, March 15, 2005. 

• Final Rule – Regulatory Text - Standards of Performance for New and Existing 

Stationary Sources: Electric Utility Steam Generating Units, March 15, 2005. 

• Final Rule - Revision of December 2000 Regulatory Finding on the Emissions of 

Hazardous Air Pollutants from Electric Utility Steam Generating Units and the Removal 

of Coal- and Oil-fired Electric Utility Steam Generating Units from the Section 112 (c) 

List, March 15, 2005. 

• Reconsideration: Revision of December 2000 Regulatory Finding on the Emissions of 

Hazardous Air Pollutants from Electric Utility Steam Generating Units and the Removal 

of Coal- and Oil-fired Electric Utility Steam Generating Units from the Section 112 (c) 

List, October 21, 2005. 

• Reconsideration: Standards of Performance for New and Existing Stationary Sources: 

Electric Utility Steam Generating Units, October 21, 2005. 

22 
Prior Coverage 

In a prior EPA OIG report, Additional Analyses of Mercury Emissions Needed Before EPA 

Finalizes Rules for Coal-Fired Electric Utilities (2005-P-00003), dated February 3, 2005, we 

cited concerns about EPA’s limited assessment of the potential for mercury hotspots resulting 

from its (then proposed) cap-and-trade program under CAMR. In that report, we recommended 

that EPA further assess the risk of hotspots and, if necessary, identify how the Agency would 

reassess the hotspot issue. In response to our recommendation, EPA stated that it did not 

believe utility emissions would result in hotspots based on additional analyses it had performed, 

particularly after implementation of the Clean Air Interstate Rule and CAMR, but it would 

monitor the situation and take action if necessary. For this current review, we evaluated EPA’s 

analysis of hotspots, its conclusion that there will be no “utility-attributable” hotspots after 

implementation of the Clean Air Interstate Rule and CAMR, and plans the Agency may have in 

place to continue to monitor the issue. Details on what we found, including recommendations, 

are in Chapters 2 and 3 of this current report. 

Internal Controls 

Government Auditing Standards require that auditors obtain an understanding of internal 

control significant to the audit objectives and consider whether specific internal control 

procedures have been properly designed and placed in operation. This evaluation was a 

limited-scope assessment of certain analyses pertaining to a rulemaking. Thus, we determined 

whether the Agency's hotspots analysis and conclusions were peer reviewed, and if the key 

model used in this analysis was separately peer reviewed. Peer review is a key internal control 

for ensuring the acceptability of scientific data and processes. We found that CMAQ, the main 

model used by EPA in its hotspots analysis, was peer reviewed; however, we found no evidence 

that the Agency’s overall hotspots analysis, described in the document Methodology to 

Generate Deposition, Fish Tissue Methylmercury Concentrations, and Exposures for 

Determining Effectiveness of Utility Emission Controls, was peer reviewed. The Agency’s 

Ecosystem Scale Modeling for Mercury Benefits Analysis, part of the Regulatory Impact 

Analysis for the CAMR, was peer reviewed. The benefits analysis was similar to the hotspots 

analysis, but it assessed the impact of CAMR on a national scale, as opposed to identifying 

localized hotspots or local-scale impacts. 

Limitations 

Our work had several limitations. Specifically, we did not: 

• Review every model that contributed to EPA’s analysis of the potential for “utilityattributable” hotspots under CAMR. 

• Evaluate all of the inputs and assumptions associated with EPA’s mercury hotspots 

analysis. 

• Evaluate the adequacy of EPA’s water quality criterion to protect human health. 

23 
Appendix B 

Models Used in CAMR Analysis 

The following diagram depicts how data from each model were used in EPA’s hotspot analysis. 

Details on each model follow the diagram. 

MM5 

↓

 IPM → CMAQ → MMaps ≈ Changes in methylmercury levels in fish tissue 

↑

 GEOS-CHEM 

Model Purpose 

Contribution to 

Hotspot Analysis 

Integrated Planning 

Model (IPM) 

• To analyze future cost and emissions 

impacts of proposed environmental 

regulations upon utilities. 

Estimates mercury emissions 

from utilities after implementation 

of Clean Air Interstate Rule and 

CAMR. 

Mesoscale Model (MM5) • To provide meteorological information, 

such as wind, temperature, precipitation, 

and sea level pressure. 

Simulates weather patterns, 

which affect where mercury 

deposits. 

Goddard Earth Observing 

System-CHEMistry 

(GEOS-CHEM) Global 

Model 

• To provide a global three-dimensional 

model of atmospheric chemistry driven by 

meteorology. 

Uses global chemistry and 

transport information to provide 

global/background mercury 

concentrations. 

Community Multiscale Air 

Quality (CMAQ) Model 

• To estimate mercury deposition. 

• To simulate various chemical and physical 

processes thought to be important in the 

atmospheric transformation and distribution 

of mercury. 

Estimates amount of mercury 

deposition occuring within 36 km2 

grid cells after implementation of 

Clean Air Interstate Rule and 

CAMR. 

Mercury Maps (MMaps) • To relate changes in mercury air deposition 

rates to changes in mercury fish tissue 

concentrations on a national scale. 

Uses CMAQ deposition data to 

estimate fish tissue 

concentrations of methylmercury 

based on the assumption of a 

1-to-1 ratio between reductions in 

air deposition and reductions in 

average methylmercury fish 

tissue concentrations. 

24 
Appendix C 

Agency Response to Draft Report 

Bill Roderick, Acting Inspector General 

Office of the Inspector General 

Office of Program Evaluation 

1301 Constitution Ave. NW (2400 T) 

EPA West Building 

Washington, DC 20004 

Dear Mr. Roderick: 

Thank you for the opportunity to comment on the draft Office of the Inspector General 

(OIG) report entitled “Monitoring Needed to Assess Impact of EPA’s Clean Air Mercury Rule 

on Potential Hotspots.” In reviewing the draft report, we acknowledge your acceptance of the 

majority of the issues we identified in our earlier review. We have also recently supplied your 

office with additional written comments pertaining to the modeling analyses associated with the 

Steubenville project. We believe that the collective scientific and engineering expertise within 

EPA’s Offices of Air and Radiation (OAR) and Research and Development (ORD) puts our 

offices in a unique position to assess the current state-of-the-science with respect to mercury 

transport, deposition, and fate, and its impact on the creation of utility-attributable hotspots. 

We continue to have concerns about the portrayal of some scientific issues in the report, 

and note three areas where we would like to provide clarifying remarks. First, with respect to the 

potential changes in the atmospheric reaction rates within the Community Multiscale Air Quality 

(CMAQ) model (see pages 9 and 10), such changes would be made uniformly in all mercury 

transport/deposition models, not just CMAQ. Thus, the enhancements would create different 

results in any assessment using these numerical simulation technologies. Second, regarding our 

need to improve ambient monitoring (see page 13), the report should acknowledge that the 

Mercury Deposition Network (MDN) currently measures only wet deposition because there is no 

adequate field methodology available for dry deposition. 

25 
Finally, in terms of how EPA addressed the uncertainties in methylation and 

bioaccumulation rates between different fresh water bodies, our supporting health benefits 

assessment materials describe in great detail our complete understanding of these processes. 

You are correct to point out, and we clearly acknowledge in our documents, the uncertainties 

associated with mercury transport, deposition, and effects. At the same time, it should be 

acknowledged that the magnitude of uncertainties and their effect on our ability to inform 

mercury control policies is highly variable. We believe we have clearly explained the science 

and the uncertainties and provided a solid foundation for the Clean Air Mercury Rule (CAMR). 

In your draft report, you recommend two specific follow-up actions for the Agency. 

Below we address each of these recommendations. 

Recommendation 2-1: Work with the Assistant Administrator for the Office of Research 

and Development to develop and implement a mercury 

monitoring plan, including milestones and responsible program 

offices for implementing each component of the plan, to: (1) 

assess the impact of CAMR, if adopted, on mercury deposition 

and fish tissue, and, (2) evaluate and refine, as necessary, 

mercury estimation tools and models. This effort should 

consider the suitability of the Office of Research and 

Development’s mercury research plan for addressing these 

objectives. 

EPA currently operates the MDN, which is located predominantly in the eastern U.S. and 

monitors only wet deposition. In the technical support documents supporting CAMR, EPA 

has continually highlighted the need for and the willingness to support additional ambient 

monitoring, including the development of dry deposition monitoring, to enhance our ability to 

assess the numerical accuracy of our sophisticated simulation tools – e.g., the CMAQ model. 

As you are aware, ORD has been heavily involved over the past decade in developing the 

CMAQ model, and is actively engaged in utilizing ambient data and the latest scientific 

information to update the model to reflect the best possible chemistry and physics. OAR and 

ORD will continue to work together to ensure that we are using the best possible information 

to assess the transport, transformation, deposition, and fate of mercury emissions in the U.S. 

Recommendation 3-1: If EPA decides to adopt CAMR after the rule reconsideration 

process, we recommend that the Acting Assistant Administrator 

for Air and Radiation: Specifically explain what role the “utilityattributable” hotspot definition has in determining whether to 

make any future changes to the performance standards under 

CAMR. 

EPA has explained to your staff that while information regarding utility-attributable 

hotspots would be relevant to future possible revisions to CAMR, such hotspots are not a 

prerequisite. CAMR controls are based on the new source performance standards (NSPS) as 

set forth in section 111 of the Clean Air Act. To this end, the Agency is required by law to 

review and revise, as necessary, these limits every eight years. In conducting such a review, 

26 
we will analyze and evaluate the availability of new mercury control technologies installed 

since the previous review, and to the extent they provide additional cost-effective control, the 

Agency can move to change the existing NSPS limits. Additionally, the Agency continues to 

update its understanding of the science associated with mercury emissions, transport, 

transformation, and deposition, both from ambient data collection and monitoring and 

through continued enhancements to our analytical tool box. Thus, we feel that OAR and 

ORD are uniquely positioned to monitor this situation and provide the best possible solution 

for the protection of public health and the environment. 

In closing, we direct the OIG staff to the numerous technical documents supporting 

the final CAMR, particularly the benefits assessment materials in which we outline in detail 

the variability associated with methylation and bioaccumulation rates in different water 

bodies. In these documents, EPA has demonstrated that the conclusions reached in the 

CAMR are based firmly in sound scientific principles, utilizing the best information 

available. If your staff have additional questions in researching these documents, our 

scientists, engineers, and modelers would be happy to assist them. 

Sincerely, 

William Wehrum 

Acting Assistant Administrator 

Office of Air and Radiation 

George Gray 

Assistant Administrator 

Office of Research and Development 

27 
Appendix D 

Distribution 

Office of the Administrator 

Acting Assistant Administrator for Air and Radiation 

Assistant Administrator for Research and Development 

Deputy Assistant Administrator for Air and Radiation 

Acting Deputy Assistant Administrator for Science, Office of Research and Development 

Acting Deputy Assistant Administrator for Management, Office of Research and Development 

General Counsel 

Agency Followup Official (the CFO) 

Agency Followup Coordinator 

Audit Followup Coordinator, Office of Air and Radiation 

Audit Followup Coordinator, Office of Research and Development 

Associate Administrator for Congressional and Intergovernmental Relations 

Associate Administrator for Public Affairs 

Director, Office of Air Quality Planning and Standards 

Deputy Director, Office of Air Quality Planning and Standards 

Audit Liaison, Office of Air Quality Planning and Standards 

Acting Inspector General 

28