Technical Support Document: National-Scale 

Mercury Risk Assessment Supporting the 

Appropriate and Necessary Finding for Coaland Oil-Fired Electric Generating Units
ii

EPA-452/D-11-002

March 2011

Technical Support Document: National-scale Mercury Risk Assessment 

Supporting the Appropriate and Necessary Finding for Coal- and Oilfired Electric Generating Units

U.S. Environmental Protection Agency

Office of Air Quality Planning and Standards

Health and Environmental Impacts Division

Research Triangle Park, North Carolina
iii

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.

Zachary Pekar, U.S. Environmental Protection Agency, Office of Air Quality Planning

and Standards, C504-06, Research Triangle Park, North Carolina 27711 (email:

pekar.zachary@epa.gov).
iv

Table of Contents:

Executive Summary..............................................................................................................1

1. Review of Analysis Approach .............................................................................................. 13

1.1. Purpose and Scope of Analysis...................................................................................... 13

1.2. Overview of Risk Metrics and the Risk Characterization Framework .......................... 17

1.3. Overview of Analytical Approach ................................................................................. 19

1.4. Discussion of key sources of uncertainty and variability............................................... 26

1.5. Differences between the 2005 Section 112(n) Revision Rule analysis and the current 

analysis in support of the Propose U.S. EGU Toxics Rule....................................................... 26

2. Discussion of Analytical Results.......................................................................................... 29

2.1. Key design elements to consider when reviewing the risk assessment results.............. 29

2.2. Mercury Emissions from U.S. EGUs............................................................................. 30

2.3. Mercury Deposition from U.S. EGUs as Modeled Using CMAQ................................. 31

2.4. Fish Tissue MeHg Concentrations................................................................................. 35

2.5. Comparing Patterns of Hg Deposition with Hg Fish Tissue Data for the 2,461 

Watersheds Included in the Risk Assessment........................................................................... 44

2.6. Overview of Risk Estimates........................................................................................... 49

2.6.1. Overview of percentile risk estimates..................................................................... 50

2.6.2. Overview of number (and frequency) of watersheds with populations potentially atrisk due to U.S. EGU mercury emissions............................................................................. 55

2.7. Sensitivity Analyses....................................................................................................... 58

2.8. Summary of Key Observations...................................................................................... 62

Appendices: Additional Technical Detail on Modeling Elements ............................................... 69

List of Tables

Table 2-1. Comparison of total and U.S. EGU-attributable mercury deposition (ug/m2) for the 

2005 and 2016 scenarios.*............................................................................................................ 34

Table 2-2. Comparison of percent of total mercury deposition attributable to U.S. EGUs for 

2005 and 2016.*............................................................................................................................ 34

Table 2-3. Comparison of percent reduction of total mercury deposition, and U.S. EGUattributable deposition, based on comparing the 2016 scenario against the 2005 scenario.*....... 34

Table 2-4. Comparison of total and U.S. EGU-attributable Hg fish tissue concentrations 

(including % change) for the 2005 and 2016 scenarios................................................................ 43

Table 2-5. Comparison of U.S. EGU fraction of total Hg deposition (used to apportion Hg fish 

tissue concentrations and risk) between the 2005 and 2016 scenarios. Note, that these values are 

specifically for the 2,461 watersheds included in the risk assessment. ........................................ 43

Table 2-6. Percentile risk estimates for the high-end female consumer population assessed 

nationally (2005 scenario) (for both total and U.S. EGU incremental risk, including IQ loss and 

MeHg RfD-based HQ estimates).................................................................................................. 51
v

Table 2-7. Percentile risk estimates for the high-end female consumer population assessed 

nationally (2016 Scenario) (for both total and U.S. EGU incremental risk, including IQ loss and 

RfD-based HQ estimates) ............................................................................................................. 51

Table 2-8. Percentile risk estimates for the full set of fishing populations included in the analysis 

(2005 scenario) (for both total and U.S. EGU incremental risk, only for RfD-based HQ 

estimates) ...................................................................................................................................... 52

Table 2-9. Watersheds with potentially at-risk populations based on consideration for various 

degrees of U.S. EGU contribution to total risk (for IQ loss and HQ)........................................... 56

Table 2-10. Watersheds with potentially at-risk populations based on consideration for risk (both 

IQ loss and HQ) based on U.S. EGU mercury deposition and resulting exposure before 

considering other sources of mercury deposition ......................................................................... 57

Table 2-11. Combination of watersheds with potentially at-risk populations based on either 

consideration for (a) U.S. EGU percent contribution to total risk OR (b) risk when U.S. EGU 

mercury deposition is considered alone........................................................................................ 57

Table 2-12. Sensitivity analysis results based on constraining analysis to (a) watersheds in the 

top 25th percentile with regard to total Hg deposition and (b) exclude watersheds located in MN, 

LA, SE or ME) – Results for watersheds with potentially at risk populations based on U.S. EGUs 

making a specified contribution to total risk (Stage 1 of the 3 stage framework)........................ 59

Table 2-13. Sensitivity analysis results based on constraining analysis to (a) watersheds in the 

top 25th percentile with regard to total Hg deposition and (b) exclude watersheds located in MN, 

LA, SE or ME) – Results for watersheds with potentially at risk populations based on U.S. EGUincremental contribution to total risk (Stage 2 of the 3 stage framework) ................................... 60

Table 2-14. Sensitivity analysis results based on constraining analysis to (a) watersheds in the 

top 25th percentile with regard to total Hg deposition and (b) exclude watersheds located in MN, 

LA, SE or ME) – Results for watersheds with potentially at risk populations based on combining 

both Stage 1 and Stage 2 results (Stage 3 of the 3 stage framework)........................................... 60

Table 2-15. Sensitivity analysis results based on estimating risk for watersheds with Hg fish 

tissue levels based only on stationary waterbodies (i.e., excluding samples taken from flowing 

waterbodies) – Results for both total risk and U.S. EGU-attributable risk................................... 61

Table C-1. Fisher Populations Included in the Analysis for Hg Exposure and Risk.................... 75

Table F-1. Key sources of variability associated with the analysis and degree to which they are 

reflected in the design of the analysis........................................................................................... 80

Table F-2. Key sources of uncertainty associated with the analysis, the nature of their potential 

impact on risk estimates, and degree to which they are characterized ......................................... 81

List of Figures

Figure 1-1 Flow Diagram of Risk Analysis Including Major Analytical Steps and Associated 

Modeling Elements....................................................................................................................... 21

Figure 2-1. Total mercury deposition by watershed (2005) ......................................................... 32

Figure 2-2. Total mercury deposition by watershed (2016) ......................................................... 32

Figure 2-3. U.S. EGU-attributable mercury deposition by watershed (2005).............................. 33

Figure 2-4. U.S. EGU-attributable mercury deposition by watershed (2016).............................. 33

Figure 2-5. Location of 2,461 watersheds with mercury fish tissue data included in the risk 

assessment..................................................................................................................................... 38
vi

Figure 2-6. Subset of 2,170 watersheds (from the larger set of 2,461 included in the risk 

assessment) located in the eastern half of the country. (this maps also illustrates limitations with 

using color-coding at the watershed-level to explore trends in Hg fish tissue concentrations – see 

text)............................................................................................................................................... 38

Figure 2-7. Total Hg fish tissue concentrations (for the 2005 Base Case) for the subset of 

watersheds included in the risk assessment located in the eastern U.S. ....................................... 39

Figure 2-8. Total Hg fish tissue concentrations (for the 2016 Base Case) for the subset of 

watersheds included in the risk assessment located in the eastern U.S. ....................................... 39

Figure 2-9. U.S. EGU-attributable Hg fish tissue concentrations (for the 2005 Base Case) for the 

subset of watersheds included in the risk assessment located in the eastern U.S......................... 40

Figure 2-10. U.S. EGU-attributable Hg fish tissue concentrations (for the 2016 Base Case) for 

the subset of watersheds included in the risk assessment located in the eastern U.S. (Note, this 

map uses the same scale as Figure 2-7, thereby supporting direct comparison between these two 

time periods) ................................................................................................................................. 40

Figure 2-11. Top 10th percentile of watersheds based on total Hg fish tissue concentrations (for 

the 2005 simulation). (ranking is based on full national set of watersheds included in the risk 

assessment, but map focuses on locations in the eastern U.S.)..................................................... 41

Figure 2-12. Top 10th percentile of watersheds based on total Hg fish tissue concentrations (for 

the 2016 simulation). (ranking is based on full national set of watersheds included in the risk 

assessment, but map focuses on locations in the eastern U.S.)..................................................... 41

Figure 2-13. Top 10th percentile of watersheds based on U.S. EGU-attributable Hg fish tissue 

concentrations (for the 2005 simulation). (ranking is based on full national set of watersheds 

included in the risk assessment, but map focuses on locations in the eastern U.S.)..................... 42

Figure 2-14. Top 10th percentile of watersheds based on U.S. EGU-attributable Hg fish tissue 

concentrations (for the 2016 simulation). (ranking is based on full national set of watersheds 

included in the risk assessment, but map focuses on locations in the eastern U.S.)..................... 42

Figure 2-15. For the 2005 scenario, comparison of coverage of watersheds with Hg fish tissue 

data (used in the risk assessment) for areas in the eastern U.S. with relatively elevated U.S. EGUattributable Hg deposition............................................................................................................. 46

Figure 2-16. For the 2016 Scenario, comparison of coverage of watersheds with Hg fish tissue 

data (used in the risk assessment) for areas in the eastern U.S. with relatively elevated U.S. EGUattributable Hg deposition............................................................................................................. 46

Figure 2-17. For the 2005 scenario, plot of total Hg fish tissue concentrations versus total Hg 

deposition for the 2,366 watersheds included in the risk assessment for the high-end female 

consumer population..................................................................................................................... 47

Figure 2-18. Cumulative distribution plots of U.S. EGU-attributable Hg deposition over the 

2,366 watersheds used in modeling the high-end female consumer population as contrasted with 

all 88,000 watersheds (plots provided both for the 2005 and 2016 Scenarios). ........................... 47
1

Executive Summary

The EPA has completed a national-scale risk assessment for mercury to inform the 

appropriate and necessary determination for electric utility steam generating units in the United 

States (U.S. EGUs), pursuant to Section 112(n)(1)(A) of the Clean Air Act (CAA). See section 

III of the Preamble to the proposed U.S. EGU Toxics Rule.

This document (the “Mercury Risk TSD”) describes this national-scale mercury risk 

analysis. This executive summary provides an overview of the risk assessment including the 

design of the risk assessment and the risk estimates that were generated. Additional technical 

detail on the risk assessment design as well as more in-depth presentation and interpretation of 

the risk estimates generated are provided in the body of this document. 

Scope of the analysis

The national-scale risk assessment for Hg focuses on risk associated with Hg released 

from U.S. EGUs that deposits to watersheds within the continental U.S., bioaccumulates in fish 

and then is consumed as MeHg in fish eaten by subsistence fishers and other freshwater selfcaught fish consumers. The risk assessment considered the nature and magnitude of the risk to 

public health posed by current U.S. EGU Hg emissions and the remaining risk posed by U.S. 

EGU Hg emissions once CAA requirements potentially reducing Hg from U.S. EGUs are in 

place. In both cases, we also assess the contribution of U.S. EGUs to potential risks from MeHg 

exposure relative to total MeHg risk associated with Hg deposited by other sources both 

domestic and international.

The overall design and scope of the risk assessment reflect the following factors related 

to exposure to air emissions of Hg: (a) the dominant pathway associated with ambient air Hg 

releases is through the consumption of fish that have bioaccumulated MeHg originally deposited 

to watersheds following atmospheric release and transport; (b) the primary focus in quantifying 

risk associated with consumption of fish containing MeHg is risk to children born to mothers 

who were exposed to MeHg during pregnancy through fish consumption; (c) because U.S. EGU 

Hg is likely to make a very small contribution to Hg in non-U.S. sourced bought fish consumed 

in the U.S. and in bought fish sourced from further off the U.S. coast, it is not useful to assess 

risks due to consumption of those bought fish;1

Current conditions with regard to U.S. EGU emissions based on the 2010 ICR show HAP 

emissions are closer to the 2016 emissions than to the 2005 emissions (due in part to Hg 

reduction co-benefits of existing state and Federal actions).

and (d) the type of fish consumption likely to 

experience the greatest contributions from U.S. EGU-sourced Hg is associated with fishing 

activity at inland freshwater rivers and lakes located in regions with elevated U.S. EGU Hg 

deposition.

2

 1 Furthermore, although commercial fish sourced closer to the U.S. coast (including estuarine areas) may have 

greater U.S. EGU impacts in some cases, relative to the average U.S. EGU impact nationally, because of uncertainty 

in modeling the linkage between U.S. EGU deposition and the apportionment of mercury in these fish, this 

commercial consumption pathway is not included in the quantitative risk assessment.

 For this reason, in discussing risk 

estimates, we focus on the 2016 results.

2 A number of air quality modeling runs were completed in support of this rule. For this risk assessment, we 

modeled risk for a 2005 and 2016 scenario, reflecting emissions of 52.9 and 29 tons of total mercury from U.S. 

EGUs, respectively. We also developed a current estimate of Hg emissions from EGUs based on the 2010 ICR data 

and that estimate was 29 tons of mercury from U.S. EGUs.
2

The risk assessment calculated both the magnitude of the U.S. EGU incremental 

contribution to total potential exposure and risk and the percent of total Hg exposures and risk 

contributed by U.S. EGUs (i.e., the fraction of total risk associated with U.S. EGUs) to 

individual watersheds for which we have fish tissue MeHg data.

Given the goal of determining whether a public health hazard is associated with U.S. 

EGU emissions, we have focused this analysis on those populations likely to experience the 

greatest risk when fishing at inland (freshwater) locations including subsistence-fishers.3

Although Hg-related risks associated with commercial fish consumption are a public 

health concern, the relatively low contribution of U.S. EGU Hg to this source of dietary fish 

(relative to non-U.S. Hg emissions), and the high levels of uncertainty in mapping U.S. EGU Hg 

emissions to concentrations of MeHg in ocean-going fish, precludes our assessment of this 

consumption pathway as part of the risk assessment. In the specific case of commercial fish 

sourced from near the U.S. coast and the Great Lakes, although there is the potential for U.S. 

EGUs to have a greater role in affecting Hg levels in these fish, uncertainty associated with 

modeling the linkage between U.S. EGU Hg deposition and Hg exposure and risk for this dietary 

pathway precludes us from including it in the risk assessment. However, it is likely that the 

range of potential exposures to U.S. EGU Hg deposition across inland watersheds captures the 

types of potential exposures that occur in near-coastal environments.

 In 

defining these high-end populations, we have included behavior that places them at greater risk 

(e.g., focusing on somewhat larger fish in supplementing their diet and focusing their fishing 

activity at individual watersheds - see Section 1.1 for additional detail). While these attributes 

define a subset of subsistence fishers, we think that they are reasonable and that fishing 

populations with these attributes are likely to exist and be active to some extent at the watersheds 

included in this risk assessment. Because the general recreational angler population is likely to 

experience individual risk levels well-below these high-consuming fisher populations, we have 

not quantified risk for this more generalized population, although the consumption rates for 

subsistence fishers are likely to be similar to consumption rates for high-end recreational anglers. 

Consumption rates for the high-end fishing populations included in the risk assessment are based 

on studies in the published literature, and are documented in Section 1.3 and Appendix C.

Risk Characterization Framework

EPA assessed risk from potential exposure to MeHg through fish consumption at a subset 

of watersheds across the country for which we have measured fish tissue MeHg data. This risk 

assessment uses estimates of potential exposure for subsistence fisher populations to generate 

risk metrics based on comparisons of MeHg exposure to the reference dose.4

 3 As noted in Section 1.2, subsistence fish consumption is defined by the EPA as individuals who rely on 

noncommercial fish as a major source of their protein (USEPA, 2006). This definition does reflect a degree of 

subjectivity in terms of what is meant by "major source of their protein". For this risk assessment, we consider our 

high-end consumption rates (i.e., a meal every 1-2 days) as clearly subsistence.

Because of 

4 We also generated estimates of the loss in intelligence quotient (IQ) points for the same populations, however, we 

are focusing on the reference dose because it represents a more sensitive risk metric that potentially captures a wider 

range of neurobehavioral health effects (see Section 1.2 for additional discussion). .
3

limitations in quantifying the number of high-consumption fishers active across the set of 

modeled watersheds, it is not possible to generate population-weighted distributions of risk.

For the analysis, we have developed a risk characterization framework for integrating two 

types of U.S. EGU-attributable risk estimates. This framework estimates the total percent of 

watersheds with fish tissue MeHg samples (approximately 2,400 out of 88,000 watersheds) that 

are at risk due to potential exposures to MeHg attributable to U.S. EGU. This total percent of 

watersheds where populations may be at risk from EGU-attributable Hg includes watersheds that 

either have deposition of Hg from U.S. EGUs that is sufficient to lead to potential exposures that 

exceed the reference dose even without considering the contributions from other U.S. and nonU.S. sources, or have deposition of Hg from U.S. EGUs that represents a fraction (e.g., 5, 10, 15, 

or 20 percent) of total Hg deposition from all sources, in watersheds where potential exposures to 

MeHg from all sources (U.S. EGU, U.S. non-EGU, and non-U.S.) exceed the RfD.5

The results of the analysis include the total percent of watersheds where populations may 

be at risk from EGU-attributable Hg, as well as the two component estimates. This framework 

allows us to consider whether U.S. EGUs alone or in combination with other sources of Hg, pose 

a potential public health hazard. The framework also allows us to evaluate the impacts of the 

proposed regulation on this potential public health hazard.

Analytical Approach

Watersheds can be defined at varying levels of spatial resolution. For the purposes of this 

risk analysis, we have selected to use watersheds classified using 12-digit Hydrologic Unit Codes 

(HUC12) (USGS, 2009), representing a fairly refined level of spatial resolution with watersheds 

generally 5 to 10 km on a side, which is consistent with research on the relationship between 

changes in Hg deposition and changes in MeHg levels in aquatic biota.

After estimating total MeHg risk based on modeling consumption of fish at each of these 

watersheds, the ratio of U.S. EGU to total Hg deposition over each watershed (estimated using 

Community Multi-scale Air Quality, CMAQ, modeling) is used to estimate the U.S. EGUattributable fraction of total MeHg risk. This apportionment of total risk between the U.S. EGU 

fraction and the fraction associated with all other sources of Hg deposition is based on the EPA’s 

Office of Water’s Mercury Maps (MMaps) approach that establishes a proportional relationship 

between Hg deposition over a watershed and resulting fish tissue Hg levels, assuming a number 

of criteria are met (USEPA, 2001). Each of the steps in the analysis is briefly described below.

Methodology for Assessing MeHg Levels in Fish Tissues

The fish tissue dataset for the risk assessment includes fish tissue Hg samples from years 

1995 to 2009, with approximately 50,000 unique samples from 4,115 HUC12s out of 

approximately 88,000 HUC12s in the continental U.S. The samples are more heavily focused on 

locations east of the Mississippi River. For this risk assessment, a subset of the data from 2000 

and later was selected, with samples distributed across 2,461 HUC12s, which provided samples 

more representative of current conditions with regard to patterns of mercury deposition.

 5 Any contribution of Hg emissions from EGUs to watersheds where potential exposures from total Hg deposition 

exceed the RfD is a hazard to public health, but for purposes of our analyses we evaluated only those watersheds 

where we determined EGUs contributed 5 percent or more to deposition to the watershed. EPA believes this is a 

conservative approach given the increasing risks associated with incremental exposures above the RfD.
4

The fish tissue samples in the master dataset come primarily from three sources: the 

National Listing of Fish Advisory (NLFA) database is managed by EPA;6

http://emmma.usgs.gov/datasets.aspx

the U.S. Geologic 

Survey (USGS), which manages a compilation of Hg datasets as part of its Environmental 

Mercury Mapping and Analysis (EMMA) program, and compiles Hg fish tissue sample data 

from a wide variety of sources (including the NLFA) and posts these data at 

; and EPA’s National River and Stream Assessment 

(NRSA) study data, which includes nearly 600 fish tissue Hg samples collected at randomly 

selected freshwater sites across the U.S. during the period 2008 to 2009. Additional detail on 

these fish tissue MeHg data are provided in Section 1.3 and Appendix B.

Data from these three datasets were combined into a single master fish tissue dataset 

covering the period 1995 to 2009. As noted above, only a fraction (2,461) of the approximately 

88,000 watersheds in the continental U.S. had fish tissue concentration data and, therefore, could 

be included in risk characterization. Most of the watersheds with measured fish tissue MeHg 

data had multiple measurements (the average number of fish tissue measurements for the period 

2000 to 2009 for each of the 2,461 watersheds is 10, although some watersheds had up to 270 

measurements). The assessment used the 75th percentile fish tissue value at each watershed as 

the basis for exposure and risk characterization, based on the assumption that subsistence fishers 

would favor larger fish which have the potential for higher bioaccumulation (i.e., use of a median 

or mean value could bias low the likely MeHg levels in typically consumed fish). The 75th 

percentile represents the upper bound of the interquartile range, which is generally seen as a 

reasonable limit on the central tendency of a distribution. Selection of the 75th percentile 

represents a reasonable assumption that acknowledges the median or mean fish may give too 

much weight to smaller, less likely to be eaten fish, while avoiding assumptions that consumers 

would always be able to catch and eat the largest fish with the highest MeHg levels.

Air Quality Modeling of Hg Deposition over Watersheds

Deposition of Hg was estimated using the CMAQ model v4.7.1 (www.cmaq-model.org). 

The CMAQ v4.7.1 is a state of the science three-dimensional Eularian “one-atmosphere” 

photochemical transport model used to estimate air quality (Byun et al., 2006, Appel et al., 2007, 

Appel et al., 2008). The CMAQ simulates the formation and fate of photochemical oxidants, 

ozone, primary and secondary PM concentrations, and air toxics at a 12 km gridded spatial 

resolution over regional and urban spatial scales for given input sets of meteorological conditions 

and emissions. Mercury oxidation pathways are represented for both the gas and aqueous phases 

in addition to aqueous phase reduction reactions (Bullock et al., 2002). Because measurements 

for the dry deposition of Hg do not currently exist, the modeled dry deposition performance 

could not be evaluated. In EPA’s view, CMAQ model wet deposition estimates agree well with 

the Mercury Deposition Network (MDN) monitoring sites with a minimal seasonal bias. 

Additional information on the CMAQ modeling is provided in Section 1.3 and Appendix E and 

Appendix F.

CMAQ modeling at a 12 km resolution was used to estimate total annual Hg deposition 

from U.S. and non-U.S. anthropogenic and natural sources over each watershed. In addition, 

CMAQ simulations were conducted where U.S. EGU Hg emissions were set to zero to determine 

the contribution of U.S. EGU Hg emissions to total Hg deposition. U.S. EGU-related Hg 

deposition characterized at the watershed-level for the two scenarios assessed (2005 and 2016) is 

 6 http://water.epa.gov/scitech/swguidance/fishshellfish/fishadvisories/
5

summarized in Table ES-1 for the complete set of 88,000 HUC12 watersheds, while Table ES-2

summarizes the percent of total Hg deposition attributable to U.S. EGUs (by percentile).

Table ES-1. Comparison of Total and U.S. EGU-Attributable Hg Deposition (μg/m2

) for 

the 2005 and 2016 Scenarios.*

Statistic

2005 Scenario 2016 Scenario**

Total Hg 

Deposition

U.S. EGUattributable Hg 

Deposition

Total Hg 

Deposition

U.S. EGUattributable Hg 

Deposition

Mean 19.41 0.89 18.66 0.34

Median 17.25 0.24 16.59 0.15

75th percentile 23.69 1.07 22.83 0.46

90th percentile 30.78 2.38 29.90 0.85

95th percentile 36.85 3.60 35.16 1.18

99th percentile 58.32 7.77 56.23 2.41

* Statistics are based on CMAQ results interpolated to the watershed –level and are calculated using all ~88,000 

watersheds in the U.S.

Table ES-2. Comparison of Percent of Total Hg Deposition Attributable To U.S. EGUs for 

2005 and 2016.*

Statistic 2005 Scenario 2016 Scenario

Mean 5% 2%

Median 1% 1%

75th percentile 6% 3%

90th percentile 13% 5%

95th percentile 18% 6%

99th percentile 30% 11%

* Values are based on CMAQ results interpolated to the watershed –level and reflect trends across all ~88,000 

watersheds in the U.S.

We note the following observations regarding estimated deposition based on information 

presented in Table ES-2: (a) U.S. EGUs are estimated to contribute up to 30 percent of total Hg 

deposition for the 2005 scenario and up to 11 percent for the 2016 scenario (99th percentile 

values – see Table ES-2); (b) on average, U.S. EGUs contribute a substantially smaller fraction 

of total Hg deposition (2 to 5 percent for the 2016 and 2005 scenarios, respectively – see Table 

ES-2), this reflecting contributions made by other U.S. air emissions sources and more 

importantly, by non-U.S. sources; and (c) U.S. EGU-related deposition is predicted to decrease 

substantially between the 2005 and 2016 scenarios.7

 7 The estimated decrease in U.S. EGU Hg emissions between 2005 and 2016 is due, in part, to decreases in SO2 and 

other criteria pollutant emissions pursuant to Federal requirements and enforcement actions. If those controls were 

not maintained, and Hg emissions were to increase from current levels, the U.S. EGU attributable deposition, and 

fraction of deposition would be somewhere between the 2005 scenario and 2016 scenario.
6

EPA also evaluated the potential for “hot spot” deposition near U.S. EGU emission 

sources on a national scale, based on the CMAQ modeled Hg deposition for the 2005 and 2016 

scenarios. We calculated the excess deposition within 50 km of U.S. EGU sources by first 

calculating the average U.S. EGU attributable Hg deposition within a 500 km radius around the 

U.S. EGU source. This deposition represents the likely regional contribution around the EGU. 

We then calculated the average U.S. EGU attributable Hg deposition within 50 km of the U.S. 

EGUs to characterize local deposition plus regional deposition near the EGU. Excess local 

deposition is then the 50 km radius average deposition minus the 500 km radius average 

deposition. Summary statistics for the excess local deposition are provided in Table ES-3. Table 

ES-3 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 ES-3 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 around 120 

percent of the average deposition, while for the top 10 percent of Hg emitting U.S. EGU, local 

deposition was around 3.5 times the regional average. By 2016, the absolute excess deposition 

falls, however, the local excess still remains around 3 times the regional average for the highest 

10 percent of Hg emitting U.S. EGUs (note, additional detail on this hot spot assessment is not 

provided in the main body of the TSD, since it is clearly laid out here).

Table ES-3. Excess Local Deposition of Hg Based on 2005 CMAQ Modeled Hg Deposition

Category of Results

50km-Radius-Average Excess Local 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%) 0.36 (93%)

Top ten percent U.S. EGU in Hg 

emissions

(67 sites)

4.89 (352%) 1.18 (302%)

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 chance that Hg from U.S. EGUs will impact local 

watersheds.

Estimating the Proportion of Total Hg Exposure Associated with U.S. EGUs and 

Projecting Changes in Fish Hg concentrations

The MMaps approach specifies that, under certain conditions (e.g., Hg deposition is the 

primary loading to a watershed and near steady-state conditions have been reached), a fractional 

change in Hg deposition to a watershed will ultimately be reflected in a matching proportional 

change in the levels of MeHg in fish. This proportionality assumption between deposition 

changes and fish tissue MeHg concentrations can be used to both estimate the portion of total Hg 

exposure that is associated with U.S. EGUs and project changes in fish Hg concentrations (and 
7

consequently total exposure) associated with changes in total Hg deposition in the future.8

For the 2005 analysis, CMAQ modeling results for a particular watershed allow U.S. to 

estimate the proportion of total exposure (estimated for that watershed) that is associated with 

U.S. EGU deposition (i.e., based on the ratio of U.S. EGU Hg deposition to total Hg deposition 

over the watershed as specified by the MMaps approach). In the case of the 2016 future 

simulation, we can first project changes in total fish tissue Hg levels (for that watershed) by 

comparing estimates of total Hg deposition in 2005 to estimates for 2016 generated by CMAQ 

and then again, apportion that adjusted total risk between U.S. EGUs and all other sources, based 

on comparing U.S. EGU Hg deposition to total Hg deposition in 2016.

 This 

assumption holds in watersheds where air deposition is the primary source of Hg loadings, and as 

a result, watersheds where this is not the case are removed from the risk analysis. MMaps is 

discussed in greater detail in Section 1.3 and in Appendix E and Appendix F.

Patterns of U.S. EGU-attributable fish tissue MeHg concentrations are summarized in 

Tables ES-4 and ES-5. Table ES-4 compares total and U.S. EGU-attributable fish tissue MeHg 

concentrations for the 2016 and 2005 scenarios by watershed percentile (including the percent 

reduction between 2005 and 2016). Table ES-5 summarizes the percent of total fish tissue 

MeHg concentrations attributable to U.S. EGUs (also by watershed percentile).

Table ES-4. Comparison of Total and U.S. EGU-Attributable Fish Tissue MeHg 

Concentrations for the 2005 and 2016 Scenarios

Statistic

Fish tissue MeHg concentration (ppm) % change (2016 

versus 2005) in fish 

tissue MeHg 

concentration

2005 Scenario 2016 Scenario

Total U.S. EGUattributable Total U.S. EGUattributable Total U.S. EGUattributable

mean 0.31 0.024 0.29 0.008 -6.7% -65%

50th Percentile 0.23 0.014 0.20 0.005 -10.0% -61%

75th Percentile 0.39 0.032 0.36 0.011 -6.4% -67%

90th Percentile 0.67 0.056 0.63 0.019 -5.9% -66%

95th Percentile 0.91 0.079 0.87 0.026 -4.7% -67%

99th Percentile 1.34 0.150 1.29 0.047 -3.7% -68%

Table ES-5. Comparison of U.S. EGU Fraction of Total Fish Tissue MeHg Levels*

Statistic U.S. EGU-attributable percent of total fish tissue MeHg levels

2005 Scenario 2016 Scenario

Mean 9% 4%

50th Percentile 6% 3%

75th Percentile 14% 5%

 8 The risk assessment estimates risk for future points in time once near steady state conditions have been reached 

following simulated changes in mercury deposition and does not attempt to simulate the temporal profile of that 

response. As noted in Section 1.3 and Appendix E, the amount of time required for MeHg levels in fish to fully 

respond following a change in mercury deposition can range from years to decades depending on the nature of the 

watershed involved (e.g., methylation potential, importance of watershed runoff to load mercury into the 

waterbody). 
8

Statistic U.S. EGU-attributable percent of total fish tissue MeHg levels

2005 Scenario 2016 Scenario

90th Percentile 20% 7%

95th Percentile 26% 9%

99th Percentile 40% 18%

* These values are specifically for the 2,461 watersheds included in the risk assessment.

We note the following observations regarding fish tissue MeHg levels based on 

information presented in Tables ES-4 and ES-5: (a) U.S. EGUs can contribute up to 18 percent 

of total fish tissue MeHg levels for a subset of the watersheds with fish tissue data (99th 

percentile watershed for the 2016 Scenario – see Table ES-4); (b) on average, U.S. EGUs 

contribute 4 percent of total fish tissue MeHg levels (for the 2016 Scenario); and (c) reflecting 

the pattern seen with Hg deposition, the U.S. EGU-attributable fraction of fish tissue MeHg 

levels is estimated to decrease significantly between the 2005 and 2016 scenarios.

Defining Subsistence Fisher Scenarios

As discussed above, this analysis focuses on higher-consumption self-caught fisher 

populations active at inland freshwater locations, because these populations are expected to 

experience the greatest U.S. EGU-attributable risks. Therefore, in reviewing studies of fishing 

populations, emphasis was placed on identifying surveys of higher consumption fishing 

populations active at inland freshwater rivers and lakes within the continental U.S. Information 

on the studies used to develop the high end fish consumption scenarios for the risk analysis is 

provided in Section 1.3 and in Appendix C.

Based on EPA’s review of the fish consumption literature, EPA defined consumption 

rates for the subsistence fisher populations modeled across the 2,461 watersheds included in the 

risk assessment. We used the studies referenced above as a guide to characterize high-end 

consumption behavior for a scenario that could be assessed broadly across the 2,461 watersheds. 

Generally all of the studies identified high-end percentile consumption rates (90th to 99th 

percentiles for the populations surveyed) ranging from approximately one fish meal every few 

days to a larger fish meal very day (i.e., 120 grams per day (g/day) to greater than 500 g/day fish 

consumption). We used this trend across the studies to support application of a generalized 

female high-end fish consumption scenario (high-end female consumer scenario) across most of 

the 2,461 watersheds.9

Consumption rates for this high-end female consumer were based on values presented for 

female fishers in South Carolina. Values from the South Carolina study were used because they 

specifically covered high-end consumption by women, which is the population of interest in this 

risk assessment. Furthermore, the consumption rates identified in the South Carolina study (123 

g/day to 373 g/day for the 90th and 99th percentiles, respectively) are in the range of values seen 

across the other studies reviewed in designing this analysis and therefore are considered to be 

generally representative of subsistence consumption (these consumption rates translate into 

approximately one 8oz fish meal every other day). 

 9 Reflecting the fact that higher levels of self-caught fish consumption (approaching subsistence) have been 

associated with poorer populations, we only assessed this generalized high-end female consumer scenario at those 

watersheds located in U.S. Census tracts with at least 25 individuals living below the poverty line (this included the 

vast majority of the 2,461 watersheds and only a handful were excluded due to this criterion).
9

In addition to the studies used to define the high-end fish consuming populations 

modeled in the risk assessment, we also reviewed a large number of additional studies 

characterizing higher-level self-caught fish consumption in the U.S. While these studies had 

limitations that prevented their use as the basis for defining high-end fisher scenarios to include 

in the analysis, in several instances, they did support the levels of self-caught fish consumption 

modeled in the analysis. These studies are described in Appendix C.

Risk Related to Exposure to MeHg in Fish and Assessment of Contribution of U.S. EGUs to 

MeHg Exposure and Risk

This section provides an overview of risk estimates generated for the 2,461 watersheds 

included in the risk assessment.10

We estimated total exposure to MeHg at each of the 2,461 watersheds.

 As noted above, we focus on risk estimates generated for the 

high-end female consumer assessed at the national level, since this population, as defined for this 

analysis, provides the most comprehensive coverage for watersheds with fish tissue MeHg data 

across the U.S. and because the consumption rates used to model this population represent 

subsistence levels that would characterize populations most likely to experience high levels of 

exposure to MeHg and thus experience higher risk. 

11

We estimate the fraction of total potential exposure that is associated with U.S. EGU Hg 

deposition at each watershed using the proportionality assumption supported by MMaps (see 

above). Once total potential exposure to MeHg has been estimated and the U.S. EGUattributable portion of that exposure has been estimated, we then estimate risk based on 

exposures above the RfD, both due to total MeHg, and MeHg attributable to U.S. EGU without 

consideration of other U.S. and non-U.S. sources.

Estimates of 

total Hg exposure were generated by combining 75th percentile fish tissue values with the 

consumption rates for female subsistence fishers. A cooking loss factor (actually reflecting the 

fact that the preparation of fish can result in increased Hg concentrations) was also included in 

exposure calculations (Morgan et al., 2007).

A summary of risk estimates is presented here and detailed summaries of the risk 

estimates are presented in Section 2.6. Our estimates of total percent of watersheds where 

populations may be at risk from EGU-attributable Hg are as high as 28 percent.12

 10 Each fish consumption rate scenario was assessed for a subset of the 2,461 watersheds based on consideration for 

where “source populations” for each scenario were located (i.e., a watershed was modeled if it fell within a U.S. 

Census tract containing the source population for a particular fisher scenario). The high-end female consumption 

rate scenario is the scenario with the broadest spatial coverage since it was applied to all watersheds intersecting 

U.S. Census tracts with at least 25 poor white individuals (i.e., the “source population” for high-end female selfcaught fish consumers). This meant that this scenario was assessed for 2,366 of the 2,461 watersheds with fish tissue 

MeHg data.

 The upper end 

estimate of 28 percent of watersheds reflects the 99th percentile fish consumption rate for that 

population modeled, and a benchmark of 5 percent U.S. EGU contribution to total Hg deposition 

in the watershed. Any contribution of Hg emissions from EGUs to watersheds where potential 

11 As noted earlier, each high-end fish consuming population included in the analysis was assessed for a subset of 

these watersheds, depending on which of those watersheds intersected a U.S. Census tract containing a “source 

population” for that fish consuming population. Of the populations assessed, the high-end female consumer scenario 

was assessed for the largest portion (2,366) of the 2,461 watersheds.

12 As noted earlier, the determination of whether U.S. EGUs make a significant contribution to total Hg deposition 

is only considered for watersheds where total risk is considered to represent a potential hazard to public health (e.g., 

Potential MeHg exposure from all U.S. and non-U.S. sources exceeds the RfD).
10

exposures from total Hg deposition exceed the RfD is a hazard to public health, but for purposes 

of our analyses we evaluated only those watersheds where we determined EGUs contributed 5 

percent or more to deposition to the watershed. EPA believes this is a conservative approach 

given the increasing risks associated with incremental exposures above the MeHg RfD. Scenario 

The percent of where populations may be at risk from EGU-attributable Hg based on the 2010 

ICR would be somewhat higher due to the greater level of Hg emissions in that case (35 tons in 

2010 compared with 29 tons in 2016).

Of the total percent of watersheds where populations may be at risk from EGUattributable Hg, we estimate that up to 22 percent of watersheds included in this analysis could 

be potentially at risk based on consideration of the U.S. EGU attributable fraction (e.g., 5, 10, 15, 

or 20 percent) of total Hg deposition over watersheds with total risk judged to represent a public 

health hazard (MeHg total exposure greater than the RfD). The 22 percent estimate is also 

generated for the 2016 scenario and is based on the same assumptions used in the estimate of 

total risk provided earlier (i.e., 99th percentile fish consumption rate, and a 5 percent U.S. EGU 

contribution to total Hg deposition over a given watershed). We do note however, that, 

specifically with regard to the HQ estimates, any contribution of mercury from EGUs to 

watersheds with exposures exceeding the MeHg RfD represents a potential hazard to public

health, but for purposes of this analysis we have focused on those waterbodies where we 

determined EGUs contributed 5% or more to the hazard. We think this is a conservative 

approach given the increasing risks associated with incremental exposures above the MeHg RfD.

Of the total percent of watersheds where populations may be at risk from EGUattributable Hg, we estimate that up to 12 percent of the watersheds could potentially be at risk 

based on watersheds where the U.S. EGU incremental contribution to exposure exceed the 

MeHg RfD, without consideration of contributions to exposures from U.S. non-EGU and nonU.S. sources. The upper end estimate of 12 percent of watersheds is based on the 2016 Scenario 

and reflects a scenario using the 99th percentile fish consumption rate.

The two component estimates of percent of watersheds where populations may be at risk 

from EGU-attributable Hg do not sum to the total percent watersheds where populations may be 

at risk from EGU-attributable Hg of 28 percent due to overlap in the risk estimates – some 

watersheds where U.S. EGUs contribute greater than 5 percent to total Hg deposition also have 

U.S. EGU attributable exposures that exceed the RfD without consideration of exposures from 

other U.S. and non-U.S. Hg sources.

The percentage of watersheds where U.S. EGUs contribute to exposures of concern 

increases dramatically as we consider higher fish consumption scenarios. Exposures based on 

the 99th percentile consumption rate represent close to maximum potential individual risk 

estimates. These consumption rates are based on data reported by fishers in surveys, and, thus, 

represent actual consumption rates in U.S. populations. However, EPA does not have data on 

the locations where these high self-caught fish consuming populations reside and fish, and as a 

result, there is also increased uncertainty about the percent of watersheds that might experience 

potential exposures at the highest levels.

With regard to the other fisher populations included in the full risk assessment 

(Vietnamese, Laotians, Hispanics, blacks and whites in the southeast, and tribes in the vicinity of 

the Great Lakes), our risk estimates suggests that the high-end female consumer assessed at the 
11

national-level generally provides coverage (in terms of magnitude of risk) for all of these fisher 

populations except blacks and whites in the southeast.13

Variability and Uncertainty (Including Discussion of Sensitivity Analyses)

The risk assessment has been designed to reflect consideration for key sources of 

variability associated with the exposure scenario (e.g., spatial pattern of total and U.S. EGUrelated Hg deposition, spatial variation in fish tissue MeHg levels, variation in the location and

behavior of high-consuming fisher populations). The degree to which critical sources of 

variability have been reflected in the design of the analysis is discussed in Appendix F, Table F1.

Key sources of uncertainty potentially impacting the risk analysis include: (a) 

uncertainty in predicting Hg deposition over watersheds using CMAQ; (b) uncertainty in 

predicting which watersheds will be subject to high-end fishing activity an the nature of that 

activity (e.g., frequency of repeated activity at a given watershed and the types/sizes of fish 

caught); and (c) uncertainty in using MMaps to apportion exposure and risk between different 

sources including U.S. EGUs and predict changes in fish tissue MeHg levels for future scenarios. 

We describe key sources of uncertainty impacting the risk analysis, including their potential 

impact on the risk estimates and the degree to which their potential impact is characterized as 

part of the analysis in Appendix F, Table F-2.

As part of the risk assessment, we have also completed a number of sensitivity analyses 

focused on exploring the impact of uncertainty related to the application of the MMaps approach 

in apportioning exposure and risk estimates between sources (U.S. EGU and total) and in 

predicting changes in fish tissue MeHg levels. These sensitivity analyses have explored: (a) 

concerns over including watersheds that may be disproportionately impacted by non-air Hg 

sources;14 and (b) concern that the MMaps approach may be more representative when applied to 

stationary waterbodies (in the analysis, the MMaps was applied to watersheds including a 

mixture of flowing and stationary waterbodies). The results of the sensitivity analyses, when 

considered in aggregate, suggest that uncertainties due to application of MMaps are unlikely to 

have a substantial effect on the risk estimates discussed here.

Key Observations

The following key observations result from consideration of the risk estimates generated 

(additional detail on these observations is presented in Section 2.8):

• Reflecting current emissions, U.S. EGUs can contribute up to 11 percent of total Hg 

deposition (for the 99th percentile watershed in the 2016 Scenario). U.S. EGUs (for the 2016 

scenario) contribute on average, about 2 percent of total Hg deposition across the country. 

 13 Specifically, upper percentile risk estimates for the high-end female consumer assessed at the national level were 

notably higher than matching percentile estimates for the Hmong, Vietnamese, Hispanic and Tribal populations. By 

contrast, risk estimates for whites in the southeast were somewhat higher than the high-end female consumer, while 

risk estimates for blacks in the southeast were notably higher (see summary of risk estimates in the TSD supporting 

the A&N Determination (see section 2.6.1 for additional detail on these risk estimates). 14 In addition to non-air Hg sources of loadings, some regions of concern may also have longer lag period 

associated with the linkage between Hg deposition such that the fish tissue MeHg levels we are using are actually 

associated with older historical Hg deposition patterns.
12

The average U.S. EGU deposition decreased from approximately 5 percent of total to 

approximately 2 percent of total for the 2005 and 2016 Scenarios, respectively. 

• Although U.S. EGUs contribute on average, about 2 percent of total Hg deposition for the

2016 Scenario, they contribute about 4 percent of the fish tissue MeHg levels at watersheds 

included in this analysis. This discrepancy reflects the fact that fish tissue MeHg sampling is 

focused in the eastern half of the country which has higher U.S. EGU Hg deposition levels, 

compared with the national average, and, therefore, the fraction of MeHg in fish tissue 

attributable to U.S. EGUs will also be larger given that it is driven by estimates of U.S. EGU 

Hg deposition over these watersheds. U.S. EGUs are estimated to contribute up to 18 percent 

of fish tissue MeHg levels in the 2016 scenario (for the 99th percentile watershed).

• Comparing the pattern of U.S. EGU-attributable Hg deposition with watersheds containing 

fish tissue MeHg data results in our concluding that, while we have some degree of coverage 

for high U.S. EGU impact areas, this coverage is limited. For this reason, we believe that the 

actual number of where populations may be at risk from EGU-attributable Hg (i.e., 

watersheds where U.S. EGUs could contribute to a public health hazard) could be 

substantially larger than estimated.

• We estimate that up to 28 percent of the watersheds included in this risk assessment could 

have populations potentially at-risk under the 2016 scenario. This total risk estimate reflects 

a combination of watersheds where the U.S. EGU incremental contribution alone is 

considered to represent a potential public health hazard or where U.S. EGUs make at least a 

5% contribution to total Hg deposition over watersheds where total risk is considered to pose 

a public health hazard. The 28 percent total risk estimate is also based on application of the 

99th percentile consumption rate for the high-end female consumer.

• Reductions in U.S. EGU-attributable Hg will reduce the magnitude of US EGU-attributable 

risk, although substantial risk from Hg deposition will likely remain as a results of these 

sources.

• Sensitivity analyses were conducted primarily to examine uncertainty in applying the MMaps 

approach for linking Hg deposition to fish tissue MeHg levels. These analyses suggest that 

uncertainty related to the MMaps approach is unlikely to substantially affect our assessment 

of the public health hazard posed by Hg emissions from U.S. EGUs.
13

1. Review of Analysis Approach

1.1. Purpose and Scope of Analysis

This document (the “Mercury Risk TSD”) describes the national-scale risk assessment 

for mercury completed to inform the appropriate and necessary determination for electric utility 

steam generating units in the United States (U.S. EGUs), pursuant to Section 112(n)(1)(A) of the 

Clean Air Act (CAA). See Section III of the preamble to the proposed U.S. EGU Toxics Rule. 

This risk assessment focuses on risk associated with mercury released from U.S. EGUs that has 

deposited to freshwater watersheds within the continental U.S., bioaccumulated in fish and is 

consumed as methylmercury (MeHg) in dietary fish by the public. 

The following policy-related questions were developed to help guide the design of the 

risk assessment: (a) what is the nature and magnitude of the potential risk to public health posed 

by current U.S. EGU mercury emissions, (b) what is the nature and magnitude of the potential 

risk posed by U.S. EGU mercury emissions in 2016 considering potential reductions in EGU Hg 

emissions attributable to CAA requirements,

15

In addition to the above policy-related questions, the overall design and scope of the risk 

assessment reflects consideration of important technical factors related to air-sourced mercury, 

including, in particular, mercury released from U.S. EGUs (Note, a number of these technical 

factors are discussed in greater length, including provision of relevant citations, later in Section

1.3):

and (c) how is risk estimated for both the current 

and future scenario apportioned between the incremental contribution from U.S. EGUs and other 

sources of mercury? The last policy-related question reflects the fact that mercury emitted from 

U.S. EGUs does not result in a distinct and isolated exposure pathway, but rather is combined 

with mercury emitted from other sources (domestic and international) in contaminating fish. 

Therefore, to consider U.S. EGU contributions to exposure and risk associated with the 

consumption of fish containing MeHg, we determine what share of total exposure is attributable 

to U.S. EGUs. 

• While mercury exposure and risk can occur through a variety of pathways, the dominant 

pathway associated with ambient air releases is through the consumption of fish that have 

bioaccumulated mercury originally deposited to watersheds following atmospheric 

release and transport. Deposition of mercury to watersheds includes mercury originating 

from local/regional sources, combined with mercury that has been transported over 

greater distances, including mercury released outside of the U.S.. Generally oxidized 

(divalent) and particle-bound mercury will deposit relatively closer to the release source, 

while elemental mercury will travel further, often becoming part of the global pool, 

before being deposited.16

 15 For purposes of this analysis, we focus on 2016 as this is the first year after compliance would be required to 

occur.

16 Mercury is a persistent, bioaccumulative toxic metal that is emitted from power plants in three forms: Gaseous 

elemental Hg (Hg0

), oxidized Hg compounds (Hg+2), and particle-bound Hg (HgP). Elemental Hg does not quickly 

deposit or chemically react in the atmosphere, resulting in residence times that are long enough to contribute to 
14

• Available information supporting the quantification of mercury-related health effects

provides the strongest support for modeling neurological deficits in children who were 

exposed to MeHg during pregnancy through maternal fish consumption.17

• U.S. EGU emitted mercury is likely to make a very small contribution to MeHg in 

foreign-sourced commercial fish consumed in the U.S. and in commercial fish sourced 

form further off the U.S. coast. Therefore, the risk assessment, while acknowledging 

these sources of exposure to U.S. EGU-sourced mercury, does not quantify these risks 

since the U.S. EGU-attributable portion of these risks is likely to be very small and any 

quantitative estimates of U.S. EGU-attributable risk would be highly uncertain.

18

• While areas closer to the US coast (including estuarine areas) and from the Great Lakes 

may have elevated U.S. EGU impacts in some cases, because of uncertainty in modeling 

the linkage between U.S. EGU deposition and the apportionment of mercury in fish, we 

have not included this commercial consumption pathway in the quantitative risk 

assessment.19

• The type of fish consumption likely to experience the greatest contributions from U.S. 

EGU-sourced mercury is associated with fishing activity at inland freshwater rivers and 

lakes located in regions experiencing relatively elevated U.S. EGU mercury deposition. 

While the average U.S. EGU percent contribution to total mercury deposition in the U.S.

in 2005 was estimated at ~5% and in 2016 at ~2%, some watersheds had U.S. EGU

contributions ranging up to 30% and higher in 2005 (see Section 2.3). Therefore, efforts 

to identify areas with likely high U.S. EGU attributable MeHg exposures and risk are 

focused on assessing risk for those areas which have (a) relatively elevated fish tissue 

MeHg levels and (b) relatively elevated levels of U.S. EGU mercury deposition (with the 

assumption that the elevated U.S. EGU deposition in these regions translates into a larger 

 

 

global scale deposition. Hg(2+) and Hg(p) deposit quickly from the atmosphere impacting local and regional areas 

in proximity to sources. 

17 The EPA’s health benchmark for methylmercury exposure (the reference dose or RfD) is based on three 

epidemiological studies. These studies relate hair mercury levels in mothers (a surrogate for exposure during 

pregnancy) or mercury in cord blood (a direct measure of fetal exposure) to deficits in children’s performance on a 

range of neuro-cognitive tests (see section 1.3). 18 While mercury released from U.S. EGUs does contribute to contamination of foreign-sourced commercial fish, 

the fraction contributed by U.S. EGUs is extremely small. Current estimates of U.S. EGU mercury emissions are 

~29 tons per year (see section 2.2), compared with global anthropogenic mercury emissions (for 2005), excluding 

biomass burning, estimated at approximately 2,120 tons with a range of 1,347 to 3,255 tons/year (Pirrone et al., 

2010, UNEP, 2010). Based on these estimates, we would expect U.S. EGUs to contribute less than 1% of the 

mercury in commercially (foreign) sourced fish. Therefore, particularly in the context of estimating individual risk, 

U.S. EGU contributions to risk that residents in the US experience through consumption of foreign-sourced 

commercial fish is expected to be to small to characterize. This observation would also likely hold for the U.S. EGU 

contribution to commercial fish sourced from further off the U.S. coast, where total mercury loading is likely to also 

be dominated by non-US anthropogenic emissions which are globally transported.

19 While air quality modeling does suggest that some near coastal areas (e.g. the Chesapeake Bay) and portions of 

the Great Lakes may have elevated U.S. EGU deposition relative to the average levels in the continental U.S., a 

number of factors make the simulation of the linkage between mercury deposition and fish tissue MeHg levels in 

these near-coastal areas and the Great Lakes challenging and uncertain. Specifically, the size of the waterbody 

involved (i.e., inner coastal waterways, near coastal areas and the Great Lakes) combined with the potential for fish 

to have larger habitats in these locations, relative to inland lakes and rivers, means we cannot adequately quantify 

the EGU contribution to fish tissue MeHg levels. Given the greater uncertainty associated with simulating the 

linkage between near coastal U.S. EGU deposition and fish tissue MeHg levels, we have elected not to simulate this 

pathway in the risk assessment. 
15

fraction of the MeHg levels being attributed to this source category). Furthermore, while 

recreational angler activity is likely to occur broadly across the U.S., high self-caught fish 

consumers (i.e., the subsistence populations) will experience the greatest degree of U.S. 

EGU-attributable risk if they are active at high U.S. EGU-impact watersheds.

In consideration of the policy questions and technical factors discussed above, the 

national-scale Hg risk assessment was designed as follows. 

• Evaluate risk for two scenarios - a 2005 Scenario and a 2016 Scenario. Risk is estimated 

both for a 2005 Scenario and a 2016 Scenario, the latter reflecting consideration of 

potential HAP emission reductions from CAA requirements. The latest emissions data 

(see Section 2.2) suggest that current 2010 U.S. EGU emissions are closer to levels 

reflected in the 2016 Scenario and substantially lower than levels reflected in the 2005

Scenario. As a result, the 2016 Scenario analysis is most relevant for this rulemaking. 

Further modeling of future emissions indicates that in the absence of binding federal 

regulations U.S. EGU emissions are not likely to be substantially reduced between 2010 

and 2016, as the CAA directs the Agency to consider only Federal CAA requirements in 

estimating future HAP emissions (and attendant risks) associated with EGUs. Thus, we 

conclude that if we find there exists a public health hazard from current U.S. EGU 

mercury emissions based on the 2016 Scenario, we will also find that a public health 

hazard will continue to exist in 2016. 

• Include estimates of total (all mercury deposition sources) risk as well as the U.S. EGU

incremental contribution to total risk. As discussed below (Section 1.2), we focus on two 

aspects of MeHg-related risk: (a) total mercury risk with an estimate of the percent of that 

total risk contributed by U.S. EGUs (i.e., the fraction of total risk associated with U.S. 

EGUs) and (b) risk when deposition from U.S. EGUs is considered before taking into 

account deposition and exposures resulting from other sources of Hg. These two risk 

metrics reflect the cumulative burden of mercury exposures and incremental contribution 

that the U.S. EGU attributable deposition makes to the overall exposures to MeHg.20

• Focus on assessing risk to subsistence fishers active at inland watersheds. Given the goal 

of determining whether a public health hazard is associated with U.S. EGU emissions, we 

have assessed risk for a set of subsistence populations active at inland (freshwater) 

watersheds. By focusing on inland watersheds, we are focusing on those locations with 

the greatest U.S. EGU-attributable mercury deposition and consequently the greatest U.S. 

EGU-attributable fish tissue MeHg levels. Furthermore, by focusing on subsistence fisher 

scenarios, we focus on those self-caught fish consumers with the highest intake rates and 

therefore, those who will experience the greatest MeHg exposures at a given watershed. 

In defining the high-end fisher populations to include in the analysis, we have used peerreviewed study data characterizing behavior for a variety of high-end fisher populations 

 20 When exposures are to be compared to the EPA’s reference dose (RfD) for MeHg in order to generate a hazard 

quotient (HQ), we must first consider total MeHg exposure given the definition of the RfD, which is intended to be 

compared against total exposure to a given hazardous air pollutant. Once an HQ reflecting total exposure is 

calculated, we can then consider the U.S. EGU incremental contribution to that total risk. However, U.S. EGU

incremental risk in the form of an HQ should not be considered in isolation without placing it in context with regard 

to risk associated with total MeHg in the fish being consumed. 
16

active in different regions of the country (e.g., Laotians, Great Lakes Tribal populations, 

Black and White anglers active in the Southeast - see Section 1.3). 

• Generate watershed-level estimates of risk for a representative fisher for each fisher 

population (these estimates are not population-weighted): In modeling risk for these 

populations, we generate watershed-level estimates based on the subset of watersheds in 

the U.S. where we have fish tissue MeHg data and where we believe a given high-end 

fisher population could be active.21 Because it is not possible to enumerate these high-end 

fisher populations, we cannot develop population-weighted risk distributions.22

• Exclude commercial fish consumption from the quantitative risk analysis. Although risk 

associated with commercial fish consumption may be a potential public health concern

under certain circumstances, the relatively low contribution of U.S. EGU mercury to this 

source of dietary fish (relative to non-US mercury emissions), leads us to exclude this 

consumption pathway from the risk assessment. In the specific case of commercial fish 

sourced from near the US coast (e.g. Chesapeake Bay) and the Great Lakes, while there is 

the potential for U.S. EGUs to have a greater role in effecting mercury levels in these 

fish, as noted earlier, uncertainty associated with modeling the linkage between U.S. 

EGU mercury deposition and mercury exposure and risk for this dietary pathway 

precludes us from including this pathway in the risk assessment.

Therefore, in modeling risk, we generate a risk estimate for each high-end fisher 

population for each watershed where we believe that population could be active. We are 

then able to generate percentile risk estimates, based on the set of watershed-level risk 

estimates generated for each fisher population (i.e., assuming each watershed gets equal 

weight in deriving that risk distribution).

 21 The potential for a high-end fisher population to be active at a given watershed is based on consideration for 

whether members of the demographic group from which that fishing population originates are located in the US 

Census tract(s) intersecting that watershed. For example, if we are considering Hispanic high-end fishers, we would 

only assess that scenario at watersheds located in U.S. Census tracts with at least 25 poor Hispanics (in this case, 

poor Hispanics represent a “source population” for this category of high-end fisher – see Section 1.3 for additional 

detail).

22 In order to enumerate risk estimates generated for the female high-end consumer scenario used in this risk 

assessment, we would need to have the following types of specific information: (a) the fraction of anglers who 

consume at the subsistence-levels modeled for this population specifically at inland freshwater waterbodies, (b) for 

this population, the fraction that focus their activity at individual watersheds, and target somewhat larger fish to 

supplement their diet, and (c) for this subgroup, the fraction of consumers of childbearing age who either fish 

themselves and consume at this level, or are associated with male fishers who fish at this level (with that female in 

turn consuming at a subsistence rates). However, currently available information does not allow us to estimate each 

of these subgroups of high-end fishers. Specifically, while we have data on frequency of recreational angling within 

the U.S., this covers general recreational fishing and not subsistence fishing. Furthermore, there are concerns as to 

whether surveys of recreational activity would effectively capture subsistence fishers who include poorer individuals 

who traditionally have lower survey response rates. We do have surveys like the Burger et al., 2002 study which 

provide fish consumption rates for percentiles of the survey populations, with the upper percentiles (i.e., 95th and 

99th percentiles) approaching subsistence levels (we could interpret this as suggesting that 1 to 5% of the surveyed 

fishing population at these shows consumes at a subsistence level). However, we would still be concerned as to the 

degree that this type of fishing show population accurately captures rates of subsistence fishing by poorer 

individuals who may not frequent a show like this in proportion to their prevalence in the general population. And 

finally, we have focused on a subset of female subsistence consumers that we believe (a) are reasonably likely to 

exist at a subset of our watersheds and (b) are likely to experience higher risk due to their behavior (i.e., favor larger 

fish as a dietary source, focus their activity at individual watersheds). While we believe it is reasonable to assume 

that a subset of high-end fishers would have these attributes, this introduces additional uncertainty into any effort to 

enumerate this female high-end consuming population.
17

1.2. Overview of Risk Metrics and the Risk Characterization Framework

The risk assessment uses estimates of exposure for subsistence fisher populations23

Both the IQ loss and HQ risk metrics are further stratified to consider both total risk as 

well as U.S. EGU-attributable risk. In considering U.S. EGU-attributable risk we generate two 

types of risk estimates:

to 

generate two categories of risk metrics including (a) IQ loss in children born to mothers from 

these high-consuming fisher populations and (b) hazard quotient (HQ) estimates generated by 

comparing exposure estimates for these populations to the MeHg RfD. As discussed in greater 

detail in Section 1.3, risk estimates (for these populations) are generated for the subset of 

watersheds in the US where we have sampled fish tissue data. Because of limitations in 

quantifying the number of high-consumption fishers active across the set of modeled watersheds, 

it is not possible to generate population-weighted distributions of risk (see previous section). 

However, we use the watershed-level estimates of risk to consider the percentage of watersheds 

modeled that fall within specific risk ranges.

• The percent or fraction of total risk at a given watershed that is associated with U.S. 

EGU’s. We consider the magnitude of total risk (IQ loss or HQ) and then estimate the 

fraction (or percent) of that total risk estimates that is attributable to U.S. EGUs.

• Risk when deposition from U.S. EGUs is considered before taking into account 

deposition and exposures resulting from other sources: Here, we estimate risk based on 

the U.S. EGU incremental contribution to total exposure. Specifically, for IQ loss, we are 

using the U.S. EGU-portion of exposure at each watershed to generate an estimate of 

U.S. EGU attributable IQ loss and for HQ, we are comparing U.S. EGU-attributable 

exposure against the MeHg RfD. 

In assessing the potential public health significance of the IQ loss risk estimates, based on 

recommendations provided by the Clean Air Science Advisory Committee (CASAC) in the 

context of the last National Ambient Air Quality Standard (NAAQS) review for lead completed 

in 2008 (US EPA, 2007a), we interpreted IQ loss estimates of 1-2 points as being clearly of 

public health significance. With regard to HQ estimates based on the MeHg RfD, we considered 

exposures above the RfD to represent a potential public health hazard.

24

 23 Subsistence fishers are individuals who rely on noncommercial fish as a major source of protein (US EPA, 2000). 

For purposes of this risk assessment, we have interpreted this as representing self-caught fish consumption ranging 

from a fish meal (8 ounce) every few days to a large fish meal (12 ounces or more) every day.

Because HQ estimates, 

by convention, have been reported to one significant figure, reflecting precision in the underlying 

24 EPA’s interpretation for this assessments is that any exposures to MeHg above the RfD are of concern given the 

nature of the data available for mercury that is not available for many other chemicals, where exposures have often 

had to be significantly above the RfD before they might be considered as causing a hazard to public health. The 

scientific basis for the mercury RfD includes extensive human data and extensive data on sensitive subpopulations 

including pregnant mothers; therefore, the RfD does not include extrapolations from animals to humans, and from 

the general population to sensitive subpopulations. In addition, there is no evidence for a threshold observed for 

critical effect of neurological deficits in children studied in the prinicipal studies of the IRIS assessment for MeHg. 

This additional confidence in the basis for the RfD suggests that all exposures above the RfD can be interpreted with 

more confidence as causing a potential hazard to public health.
18

RfD, mathematically, we considered exposures that were at least 1.5 times the RfD (i.e., an HQ ≥ 

1.5), to represent a potential public health hazard since these would round to an HQ of 2. 

Risk Characterization Framework

We have developed a 3-stage framework for integrating the risk metrics described above

(throughout this document, this will be referred to as the “risk characterization framework”):

Stage 1 – consider various degrees of U.S. EGU contribution to total risk at watersheds 

where total risk is considered to pose a potential public health hazard: Here we 

identify watersheds with populations potentially at-risk due to U.S. EGU mercury by:

(a) identifying those watersheds where total risk meets or exceeds levels considered 

to represent a potential public health hazard (i.e., HQ ≥ 2 or IQ loss estimates of 1 to 

2 points or greater) and (b) U.S. EGUs contribute to total risk at this subset of 

watersheds with elevated risk (we have considered various increments of U.S. EGU 

contribution ranging from 5 to 15% [20%?]). We note that, any contribution of 

mercury emissions from U.S. EGUs to watersheds where potential exposures from 

total mercury deposition exceed the RfD is a hazard to public health, but for purposes 

of our analyses we evaluated only those watersheds where we determined U.S. EGUs 

contributed 5 percent or more to deposition to the watershed. EPA believes this is a 

conservative approach given the increasing risks associated with incremental 

exposures above the RfD..

Stage 2 – identify watersheds where risk based on considering deposition from U.S. EGUs 

before taking into account deposition and exposures resulting from other sources of 

Hg represents a potential public health hazard? Here we identify watersheds with 

populations potentially at-risk due to U.S. EGU-attributable risk (prior to considering 

mercury contributed by other sources). Although this stage focuses on U.S. EGU

exposure, it is important to keep this incremental exposure in perspective with regard 

to total MeHg exposure which typically dominates the U.S. EGU increment across 

watersheds.

Stage 3 – what is the combined total number of watersheds (and percentage) where 

populations may be at risk from U.S. EGU-attributable Hg? Here we combine 

estimates from Stages 1 and 2 to consider watersheds where populations may be at 

risk due to (a) U.S. EGUs contributing to exposures at watersheds where total risk 

potentially poses a potential public health hazard or (b) U.S. EGUs making an 

incremental contribution to total Hg exposure which, when considered alone, 

represents a potential public health hazard.

This framework allows us to consider whether U.S. EGU-related exposure when 

considered alone, or as a portion of total risk, represents a potential public health hazard. More 

specifically, it allows us to estimate the number and percentage of watersheds where populations 

may be at risk due to U.S. EGU-related mercury emissions.

Note, that while we present both MeHg RfD-based HQ and IQ loss-based risk metrics in 

section 2.6, in discussing risk estimates in the context of determining whether a potential public 
19

health hazard exists due to U.S. EGU mercury emissions, we focus on the HQ estimates. This 

reflects concerns that the IQ loss endpoint may not capture all of the neurodevelopmental effects 

associated with MeHg exposure. Specifically, concerns have been raised in the literature that if 

mercury affects a set of specific neurological functions, then use of full-scale IQ as the modeled 

health endpoint, could underestimate the neurdevelopmental impacts on other targeted functions 

(Axelrad et al., 2007). In addition, two of the most sensitive endpoints in the Faroe Islands study 

were the Boston Naming Test and California Verbal Learning Test, both of which can represent 

a significant educational risk depending on severity, and those tests are not directly assessed as 

part of measuring IQ in children. In addition IQ does not cover other neurologic domains such as 

motor skills and attention/behavior and therefore, risk estimates based on IQ will not cover these 

additional endpoints and therefore could further underestimate overall neurodevelopmental 

impacts (Axelrad et al., 2007).

1.3. Overview of Analytical Approach

This section describes the analytical approach used in conducting the national-scale 

mercury risk assessment (note, additional detail on specific modeling elements can be found in 

the appendices).

Figure 1-1 provides a flow diagram of the risk analysis identifying the major analytical 

steps and associated modeling elements. The risk assessment is based on estimating a set of 

subsistence fisher scenarios at watersheds across the U.S. where we have measured fish MeHg 

concentration data. After we have estimated total MeHg risk based on modeling consumption of 

fish at each of these watersheds, we use the ratio of U.S. EGU to total Hg deposition over each 

watershed (estimated using Community Multi-scale Air Quality (CMAQ) modeling) to estimate 

the U.S. EGU incremental contribution to total Hg risk. This apportionment of total risk between 

the U.S. EGU fraction and the fraction associated with all other sources of Hg deposition is

based on the EPA’s Office of Water’s Mercury Maps approach (MMaps) that establishes a 

proportional relationship between Hg deposition over a watershed and resulting fish tissue Hg 

levels, assuming a number of criteria are met. Each of the steps in the analysis is briefly 

described below.

Specifying the spatial scale of watersheds

The fist step in designing the analysis was to specify the spatial scale of the watersheds to 

use as the basis for risk characterization. As noted above, this risk assessment is based on 

estimating risk at watersheds for which we have measured Hg fish tissue data. A number of 

studies (Knights et al., 2009, Harris et al., 2007), examining the response of aquatic freshwater 

ecosystems to changes in Hg deposition focused on watersheds with dimensions closest to 12-

digit Hydrologic Unit Code classifications (HUC- 12’s) (representing a fairly refined level of 

watersheds approximately 5-10 km on a side). This suggests that, at least in the context of these 

studies, researchers believed that the relationship between changes in mercury deposition and 

changes in MeHg levels in aquatic biota could be effectively explored at the level of these more 

spatially refined watersheds. In addition, use of a more refined spatial scale (i.e., use of HUC12s 

rather than a coarser scale of watershed) in linking changes in mercury deposition to changes in 

fish tissue Hg levels also reduces the potential for averaging out areas of high Hg deposition. The 

HUC12 represents the most refined scale of watershed currently available at the national level 
20

and therefore was chosen as the basis for linking changes in Hg deposition to changes in fish 

tissue MeHg levels. As discussed later in this section, this linkage is central to generating risk 

estimates and determining the fraction of total risk associated with U.S. EGUs. Note, the term 

“watershed” when used in this document refers to HUC12s unless otherwise noted (see 

Appendix A for additional detail on the rationale for selecting HUC12s as the watershed spatial 

scale to use in the analysis).
21

Figure 1-1 Flow Diagram of Risk Analysis Including Major Analytical Steps and 

Associated Modeling Elements

Specifying 

spatial scale of 

watersheds

Characterizing 

measured fish tissue 

Hg concentrations at 

the watershed-level

Defining nearsubsistence and 

subsistence fisher 

populations by 

watershed

Estimating total fish 

consumption-related 

Hg exposure for fishers 

active at each 

watershed

US Demographic data 

characterizing source 

populations for highend fishers at the US 

Census tract-level

Studies characterizing fish 

consumption rates for 

high-consumption fisher 

populations

Defining near-subsistence and 

subsistence fisher scenarios

Apportioning total 

exposure between 

total and US EGUattributable at each 

watershed

CMAQ 

modeling 

results for total 

and US EGU Hg 

deposition at 

each watershed 

Estimation of HQ and 

IQ loss at each 

watershed including total 

risk and US EGUattributable fraction

Mercury RfD for 

HQ estimation

IQ loss concentrationresponse function

Factor translating Hg 

ingestion dose into Hg hair 

concentrations (in ppm) 

Modeling IQ loss

Exposure modeling

Risk modeling

KEY: decision data input analysis step
22

Characterizing measured fish tissue Hg concentrations

The next step was to identify which of the approximately 88,000 HUC12s in the 

continental U.S. had fish tissue concentration data and therefore, could be included in the risk 

characterization. Although we had compiled fish tissue Hg sampling data for the period 1990 to 

2009 from a variety of sources (see Appendix B), we decided to use a subset of these data from 

the period 2000 to 2009 in the risk assessment in order to exclude fish tissue samples that likely 

reflected Hg deposition levels from the 1990’s when anthropogenic emissions in the U.S. were 

higher than for period after 2000. We recognizes the complex spatial and temporal nature of the 

response of fish tissue Hg levels to changes in Hg deposition and loading and acknowledge that a 

portion of the sampling data from 2000 to 2009 could still reflect higher Hg loading rates from 

earlier periods (see Appendix B). Inclusion of fish tissue Hg sampling data collected between 

2000 and 2009 resulted in the ability to characterize fish tissue Hg levels for 2,461 of the 88,000 

HUC12’s in the continental U.S. These watersheds were not randomly sampled across the 88,000 

HUC12s, and watersheds in the eastern U.S. are more heavily represented. In addition, because 

the samples were often based on state sampling, the samples are not evenly distributed across 

states, and some states have very few samples, while others have a large number of samples. 

Uultimately, the risk assessment includes estimates of risk for up to 2,461 watersheds depending 

on the fisher population being considered (fisher populations that are less ubiquitous such as the 

Vietnamese or Great Lakes Tribal populations will not be active across the entire set of 2,461

watersheds and would therefore, only be assessed for a subset of the watersheds– see discussion 

below).25

Most of the watersheds with measured Hg fish tissue data had multiple fish tissue MeHg 

measurements. This necessitated selecting a fish tissue MeHg statistic to use as the basis for 

generating risk estimates. This analysis uses the 75th percentile fish tissue value (computed 

separately for each watershed) to model risk. The selection of this statistic reflected the potential 

for subsistence-like fishers to favor larger fish, which would likely have relatively higher 

mercury levels due to greater bioaccumulation (see Appendix B for additional detail).

Defining subsistence fisher scenarios

Next, we identified the suite of subsistence fishing populations to evaluate in the risk 

assessment. As discussed in the introduction, this analysis focuses on populations with higher 

consumption rates of self-caught fish who have the potential to fish at inland freshwater 

locations, since these populations are expected to experience the greatest U.S. EGU-attributable 

risks. Therefore, in reviewing studies of fishing populations, emphasis was placed on identifying 

surveys of higher consumption fishing populations active at inland freshwater rivers and lakes

within the continental U.S. 

A number of studies were identified that characterized activity for a selection of high-end 

fishing populations that met our criteria. These populations included: (a) white and black 

populations (including female and poor strata) surveyed in South Carolina (Burger et al., 2002), 

(b) Hispanic, Vietnamese and Laotian populations surveyed in California (Shilling et al., 2010) 

 25 As discussed in Appendix B, in identifying the 2,461 watersheds to include in the risk assessment, we excluded 

those that contained gold mines or non-EGU sources of mercury emissions meeting specific criteria we identified as 

potentially representing a significant contribution to mercury loading within a watershed. 
23

and (c) Great Lakes Tribal populations (Chippewa and Ojibwe) active around the Great Lakes

(Dellinger et al., 2004). For most of these fisher populations (with the exception of the Tribal 

populations near the Great Lakes) we assumed that high-end fisher population could be 

generalized beyond the specific areas covered in a particular study. This type of generalization 

was necessary to provide greater coverage for the continental U.S. and in particular, the eastern 

part of the U.S. where U.S. EGU deposition is higher and where we have more measured Hg fish 

tissue data. In deciding how to extend coverage for each fisher population, we have considered 

several factors including (a) the degree to which high-end fishing activity might be culturally 

related and therefore more likely to be followed by populations (e.g., of a given ethnicity) living 

across the U.S. and (b) the degree to which high-end fishing activity might be driven by 

economic need (i.e., to truly supplement diet) and therefore linked to groups of individuals living 

below the poverty line. In each case, we have described our rationale for the regional extension 

that has been applied for each of the high-end fisher populations included in the analysis (see 

Appendix C and Table C-1 for additional detail on the regions used in modeling each fisher 

population). Note, that in identifying the specific subset of the 2,461 watersheds to model for 

each fisher population, we considered whether there was a “source population” for that fishing 

population within the US Census tract that intersected each watershed. For example with 

Vietnamese fishers, we would require that at least 25 Vietnamese were located in a US Census 

tract intersecting a watershed in order for that watershed to be included in the set modeled for 

this fisher population (additional detail on “source populations” is presented in Appendix C).

The studies used to characterize high-end fishing behavior for these populations also 

included either (a) high-end percentile self-caught fish consumption rates (90th to 99th percentile 

values generally on the order of 100g/day to ~400 g/day or more) or (b) the statistical parameters 

necessary to calculate those high-end percentiles (e.g., median and standard deviations). These 

subsistence-level consumption rates were used in modeling risk for these populations (see 

Appendix C and specifically, Table C-1 for specific consumption rates for each of the fisher 

populations included in the analysis. 

As part of the analysis, in addition to the high-end fisher populations listed above, we 

also included a high-end female consumer scenario that was applied more broadly to most of the 

watersheds included in the risk assessment.26

 26 We applied this high-end fish consuming population to watersheds located in U.S. Census tracts which had at 

least 25 people living below the poverty line (this results in this scenario being applied to most of the watersheds 

where we have fish tissue MeHg data, since this is a less stringent criterion for inclusion). This requirement of 

having at least 25 people below the poverty line reflects the assumption that near-subsistence levels of fishing 

activity is more likely among individuals who are economically disadvantaged. 

This fisher scenario was based on fish consumption 

rate data provided in the study by Burger et al., 2002 (see Appendix C, Table C-1) and the 

consumption rates involved are generally supported by a number of the studies reviewed (see 

Appendix C for additional detail). Because this high-end female consumer population (a) covers 

the population of greatest concern from a MeHg exposure standpoint (women of child-bearing 

age) and (b) was fairly widely applied (increasing the potential for including high U.S. EGUimpacted watersheds), we have focused the discussion of risk results provided later in Section

2.6 on this fisher population. With regard to this fisher population, as well as the other 

populations considered in the analysis, we would point out that the high-end fish consumption 

rates considered, while representing high-end (near bounding) levels, are still reasonable in terms 

of subsistence consumption. Most of the rates used (see Appendix C, Table C-1), translate into 
24

between one fish meal every few days to a large fish meal every day. Viewed from the 

perspective of the type of subsistence fishing activity that is the focus of this analysis, these 

consumption rates are reasonable (i.e., they represent consumption rates in line with subsistence 

behavior). 

Estimating total fish consumption-related Hg exposure

The next step in completing the analysis was to estimate total exposure to Hg at each of 

the 2,461 watersheds for the set of high-end fish consumption populations identified for each 

watershed. Estimates of total mercury exposure were generated by combining 75th percentile fish 

tissue value with the consumption rates for a particular fisher populations. Note, that a cooking

loss factor (actually reflecting the fact that the preparation of fish can result in increased mercury 

concentrations) was also included in exposure calculations (see Appendix D for additional detail 

on the calculation). 

Apportioning total MeHg exposure between total and U.S. EGU-attributable exposure

Next we needed to estimate the fraction of total exposure that is associated with U.S. 

EGU Hg deposition at each watershed. U.S. EGU apportionment of total Hg exposure is based 

on application of the MMaps assumption (see Appendix E). Essentially this approach assumes 

that under near steady state conditions, a fractional change in mercury deposition to a watershed 

will be reflected in a matching proportional change in the levels of MeHg in fish. We have 

extended this proportionality assumption to allow us to apportion MeHg levels in fish between 

mercury sources based on the associated apportionment of mercury deposition within a given 

watershed between these sources. Of course, the process of mercury loading and impacts on 

mercury bioaccumulated in fish is complex and involves varying temporal profiles depending on 

a variety of factors (e.g., methylation potential of the waterbody, role of watershed sediment 

erosion and runoff in mercury loading to the watershed etc). However, for purposes of this 

analysis, we make the assumption that given sufficient time to achieve near steady-state 

conditions, a given ratio of total Hg deposition to U.S. EGU deposition will ultimately be 

reflected in the fish tissue MeHg levels. In addition, we note that the MMaps assumption does 

require that certain criteria be met (e.g., atmospheric deposition is the primary source of mercury 

loading to the watershed and that factors related to methylation potential in watersheds be held 

constant for a sufficient time to allow near-steady state conditions to be reached). These criteria 

and the degree to which they were considered in our analysis are further described in Appendix 

E. For example, as discussed in greater detail in Appendix E, in identifying the 2,461 watersheds 

to include in the risk assessment, we excluded those that contained gold mines or had significant 

non-aerial sources of mercury loading.

CMAQ modeling completed at the 12km grid cell resolution was used to estimate total 

annual mercury deposition from US and foreign anthropogenic and natural sources over each 

watershed, including the fraction of deposition contributed by U.S. EGUs. As noted in the 

discussion of the scope of the analysis presented in Section 1.1, we are modeling two temporal 

period in the analysis: (a) a 2005 scenario representing 2005 conditions as reflected in the 2005 

NEI mercury emissions inventory and (b) an 2016 based case scenario after CAA-related 

regulations potentially reducing Hg emissions from U.S. EGUs (e.g., the Transport Rule) are in 

place. In the context of these two temporal scenarios, the CMAQ modeling results together with 
25

the MMaps assumption can be used to both estimate the portion of total Hg exposure that is 

associated with U.S. EGUs and project changes in fish Hg concentrations (and consequently total 

exposure) associated with changes in total Hg deposition in the future. Specifically, for the 2005 

analysis, CMAQ modeling results for a particular watershed allow us to estimate the proportion 

of total exposure (estimated for that watershed) that is associated with U.S. EGU deposition (i.e., 

based on the ratio of U.S. EGU Hg deposition to total Hg deposition over the watershed). In the 

case of the 2016 simulation, we can first project changes in total fish tissue Hg levels (for that 

watershed) by comparing estimates of total Hg deposition in 2005 to estimates for 2016 

generated by CMAQ and then again, apportion that adjusted total risk between U.S. EGUs and 

all other sources, based on comparing U.S. EGU Hg deposition to total Hg deposition in 2016. 

See Appendix E for additional detail on CMAQ modeling.

Estimate risk including HQ and IQ loss

Once both total exposure and the U.S. EGU incremental contribution to that total 

exposure have been estimated, we can then estimate risk, including both HQ and IQ points lost. 

The HQ estimate is generated by comparing total Hg exposure (as annual-average bodyweightadjusted ingested MeHg dose) to the MeHg RfD.27 Similarly, the U.S. EGU incremental 

contribution to total HQ is estimated by comparing the U.S. EGU increment of exposure to the 

MeHg RfD. As noted earlier, an HQ > 1.5 (which rounds to an HQ of 2) is considered to 

represent a potential public health hazard, since it signifies that exposure has been assessed to 

exceed the MeHg RfD.28

In the case of IQ loss, we first covert annual-average ingested dose estimates for MeHg 

into equivalent maternal hair mercury levels, since the CR function for IQ loss is based on 

estimated exposure characterized as maternal hair mercury levels. This conversion is 

accomplished using a factor based on a one compartment toxicokinetic model used for deriving 

the MeHg RfD by Swartout and Rice (2000). Then a CR function relating hair mercury levels to 

IQ points lost in children born to mothers whose exposure is modeled in this analysis is used to 

predict IQ points lost for those children. This CR function was published in Axelrad et al.,

2007and is based on application of a Bayesian hierarchical model which integrates data from the 

three key epidemiological studies (Seychelles, New Zealand and Faroe Islands).29

Since the CR function was published in the Axelrad et al., 2007 study, a number of 

authors have raised the possibility that neurological deficits related to Hg exposure through fish 

consumption could be masked to some degree by the neurologically-beneficial effects of fish oil 

consumption. Some authors have suggested that the IQ loss factor should be adjusted upward to 

compensate for this masking effect (see Rice et al., 2010 and Oken, 2008). However, no rigorous 

 27 The MeHg RfD is 0.0001 ug/kg-day (equivalent to 0.1 ug/kg-day) and was published by EPA in the Integrated 

Risk Information System in 2001 (http://www.epa.gov/iris/subst/0073.htm) 28 Note, that for the U.S. EGU incremental contribution analysis, an HQ of less than 1.5 does not necessarily 

indicate there is no public health hazard related to U.S. EGU emissions. Rather, it suggests that, for those specific 

watersheds, we need to also consider whether total risk (i.e., the HQ reflecting total MeHg exposure) exceeds 1.5

and therefore represents a potential public health hazard. If that is the case, then we would consider the degree to 

which U.S. EGUs contribute to that total exposure because incremental exposure above the RfD increase the risk. 29 The IQ loss model uses a linear slope of 0.18 IQ points per ppm hair Hg concentration (Axelrad et al., 2007).
26

basis for a specific adjusted estimate has been provided to-date and therefore, we address this 

potential for low-bias as part of our qualitative uncertainty discussion.

1.4. Discussion of key sources of uncertainty and variability

The risk assessment has been designed to reflect critical sources of variability to the 

extent allowed by available methods and data and given the resources and time available. The 

key sources of variability associated with the analysis include: (a) variation in the pattern of total 

and U.S. EGU-attributable mercury deposition across watersheds in the US, (b) variation in the 

patterns of fish tissue MeHg levels across the US, and (c) variation of the types of high-end 

fishing activity likely to occur in different parts of the country. These sources of variability and 

the degree to which they are reflected in the design of the analysis are identified and described in 

Appendix F, Table F-1. 

Regarding uncertainty, in Appendix F, Table F-2, we have identified sources of 

uncertainty impacting the analysis and attempted to characterize (a) the nature of the impact of 

each source on risk estimates and (b) the degree to which the potential impact of the source of 

uncertainty is characterized as part of the analysis (including whether sensitivity analyses 

completed for the risk assessment address a particular source of uncertainty). 

In addition to the sources of uncertainty addressed in Table G-2, which focus on factors 

related directly to the exposure scenarios modeled in the risk assessment, our decision not to 

model risk associated with consumption of Great Lakes fish and fish sourced from near U.S. 

coastal locations (including estuarine areas) also adds additional uncertainty into the analysis. As 

explained above in Section 1.1, these sources of self-caught and commercial fish were not 

modeled primarily due to challenges associated with linking specific areas of U.S. EGU mercury 

deposition to fish in these waterbodies. A related concern is that the greater dilution and potential 

mobility of fish in these larger waterbodies, could reduce the impact of elevated U.S. EGU

mercury deposition of portions of these waterbodies. Despite these considerations, the risk 

assessment may have overlooked elevated U.S. EGU-attributable risks for high-consuming fisher 

populations active in these locations. However, we would point out that we still expect the 

greatest U.S. EGU-attributable risk to occur at inland freshwater bodies with (a) relatively 

elevated fish tissue Hg levels and (b) relatively elevated levels of U.S. EGU mercury deposition. 

1.5. Differences between the 2005 Section 112(n) Revision Rule analysis and the current 

analysis in support of the Propose U.S. EGU Toxics Rule

In 2005, EPA conducted a set of technical analyses to support revision of the 2000 

appropriate and necessary finding.30 This section identifies key differences between the 

watershed-level risk assessment completed in support of the 2005 revision rule and the current 

risk assessment. These differences include both technical factors related to the design of the 

assessments, as well as differences in the interpretation of potential public health significance of 

the risk estimates generated. Key differences between the two analyses include:

 30 U.S. EPA. 2005. Technical Support Document: Methodology Used to Generate Deposition, Fish Tissue 

Methylmercury Concentrations, and Exposure for Determining Effectiveness of Utility Emission Controls.
27

Higher spatial resolution through use of CMAQ 12km grid cells: We are now using 12 km grid 

cells in estimating mercury deposition using CMAQ, whereas in the 2005 analysis, we used 36 

km grid cell modeling. The more refined grid cells used in the current analysis are more 

appropriate for representing areas of elevated U.S. EGU deposition (and total Hg deposition in 

general) compared with the 36 km grid cells used in the 2005 analysis. The 12 km grid cell also 

matches up with the more refined HUC12 watersheds now being used in the analysis, thereby 

allowing a more refined treatment of the intersection of aerial mercury deposition and measured 

fish tissue concentrations at the watershed level.

Application of more refined HUC12 watersheds: The current analysis uses HUC12 watersheds as 

the basis for risk estimation (these watersheds typically are 5-10 km on a side). By contrast, the 

the 2005 analysis used HUC8s which are much larger (averaging 40km on a side). The use of 

more spatially refined watersheds increases the potential for capturing areas of elevated aerial Hg 

deposition (combined with measured fish tissue levels). 

Inclusion of updated fish tissue data: For this analysis, we included measured fish tissue data 

collected between 2000 and 2009. By contrast, the the 2005 analysis used data collected 

between 1999 and 2003 (in that case to support an analysis completed in 2005). 

Subsistence fisher activity better defined and considered more ubiquitous: Based on an extensive 

review of available literature, we have identified studies characterizing high-end self-caught fish 

consumption for a wide variety of source populations (e.g., Hispanic, Vietnamese, Whites and 

Blacks in the southeast, Tribal populations). Although it was necessary to extrapolate high-end 

fishing activity to regions beyond those covered in the underlying studies, we do believe that the 

literature generally supports the plausibility of high-end subsistence-like fishing activity existing 

across to some extent across the watersheds included in the analysis. Additionally, the variety of 

studies identifying self-caught fishing activity at subsistence levels (i.e., a meal every few days 

to a meal every day) for a variety of diverse populations in different regions of the country, adds 

support to assessing this type of fishing behavior across the modeled watersheds (for additional 

detail on the fishing populations included in this risk assessment, including consumption rates 

see Section 1.4 and Appendix C). 

By contrast, in the 2005 analysis, we concluded that the study data characterizing fishing activity 

available at that time was limited in its ability to support modeling of subsistence fisher activity 

for the following reasons: (a) it characterized regional or local activity that could not be readily 

extrapolated more broadly, (b) fishing activity queried included consumption of saltwater 

species, or (c) specific high-end percentiles were not identified (or if they were, they only 

applied during specific harvesting periods - e.g., spearfishing months for Great Lakes Tribes). 

Therefore, in the 2005 analysis, we ended up applying a high-end self-caught percentile values 

(95th and 99th percentiles) based on Tribal fishing practices in the Northwest to watersheds across 

the country.31

 31 Note, that these NW Tribal fishing estimates are subject to considerable uncertainty when they, in particular, are 

extrapolated to cover other areas in the U.S. These specific high-end fish consumption rates were derived for Tribes 

active in the Northwest who engage in specific cultural practices focused around salmon fishing. There is significant 

uncertainty in extrapolating this type of highly-specific cultural-based fishing activity to other Tribes, let alone to 

The updated literature review we have done for the current analysis, has lead us to 
28

revise several of our earlier conclusions regarding high-end fishing activity. Specifically, while 

many of the studies of subsistence-like activity are regional in nature, when considered together, 

we now conclude that they support modeling subsistence-like fishing activity more broadly 

across the entire study area. Additionally, while some of the studies may include saltwater 

fishing in addition to freshwater (e.g., Burger, 2002), when those studies clearly covered both 

saltwater and freshwater self-caught fish consumption, we concluded that it was reasonable to 

assume that subsistence-like fishing activity could occur both at the coast and inland at 

freshwater bodies.32

For the current analysis, we are also using the 75th percentile fish tissue MeHg level reflecting 

the potential for high-end subsistence fishers to target larger fish which would have greater 

bioaccumulation potential relative to the average fish. By contrast, in the 2005 risk assessment, 

we used the maximum fish tissue MeHg level across species of fish in a given HUC8, which is a 

more conservative approach (i.e., resulting in higher risk, other factors equal). 

Calculation of RfD-based HQ estimates including total and U.S. EGU-attributable risk and 

calculation of IQ loss: For this analysis, we have compared total exposure to the MeHg RfD to 

generate an HQ estimate based on total mercury exposure for fishers at a given watershed. 

Furthermore, to focus on the U.S. EGU component of that total risk, we have generated two 

related risk metrics: (a) U.S. EGU incremental contribution to total risk which essentially 

considers the magnitude of the HQ when deposition from U.S. EGUs is considered before taking 

into account deposition and exposures resulting from other sources of Hg and (b) the percent of 

total HQ risk attributable to U.S. EGUs. The calculation of U.S. EGU incremental contribution 

to total HQ is identical to the IDI (index of daily intake) metric used in the 2005 analysis. 

However an important distinction is that in the current analysis, we highlight the fact that this 

U.S. EGU-related risk is always associated with a total HQ which is generally substantially 

larger (i.e., the US-EGU-attributable HQ should not be considered in isolation as was done in the 

2005 analysis with the IDI). By contrast, for the 2005 analysis both of the risk metrics used (i.e., 

the IDI and the comparison of U.S. EGU-related fish tissue concentrations against EPA’s water 

quality criterion expressed as a mercury fish tissue value) essentially considered the U.S. EGU

portion of risk in isolation. These risk metrics in the 2005 analysis were not contrasted with the 

much larger fraction of total mercury-related risk associated with the non-U.S. EGU portion of 

risk. 

For the current analysis, we also generated estimates of IQ loss (these were not generated for the 

the 2005 analysis). Estimates of a specific health endpoint associated with the U.S. EGUattributable fraction of mercury in fish provides a risk metric that can be more appropriately 

considered in isolation (i.e., it is more reasonable to consider the U.S. EGU-attributable IQ loss 

 

other fish consumers in the U.S. By contrast, extrapolation of more generalized high-consumption rates (for ethnic 

groups and whites and blacks) to cover portions of the U.S. as was done in the current analysis is subject to far less 

uncertainty. 

32 Note, that particularly in the situation where a study specifically characterized poor high-end fishing populations, 

as is done in the Burger 2010 study of activity in SC, we considered it reasonable to assume that poor individuals 

would likely conduct their frequent fishing activity near home. In that case, some of these high-end fishers would 

likely be located near the coast and some inland. In the case of subsistence-like fishing activity in the southeast, 

other studies from rivers in that area also showed subsistence-like fish consumption rates when only freshwater 

rivers were considered (e.g., Burger et al., 1999 focusing on fishing activity on the Savannah river in GA). 
29

than to focus on the U.S. EGU-attributable fraction of an HQ). This reflects the fact that IQ loss

quantifies a discrete increment of a public health effect, while with the RfD, it is more difficult to 

characterize what a “fraction” of an HQ value actually represents in terms of potential health 

significance. However, as discussed in Section 1.2, there is concern that the IQ endpoint may not 

fully capture all of the neurodevelomental effects associated with MeHg exposure and for this 

reason, in presenting risk estimates in the context of determining whether there is the potential 

for a public health hazard associated with exposure to U.S. EGU-sourced mercury, we focus on 

the MeHg RfD-based HQ estimates and not on the IQ loss estimates, although both are 

presented. 

2. Discussion of Analytical Results

This section provides a discussion of the results from the various analyses completed as 

part of the risk assessment for the 2005 simulation and the 2016 simulation. Prior to discussing 

these results, a brief overview of critical design elements of the risk analysis that the reader 

should keep in mind when reviewing the results (Section 2.1). The specific sets of analyses 

described in this section include: (a) mercury emissions from U.S. EGUs (Section 2.2), (b) 

mercury deposition from U.S. EGUs as modeled using CMAQ (Section 2.3), (c) fish tissue 

MeHg concentrations (Section 2.4), (d) relationship between mercury deposition and 

methlymercury fish tissue concentrations (Section 2.5) and (d) risk assessment results, including 

MeHg RfD-based HQ estimates and IQ loss estimates (Section 2.6). In discussing each of these 

category of results, emphasis is placed on identifying key policy-relevant observations. In 

Section 2.7, we discuss the results of several sensitivity analyses conducted to characterize the 

potential impact of specific sources of uncertainty on the risk estimates. In Section 2.8, we 

provide a summary of critical observations from the analysis. 

2.1. Key design elements to consider when reviewing the risk assessment results

The following design elements of the analysis should be considered when reviewing the 

results discussed in the following sections (note, that this only highlights portions of the design 

of the analysis – the reader is referenced to Section 1.3 and associated Appendices for a more indepth discussion of the analysis design):

• The analysis focuses on subsistence-like fishing activity at inland freshwater bodies. The 

analysis is not intended to capture more generalized recreational fishing activity or to 

reflect self-caught fisher exposure associated with saltwater fishing or fishing in the Great

Lakes. In comparing any risk profiles generated in this analysis to risks estimated in other 

contexts, the specific focus on this analysis on these high-end populations needs to be 

considered (e.g., risks in this analysis will generally be substantially higher than those 

estimated for recreational fishers).

• The analysis is watershed-focused and risks are generated for subsets of the 2,461

watersheds for which we have measured fish tissue MeHg data. This watershed coverage 

(which is only about 4% of the watersheds in the U.S.), leaves much of the country not 

covered by the analysis, including a substantial number of watersheds with relatively 

elevated levels of U.S. EGU-related mercury deposition. Further, we note that the 

watersheds with fish tissue MeHg data are concentrated in the eastern part of the country 
30

and therefore, this portion of the continental U.S. is more heavily represented in the 

watershed-level estimates of risk that are generated. Given that U.S. EGU mercury 

deposition is generally higher in the eastern part of the U.S., the fact that the risk 

assessment is focused on this part of the country is considered a strength of the analysis. 

• The analysis uses the MMaps approach to relate changes in mercury deposition over 

particular watersheds to resulting changes in mercury fish tissue concentrations. 

Similarly, this approach allows us to reflect source-apportionment of mercury deposition 

(i.e., between U.S. EGU and all other sources) in the underlying fish tissue levels. It is 

then possible to translate those changes in fish tissue levels (or source-apportioned fish 

tissue levels) into equivalent changes in exposure and risk. This approach assumes that 

near steady state conditions are met in the fish tissue MeHg concentrations, which may 

take years to decades at a given watershed following changes in mercury deposition. 

• The analysis generates a series of risk metrics based on (a) estimates of MeHg RfD-based 

HQ and (b) estimates of IQ loss in children born to mothers exposed to MeHg through 

high-end fish consumption. We do consider the U.S. EGU-related contribution to both 

types of risk. However, particularly in the context of HQ, U.S. EGU incremental 

contributions to total risk should always be considered in the context of total HQ which is 

typically substantially larger than the U.S. EGU incremental contribution when 

considered in isolation.

• Because it is not feasible to enumerate the subsistence-like fisher populations modeled in 

this analysis, we could not generate distributions of population-weighted risk for specific 

fishing populations assessed (e.g., poor Hispanic fishers, or Tribal fishers in the vicinity 

of the Great Lakes). To reiterate, this reflects the fact that we are focusing on fishers 

engaging in high fish consumption and it is difficult to count individuals in this subset of 

each fishing population (and assign them to specific watersheds). While we can not 

enumerate these populations, we do believe that, based on surveys of their behavior, that 

this type of subsistence-like activity could reasonably be expected to occur across some 

fraction of the 2,461 watersheds included in the analysis.33

2.2. Mercury Emissions from U.S. EGUs

Therefore, we have assessed 

high-end fisher risk for each watershed. We then consider the fraction of watersheds with 

simulated high-end risk within specific categories of interest. While not a populationrepresentative characterization of risk, this approach does allow us to consider percentiles 

of watersheds based on a reasonable assumption that this kind of high-end fishing activity 

could occur across the watersheds modeled. 

The most recent data on U.S. EGU emissions based on information collected from 

industry through the Information Collection Request (ICR) show total mercury emissions of 29

tons in 2010. This shows a significant reduction in U.S. EGU mercury emissions from 2005, 

when mercury emissions were estimated to be 52.9 tons. The reductions between 2005 and 2010 

 33 As discussed in section 1.3 and in additional detail Appendix C, we only considered specific high-end populations 

for those watersheds located in US Census tracts with a “source population” greater than 25 for the fishing 

population being assessed.
31

are largely due to state mercury regulations and federal enforcement actions that achieve 

mercury reductions as a co-benefit of controls for NOx and SO2 emissions. The EPA projection 

of total mercury emissions from U.S. EGUs in the 2016 (once a number of the CAA-related 

regulations are fully in effect) is 29 tons. Given these estimates of total mercury emissions,

characterization of “current conditions” would be better represented by our 2016 Scenario than 

the 2005 Base Case, since total emission for the former (at 29 tons) is closer to our projection of 

current 2010 emissions (at 29 tons). By contrast, the 2005 analysis reflects total mercury 

emissions (52.9 tons) which are significantly higher than our estimate of current emissions in 

2010. For this reason, as mentioned earlier, we emphasis risk estimates for the 2016 Scenario in 

presenting and interpreting risk estimates.

2.3. Mercury Deposition from U.S. EGUs as Modeled Using CMAQ

This section characterizes patterns of U.S. EGU-related mercury deposition for the two 

scenarios assessed (2005 and 2016 Scenario) using CMAQ. In presenting and discussing these 

results, we contrast U.S. EGU-attributable deposition with deposition from all sources combined. 

This discussion is based around a series of figures and tables conveying relevant information, 

which are described below. After presenting these figures, a set of bulleted observations is 

presented at the end of the section that draws on information conveyed in the figures and tables. 

The set of figures and tables presented include:

• Figure 2-1 and 2-2: Maps presenting CMAQ modeling results for total mercury 

deposition (ug/m2

) at the watershed-level, for the 2005 and 2016 scenarios respectively.

• Figures 2-3 and 2-4: Maps presenting CMAQ modeling results for U.S. EGU-attributable 

mercury deposition (ug/m2) at the watershed-level, again for the 2005 and 2016

scenarios, respectively.

• Table 2-1: Summary of statistics (mean, 50th, 75th, 90th, 95th and 99th percentiles) for total 

mercury deposition and U.S. EGU-attributable deposition for the 2005 and 2016

scenarios. 

• Table 2-2: Summary of statistics (mean, 50th, 75th, 90th, 95th and 99th percentiles) for U.S. 

EGU-deposition as a percent of total deposition for the 2005 and 2016 scenarios. 

• Table 2-3: Summary of statistics (mean, 50th, 75th, 90th, 95th and 99th percentiles) for 

percent reduction of (a) total mercury deposition, and (b) U.S. EGU-attributable 

deposition, based on comparison of the 2016 scenario against the 2005 scenario. 
32

Figure 2-1. Total mercury deposition by watershed (2005)

Figure 2-2. Total mercury deposition by watershed (2016)
33

Figure 2-3. U.S. EGU-attributable mercury deposition by watershed (2005)

Figure 2-4. U.S. EGU-attributable mercury deposition by watershed (2016)
34

Table 2-1. Comparison of total and U.S. EGU-attributable mercury deposition (ug/m2) for 

the 2005 and 2016 scenarios.* 

Statistic

2005 scenario 2016 scenario

Total Hg 

Deposition

U.S. EGU-attributable 

Hg Deposition

Total Hg 

Deposition

U.S. EGU-attributable 

Hg Deposition

Mean 19.41 0.89 18.66 0.34

Median 17.25 0.24 16.59 0.15

75th percentile 23.69 1.07 22.83 0.46

90th percentile 30.78 2.38 29.90 0.85

95th percentile 36.85 3.60 35.16 1.18

99th percentile 58.32 7.77 56.23 2.41

* Values are based on CMAQ results interpolated to the watershed –level and reflect trends across all ~88,000 

watersheds in the U.S.

Table 2-2. Comparison of percent of total mercury deposition attributable to U.S. EGUs 

for 2005 and 2016.* 

Statistic 2005 scenario 2016 scenario

Mean 5% 2%

Median 1% 1%

75th percentile 6% 3%

90th percentile 13% 5%

95th percentile 18% 6%

99th percentile 30% 11%

* Values are based on CMAQ results interpolated to the watershed –level and reflect trends across all ~88,000 

watersheds in the U.S.

Table 2-3. Comparison of percent reduction of total mercury deposition, and U.S. EGUattributable deposition, based on comparing the 2016 scenario against the 2005 

scenario.* 

Statistics

Percent Change in Total 

Hg Deposition

Percent Change in U.S. 

EGU-attributable Hg 

Deposition

Mean -4% NC**

Median -1% -41%

75th percentile -5% -70%

90th percentile -12% -80%

95th percentile -16% -85%

99th percentile -27% -91%

* Values are based on CMAQ results interpolated to the watershed –level and reflect trends across all ~88,000 

watersheds in the U.S.

** A mean value was not calculated for this category due to presence of a number of watersheds with very small 

U.S. EGU-attributable deposition values which skewed this distribution.
35

Consideration of information presented above in Figures 2-1 through 2-4 and in Tables 2-

1 through 2-3 resulted in the following observations regarding estimates of total and U.S. EGUattributable mercury deposition for the 2005 and 2016 scenarios (note, all observations 

referencing the U.S. are for the continental U.S.):

• Patterns of total and U.S. EGU-related Hg deposition differ considerably: The pattern of 

total Hg deposition across the country is different from the pattern of U.S. EGU

deposition. There are areas of elevated total Hg deposition distributed around the country 

(e.g., west coast, areas in Nevada, southern Mississippi, West Virginia, southeastern 

Georgia) (see Figures 2-1 and 2-2). By contrast, U.S. EGU Hg deposition is concentrated 

in the eastern half of the country with one of the main regions of elevated deposition 

being in the Ohio River Valley (see Figures 2-3 and 2-4). Figures 2-3 and 2-4 also 

illustrate that while some near-coastal areas and portions of the Great Lakes do have 

elevated U.S. EGU mercury deposition, many of the highest areas (and largest expanses) 

of U.S. EGU deposition occur inland (e.g., Ohio River Valley, areas in northeast Texas 

and along the Mississippi River).

• US Hg deposition is generally dominated by sources other than U.S. EGUs (with the 

contribution from U.S. EGUs decreasing between the 2005 and 2016 scenarios): On 

average across the U.S., U.S. EGUs contribute 5% of total Hg deposition under the 2005

scenario with this level decreasing to 2% under the 2016 scenario (see Table 2-3). The 

remaining Hg deposition (i.e., ~95% and ~98%, respectively for the two scenarios) 

originates from other U.S. sources of mercury emissions and from foreign sources (both 

anthropogenic and natural). There is a considerably decrease in U.S. EGU Hg deposition 

between the 2005 and 2016 scenarios, with this resulting primarily from implementation 

of the Transport Rule, state mercury regulations and Federal enforcement actions.

34

• The contribution of U.S. EGU deposition to total deposition does vary across watersheds 

and can represent a relatively large fraction in some (more limited) instances: In the 

2005 scenario, while on average, U.S. EGUs only represented 5% of total Hg deposition 

in the U.S., values ranged up to 30% for the 99th% watershed (see Table 2-2). While 

overall U.S. EGU Hg deposition decreased substantially for the 2016 scenario, still, U.S. 

EGUs contributed 11% of total Hg deposition for the 99th% watershed (ranked according 

to U.S. EGU deposition) (see Table 2-2). 

The 

median reduction in U.S. EGU Hg deposition was 41% with reductions ranging up to 

85% for the 95th% watershed (when ranked according to magnitude of reduction in U.S. 

EGU Hg deposition) (see Tables 2-2 and 2-3). 

2.4. Fish Tissue MeHg Concentrations

 34 Controls on PM precursors, including directly emitted PM and SO2, can have significant secondary reductions on 

divalent and particle-bound mercury, both of which produce much of the local and regional deposition.
36

This section characterizes the set of 2,461 watershed-level fish tissue MeHg samples used 

in the analysis. As noted earlier in Section 1.3, the 75th percentile of the samples falling within a 

given watershed is used as the basis for the risk estimates generated for that watershed. 

Consequently we have used the 75th percentile statistic (at the watershed level) as the basis for 

summarizing the fish tissue MeHg data presented in this section. Recall also, that as discussed in 

Section 1.3, the MMaps approach was used to estimate the U.S. EGU-attributable portion of each 

75th percentile fish tissue value within each watershed (based on the fraction of total mercury 

deposition associated with U.S. EGUs for each watershed). Similarly, for the 2016 scenario, 

baseline fish tissue sampling data used in 2005 was first adjusted to reflect changes in total 

deposition (between the 2005 and 2016 Base Cases) for a given watershed (also using the 

MMaps approach), and then the U.S. EGU-attributable fraction was estimated, again based on 

the fraction of total mercury deposition over that watershed that is associated with U.S. EGUs. 

The summary of fish tissue MeHg data is based around a series of figures and tables 

conveying relevant information. The figures and tables used in summarizing the fish tissue data 

are described below. Note, that, as discussed in Section 2.3, most of the areas experiencing 

elevated U.S. EGU-attributable mercury deposition are located in the eastern half of the country. 

We also have greater coverage with mercury fish tissue data in the eastern part of the country. In 

addition, because the Transport Rule primarily affects U.S. EGUs in the eastern half of the 

country, we mainly see reductions in U.S. EGU-attributable risk (in comparing the 2005 to 2016

scenarios) for the eastern portion of the country. For these reasons, in illustrating spatial 

trends/patterns in fish tissue data in this section through figures, we focus primarily on the 

eastern half of the country (note, however that data presented in tables are for the whole 

continental U.S.). After presenting these figures and tables, a set of bulleted observations is 

presented at the end of the section that draws on information conveyed in the figures and tables. 

The set of figures and tables presented include:

• Figure 2-5: Map of 2,461 watersheds with fish tissue sampling data used in the risk 

assessment. This map not only illustrates general coverage of the fish tissue data for 

different regions of the country, it also illustrates the relatively small size of the HUC12 

watersheds used in the analysis.

• Figure 2-6: Map of the subset (approximately 2,170) of the 2,461 watersheds falling in 

the eastern half of the U.S.. This map uses color gradients to illustrate spatial variation in 

the total mercury fish tissue concentrations (for the 2005 Base Case) across the 

watersheds and as such, clearly illustrates how difficult it is to identify any discernable 

patterns with this approach given the small size of the watersheds. Because of this, we 

decided instead, to use graduated circles (with circle size tracking 75th percentile fish 

tissue concentrations at each watershed) in the remainder of maps presented in this 

section, since this approach allows spatial patterns to be more readily discerned.

• Figure 2-7 and 2-8: Maps presenting CMAQ modeling results for total mercury 

deposition (ug/m2) at the watershed-level in the eastern U.S., for the 2005 and 2016

scenarios respectively. 
37

• Figures 2-9 and 2-10: Maps presenting CMAQ modeling results for U.S. EGUattributable mercury deposition (ug/m2

) at the watershed-level in the eastern U.S., again 

for 2005 and 2016 scenarios, respectively.

• Figures 2-11 and 2-12: Maps of the upper 10th percentile of watersheds based on total 

mercury fish tissue levels for the 2005 and 2016 scenarios, respectively. These maps 

allow us to consider how spatial patterns (and overall magnitude) of watersheds with the 

highest fish tissue levels change between the 2005 and 2016 scenarios.

• Figures 2-13 and 2-14: Maps of the upper 10th percentile of watersheds based on U.S. 

EGU-attributable mercury fish tissue levels for the 2005 and 2016 scenarios, 

respectively. These maps allow us to consider how spatial patterns (and overall 

magnitude) of watersheds with the highest U.S. EGU-attributable fish tissue levels 

change between the 2005 and 2016 Base Cases. Note, that these maps are particular 

relevant to consideration of changes in the patterns of U.S. EGU-attributable risk between 

the 2005 and 2016 Base Cases. 

• Table 2-4: Summary of statistics (min, max, mean, 50th, 75th, 90th, 95th and 99th

percentiles) for both total and U.S. EGU-attributable Hg fish tissue levels (for the 2005 

and 2016 scenarios). These statistics are based on watershed-level data. In addition, this 

table also presents the percent reduction (between the 2005 and 2016 scenarios) for both 

total and U.S. EGU-attributable Hg fish tissue levels.

• Table 2-5: Summary of statistics (min, max, mean, 50th, 75th, 90th, 95th and 99th

percentiles) for U.S. EGU-attributable fraction of total Hg fish tissue (these results are 

reflected directly in total and U.S. EGU-attributable risk calculations).
38

Figure 2-5. Location of 2,461 watersheds with mercury fish tissue data included in the risk 

assessment

Figure 2-6. Subset of 2,170 watersheds (from the larger set of 2,461 included in the risk 

assessment) located in the eastern half of the country. (this maps also illustrates 

limitations with using color-coding at the watershed-level to explore trends in Hg fish 

tissue concentrations – see text)
39

Figure 2-7. Total Hg fish tissue concentrations (for the 2005 Base Case) for the subset of 

watersheds included in the risk assessment located in the eastern U.S. 

Figure 2-8. Total Hg fish tissue concentrations (for the 2016 Base Case) for the subset of 

watersheds included in the risk assessment located in the eastern U.S. 
40

Figure 2-9. U.S. EGU-attributable Hg fish tissue concentrations (for the 2005 Base Case) 

for the subset of watersheds included in the risk assessment located in the eastern 

U.S. 

Figure 2-10. U.S. EGU-attributable Hg fish tissue concentrations (for the 2016 Base Case) 

for the subset of watersheds included in the risk assessment located in the eastern 

U.S. (Note, this map uses the same scale as Figure 2-7, thereby supporting direct 

comparison between these two time periods)
41

Figure 2-11. Top 10th percentile of watersheds based on total Hg fish tissue concentrations 

(for the 2005 simulation). (ranking is based on full national set of watersheds included 

in the risk assessment, but map focuses on locations in the eastern U.S.) 

Figure 2-12. Top 10th percentile of watersheds based on total Hg fish tissue concentrations 

(for the 2016 simulation). (ranking is based on full national set of watersheds included 

in the risk assessment, but map focuses on locations in the eastern U.S.) 
42

Figure 2-13. Top 10th percentile of watersheds based on U.S. EGU-attributable Hg fish 

tissue concentrations (for the 2005 simulation). (ranking is based on full national set of 

watersheds included in the risk assessment, but map focuses on locations in the eastern 

U.S.) 

Figure 2-14. Top 10th percentile of watersheds based on U.S. EGU-attributable Hg fish 

tissue concentrations (for the 2016 simulation). (ranking is based on full national set of 

watersheds included in the risk assessment, but map focuses on locations in the eastern 

U.S.) 
43

Table 2-4. Comparison of total and U.S. EGU-attributable Hg fish tissue concentrations 

(including % change) for the 2005 and 2016 scenarios.

Statistic

Hg fish tissue concentration (ppm) % change (2016 versus 

2005) in Hg fish tissue 

2005 scenario 2016 scenario concentration

Total 

U.S. EGUattributable 

U.S. EGU

as percent 

of total Total 

U.S. EGUattributable 

U.S. EGU

as percent 

of total Total

U.S. EGUattributable 

Mean 0.31 0.024 7.7% 0.29 0.008 2.9% -6.7% -65%

Median 0.23 0.014 6.2% 0.20 0.005 2.7% -10.0% -61%

75th % 0.39 0.032 8.3% 0.36 0.011 2.9% -6.4% -67%

90th % 0.67 0.056 8.3% 0.63 0.019 3.0% -5.9% -66%

95th % 0.91 0.079 8.7% 0.87 0.026 3.0% -4.7% -67%

99th % 1.34 0.150 11.2% 1.29 0.047 3.7% -3.7% -68%

Table 2-5. Comparison of U.S. EGU fraction of total Hg deposition (used to apportion Hg 

fish tissue concentrations and risk) between the 2005 and 2016 scenarios. Note, that 

these values are specifically for the 2,461 watersheds included in the risk assessment.

statistic

U.S. EGU-attributable fraction of 

total Hg fish tissue levels 

2005 scenario 2016 scenario

Mean 0.09 0.04

Median 0.06 0.03

75th % 0.14 0.05

90th % 0.20 0.07

95th % 0.26 0.09

99th % 0.40 0.18

Consideration of information presented above in Figures 2-5 through 2-14 and in Tables 

2-4 through 2-5 resulted in the following observations regarding estimates of total and U.S. 

EGU-attributable fish tissue MeHg concentrations across the 2,461 watersheds included in the 

risk assessment:

• Focus on U.S. EGU-attributable Hg fish tissue levels is in the eastern half of the U.S.:

Given (a) that the number of watersheds with measured fish tissue MeHg data is 

substantially greater in the east (see Figure 2-5) and (b) more importantly, that the levels 

of U.S. EGU Hg deposition (that largely drives U.S. EGU-attributable Hg fish tissue 

levels) are much higher in the east (see Figures 2-3 and 2-4), trends in U.S. EGUattributable Hg fish tissue levels discussed here are driven by data in the eastern half of 

the U.S. 

• U.S. EGUs contribute a larger fraction to total Hg fish tissue levels in the U.S. than they 

do to total Hg deposition (in terms of percent), this reflects the fact that Hg fish tissue 

samples are focused in the east where U.S. EGU deposition is greater. While U.S. EGUs 
44

contribute ~5% of total Hg deposition in the U.S. (for the 2005 scenario – see Table 2-2), 

their contribution to Hg fish tissue levels (summarized at the watershed-level) for the 

2005 scenario is larger at ~9% (see Table 2-5). This reflects the fact that Hg fish tissue 

samples are heavily weighted in the eastern portion of the U.S. where U.S. EGU Hg 

deposition is typical higher than in the west.35

• Relative to the combined impact of other sources, U.S. EGUs represent a smaller, but 

still potentially important contributor to total fish tissue MeHg levels: U.S. EGUs 

contribute ~9% of Hg fish tissue levels on average under the 2005 scenario (see Table 2-

5). Under the 2016 scenario, the U.S. EGU contribution decreases to ~ 4% on average 

(see Table 2-5). While U.S. EGU-attributable Hg fish tissue decreases notably between 

the 2005 and 2016 scenarios, the impact on total Hg fish tissue levels is not that 

noticeable given that U.S. EGUs contribute a relatively small fraction on total Hg fish 

tissue levels in general (contrast the pattern of reduction seen in Figures 2-9 and 2-10 for 

U.S. EGU-attributable Hg fish tissue levels with the relatively smaller changes seen in 

Figures 2-7 and 2-8 for total Hg fish tissue levels). 

 By providing greater coverage for the 

eastern half of the country, the Hg fish tissue sampling data generally provides greater 

coverage for regions with potentially greater U.S. EGU-attributable risk.

• Despite the relatively small fraction of total fish tissue MeHg associated with U.S. EGUs 

on average, for a subset of watersheds, they can make a substantially larger contribution: 

Under the 2005 scenario, U.S. EGUs can range up to 40% of total Hg fish tissue levels 

(for the 99th% watershed). Under the 2016 Scenario, this pattern is reduced, but U.S. 

EGUs can still contribute up to 18% of total Hg fis tissue levels (again, for the 99th% 

watershed) (see Table 2-5).

2.5. Comparing Patterns of Hg Deposition with Hg Fish Tissue Data for the 2,461 

Watersheds Included in the Risk Assessment

In addition to the observations provided in the last two sections based on consideration of 

the CMAQ-based Hg deposition estimates and Hg fish tissue data separately, it is also possible to 

directly compare spatial patterns between these two sets of data. This comparison provides

information that can help in interpreting risk estimates discussed in Section 2.6. Specifically we 

can consider: (a) whether the watershed-level Hg fish tissue levels are positively correlated with 

total Hg deposition, (b) how patterns of Hg deposition for the 2,461 watersheds where we have 

Hg fish tissue data compare with patterns for the full set of 88,000 watersheds in the U.S. and (c)

to what extent the watersheds for which we have Hg sampling data provide coverage for areas of 

elevated U.S. EGU deposition. To address these questions, we have presented a series of figures 

and tables below including:

 35 As discussed in section 1.3, U.S. EGU-attributable Hg fish tissue levels are directly based on U.S. EGU Hg 

deposition (at the watershed-level) together with application of the MMaps approach.
45

• Figures 2-15 and 2-16: For the 2005 and 2016 scenarios respectively, maps showing 

areas of elevated U.S. EGU-related Hg deposition36

• Figure 2-17: Provides plot for the 2005 Scenario of Hg fish tissue levels versus total Hg 

deposition by watershed. This plot allows consideration for whether there appears to be a 

correlation between these two factors at the watershed level.

and the degree to which the 2,461 

watersheds with fish tissue sampling data used in the risk assessment provide coverage 

for these areas.

• Figure 2-18: Presents cumulative distribution plots comparing U.S. EGU-attributable 

deposition for the 2,461 watersheds used in the risk assessment with U.S. EGUattributable deposition of the entire set of ~88,000 watersheds in the U.S. Separate sets 

of plots are provided for the 2005 and 2016 Scenarios, allowing trends for each scenario 

to be compared to each other. These plots allow us to consider whether the watersheds 

with fish tissue MeHg data tended to fall in regions with higher U.S. EGU-attributable 

Hg deposition and the degree to which this subset of watersheds provided coverage for 

areas with relatively elevated U.S. EGU mercury deposition across the country. 

 36 Areas of “elevated U.S. EGU-related Hg deposition” refer to areas that are at or above the average deposition 

level seen in watersheds with U.S. EGU-attributable exposures above the MeHg RfD. Specifically, we used 

exposure estimates based on the 95th percentile fish consumption rate (for the female high consumer scenario 

assessed nation-wide) to identify watersheds with U.S. EGU-attributable exposures above the MeHg RfD and then 

queried for the average U.S. EGU-related Hg deposition across that subset of watersheds. This average deposition 

rate differed for the 2005 and 2016 Scenarios (i.e., 3.79 and 1.28 ug/m2

, respectively). These values were used as the 

basis for identifying watersheds with levels of U.S. EGU-related Hg deposition for the 2005 and 2016 Scenarios 

presented in Figures 2-13 and 2-14.
46

Figure 2-15. For the 2005 scenario, comparison of coverage of watersheds with Hg fish 

tissue data (used in the risk assessment) for areas in the eastern U.S. with relatively 

elevated U.S. EGU-attributable Hg deposition. 

Figure 2-16. For the 2016 Scenario, comparison of coverage of watersheds with Hg fish 

tissue data (used in the risk assessment) for areas in the eastern U.S. with relatively 

elevated U.S. EGU-attributable Hg deposition. 
47

Figure 2-17. For the 2005 scenario, plot of total Hg fish tissue concentrations versus total 

Hg deposition for the 2,366 watersheds included in the risk assessment for the highend female consumer population. 

Figure 2-18. Cumulative distribution plots of U.S. EGU-attributable Hg deposition over 

the 2,366 watersheds used in modeling the high-end female consumer population as 

contrasted with all 88,000 watersheds (plots provided both for the 2005 and 2016

Scenarios). 

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

5 10 15 20 25 30 35 40

2005 Hg Deposition 

2,366 Hg Fish Tissue Concentrations Used in Risk Assessment (ppm)

Cumulative Distribution of Mercury Deposition by Watershed

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 2 4 6 8 10 12 14 16 18 20

Hg deposition (ug/m2

)

Cumulative Percentile Distribution

2005 EGU fish tissue waterheds 2005 EGU all watersheds

2016 EGU fish tissue watersheds 2016 EGU all watersheds

2005 EGU fish tissue watersheds

2005 EGU all watersheds

2016 EGU fish tissue watersheds

2016 EGU all watersheds
48

Consideration of information presented above in Figures 2-15 through 2-18 resulted in 

the following observations regarding how estimates of Hg deposition estimates relate to 

measured fish tissue MeHg levels, when considered at the watershed-level:

• The fish tissue MeHg sampling data (summarized at the watershed-level) provides limited 

coverage for areas with elevated U.S. EGU Hg deposition. Therefore, the number of “at 

risk” watersheds as characterized in this risk assessment may be substantially higher 

than estimated: As depicted in Figures 2-15 and 2-16 (for the 2005 and 2016 Scenarios, 

respectively), while the 2,461 watersheds used in the risk assessment to fall into regions 

in the east with elevated U.S. EGU Hg deposition, the degree of coverage is limited. This 

can be seen by noting in these figures the wide expanses of areas of elevated U.S. EGU

Hg deposition (shown in red) and therefore, not covered by watersheds modeled for risk 

in the analysis.37

• Hg fish tissue levels are not correlated with total Hg deposition (the relationship is highly 

dependent on methylation potential of individual waterbodies): As shown in Figure 2-17, 

total Hg fish tissue levels (summarized at the watershed-level) are not correlated with 

levels of total Hg deposition when looking across watersheds (i.e., the highest total 

mercury deposition watersheds does not always have the highest fish tissue MeHg 

levels). This is not unexpected given that the relationship between total Hg deposition 

and total Hg fish tissue levels is highly dependent on the methylation potential at the 

waterbody-level. As discussed above in Appendix E, a variety of factors that display 

spatial variability are associated with methylation potential (e.g., pH, sulfate deposition, 

turbidity etc). Therefore, we would anticipate that there would not be a direct correlation 

between total Hg deposition and Hg fish tissue levels, again looking across watersheds. 

The MMaps approach and underlying analyses (see Section 1.3 and Appendix E), support 

a proportional relationship between mercury deposition and fish tissue MeHg levels 

within a given watershed, such that changes in deposition will be reflected in changes in 

fish tissue levels. In other words, the correlation between Hg deposition and fish tissue 

MeHg concentrations does not appear to hold between watersheds, but is espected tos 

hold within a given watershed. 

• Hg fish tissue samples were generally collected in regions with elevated total Hg 

deposition: As demonstrated in Figure 2-18, Hg fish tissue sampling appears to have 

 37 We completed a follow-on assessment to help interpret the significance of the group of watersheds with “elevated 

U.S. EGU deposition” that were not covered in the risk assessment (i.e., areas shown in red in Figures 2-15 and 2-

16). Specifically, we were interested in knowing, for the subset of 2,366 watersheds we did assess for risk for the 

female high-end consumer, what is the percentage that (a) had Hg deposition above the threshold identified here for 

“elevated deposition” (i.e., 3.79 and 1.28 ug/m2, respectively for the 2005 and 2016 Scenarios) and (b) had U.S. 

EGU incremental risk of an HQ =1.5. This percentage lets us know, for the watersheds we modeled, what fraction 

of watersheds with elevated U.S. EGU Hg deposition also had U.S. EGU-incremental risk representing a potential 

public health hazard. The results of the assessment are 37% and 9%, respectively for the 2005 and 2016 Scenarios. 

We can consider use these estimates to help interpret the areas in Figures 2-15 and 2-16 that have elevated U.S. 

EGU Hg deposition and are NOT included in the risk assessment. If these watersheds tracked the pattern seen in the 

watersheds we modeled, then we would expect to see ~40% of the red highlighted areas in Figure 2-15 translate into 

relatively elevated U.S. EGU incremental risk watersheds and ~10% of the red highlighted watersheds in Figure 2-

16. Note, however, that there is substantial uncertainty in extrapolating trends seen across our modeled watersheds 

to the non-modeled watersheds with elevated U.S. EGU Hg deposition.. 
49

favored areas with relatively higher total Hg deposition. This can be seen by comparing 

cumulative plots of modeled watersheds (where we have fish tissue MeHg data) against 

plots for the entire set of 88,000 watersheds. This comparison suggests that watersheds 

where fish tissue MeHg data were collected tended to have higher total Hg deposition, 

then the full set of watersheds. This likely reflects to some extent, the fact that fish tissue 

sample are focused in the eastern half of the country, which does have elevated total Hg 

deposition compared to the broad central region (see Figure 2-1 and 2-2).

2.6. Overview of Risk Estimates

This section provides an overview of risk estimates generated for the 2,461 watersheds 

included in the risk assessment. As noted earlier in Section 1.2, presentation of risk estimates 

will focus on the high-end female consumer population assessed at the national-level, since this 

population provides the most comprehensive coverage for watersheds with Hg fish tissue data 

across the U.S. and because the consumption rates used to model this population represent 

subsistence levels and are supported by a number of studies (see Section 1.3 and Appendix D). 

While this fisher population is emphasized in summarizing risk estimates, we do provide risk 

estimates generated for the other populations covered in the analysis (e.g., blacks in the 

southeast, Tribal populations near the Great Lakes, Hispanics). In summarizing risk estimates, 

we will provide estimates for both the 2005 and 2016 Scenarios, while placing the most 

emphasis on the 2016 estimates. The remainder of this section is organized as follows: 

• Overview of percentile risk estimates generated for the different fisher populations 

evaluated (Section 2.6.1): In this section, we provide percentile risk estimates (for HQ 

and IQ loss risk) for the high-end female consumer population assessed at the national 

level. We then summarize percentile risk estimates (HQ only) for the broader set of fisher 

populations assessed in the analysis. The percentile risk estimates provided in this section 

allow us to (a) consider Stage 2 of the 3-Stage framework developed to support the 

interpretation of risk estimates (i.e., consider the U.S. EGU-related increment of total risk

– see Section 1.2) and (b) consider the magnitude of risk across the set of fisher 

populations assessed in the risk assessment. 

• Overview of the number (and frequency) of watersheds with populations potentially at 

risk due to U.S. EGU-sourced mercury (Section 2.6.2): This set of risk estimates provides 

the main input to the risk characterization framework (see Section 1.2). Specifically, 

watersheds with populations potentially at risk comprise:

o Watersheds where total risk is considered to represent a public health concern and 

where U.S. EGUs contribute to that total risk (in the analysis, we considered 

various increments of U.S. EGU contribution including 5%, 10%, 15% and 20%, 

although as noted in Section 1.2, we focus on 5%). This represents Stage 1 of the 

rick characterization framework AND/OR, 

o Watersheds where risk when considering U.S. EGUs mercury emissions before 

considering other sources of mercury represents a potential public health hazard 

(this is Stage 2 in the risk characterization framework)
50

To support the discussion of risk estimates, a series of tables summarizing those estimates 

are presented in the subsections below. A list of observations based on consideration for the risk 

estimates summarized is presented at the end of each subsection. 

2.6.1. Overview of percentile risk estimates

In presenting percentile risk estimates in this section, we have sorted risk estimates by 

U.S. EGU-attributable risk in order to track trends in the magnitude of this category of risk.38

The percentile estimates themselves, are based on risk bands around the percentile value (rather 

than being based on the specific watershed at that percentile). Specifically, we have taken the 

average of the 5% of watershed values surrounding the specific percentile estimate in the table. 

So, for example, for a 50th% risk value presented in the table, we have actually taken the average 

of the 47.5th through 52.5th percentile watershed-level risk estimates (after ranking them by U.S. 

EGU-attributable risk, as mentioned earlier). We used this risk band approach, rather than using 

the estimates from the singe watershed located specifically at that percentile, because we wanted 

to capture general trends in the patterns of U.S. EGU-attributable and total risk for watersheds 

around that percentile.39

• Tables 2-6 and 2-7: Presents risk percentiles for both IQ loss and RfD-based HQs for the 

high-end female consumer assessed at the national-level (for 2,366 watersheds) for the 

2005 and 2016 Scenarios, respectively. We do note that overall confidence in IQ loss 

estimates above approximately 7 points decreases because we begin to apply the 

underlying IQ loss function at exposure levels (ppm hair levels) above those reflected in 

epidemiological studies used to derive those functions.

Note also, that while this risk assessment does focus on subsistence 

levels of fish consumption, the risk tables summarized here do include risk estimates based on 

mean fish consumption rates for these higher consuming populations, which are relatively high 

compared to general recreational angler rates. The following tables are used to summarize risk 

estimates for the fisher populations included in the analysis. 

40

• Table 2-8: Provides risk percentiles for RfD-based HQs for the remaining fisher 

populations assessed in this analysis specifically for the 2005 scenario. We did not 

evaluate these populations for the 2016 Scenario, since the relative magnitude of risks for

these additional populations can be inferred by comparing risks for these populations 

against the risk generated for the high-end female consumer population (for the 2005 

We have flagged IQ loss 

estimates above 7 as subject to greater uncertainty. 

 38 This means that when reviewing the risk estimates presented in tables in this section, the trend in risk across 

watershed percentiles (i.e., higher percentiles will have higher risk) will be seen for the U.S. EGU-attributable 

portion of the risk and not necessarily for total risk, since the watersheds were ranked on U.S. EGU-attributable risk 

and on total risk, prior to generating the percentile summaries provided in the tables. 

39 Note, that watersheds can display considerable variation in the relationship between total and U.S. EGUattributable risk. Therefore, if we had selected the specific watershed associated with a given percentile, that 

watershed could misrepresent the general trend (in terms of the relationship between total and U.S. EGU-attributable 

risk) for watersheds in the vicinity of that percentile. By taking average values for these “risk bands” around each 

percentile, we get more stable and meaningful results in terms of capturing trends in risk estimates across the 

percentiles presented in the risk tables. 40 The 39.1 ppm was the highest measured ppm level in the Faroes Island study, while ~86 was the highest value in 

the New Zealand study (USEPA, 2005) (a 7 IQ points loss is approximately associated with a 40 ppm hair level 

given the concentration-response function we are using). 
51

scenario) as summarized in Table 2-8. Similarly, we did not calculate IQ loss estimates, 

since these are linearly related to HQ loss and can be inferred from the estimates 

presented in Table 2-8 (based on comparison of magnitude of HQ estimates between 

different fisher populations).

Table 2-6. Percentile risk estimates for the high-end female consumer population assessed 

nationally (2005 scenario) (for both total and U.S. EGU incremental risk, including IQ 

loss and MeHg RfD-based HQ estimates)

Fisher 

consumption 

rate percentile 

and rate (g/day)

Watershed percentile

50th percentile 75th percentile 90th percentile 95th percentile 99th percentile

total

U.S. 

EGU total

U.S. 

EGU total

U.S. 

EGU total

U.S. 

EGU total

U.S. 

EGU

IQ loss (points)

mean (39) 0.8 - 0.7 - 1.0 0.1 1.2 0.2 1.6 0.3

90th (123) 2.5 0.2 2.3 0.3 3.0 0.4 3.9 0.5 5.1 1.0

95th (173) 3.5 0.3 3.2 0.4 4.2 0.5 5.4 0.7 7.2* 1.4

99th (373) 7.6 0.7 6.9 0.9 9.1* 1.1 11.7* 1.6 15.5* 2.9

RfD-based HQ 

mean (39) 3.5 0.3 3.2 0.4 4.2 0.5 5.4 0.7 7.2 1.4

90th (123) 11.1 1.1 10.1 1.3 13.3 1.6 17.1 2.3 22.8 4.3

95th (173) 15.6 1.5 14.2 1.8 18.7 2.3 24.1 3.2 32.0 6.0

99th (373) 33.6 3.3 30.7 3.9 40.3 4.9 51.9 6.9 69.0 13.0

- IQ loss is <0.1 point 

* IQ loss estimate subject to greater uncertainty due to application of the underlying concentration-response function 

for IQ loss at levels of exposure above those in the underlying epidemiological studies (see text)

Table 2-7. Percentile risk estimates for the high-end female consumer population assessed 

nationally (2016 Scenario) (for both total and U.S. EGU incremental risk, including IQ 

loss and RfD-based HQ estimates)

Fisher 

consumption 

rate percentile 

and rate (g/day)

Watershed percentile

50th percentile 75th percentile 90th percentile 95th percentile 99th percentile

total

U.S. 

EGU total

U.S. 

EGU total

U.S. 

EGU total

U.S. 

EGU total

U.S. 

EGU

IQ loss (points)

mean (39) 0.6 - 0.6 - 1.0 - 1.3 - 1.8 0.1

90th (123) 1.7 - 2.0 - 3.2 0.1 4.0 0.2 5.6 0.4

95th (173) 2.4 - 2.8 - 4.5 0.2 5.6 0.2 7.9* 0.5

99th (373) 5.3 0.1 6.0 0.2 9.8* 0.4 12.1* 0.5 16.9* 1.1

RfD-based HQ 

mean (39) 2.5 - 2.8 - 4.6 0.2 5.6 0.2 7.9 0.5

90th (123) 7.7 0.2 8.8 0.3 14.4 0.5 17.7 0.8 24.8 1.6
52

95th (173) 10.9 0.2 12.3 0.4 20.2 0.8 24.9 1.1 34.9 2.2

99th (373) 23.4 0.5 26.6 0.9 43.6 1.7 53.6 2.3 75.3 4.8

- IQ loss is <0.1 point 

* IQ loss estimate subject to greater uncertainty due to application of the underlying concentration-response function 

for IQ loss at levels of exposure above those in the underlying epidemiological studies (see text)

Table 2-8. Percentile risk estimates for the full set of fishing populations included in the 

analysis (2005 scenario) (for both total and U.S. EGU incremental risk, only for 

RfD-based HQ estimates)

Fisher 

consumption 

rate percentile 

and rate (g/day)

Watershed percentile

50th percentile 75th percentile 90th percentile 95th percentile 99th percentile

total

U.S. 

EGU total

U.S. 

EGU total

U.S. 

EGU total

U.S. 

EGU total

U.S. 

EGU

High-end female consumer assessed nationally

mean (39) 3.5 0.3 3.2 0.4 4.2 0.5 5.4 0.7 7.2 1.4

90th (123) 11.1 1.1 10.1 1.3 13.3 1.6 17.1 2.3 22.8 4.3

95th (173) 15.6 1.5 14.2 1.8 18.7 2.3 24.1 3.2 32.0 6.0

99th (373) 33.6 3.3 30.7 3.9 40.3 4.9 51.9 6.9 69.0 13.0

Poor white fishers in the Southeast 

mean (39) 5.0 0.5 4.9 0.6 7.3 0.7 7.3 1.0 8.8 1.6

90th (93) 12.0 1.1 11.6 1.4 17.6 1.7 17.5 2.5 21.1 3.9

95th (129) 16.6 1.6 16.1 1.9 24.4 2.4 24.3 3.4 29.2 5.5

99th (286) 36.9 3.5 35.8 4.2 54.1 5.3 53.9 7.5 64.7 12.1

Poor black fishers in the Southeast 

mean (171) 19.0 1.9 24.7 2.3 26.2 2.9 39.6 4.2 40.0 7.2

90th (446) 49.6 4.9 64.4 5.9 68.2 7.6 103.3 10.9 104.3 18.8

95th (557) 61.9 6.1 80.5 7.4 85.2 9.5 129.0 13.6 130.3 23.4

99th NC*

Poor Hispanic nationally 

mean (26) 1.5 - 2.3 0.2 3.0 0.3 3.6 0.5 6.2 0.9

90th (98) 5.5 0.3 8.6 0.7 11.6 1.2 13.6 1.8 23.4 3.5

95th (156) 8.8 0.5 13.6 1.1 18.4 2.0 21.7 2.8 37.2 5.5

99th NC*

Vietnamese 

mean (27) 1.4 - 3.0 0.2 1.7 0.3 2.8 0.4 4.6 0.9

90th (99) 5.2 0.3 10.9 0.6 6.2 1.0 10.4 1.4 16.8 3.1

95th (152) 8.0 0.5 16.8 1.0 9.5 1.6 15.9 2.2 25.8 4.8

99th NC*

Laotians 

mean (47) 2.2 0.1 3.3 0.3 2.0 0.4 3.9 0.5 6.5 1.5

90th (145) 6.6 0.4 10.2 0.9 6.0 1.3 12.0 1.5 20.0 4.6

95th (266) 12.2 0.7 18.8 1.6 11.1 2.4 22.0 2.7 36.7 8.5

99th NC*

Tribal (near Great Lakes) 

mean (62) 4.1 0.1 7.4 0.2 9.4 0.4 8.7 0.6 6.9 0.9

90th (136) 8.9 0.2 16.2 0.4 20.6 0.9 19.1 1.3 15.0 2.0
53

Fisher 

consumption 

rate percentile 

and rate (g/day)

Watershed percentile

50th percentile 75th percentile 90th percentile 95th percentile 99th percentile

total

U.S. 

EGU total

U.S. 

EGU total

U.S. 

EGU total

U.S. 

EGU total

U.S. 

EGU

95th (213) 13.9 0.4 25.4 0.6 32.2 1.4 29.9 2.0 23.5 3.2

99th (493) 32.2 0.9 58.6 1.4 74.5 3.2 69.2 4.5 54.4 7.4

- IQ loss or HQ estimates <0.1.

NA*: It was not possible to derive a 99th% consumption rate for this population due to insufficient sample size in the 

underlying study. Consequently, risk estimates for the 99th% consumption rates were not generated.

Observations regarding the percentile risk estimates presented here reflect our 

interpretation of the potential health significance associated with both MeHg RfD-based HQ and 

IQ loss estimates (see Section 1.2). Consideration of the risk estimates summarized above in 

Tables 2-6 through 2-8 results in the following observations regarding percentile risk estimates 

generated for the fisher populations assessed:

• For the high-end female consumer assessed at the national-level, total IQ loss and total 

HQ estimates do not change in a systematic way between the 2005 and 2016 Scenarios

with these levels often being of potential health concern across a wide variety of 

consumption rates and watershed percentiles: While there are some differences in total 

IQ loss and HQ estimates generated for the high-end female consumer assessed 

nationally between the 2005 and 2016 Scenarios, there is no systematic trend between the 

scenarios (see Tables 2-6 and 2-7). Furthermore, for the high-end female consumer

assessed at the national-level, these estimates of total risk often exceed one IQ point loss 

and an HQ of 1.5 (i.e., levels of potential health concern) across most of the combinations 

of consumption rates and watersheds (see Tables 2-6 and 2-7). The absence of a 

substantial change in total risk between the two simulation years is not surprising given 

the relatively small fraction of total mercury deposition contributed by U.S. EGUs on 

average across the modeled watersheds. This means that even substantial reductions in 

U.S. EGU deposition between the simulation years is unlikely to substantially affect total 

risk (although, as noted elsewhere, it can have a substantial impact on risk at the subset of 

watersheds where U.S. EGUs do contribute a larger fraction of total deposition).

• By contrast (again focusing on the high-end female consumer assessed nationally), both 

U.S. EGU-incremental IQ loss and the U.S. EGU increment-based HQ display notable 

reductions between the 2005 and 2016 Scenarios, but U.S. EGU-attributable risk still 

exceeds potential levels of concern for a over a quarter of watersheds: Comparison of 

the U.S. EGU-attributable risk estimates presented in Tables 2-6 and 2-7, suggest that 

these categories of risk decrease significantly between the 2005 and 2016 Scenarios. As 

noted earlier, this reduction largely reflects the implementation of PM controls which 

have the co-benefit of reducing divalent and particle-bound mercury together with state 

regulations targeting mercury emissions directly. As noted in Section 2.2, because current 

(2010) emissions of mercury are likely closer to levels used in modeling the 2016

Scenario (with the 2005 scenario reflecting emission levels that are substantially larger 

than current conditions), we focus here on presenting observations based on the 2016
54

Scenario. For the high-end female consumer assessed at the national-level, under the

2016 Scenario (see Table 2-7), U.S. EGU-attributable IQ loss only meets or exceeds one 

point for the highest consumption range at the upper 1% of the watersheds. And, given 

the relatively high total exposure associated with this simulation, our calculation of IQ 

(including the U.S. EGU-attributable portion) for this combination of consumption rate 

and watershed percentile is subject to increased uncertainty.

41

• Estimates of risks generated for the high-end female consumer population (assessed at 

the national-level) are generally higher than risks estimated for the other high-end fisher 

populations, with the exception of white and black fisher populations assessed in the 

southeast: Risk estimates generated for the 2005 scenario for the set of high-end fisher 

populations assessed in this analysis suggest that risks (across all combinations of 

consumption rates and watersheds) are generally higher for the high-end female 

consumer population assessed at the national-level with the exception of black and white 

fishers assessed in the southeast (contrast estimates presented in Table 2-8). For example, 

high-end female consumer risk estimates assessed at the national-level are approximately 

twice as high (in terms of both total and EGU-attributable) as estimates generated for 

Hispanics and Vietnamese also assessed nationally (see Table 2-8). Risk estimates for the 

high-end female consumer are approximately 50% higher than estimates generated for 

Laotians assessed nationally and Tribal populations in the vicinity of the Great Lakes. 

However, risks (both total and U.S. EGU-attributable) for white fishers in the southeast 

are somewhat higher that risk estimates generated for the high-end female consumer at 

the national-level, while estimates for black populations assessed in the southeast are 

notably higher. Risk estimates for these two southeastern fisher populations are likely 

higher due to: (a) the fact that the southeast has relatively higher total and U.S. EGUattributable fish tissue concentrations compared with the full set of watersheds with fish 

tissue levels nationally (see Figures 2-1 through 2-4) (this means that upper-end 

percentiles fish tissue values will be higher in the southeast) and (b) in the case of the 

black fisher populations, percentile consumption rates are substantially higher than 

consumption rates for the high-end female consumer population (assessed at the nationallevel) see Table 2-8, “Fisher consumption rate percentile rate (g/day)” column. While 

risk estimates are higher for the two populations assessed in the southeast, we decided to 

focus the discussion of risk estimates in both Section 1.6.1 and 1.6.2, on the high-end

female consumer assessed at the national-level. As noted earlier, this reflects several 

By contrast, for the 2016

Scenario, estimates of U.S. EGU increment-based HQ for this fisher population exceed 

1.5, although these exceedences are still limited to combinations of higher percentiles of 

watersheds and consumption rates (e.g., 95th and 99th % consumption rates paired with 

the 95th and 99th % watersheds – see Table 2-7). These HQ-based risk estimates can be 

used to inform Stage 2 of the 3-Stage risk characterization framework. Specifically, for 

the 2016 Scenario, a fraction of watersheds (top 5 to 10%) have U.S. EGU incrementbased HQ’s that exceed 1.5 based on modeling subsistence-level fish consumption for the 

high-end female consumer assessed at the national-level (see Table 2-7). 

 41 As noted in section 2.6.1, estimates of total IQ loss above 7 points, involve simulation of mercury hair levels that 

exceed those used in the epidemiological studies underlying the function used in modeling of IQ loss and therefore 

are subject to greater uncertainty (not in the potential for IQ loss to occur, but rather in our ability to quantify 

degrees of loss above 7 points). 
55

factors: (a) focusing risk estimates on national-level analysis allows us to consider a 

larger number of watersheds reflecting greater regional variability in factors related to 

fish consumption exposure and risk (e.g., methylation potential, fish species), (b) risk 

estimates in the southeast are driven in part by watersheds in SC and we have concerns 

over the potential for these estimates having non-air Hg contributions (from gold mining 

– see Appendix E), and (c) fish consumption rates for both of the southeastern-focused 

populations (white and black) are based on smaller sample sizes compared with the 

estimate for the high-end female consumers and therefore, we have greater confidence in 

the consumption estimates generated for the high-end female consumer population.42

2.6.2. Overview of number (and frequency) of watersheds with populations 

potentially at-risk due to U.S. EGU mercury emissions

 

This section discusses risk estimates based on identifying the number of watersheds with 

populations potentially at risk due to mercury released from U.S. EGUs. As noted in Section 1.2, 

the “at risk population” classification is based on identifying watersheds where: (a) U.S. EGUs 

contribute to total risk at watersheds where that total risk is considered to represent a potential 

public health hazard and/or (b) risk at the watershed-level represents a potential public health 

hazard when deposition from U.S. EGUs is considered before taking into account deposition and 

exposures resulting from other sources of mercury. The estimates of watersheds with at-risk 

populations discussed in this section are used in the 3-Stage risk characterization framework 

described in Section 1.2 for interpreting risk estimates. Specifically, the first category of at risk 

populations described above maps to Stage 1 of the 3-Stage approach, while the second category 

maps to Stage 2. The combination (i.e., mathematical union) of these two groups of watersheds

with at risk populations comprises the set of watersheds represented in Stage 3 of the framework. 

We note that, specifically with regard to the HQ estimates, any contribution of mercury 

from EGUs to watersheds with exposures exceeding the MeHg RfD represents a potential hazard 

to public health, but for purposes of this analysis we have focused on those waterbodies where 

we determined EGUs contributed 5% or more to the hazard. We think this is a conservative 

approach given the increasing risks associated with incremental exposures above the MeHg RfD.

The estimates of watersheds with potentially at-risk populations discussed in this section 

are all based on the underlying risk estimates generated for the high-end female consumer

population. The decision to focus on this fisher population in discussing risk estimates is 

discussed in the previous section. The estimates of watersheds with potentially at-risk 

populations are summarized in tables described below (note, observations based on consideration 

of these risk estimates are presented in bullets following the tables). 

• Tables 2-9: Identifies watersheds with potentially at-risk populations based on 

consideration for different degrees of U.S. EGU contribution (i.e., 5, 10, 15 and 20%) at 

watersheds where total risk is considered to represent a potential public health hazard

(i.e., meet or exceed 1 IQ point or an HQ of 1.5 or higher). For reference purposes, the 

table also identifies the total number of watersheds (out of the 2,366 assessed for the 

 42 Sample size used in computing consumption rate percentiles for the female population is 149, as contrasted with 

39 and 98 for poor blacks and whites, respectively (Burger et al., 2010).
56

high-end female consumer population) with total risk exceeding the two thresholds, 

regardless of the U.S. EGU percent contribution (see the “≥ 0%” row of results in the 

table). In presenting results, the tables include both the number of watersheds meeting 

specific criteria as well as the percent (of the 2,366 watersheds assessed) that this 

represents. (Stage 1 of the 3-Stage framework)

• Tables 2-10: Identifies watersheds with potentially at-risk populations based on 

consideration for the magnitude of risk when deposition from U.S. EGUs is considered 

before taking into account deposition and exposures resulting from other sources of 

mercury. Risks are presented both for U.S. EGU-attributable IQ loss and for U.S. EGU

increment-based HQ. (Stage 2 of the 3-Stage framework)

• Table 2-11: Presents the union of the two categories of watersheds with potentially at-risk 

populations (i.e., mathematical union of the Stage 1 and 2 estimates presented in Tables 

2-9 and 2-10). As noted earlier, this analysis focuses on MeHg RfD-based HQ estimates 

rather than IQ loss estimates, since the HQ-based risk estimates generate a larger 

percentage of watersheds with populations potentially at-risk compared with the IQ loss 

estimates. Consequently, Table 2-11 considers the number and percent of watersheds that 

have (a) U.S. EGUs contributing to total risk of an HQ ≥ 1.5, OR (b) an HQ ≥ 1.5 based 

on considering U.S. EGU mercury deposition alone, before factoring in other sources of 

mercury deposition. This represents Stage 3 of the risk characterization framework.

Table 2-9. Watersheds with potentially at-risk populations based on consideration for 

various degrees of U.S. EGU contribution to total risk (for IQ loss and HQ)

EGU risk 

threshold

Number and percentage of HUCs meeting risk threshold criteria

2005 analysis 2016 analysis

90th fish 

consumption

95th% fish 

consumption

99th% fish 

consumption

90th fish 

consumption

95th% fish 

consumption

99th% fish 

consumption

Total IQ points lost = 1

≥ 0% 1667 (70%) 1948 (82%) 2224 (94%) 1558 (66%) 1858 (79%) 2202 (93%)

≥ 5% 948 (40%) 1124 (48%) 1287 (54%) 288 (12%) 394 (17%) 495 (21%)

≥ 10% 609 (26%) 726 (31%) 829 (35%) 43 (2%) 69 (3%) 94 (4%)

≥ 15% 345 (15%) 420 (18%) 473 (20%) 15 * 27 * 37 (2%)

≥ 20% 167 (7%) 202 (9%) 225 (10%) 7 * 16 * 20 *

Total RfD-based HQ = 1.5*

≥ 0% 2191 (93%) 2266 (96%) 2348 (99%) 2162 (91%) 2248 (95%) 2346 (99%)

≥ 5% 1268 (54%) 1305 (55%) 1345 (57%) 482 (20%) 505 (21%) 521 (22%)

≥ 10% 816 (34%) 834 (35%) 853 (36%) 91 (4%) 95 (4%) 97 (4%)

≥ 15% 471 (20%) 475 (20%) 480 (20%) 37 (2%) 38 (2%) 38 (2%)

≥ 20% 223 (9%) 226 (10%) 228 (10%) 20 * 20 * 20 *

* Following convention for reporting HQ estimates to one significant digit, although this query is based on 

HQ ≥ 1.5, this translates from a science policy standpoint into an HQ ≥ 2. 
57

Table 2-10. Watersheds with potentially at-risk populations based on consideration for 

risk (both IQ loss and HQ) based on U.S. EGU mercury deposition and resulting 

exposure before considering other sources of mercury deposition

EGU 

risk 

threshold

Number and percentage of 2,366 HUCs meeting risk threshold criteria*

2005 analysis 2016 analysis

90th fish 

consumption

95th% fish 

consumption

99th% fish 

consumption

90th fish 

consumption

95th% fish 

consumption

99th% fish 

consumption

IQ loss

≥ 1 pt 28 * 67 (3%) 353 (15%) 3 * 7 * 27 *

≥ 2 pts 1 * 5 * 69 (3%) 0 * 0 * 7 *

RfD-based HQ

≥ 1.5** 277 (12%) 495 (21%) 1064 (45%) 17 * 48 (2%) 284 (12%)

* 2,366 watersheds reflected in this summary are those watersheds out of the 2,461 assessed for risk for the 

high-end female consumer (at the national-level)

** Following convention for reporting HQ estimates to one significant digit, although this query is based 

on HQ ≥ 1.5, this translates from a science policy standpoint into an HQ ≥ 2. 

Table 2-11. Combination of watersheds with potentially at-risk populations based on 

either consideration for (a) U.S. EGU percent contribution to total risk OR (b) risk 

when U.S. EGU mercury deposition is considered alone

EGU risk 

threshold

Number and percentage of HUCs meeting risk threshold criteria

2005 analysis 2016 analysis

90th fish 

consumption

95th% fish 

consumption

99th% fish 

consumption

90th fish 

consumption

95th% fish 

consumption

99th% fish 

consumption

U.S. EGU-attributable risk ≥1.5* HQ OR total risk ≥1.5* HQ and U.S. EGU contribution of:

≥ 5% 1271 (54%) 1321 (56%) 1477 (62%) 484 (20%) 524 (22%) 672 (28%)

≥ 10% 879 (37%) 946 (40%) 1221 (52%) 96 (4%) 121 (5%) 325 (14%)

≥ 15% 584 (25%) 682 (29%) 1109 (47%) 49 (2%) 76 (3%) 292 (12%)

≥ 20% 391 (17%) 550 (23%) 1073 (45%) 34 * 61 (3%) 286 (12%)

* Following convention for reporting HQ estimates to one significant digit, although this query is based on 

HQ ≥ 1.5, this translates from a science policy standpoint into an HQ ≥ 2. 

Observations regarding watersheds with potentially at-risk populations due to U.S. EGU 

sourced mercury are presented below. Note, that these observations focus primarily on results 

generated for the 2016 Scenario, as explained earlier. In addition, this set of observations is 

oriented around the 3-Stage risk characterization framework.

• Less than 1% of the watersheds have an IQ loss of 1 point when deposition from U.S. 

EGUs is considered before taking into account deposition and exposures resulting from 

other sources of Hg: While total IQ loss estimates (as described earlier) can extend above 

2 IQ points for a substantial fraction of the watersheds modeled, less than 1% of the 
58

watersheds have U.S. EGU incremental IQ loss estimates extending into the 1 to 2 point 

range. However, as noted in section 1.2, IQ does not fully cover all of the neurologic 

domains such as motor skills and attention/behavior associated with MeHg exposure and 

therefore, there are concerns that risk estimates based on IQ could underestimate overall 

neurodevelopmental impacts (Axelrad et al., 2007). For this reason, in considering 

watersheds with potentially at-risk populations due to U.S. EGU-attributable mercury, we 

focus on MeHg RfD-based estimates of risk rather than IQ. Between 2 and 22% of those 

watersheds with total risk HQs ≥ 1.5 have U.S. EGUs contributing at least 5% of total 

mercury deposition: With this risk metric, we consider the degree to which U.S. EGUs 

contribute to total risk at watersheds where total risk represents a potential public health 

hazard (i.e., total risk of an HQs ≥ 1.5). Considering a 5% U.S. EGU contribution at 

watersheds where total risk is considered a potential public health hazard, we have up to 

22% of the watersheds falling into this category (with the 22% value reflecting risk 

modeled using the 99th percentile fish consumption rate for the high-end female 

consumer – see Table 2-9). It is important when considering this risk metric to reiterate 

that any exposure above the MeHg RfD represents a potential public health hazard. . 

• Between 2 and 12% of the watersheds have HQs ≥ 1.5, based on U.S. EGU mercury 

deposition before factoring in any other sources of mercury: Our analysis suggests that 

between 2 and 12% of the 2,366 watersheds modeled in the risk assessment for high-end 

female consumers could have an HQ ≥ 1.5 when U.S. EGU mercury deposition and 

resulting exposure are considered before other sources of mercury deposition. This range 

reflects the range of fish consumption rates considered for the high-end female consumer 

(i.e., 95th or 99th percentile consumption rate, respectively – see Table 2-10). 

• Combining (mathematical union) the two sets of watersheds with at-risk populations due 

to U.S. EGU mercury emissions: Combining the two categories of watersheds with 

populations at-risk due to U.S. EGU mercury emissions summarized in the last two 

bullets, we get a total estimate ranging from 2 to 28% of watersheds, with this range 

reflecting in part the U.S. EGU percent contribution that is considered (e.g., 5, 10, 15 or 

20% - see Table 2-11). Note, that this range also reflects the different fish consumption 

rates considered for the high-end female consumer (i.e., 90th, 95th and 99th percentile fish 

consumption rates). The results summarized here for total “at risk” watersheds map to 

Stage 3 of the 3-stage risk characterization framework.

2.7. Sensitivity Analyses 

This section discusses several sensitivity analyses conducted to assess the potential 

impact of key sources of uncertainty (all related to the application of the MMaps approach in our 

analysis) on risk estimates. We note that, in designing the sensitivity analysis, we focused on 

application of the MMaps approach because it represents a critical element of the analysis and is 

acknowledged as representing a potentially important source of uncertainty. The sensitivity 

analyses address two specific uncertainties related to application of the MMaps approach: (a) 

concerns over including watersheds that may be disproportionately impacted by non-air mercury 

sources and (b) application of the MMaps approach to both flowing and stationary freshwater 

bodies (if, in reality, the approach is better at predicting source-apportioning Hg fish tissue levels 

for stationary waterbodies). 
59

The sensitivity analyses addressing the first area of uncertainty (potential inclusion of 

watersheds from regions with substantial Hg contributions from non-air deposition) included two 

analyses: (a) constraining the risk analysis to only include those watersheds in the upper 25th

percentile with regards to total Hg deposition (i.e., watersheds with relatively elevated levels of 

total Hg deposition so we knew this source of loading played a larger role) and (b) excluding 

four states where we have concerns over the potential for non-air mercury playing a greater role 

(ME, MN, SC and LA).

43

The other area of uncertainty (application of the MMaps approach to both stationary and 

flowing waterbodies) was assessed by running the risk assessment only for watersheds with fish 

tissue MeHg values taken from stationary watersheds (i.e., ponds and lakes). Specifically, we 

generated a set of percentile risk estimates, again for the high-end female consumer assessed 

nationally, for the 2005 scenario (see Table 2-15). In presenting the results, we also include the 

core risk estimates generated for this scenario using fish tissue MeHg samples taken from both 

stationary and flowing waterbodies. Observations resulting from considering the results of the 

sensitivity analyses are presented at the end of this section. 

 The results of both sensitivity analyses are presented in terms of their 

impact on the “at risk” metrics described in Section 2.6.2. Specifically, the presentation of the 

sensitivity analysis results parallel the “at risk” summary table layout used in Section 2.6.2 and 

as such, present risk estimates for the high-end female consumer (2005 scenario) (see Tables 2-

12 through 2-14).

 

Table 2-12. Sensitivity analysis results based on constraining analysis to (a) watersheds in 

the top 25th percentile with regard to total Hg deposition and (b) exclude watersheds 

located in MN, LA, SE or ME) – Results for watersheds with potentially at risk 

populations based on U.S. EGUs making a specified contribution to total risk (Stage 1 of 

the 3 stage framework)

EGU risk 

threshold

Number and percentage of HUCs meeting risk threshold criteria (2005 scenario)

2005 analysis 2016 analysis

90th fish 

consumption

95th% fish 

consumption

99th% fish 

consumption

90th fish 

consumption

95th% fish 

consumption

99th% fish 

consumption

Total HQ ≥ 1.5 and U.S. EGU contribution:

ALL WATERSHEDS (2,366)

≥ 0% 2191 (93%) 2266 (96%) 2348 (99%) 2162 (91%) 2248 (95%) 2346 (99%)

≥ 5% 1268 (54%) 1305 (55%) 1345 (57%) 482 (20%) 505 (21%) 521 (22%)

≥ 10% 816 (34%) 834 (35%) 853 (36%) 91 (4%) 95 (4%) 97 (4%)

≥ 15% 471 (20%) 475 (20%) 480 (20%) 37 (2%) 38 (2%) 38 (2%)

≥ 20% 223 (9%) 226 (10%) 228 (10%) 20 * 20 * 20 *

 43 The rational for excluding the four states in this sensitivity analysis examining the MMaps approach is as follows. 

ME was excluded because Hg fish tissue levels there are fairly high, while Hg deposition is not relatively elevated 

(compared to other eastern states) – this raising the concern that some other factor may be in play (e.g., other non-air 

sources, or perhaps substantially increased methylation potential). MN was excluded for the same reason as ME 

with additional concern for taconite mining which could provide non-air Hg loading. SC was also excluded due to 

higher fish Hg levels and Hg air deposition that (while elevated in some locations) is not uniformly higher than other 

states in the region (in addition, there is a history of gold mining in SC). Finally LA was excluded as part of the 

sensitivity analysis due to concerns for the substantial industrial activity which could result in non-air Hg impacts. 
60

EGU risk 

threshold

Number and percentage of HUCs meeting risk threshold criteria (2005 scenario)

2005 analysis 2016 analysis

90th fish 

consumption

95th% fish 

consumption

99th% fish 

consumption

90th fish 

consumption

95th% fish 

consumption

99th% fish 

consumption

SENSITIVITY ANALYSIS 1 - Watersheds in top 25th % with regard to total Hg deposition (591)

≥ 0% 554 (94%) 570 (96%) 584 (99%) 548 (93%) 564 (95%) 585 (99%)

≥ 5% 323 (55%) 332 (56%) 341 (58%) 89 (15%) 90 (15%) 94 (16%)

≥ 10% 182 (31%) 185 (31%) 192 (32%) 32 (5%) 32 (5%) 33 (6%)

≥ 15% 122 (21%) 124 (21%) 126 (21%) 19 (3%) 19 (3%) 19 (3%)

≥ 20% 64 (11%) 65 (11%) 67 (11%) 10 (2%) 10 (2%) 10 (2%)

SENSITIVITY ANALYSIS 2 - Watersheds excluding those in MN, LA, SC and ME (1,844)

≥ 0% 1673 (91%) 1745 (95%) 1827 (99%) 1645 (89%) 1727 (94%) 1825 (99%)

≥ 5% 1133 (61%) 1170 (63%) 1210 (66%) 449 (24%) 472 (26%) 488 (26%)

≥ 10% 791 (43%) 809 (44%) 828 (45%) 77 (4%) 81 (4%) 83 (5%)

≥ 15% 460 (25%) 464 (25%) 469 (25%) 33 (2%) 34 (2%) 34 (2%)

≥ 20% 220 (12%) 223 (12%) 225 (12%) 19 * 19 * 19 *

Table 2-13. Sensitivity analysis results based on constraining analysis to (a) watersheds in 

the top 25th percentile with regard to total Hg deposition and (b) exclude 

watersheds located in MN, LA, SE or ME) – Results for watersheds with potentially at 

risk populations based on U.S. EGU-incremental contribution to total risk (Stage 2 of the 

3 stage framework)

EGU 

risk 

threshold

Number and percentage of HUCs meeting risk threshold criteria (2005 scenario)

2005 analysis 2016 analysis

90th fish 

consumption

95th% fish 

consumption

99th% fish 

consumption

90th fish 

consumption

95th% fish 

consumption

99th% fish 

consumption

RfD-based HQ)

ALL WATERSHEDS – core analysis (2,366)

HQ ≥ 1.5 277 (12%) 495 (21%) 1064 (45%) 17 * 48 (2%) 284 (12%)

SENSITIVITY ANALYSIS 1 - Watersheds in top 25th % with regard to total Hg deposition (591)

HQ ≥ 1.5 90 (15%) 140 (24%) 274 (46%) 8 * 25 (4%) 90 (15%)

SENSITIVITY ANALYSIS 2 - Watersheds excluding those in MN, LA, SC and ME (1,844)

HQ ≥ 1.5 225 (10%) 423 (18%) 879 (37%) 5 * 17 * 167 (7%)

Table 2-14. Sensitivity analysis results based on constraining analysis to (a) watersheds in 

the top 25th percentile with regard to total Hg deposition and (b) exclude watersheds 

located in MN, LA, SE or ME) – Results for watersheds with potentially at risk 

populations based on combining both Stage 1 and Stage 2 results (Stage 3 of the 3 stage 

framework)

EGU risk 

threshold

Number and percentage of HUCs meeting risk threshold criteria (2005 scenario)

2005 analysis 2016 analysis

90th fish 

consumption

95th% fish 

consumption

99th% fish 

consumption

90th fish 

consumption

95th% fish 

consumption

99th% fish 

consumption

U.S. EGU-attributable risk ≥1.5 HQ OR total risk ≥1.5 HQ and U.S. EGU contribution of:

ALL WATERSHEDS (2,366)
61

EGU risk 

threshold

Number and percentage of HUCs meeting risk threshold criteria (2005 scenario)

2005 analysis 2016 analysis

90th fish 

consumption

95th% fish 

consumption

99th% fish 

consumption

90th fish 

consumption

95th% fish 

consumption

99th% fish 

consumption

≥ 5% 1271 (54%) 1321 (56%) 1477 (62%) 484 (20%) 524 (22%) 672 (28%)

≥ 10% 879 (37%) 946 (40%) 1221 (52%) 96 (4%) 121 (5%) 325 (14%)

≥ 15% 584 (25%) 682 (29%) 1109 (47%) 49 (2%) 76 (3%) 292 (12%)

≥ 20% 391 (17%) 550 (23%) 1073 (45%) 34 * 61 (3%) 286 (12%)

SENSITIVITY ANALYSIS 1 - Watersheds in top 25th % with regard to total Hg deposition (591)

≥ 5% 323 (55%) 334 (56%) 377 (64%) 89 (15%) 99 (17%) 146 (25%)

≥ 10% 202 (34%) 219 (37%) 306 (52%) 34 (6%) 44 (7%) 97 (16%)

≥ 15% 152 (26%) 179 (30%) 286 (48%) 24 (4%) 37 (6%) 92 (16%)

≥ 20% 115 (19%) 153 (26%) 278 (47%) 16 (3%) 30 (5%) 90 (15%)

SENSITIVITY ANALYSIS 2 - Watersheds excluding those in MN, LA, SC and ME (1,844)

≥ 5% 1135 (62%) 1179 (64%) 1275 (69%) 449 (24%) 476 (26%) 543 (29%)

≥ 10% 811 (44%) 860 (47%) 1035 (56%) 77 (4%) 87 (5%) 208 (11%)

≥ 15% 529 (29%) 609 (33%) 923 (50%) 35 (2%) 44 (2%) 175 (9%)

≥ 20% 339 (18%) 478 (26%) 888 (48%) 22 * 30 (2%) 169 (9%)

Table 2-15. Sensitivity analysis results based on estimating risk for watersheds with Hg 

fish tissue levels based only on stationary waterbodies (i.e., excluding samples taken 

from flowing waterbodies) – Results for both total risk and U.S. EGU-attributable risk

Fisher 

consumption 

rate percentile 

and rate (g/day)

Watershed percentile

50th percentile 75th percentile 90th percentile 95th percentile 99th percentile

total

U.S. 

EGU total

U.S.

EGU total

U.S. 

EGU total

U.S. 

EGU total

U.S. 

EGU

Primary analysis (all watersheds – lakes and rivers - 2005 scenario)

mean (39) 3.5 0.3 3.2 0.4 4.2 0.5 5.4 0.7 7.2 1.4

90th (123) 11.1 1.1 10.1 1.3 13.3 1.6 17.1 2.3 22.8 4.3

95th (173) 15.6 1.5 14.2 1.8 18.7 2.3 24.1 3.2 32.0 6.0

99th (373) 33.6 3.3 30.7 3.9 40.3 4.9 51.9 6.9 69.0 13.0

SA (lakes only – 2005 scenario)

mean (39) 2.9 0.1 3.5 0.3 3.8 0.4 4.6 0.6 6.5 1.4

90th (123) 9.1 0.3 11.0 0.8 12.1 1.3 14.5 2.0 20.5 4.4

95th (173) 12.8 0.5 15.5 1.1 17.0 1.8 20.4 2.7 28.8 6.2

99th (373) 27.6 1.0 33.5 2.4 36.6 4.0 44.0 5.9 62.1 13.5

Observations regarding the sensitivity analyses completed for this analysis are presented 

below.

• Generating risk estimates including only those watersheds falling in the top 25th

percentile with regard to total Hg deposition: The percent watersheds with potentially at 

risk populations ranged from just slightly higher to as much as 50% higher compared

with the core analysis which did not exclude watersheds based on the magnitude or 
62

ranking with regard to total Hg deposition (see Tables 2-12 through 2-14). These results 

suggest that focusing on those watersheds with relatively greater total Hg deposition 

would result in a slightly larger fraction of “at risk” watersheds. 

• Generating risk estimates excluding watersheds located in four states (AL, SC, ME and 

MN): This sensitivity analysis resulted in different effects on the Stage 1 and Stage 2 risk 

estimates. Estimates of the number of watersheds with at risk populations due to U.S. 

EGUs contributing at least 5% (note, other percentage contributions are also presented) to 

total risk (Stage 1) demonstrated a mixture of moderate increases and decreases 

compared with the core analysis, depending on the “percent contribution” category 

considered (see Table 2-12). However, estimates of the percentage of watersheds with at 

risk populations when considering U.S. EGU mercury deposition (without other sources 

– Stage 2) based on dropping out the four states was 7% as compared with 12% when 

considering all states (for the 99th percentile fish consumption rate - see Table 2-13). 

While this sensitivity analysis does suggest that excluding these 4 states does reduce the 

percent of watersheds with potentially at risk populations included in Stage 2, there is 

still a notable fraction (i.e., 7% under the 99th% consumption rate based on an HQ ≥ 1.5

representing a potential health concern – see Table 2-14).

• Focusing only on stationary waterbodies (lakes and ponds) and excluding flowing 

waterbodies did result in notably lower U.S. EGU-incremental risk on average for the 

waterbodies, however risk estimates for upper end watersheds were not substantially 

effected: As can be seen in Table 2-15, running the risk model using only fish tissue 

MeHg estimates from stationary waterbodies did result in lower U.S. EGU incremental 

risk for the average watershed. However, importantly, high-end estimates - estimates for 

the 90th percentile watershed and above - of U.S. EGU incremental risk were not 

significantly different with that difference disappearing and actually reversing at the 

highest level (i.e., the risk estimates for the 99th percentile watershed actually tended to 

be higher for the simulation focusing on stationary waterbodies). Given that the primary 

focus of the risk assessment is on high-end percentile risk, the fact that this sensitivity 

analysis only showed difference in risk at mean watersheds with this difference 

diminishing and actually reversing at the highest watersheds, argues that this source of 

uncertainty (i.e., that the MMaps approach is most appropriate for stationary waterbodies) 

does not substantially affect the analysis. 

2.8. Summary of Key Observations

Key policy-relevant observations drawn from discussions presented in Sections 2.3 

through 2.7 (and including Section 1.4 – the discussion of uncertainty and variability) are 

presented below: 

• Estimates of U.S. EGU mercury emissions suggest that the 2016 Scenario is likely closer

to current (2010) emissions compared with the 2005 Base Case (which has substantially 

higher total mercury emissions for this sector). Therefore, risk estimates generated for the
63

2016 Scenario have received greater emphasis and the 2005 Base Case estimates are deemphasized (since they are likely substantially higher than current conditions).

• U.S. EGUs can contribute up to 11% of total Hg emissions over a subset of watersheds 

(for the 2016 Scenario - value cited is for the 99th% watershed). However, in general, 

other sources besides U.S. EGUs dominate Hg deposition, with U.S. EGUs contributing 

on average (again for the 2016 Base Case), about 2% of total Hg deposition across the 

country. U.S. EGU-related Hg deposition is higher in the eastern part of the country with 

specific hot spots in a number of areas, including most notably, the Ohio River valley. 

U.S. EGU-related Hg deposition estimates show a significant reduction between 2005 

and 2016 Base Cases, reflecting mainly implementation of PM controls with the average 

U.S. EGU deposition decreasing from ~5% of total to ~2% for the 2005 and 2016

scenarios, respectively. 

• U.S. EGUs can contribute up to 18% of MeHg in fish tissue (99th percentile watershed 

value for the 2016 Scenario). However, generally, U.S. EGUs contribute a much smaller 

fraction averaging 4% for the 2016 Scenario.

• Comparing the magnitude of Hg fish tissue levels with total Hg deposition (as 

characterized at the watershed-level) suggests that there is not a strong correlation. This is 

not surprising given the variety of factors which effect methylation potential; factors 

which can demonstrate substantial spatial variation. 

• Comparing the pattern of U.S. EGU-attributable Hg deposition with watersheds 

containing Hg fish tissue data (i.e., the watersheds reflected in the risk assessment) results 

in our concluding that, while we have some degree of coverage for high U.S. EGU

impact areas, this coverage is limited. For this reason, we believe that the actual number 

of “at risk” watersheds (i.e., watersheds where U.S. EGUs could contribute to a public 

health concern) could be substantially larger than estimated. 

• Estimates of total risk from all sources of mercury using the RfD based metrics do not 

show a substantial reduction between the 2005 and 2016 Scenarios, which is expected 

given that sources other than U.S. EGUs dominate Hg deposition over the vast majority 

of watersheds and emissions for these sources remain largely unchanged in the 

simulation. However, U.S. EGU-attributable risk does demonstrate a notable reduction 

between the 2005 and 2016 Scenarios, primarily reflecting implementation of PM 

precursor emissions controls, as note earlier. 

• Under the 2016 Base Case, IQ loss when we consider U.S. EGU mercury deposition 

without including other sources of mercury is below the 1 to 2 IQ point range for over 

99% of the watersheds included in the risk assessment (based on the high-end female 

consumer scenario). The 1-2 IQ point loss range was identified by the SAB as 

representing a level of clear public health significance, not that IQ loss below that is of no 

health significance. It is important to note, that for a substantial fraction of these 

watersheds, total IQ loss (reflecting all mercury sources) does reach or exceed this range 

of IQ loss. And in those instances, even a relatively small incremental reduction in IQ 

loss related to reducing U.S. EGU emissions would be considered beneficial from a 
64

public health standpoint. In addition, as noted in Section 1.2, we do not think the IQ 

endpoint fully captures the neurodevelopmental risk associated with MeHg exposure and 

for this reason, we have focused on MeHg RfD-based risk estimates in considering the 

potential public health hazard associated with U.S. EGU-attributable mercury exposure. 

Risks estimated for the high-end female consumer population at the national-level are 

generally higher than those assessed for a number of the other populations covered (e.g, 

Vietnamese, Hispanic, and Great Lakes Tribal fishers) and therefore provide coverage for 

those additional fisher populations. However, risk estimates for both white and black 

fishers in the southeast were higher than estimates for the high-end female consumers

assessed nationally. It is also important to note the uncertainty and limitations in the data 

used for the southeast fisher populations (see Section 2.6.1). Given our desire to provide 

broader coverage for the U.S. and concerns over Hg fish tissue levels in South Carolina 

(SC) potentially reflecting non-U.S. EGU loading (fish tissue values in SC play an 

important role in driving risk for the two southeastern fishing populations), we have 

placed greater emphasis in discussing risk estimates on the female high-end consumers

assessed at the national-level.44

• Based on application of the 3-stage risk characterization framework described in Section

1.2, we estimate that from 3 to 28% of the watersheds included in this risk assessment 

could be classified as potentially having at risk populations under the 2016 Scenario. This 

percent range is based on a 95th to 99th percentile consumption rates for the high-end 

female consumer assessed at the national-level and assumes further, that an HQ of ≥ 1.5

(i.e., an exposure just above the RfD – see above) represents a potential public health 

concern. These stage-3 results reflect the aggregation of results from Stages 1 and 2 of 

the 3-Stage Risk Characterization Framework (i.e., watersheds where the U.S. EGU

increment-based HQ ≥ 1.5 and watersheds where total risk is ≥ 1.5 HQ and U.S. EGUs 

make at least at 5% contribution to that risk, respectively).

• If U.S. EGU impacts to watersheds included in the risk assessment were zeroed-out, for a 

significant majority of those watersheds, total exposure would still exceed (and in most 

cases, significantly exceed) the RfD. Reductions in EGU attributable Hg will reduce the 

magnitude of the risk, although substantial total exposure and risk from Hg deposition 

will remain.

• Sensitivity analyses conducted primarily to examine uncertainty in applying the MMaps 

approach for linking Hg deposition to Hg fish tissue levels, suggest that uncertainty 

related to the MMaps approach is unlikely to substantially effect an assessment of 

whether Hg emissions from U.S. EGUs constitute a public health concern. 

 44 We note that the female high-end consumer population benefits from (a) being assessed at the national-level 

(which both increases the number of watersheds assessed for risk and more fully reflects spatial patterns of U.S. 

EGU-attributable mercury impacts and fish tissue MeHg levels across the country) and (b) has consumption rates 

based on a larger sample size compared with the more focused southeastern black and white populations (see 

Appendix C, Table C-1). For these reasons, we believe that risk estimates for the female high-end consumer 

population have higher overall confidence (in addition, they better cover on the population of concern for the 

endpoints modeled – women of childbearing age who consume relatively large amounts of self-caught fish).
65
66

Citations:

Appel, K. W., A. B. Gilliland, et al. (2007). "Evaluation of the Community Multiscale Air 

Quality (CMAQ) model version 4.5: Sensitivities impacting model performance Part I -

Ozone." Atmospheric Environment 41(40): 9603-9615.

Appel, K. W., P. V. Bhave, et al. (2008). "Evaluation of the community multiscale air quality 

(CMAQ) model version 4.5: Sensitivities impacting model performance; Part II -

particulate matter." Atmospheric Environment 42(24): 6057-6066.

Axelrad, D. A.; Bellinger, D. C.; Ryan, L. M.; Woodruff, T. J. (2007). Dose-response 

relationship of prenatal mercury exposure and IQ: an integrative analysis of 

epidemiologic data. Environmental Health Perspectives. 2007, 115, 609–615.

Bullock, O. R. and K. A. Brehme (2002). "Atmospheric mercury simulation using the CMAQ 

model: formulation description and analysis of wet deposition results." Atmospheric 

Environment 36(13): 2135-2146.

Burger, J. (2002). Daily consumption of wild fish and game: Exposures of high end 

recreationalists, International Journal of Environmental Health Research, 12:4, p. 343-

354, July, 2010.

Burger, J., Stephens, W. L., Boring, C. S., Kuklinski, M., Gibbons, J. W., Gochfeld M. Factors in 

Exposure Assessment: Ethnic and Socioeconomic Differences in Fishing and 

Soncumption of Fish Caught along the Savannah River. Risk Analysis, Vol. 19, No. 3, 

1999, p. 427.

Byun, D. and K. L. Schere (2005). "Review of the governing equations, computational 

algorithms, and other components of the models-3 Community Multiscale Air Quality 

(CMAQ) modeling system." Applied Mechanics Reviews 59(1-6): 51-77.

Harris., R. C., John W. M. Rudd, Marc Amyot, Christopher L. Babiarz, Ken G. Beaty, Paul J. 

Blanchfield, R. A. Bodaly, Brian A. Branfireun, Cynthia C. Gilmour, Jennifer A. 

Graydon, Andrew Heyes, Holger Hintelmann, James P. Hurley, Carol A. Kelly, David P. 

Krabbenhoft, Steve E. Lindberg, Robert P. Mason, Michael J. Paterson, Cheryl L. 

Podemski, Art Robinson, Ken A. Sandilands, George R. Southworth, Vincent L. St. 

Louis, and Michael T. TateRudd, J. W. M., Amyot M., et al., Whole-Ecosystem study 

Shows Rapid Fish-Mercury Response to Changes in Mercury Deposition. Proceedings of 

the National Academy of Sciences Early Edition, PNAS 2007 104 (42) pp. 16586-16591; 

(published ahead of print September 27, 2007).

Knights, D. C, Sunderland, E. M., Barber, M. C., Johnston J. M., and Ambrose, R. B., 

Application of Ecosystem-Scale Fate and Bioaccumulation Models to Predict Fish 

Mercury Response Times to Changes in Atmospheric Deposition. Environmental 

Toxicology and Chemistry, Vol 28, No. 4., pp. 881-893.

Morgan, J.N., M.R. Berry, and R.L. Graves. 1997. “Effects of Commonly Used Cooking 

Practices on Total Mercury Concentration in Fish and Their Impact on Exposure 
67

Assessments.” Journal of Exposure Analysis and Environmental Epidemiology 7(1):119-

133.

Moya. J., Itkiin, C., Selevan, S.G., Rogers, J.W., Clickner, R. P., Estimates of fish consumption 

rates for consumers of bought and self-caught fish in Connecticut, Florida, Minnesota, 

and North Dacota. Science of the Total Environment (2008), 

doi:10.1016/j.scitotenv.2008.05.023.

NMFS, 2003a. National Marine Fisheries Service, Fisheries Statistics Division, National Oceanic 

and Atmospheric Administration. Statistical Highlights: Fisheries of the United States, 

2003, 2003.

Oken e., Radesky, J.S., Wright, R. O., Bellinger D. C., Amarasiriwardena C. UJ., Kleinman, K. 

P., Hu, H., Gillman, M.W., Maternal Fish Intake during Pregnancy, Blood Mercury 

Levels, and Child Cognition at Age 3 Years in a US Cohort. American Journal of 

Epmidemiology, Vol. 167, No. 10, p. 1171.

Pirrone N, Cinnirella S, Feng X, Finkelman RB, Friedli HR, Leaner J, et al., 2010. Global 

mercury emissions to the atmosphere from anthropogenic and natural sources. 

Atmospheric Chemistry and Physics Discussions, 10(2), 4719-4752.

Rice GE, Hammitt JK, Evans JS. (2010). A probabilistic characterization of the health benefits of 

reducing methyl mercury intake in the United States. Environmental Science Technology. 

2010 Jul 1;44(13):5216-24

Shilling, Fraser, Aubrey White, Lucas Lippert, Mark Lubell (2010). Contaminated fish 

consumption in California’s Central Valley Delta. Environmental Research 110, p. 334-

344.

Swartout, J., and G. Rice. 2000. “Uncertainty Analysis of the Estimated Ingestion Rates Used to 

Derive the Methylmercury Reference Dose.” Drug and Chemical Toxicology 23(1):293-

306. 11-41

UNEP’s Division of Technology, Industry and Economics. 2010. Study on mercury sources and 

emissions and analysis of cost and effectiveness of control measures “UNEP Paragraph 

29 study”.

USEPA, U.S. Environmental Protection Agency. (2000). Guidance for Assessing Chemical 

Contaminant Data for Use in Fish Advisories, Volume 3: Overview of Risk Management. 

Office of Science and Technology, Office of Water, U.S. Environmental Protection 

Agency, Washington, DC. EPA 823-B-00-007.

USEPA, U.S. Environmental Protection Agency, (2001). Mercury Maps - A Quantitative Spatial 

Link Between Air Deposition and Fish Tissue Peer Reviewed Final Report. U.S. EPA, 

Office of Water, EPA-823-R-01-009, September, 2001.
68

USEPA, U.S. Environmental Protection Agency, (2007). Review of the National Ambient Air 

Quality Standards for Lead: Policy Assessment of Scientific and Technical Information 

OAQPS Staff Paper. Office of Air Quality Planning and Standards, Research Triangle 

Park, NC. EPA-452/R-07-013 November 2007.

USEPA, U.S. Environmental Protection Agency. (2005), Neurobehavioral Assessments 

Conducted in the New Zealand, Faroe Islands, and Seychelles Islands Studies of 

Methylmercury Neurotoxicity in Children. David C. Bellinger, Professor of Neurology, 

Harvard Medical School, March 2005. Report to the U.S. Environmental Protection 

Agency EPA-HQ-OAR-2002-0056-6045[1]

USEPA, U.S. Environmental Protection Agency. (2007a) Review of the National Ambient Air 

Quality Standards for Lead: Policy Assessment of Scientific and Technical Information 

OAQPS Staff Paper. Office of Air Quality Planning and Standards, Research Triangle 

Park, NC. EPA-452/R-07-013 November 2007.

USEPA, U.S. Environmental Protection Agency. (2007b) Lead: Human Exposure and Health 

Risk Assessments for Selected Case Studies, Volume I. Human Exposure and Health 

Risk Assessments—Full-Scale and Volume II. Appendices. Office of Air Quality 

Planning and Standards, Research Triangle Park, NC. EPA–452/R–07–014a and EPA–

452/R– 07–014b.

USEPA, U.S. Environmental Protection Agency. (2011) Regulatory Impact Assessment: 

Benefits Associated with Reduced Mercury Air Emissions in the U.S. Utility Sector. 

Office of Air Quality Planning and Standards, Research Triangle Park, NC.

U.S. Geological Survey and U.S. Department of Agriculture, Natural Resources Conservation 

Service, 2009, Federal guidelines, requirements, and procedures for the national 

Watershed Boundary Dataset: U.S. Geological Survey Techniques and Methods 11–A3, 

55 p.
69

Appendices: Additional Technical Detail on Modeling Elements

(Note, citations for the appendices are provided in the citation list for the main document –

see above)

A. Specifying spatial scale of watersheds

As mentioned earlier in Section 1.3, the specification of the spatial scale for watersheds 

to be used as the basis for the risk assessment (i.e., HUC12’s) was based, in part, on 

consideration for the size of watersheds included in two studies examining the relationship 

between mercury deposition and changes in mercury concentrations in aquatic media and biota. 

The Knights et al., 2009 study considered a number of modeling frameworks for predicting 

changes in fish mercury levels, following changes in aerial mercury deposition. The study 

included simulation of five different types of waterbodies ranging from a seepage lake (with little 

watershed loading) in Florida to a stratified drainage lake in NH. Response times for changes in 

mercury fish tissue levels following a 50% reduction in aerial mercury deposition were simulated 

for the different watersheds. The simulations showed that all five locations had a two-phase 

response in fish tissue mercury concentrations including (a) an initial 1-3 year response linked to 

immediate reductions in aerial deposition directly to the waterbody itself and (b) a longer-term 

(decades) timeframe for a full system response that would include such factors as changes in 

watershed erosion and loading to the waterbody. The study also showed that deeper lakes with 

larger watersheds could have longer response times. The article reported that the initial “faster” 

response (taking 1-3 years) could account for 40-60% of the total steady state response in some 

instances. 

The results of this study can be interpreted as suggesting that, if we are considering the 

more immediate change in fish tissue mercury levels (occurring within a few years following a 

change in mercury loading), we should focus on assessing the fractional change in mercury 

deposition to the waterbody itself. However, if we can assume that near steady-state conditions 

are met and we are interested in a more complete simulation of changes in fish MeHg levels then 

we would want to consider changes in the level of mercury deposition over the entire watershed 

(and not just the waterbody). These observations support using the watershed (and not the 

individual waterbody) as the basis for linking Hg deposition with fish tissue MeHg levels. The 

scale of the five watersheds included in the Knights et al., 2009 study range from 20 by 100 km 

(for the coastal plain river location in GA) to 5 by 10km (for the Lake Waccamaw NC site). 

Three of the five locations had watersheds in the 10 by 10km range (see Figure 2 in the article). 

Given that the majority of locations in the study had smaller watersheds (i.e., in the 10 by 10km 

range), we conclude that this would represent a reasonable watershed spatial scale to use in 

linking changes in aerial deposition to changes in fish tissue levels (i.e., as the basis for risk 

characterization in the analysis).

An article by Harris et al., 2007, which is based on the METALLICUS study (specifically 

lake 658 catchment in northwestern Ontario Canada), also examined the temporal profile 

associated with changes in media and biota mercury levels following a change in mercury 

deposition. In this study, a 3yr loading of labeled mercury to the waterbody and watershed 

(separate labeled mercury applied to each location) was followed by measurement of mercury in 
70

various media and biota to see how long it took for the loaded mercury to impact different 

compartments. This analysis showed that impacts on aquatic biota from watershed mercury 

could be very slow (on the order of decades). While mercury deposited directly to the waterbody 

could have a more immediate effect on aquatic biota, this effect was buffered by the methylation 

of older mercury which resulted in a lag in the impact (after 3-4 years, there had been a 30-40% 

increase in biota mercury concentrations, however the actual change in deposition to the lake 

over that same time period had been ~120%, suggesting that this buffering was occurring). The 

study concludes that lakes which receive most of their mercury deposition loading directly from 

the atmosphere (e.g., catchment lakes) could see an effect on biota within a decade while lakes 

with more complex watershed loading conditions could see a two-phase temporal change with a 

longer lag. The single watershed involved in this study is relatively small (only a few km on a 

side). Therefore, the spatial scale of the watershed involved in this study also support use of a 

more refined spatial scale for watersheds in the risk assessment. 

In addition as mentioned earlier in Section 1.3, use of watersheds that are more spatially 

refined increases the potential for capturing spatial gradients in the deposition of mercury and 

resulting variation in the impact of those loadings on aquatic freshwater biota. Conversely, use of 

larger watersheds, while allowing us to model more of the country in the risk assessment, could 

result in the dilution of areas of elevated mercury deposition from U.S. EGUs (and therefore, by 

association, the dilution of U.S. EGU-attributable mercury exposure and risk). 

Consideration for the information presented above, resulted in us identifying HUC12s as 

the optimal watershed size for the risk analysis. HUC12’s vary based on topography but 

generally are from 5-10km on a side. 

B. Characterizing measured fish tissue concentrations at the watershed-level

In developing the fish tissue dataset for the risk assessment, we began with the master 

dataset that has been developed by our team to support the Regulatory Impact Analysis (RIA). 

This master datasets included fish tissue Hg samples from years 1995-2009, with approximately 

50,000 unique samples from 4,115 HUC12s across the US, although the samples are more 

heavily focused on locations east of the Mississippi. Note, that for the risk assessment, we used a 

subset of the data from 2000 and later, with samples distributed across 2,461 HUC12s. The use 

of fish tissue samples from this later time frame was intended to focus on samples more 

representative of current conditions and less likely to reflect Hg deposition levels prior to 2000 

when substantial reductions in Hg emissions and hence deposition were taking place. The fish 

tissue samples in the master dataset come primarily from three sources: 

• National Listing of Fish Advisory (NLFA) database. The NLFA, managed by EPA 

(http://water.epa.gov/scitech/swguidance/fishshellfish/fishadvisories/), collects and 

compiles fish tissue sample data from all 50 states and from tribes across the United 

States. In particular, it contains data for over 43,000 mercury fish tissue samples collected 

from 1995 to 2007. 
71

• U.S. Geologic Survey (USGS) compilation of mercury datasets. As part of its 

Environmental Mercury Mapping and Analysis (EMMA) program, USGS compiled 

mercury fish tissue sample data from a wide variety of sources (including the NLFA) and 

has posted these data at http://emmma.usgs.gov/datasets.aspx. The compilation includes 

(1) state-agency collected and reported data (including Delaware, Iowa, Indiana, 

Louisiana, Minnesota, Ohio, South Carolina, Virginia, Wisconsin, and West Virginia) 

from over 40,000 fish tissue samples, covering the period 1995 to 2007 and (2) over 

10,000 fish tissue samples from several other sources, including the National Fish Tissue 

Survey, the National Pesticide Monitoring Program (NPMP), the National Contaminant 

Biomonitoring Program (NCBP), the Biomonitoring of Environmental Status and Trends 

(BEST) datasets of the USFWS and USGS (http://www.cerc.cr.usgs.gov/data/data.htm), 

and the Environmental Monitoring and Analysis Program (EMAP) 

(http://www.epa.gov/emap/). 

• EPA’s National River and Stream Assessment (NRSA) study data. These data include 

nearly 600 fish tissue mercury samples collected at randomly selected freshwater sites 

across the United States during the period 2008 to 2009.

Data from these three datasets were combined into a single master fish tissue dataset 

covering the period 1995 to 2009. One problem encountered in combining these datasets is the 

potential duplication of samples in the NLFA and USGS state-collected data. Unfortunately, 

these two datasets do not contain directly comparable and unique identifiers that allow duplicate 

samples to be easily identified and removed. Therefore, as an alternative, the samples from these 

two datasets were subdivided into data groups according to the year and state in which they were 

collected. If both datasets contained a data group for the same year and the same state, then the 

data group with the fewer number of observations was excluded from the master data. 

In finalizing the master datasets a number of criteria were used to screen the fish tissue 

samples (e.g., include only freshwater fish species, exclude estuarine locations, exclude fish less 

than 7 inches in length). In addition, we assigned “river” and “lake” identifiers to each fish tissue 

sample (additional detail on the process used to develop the master fish tissue dataset can be 

found in USEPA, 2011, Section 5.2.2). 

As note earlier, for the risk assessment we used a subset of the Master dataset including 

fish tissue samples collected 2000 and later. This risk assessment dataset comprised a total of 

23,878 samples (12,500 from lakes and 11,478 from rivers) covering a total of 2,461 HUC12s. 

Subsets of this dataset were used in modeling risk for each of the fisher populations included in 

the analysis (i.e., those HUCs intersecting US Census tracts containing “source populations” 

associated with a particular fisher scenario – see discussion in Section 1.3).

Each watershed with measured Hg fish tissue data, tended to have multiple values (the 

average number of fish tissue measurements for the period 2000 to 2009 for the 2,461 

watersheds is 10, although some watersheds had up to 270 measurements). Therefore, we also 

needed to identify a summary statistic to use for each watershed to represent fish tissue levels in 

estimating exposure and risk. As noted earlier in Section 1.3, we selected the 75th percentile fish 

tissue value at each watershed as the basis for exposure and risk characterization. Selection of 

the 75th percentile value was based on the assumption that subsistence fishers would favor larger 
72

fish which have the potential for higher bioaccumulation (i.e., use of a median or mean value 

could low-bias likely catch-related mercury levels). There is uncertainty associated with this 

assumption and should fishers at a particular watershed favor fish that are either larger or smaller 

than the type of fish reflected in the 75th% sample, then risk estimates could be biased 

accordingly. In deriving the 75th percentile value for a given watershed, we first generated a set 

of “river” and “lake” fish tissue percentiles (e.g., a “lake” 75th percentile representing the 75th

percentile fish tissue Hg level among lake samples located within that HUC12). We then 

identified the higher of the lake-75th percentile and river-75th percentile, if both existed, and 

used that higher value in modeling risk. 

C. Defining subsistence fisher scenarios to model

A number of criteria had to be met for a study to be used in providing explicit 

consumption rates for the high-end fisher populations of interest in this analysis. For example, 

studies had to provide estimates of self-caught fish consumption and not conflate these estimates 

with consumption of commercially purchased fish. Furthermore, these studies had to focus on 

freshwater fishing activity, or at least have the potential to reflect significant contributions from 

that category, such that fish consumption rates provided in a study could be reasonably applied in 

assessing freshwater fishing activity. As noted earlier, given our interest in higher-end 

consumption rates, the studies also had to either provide upper percentile estimates, or support 

the derivation of those estimates (e.g., provide medians and a standard deviations). Studies of 

activity at specific waterbodies (e.g., creel surveys), while informative in supporting the presence 

of higher-end consumption rates, could not be used as the basis for defining our high-end 

consumption rates since there would be uncertainty in extrapolating activity at a specific river or 

lake more broadly to fishing populations in a region. Therefore, we focused on studies 

characterizing fishing activity more broadly for specific regions or states. Application of these 

criteria resulted in the selection of three studies as the basis for characterizing high-end fish 

consumption rates for the fisher populations included in the analysis. These studies, together 

with the fisher populations characterized and a description of the regional coverage assumed for 

each fisher population (see below) are presented in Table C-1.

As noted earlier in Section 1.3, with the exception of the Tribal fisher population assessed 

for the Great Lakes (which was restricted to activity on lands ceded to the Ojibwa or Chippewa), 

we did extend coverage for the other fisher populations beyond the specific areas covered in the 

surveys.45

 45 The decision to restrict activity for the Tribal fishers to the ceded territories reflects the fact that fish consumption 

rates are particular to Tribal practices and can vary considerably across Tribes, arguing against extrapolation of fish 

consumption rates across Tribes.

For example, while the Vietnamese and Laotian survey data were collected in 

California, given the ethnic/cultural nature of these high fish consumption rates, we assumed that 

this type of high-end fish consumption behavior could be associated with members of these 

ethnic groups living elsewhere in the U.S. Therefore, the high-end consumption rates referenced 

in the California study for these ethnic groups were used to model risk at watersheds elsewhere 

in the U.S. In deciding which of the 2,461 watersheds included in the risk assessment might be 

subject to fishing activity by a given fisher population, we used U.S. Census data to determine if 
73

a “source population” for that fisher population was located in the tract(s) intersecting each 

watershed. For example we modeled the Vietnamese high-end fisher population only at those

watersheds associated with U.S. Census tracts containing at least 25 Vietnamese (i.e., a “source 

population” for that fisher population). This approach was similarly done for each of the other 

fisher populations.46

Looking beyond these specific ethnic groups, we also need to establish a more 

generalized high-end (subsistence) scenario that could be assessed broadly across the 2,461 

watersheds. Generally all of the studies identified high-end percentile consumption rates (90th to 

99th percentiles for the populations surveyed) ranging from approximately one fish meal every 

few days to a fish meal a day (i.e., 120 g/day to greater than 500 g/day fish consumption). We 

used this general trend across the studies to support application of a generalized high-end female 

consumer scenario across most of the 2,461 watersheds.47

While we believe that the approach of extending coverage for these fisher populations 

beyond the regions reflected in the underlying surveys is reasonable, we do acknowledge 

uncertainty associated with this extrapolation (see Section 1.4 and Appendix F for a discussion 

of key uncertainties in the analysis). 

In addition to the studies cited in Table C-1 used to define the high-end fish consuming 

populations modeled in the risk assessment, we also reviewed a large number of additional 

studies characterizing higher-level self-caught fish consumption in the U.S. While these studies 

had limitations that prevented their use as the basis for defining high-end fisher scenarios to 

include in the analysis, they do generally support the levels of self-caught fish consumption 

modeled in the analysis. Several of these “supporting” studies are briefly described below:

• A study by Burger et al., 1999, examining recreational and subsistence fishing activity 

along the Savannah river in Georgia to determine the role played by socio-economic 

status (SES) factors (including race, education and income) and determining levels of 

self-caught fish consumption in this study area. The study suggested that all three factors 

are associated with levels of fishing activity. Specifically, in the case of race, the study 

showed that Blacks tend to have much higher rates of fish consumption than whites. 

However for both groups, the study did suggest that upper end percentile consumption 

rates could be high and certainly approach subsistence levels. For example, a ~200 g/day 

fish consumption rate represented the 98th percentile for Whites, but only the 92nd

percentile for Blacks. This study does support the presence of high-end consumption 

rates for both Blacks and Whites that approach or meet subsistence levels in this area of 

the country. 

 46 In the case of black and white fisher populations in the southeast, we further assumed that “source populations” 

for each of these fisher groups would comprise at least 25 members (of that race) below the poverty line at the tract 

level. This reflected the potential for greater subsistence fishing activity among economically disadvantaged 

individuals. 

47 Similar to the white and black fisher populations in the southeast, we only applied this fisher populations at 

watersheds located in tracts with at least 25 individuals below the poverty line, which meant that 2,366 of the 2,461 

watersheds with Hg fish tissue data were assessed for risk for this more generalized female high-consumer 

population. 
74

• A study by Moya et al., 2008 examined factors associated with regional differences in 

patterns of fish consumption, including age, ethnicity (including Tribal affiliation), 

socioeconomic status (e.g., income, education), and type/source of fish consumed 

(freshwater, marine, and estuarine obtained from commercial sources versus self-caught). 

The study examined fishing activity in four states (CT, FL, MN, and ND). The study did 

provide estimates of high-end self-caught fish consumption for populations in the four 

states. Higher, subsistence-level consumption rates were identified for fishing 

populations in FL, MN (specifically for Tribes) and CT (for Asian populations, although 

it is not clear whether the rates for Asians hold for self-caught fish consumption in 

particular). Higher-end rates reported for ND and for general fishers in CT and MN did 

not approach the range of subsistence levels of consumption. However, we would point 

out that the study designs used in these surveys may not effectively capture the relatively 

small fraction of the overall population likely engaging in high-end subsistence levels of 

self-caught fishing behavior. This study does provide added support for the existence of 

subsistence fishing populations, at least within FL and for Tribes within MN. However, 

failure of the surveys reviewed in the study to capture similar behavior in ND and CT 

does not necessarily suggest that this type of behavior is non-existent, although it may 

suggest that it is less prevalent than in FL.
75

Table C-1. Fisher Populations Included in the Analysis for Hg Exposure and Risk

Fish consuming populations 

covered by study (and 

reference information) Overview of study

Self-caught fish consumption rates 

(mean, 90th, 95th, 99th) g/day

Observations from the study relevant to the 

risk assessment

Extrapolation of study populations in 

the risk assessment

Higher self-caught fish 

consuming populations 

(white, black and female) 

surveyed in South Carolina 

Citation: Daily consumption 

of wild fish and game: 

Exposures of high end 

recreationalists, Burger et al., 

International Journal of 

Environmental Health 

Research, 12:4, p. 343-354, 

July, 2002

Random survey of 

participants in the 

Palmetto Sportsmen’s 

Classic in Columbia SC 

(1998). Population 

interested in 

fishing/hunting (not 

general population –

represents outdoor 

enthusiasts in SC)

- black: 171, 446, 557, NC * 

- white: 38.8, 93, 129, 286

- female: 39.1, 123, 173, 373

* n for this population is only 39, 

reducing overall confidence in a 99th

consumption rate (therefore, this highend percentile was not included in the 

risk assessment)

Sample size is variable – out of 458 respondents, 

39 are blacks, 149 are female and 98 are poor –

black n is relatively smaller than the other 

groups, which increases uncertainty in higher 

percentile values provided for this group. 

The authors point out that these results highlight 

the considerable spread between high-end 

consumers and more typical behavior (95th% is 

more than 10X greater than the mean or median 

intake rate for wild-caught fish).

Results are also provided for poor (0-20K$ 

annual income). These consumption rates are 

relatively high particularly for the higher 

percentiles (90th, 95th and 99th rates are: 285, 429 

and 590 g/day). This observation forms the basis 

for our decision to assess a number of the 

subsistence populations only for watersheds 

located in US Census tracts containing members 

of source populations below the poverty line for 

the white and black populations.

- the black and white fisher populations 

were extrapolated to cover all watersheds 

modeled for risk in the Southeastern 

states.a The rationale for this was that 

fishing activity by these two groups 

could be generalized in this region of the 

country. Note, however that these 

scenarios were only assessed for 

watersheds in the Southeast located 

within US Census tracts with at least 25 

individuals from that ethnic group below 

the poverty line.

- given the focus of the risk assessment 

on consumption by women (in 

considering risk to pregnant women in 

particular), we extrapolated the female 

consumption rates to all watersheds in 

the continental US with at least 25 

individuals below the poverty line (this is 

the high-end female consumer

population referenced in the risk 

assessment). 

Higher self-caught fish 

consuming ethnic populations 

including Hispanics, Laotians 

and Vietnamese surveyed in 

California 

Citation: Contaminated fish 

consumption in California’s 

Central Valley Delta (Shilling 

et al., Environmental 

Research 110, p. 334-344 

(2010)

Study looks at subsistence 

fishing activity among 

ethnic groups associated 

with more urbanized areas 

near the Sacramento and 

San Joaquin rivers in the 

Central Valley in CA. 

- Hispanic: 25.8, 98*, 155.9, NC**

- Lao: 47.2, 144.8*, 265.8, NA*

- Vietnamese: 27.1, 99.1*, 152.4, NA*

* 95th percentile values were provided 

in the study, however 90th percentile 

values were not provided and were 

calculated using Crystal Ball (based on 

the median and standard deviations 

provided) assuming a log-normality of 

the consumption rate distributions.

** 99th percentile consumption rates 

were not provided (or derived) for any 

of these populations due to small 

sample sizes of the study populations. 

The authors note that many of these ethnic 

groups relied on fishing in origin countries and 

bring that practice here (e.g., Cambodian, 

Vietnamese and Mexican). The authors also 

note that fish consumption rates reported here 

for specific ethnic groups (specifically Southeast 

Asian) are generally in-line with rates seen in 

WA and OR studies.

- the Hispanic fishing scenario was 

extrapolated to cover watersheds located 

in US Census tracts with at least 25 poor 

members of the ethnic populations (e.g., 

the Hispanic consumption rates would be 

applied to the subset of the 2,461

watersheds located in US Census tracts 

with at least 25 poor Hispanic 

individuals).

- the Laotian and Vietnamese fishing 

scenarios were extrapolated to cover 

watersheds located in US Census tracts 

with at least 25 members of the 

underlying ethnic group. 
76

Fish consuming populations 

covered by study (and 

reference information) Overview of study

Self-caught fish consumption rates 

(mean, 90th, 95th, 99th) g/day

Observations from the study relevant to the 

risk assessment

Extrapolation of study populations in 

the risk assessment

High-end self-caught fish 

consuming Chipewa and 

Ojibwa Tribal populations 

active in the vicinity of the 

Great Lakes. 

Citation: Exposure 

assessment and initial 

intervention regarding fish 

consumption of tribal 

members in the Upper Great 

Lakes Region in the United 

States. Dellinger , 

Environmental Research 95 

(2004) p. 325-340

This study contrasted selfreported fish consumption 

rates by Tribes in the 

Great Lakes area with 

“actual” fish consumption 

rates collected for a subset 

of the original study 

population (147 of 822 

from 4 Tribal 

population/location 

combinations). The study 

found that actual fish 

consumption rates were

lower than reported 

values. 

- reported value for all Tribal areas (in 

the study) combined: 62, 136.2, 213.1, 

492.8 

Note, that all higher percentiles (90th –

99th) were derived using Crytal Ball 

(based on median and standard 

deviations and an assumption of lognormally distributed variability in 

consumption rates)

While the “actual” consumption rates collected 

for a subset of the families were far lower than 

the reported values (often an order of magnitude 

smaller), a number of factors resulted in a

decision to use the reported values rather than 

the actual values in the risk assessment. First, 

and most importantly, the sample size is very 

small for the “actual” analysis with n’s ranging 

from 12 to 54 individuals (representing a smaller 

number of associated families) for the different 

survey groups. These small sampling rates 

reduce the probability of capturing individuals 

with higher consumption rates in the broader 

population. It also appears that the actual values 

may cover Walleye specifically and not include 

all fish., which could bias these values 

downward. There is concern that, even if 

consumption rates have decreased, actual 

heritage cultural practices could still exist (or 

there could be a desire to return to those rates), 

in which case, risks levels associated with those 

higher historical consumption rates could be 

important to assess. And finally, the high-end 

percentile consumption rates derived based on 

reported mean consumption rates (and standard 

deviations) are in-line with subsistence 

consumption rates seen for other populations in 

the U.S. Therefore, these Tribal high-end fish 

consumption rates would general comport with 

subsistence fish consumption activity and 

therefore are considered reasonable to include in 

the risk assessment. 

Activity only assumed to occur in areas 

ceded to the Tribes covered in the study 

(regions in the vicinity of the Great 

Lakes). Because fishing activity is 

highly variable across Tribes (and 

closely associated with heritage cultural 

practices) we have not extrapolated 

fishing behavior for these Tribes outside 

of the specific populations and regions 

covered. 

a southeast for purposes of this analysis comprises: Alabama, Arkansas, Florida, Georgia, Kentucky, Louisiana, Mississippi, North Carolina, South Carolina, Tennessee, Virginia and West Virginia
77

D. Estimating total mercury-related exposure at the watershed-level 

The following equation is used to estimate total mercury exposure at a particular 

watershed as an annual average of the daily methylmecury intake per kg body weight:

IR = FTC * FIR * CAF

BW 

IR: daily MeHg intake rate (ug/kg-day). This ingestion dose estimate can be directly 

compared with the merthylmercury RfD to generate a total HQ.

FTC: mercury fish tissue concentration (ug/g or ppm): The 75th% value provided for each 

of the 2,461 watersheds (see Section 1.3). 

FIR: fish ingestion rate (g/day). These values are specific to a given population (see 

Table 1-1 in Section 1.3)

CAF: cooking adjustment factor (unitless): Because MeHg is not volatile and is contained 

primarily in the muscle, this translates into a factor increase of 1.5 for 

concentration of mercury in the cooked fish (Morgan, Berry, and Graves, 1997).

The IR value described above can be directly compared with the MeHg RfD to generate 

HQ estimates. However, in order to estimates IQ loss, we need to convert this dose estimate into 

an equivalent maternal hair concentration since the IQ loss function uses hair mercury as the 

dose measure. To do that, we use a dose-to-hair conversion factor (DHCV) of 12.5 (units ppm 

per unit ug/kg-day) that converts ingested dose (IR) to hair mercury concentration in ppm. As 

noted earlier in 1.3, the DHCV factor is based on a one compartment toxicokinetic model used 

for deriving the MeHg RfD by Swartout and Rice (2000). 

E. Establishing the U.S. EGU-attributable fraction of total exposure

As noted earlier in 1.3, establishing the increment of total exposure estimated for a given 

fishing population active at a given watershed that is attributable to U.S. EGUs requires to key 

elements: (a) application of the MMaps assumption linking source contribution to Hg deposition 

over that watershed to source apportionment of fish tissue Hg levels and consequently exposure 

estimates and (b) use of CMAQ modeling results characterizing the fraction of mercury 

deposition over a given watershed that is attributable to U.S. EGUs (as contrasted with the 

fraction attributable to all other US and foreign-sourced anthropogenic and natural mercury 

emissions). In this section, we provide additional detail on each of these elements. 

Mercury maps assumption 

To analyze the relationship between Hg deposition and MeHg concentrations in fish 

across the 2,461 watersheds included in the risk assessment, as discussed in Section 1.3, we 

applied the EPA’s Office of Water’s Mercury Maps approach. MMaps implements a simplified 

form of the IEM-2M model applied in EPA’s Mercury Study Report to Congress (USEPA, 

1997). By simplifying the assumptions inherent in the freshwater ecosystem models that were 

described in the Report to Congress, the MMaps model showed that these models converge at a 
78

steady-state solution for MeHg concentrations in fish that are proportional to changes in Hg 

inputs from atmospheric deposition (e.g., over the long term fish concentrations are expected to 

decline proportionally to declines in atmospheric loading to a waterbody). This solution only 

applies to situations where air deposition is the only significant source of Hg to a water body, 

and the physical, chemical, and biological characteristics of the ecosystem remain constant over 

time. EPA recognizes that concentrations of MeHg in fish across all ecosystems may not reach 

steady state and that ecosystem conditions affecting mercury dynamics are unlikely to remain 

constant over time. EPA further recognizes that many waterbodies, particularly in areas of 

historic gold and Hg mining in western states, contain significant non-air sources of Hg (note, 

however, that as described below, we have excluded those watersheds containing gold mines or 

with other non-EGU related anthropogenic mercury releases exceeding specified thresholds). 

Finally, EPA recognizes that MMaps does not provide for a calculation of the time lag between a 

reduction in Hg deposition and a reduction in the MeHg concentrations in fish. Despite these 

limitations, EPA is unaware of any other tool for performing a national-scale assessment of the 

change in fish MeHg concentrations resulting from reductions in atmospheric deposition of Hg.

Given that the MMaps approach only applies in those situations where aerial deposition 

is the dominant source of mercury loading to a watershed, in identifying the 2,461 watersheds to 

include in the risk assessment, we excluded those watersheds that either contained active gold 

mines or had other substantial non-U.S. EGU anthropogenic releases of mercury. Identification 

of watersheds with gold mines was based on a 2005 USGS data set characterizing mineral and 

metal operations in the United States. The data represent commodities monitored by the National 

Minerals Information Center of the USGS, and the operations included are those considered 

active in 2003 (online link: <http://tin.er.usgs.gov/mineplant/>. The identification of watersheds 

with substantial non-EGU anthropogenic emissions was based on a TRI-net query for 2008 or

non-EGU mercury sources with total annual on-site Hg emissions (all media) of 39.7 pounds or 

more. This threshold value corresponds to the 25th percentile annual US-EGU mercury emission 

value as characterized in the 2005 NATA. The EPA team considered the 25th percentile US-EGU 

emission level to be a reasonable screen for additional substantial non-U.S. EGU releases to a 

given watershed. 

There are a number of limitations and uncertainties associated with the application of the 

MMaps approach in the context of this risk assessment. Several of these limitations are briefly 

discussed here, but a more complete discussion is presented in Section 5.3.2 or the RIA TSD 

supporting this regulatory review (USEP, 2011). The application of MMaps in apportioning fish 

tissue mercury levels and consequently exposure and risk between U.S. EGUs and all other 

sources of mercury at the watershed-level, assumes that the relationship between fish tissue 

levels and mercury deposition has remained fairly consistent such that near steady-state 

conditions have been reached. However, in reality, patterns of mercury deposition for the period 

during which the fish tissue samples were collected (2000 to 2009) have not remained constant. 

In addition, those fish tissue concentrations may actually reflect patterns of mercury deposition 

from earlier time periods (e.g., the 1990s) when mercury emissions from US sources were 

experiencing substantial decreases. In addition other factors that can impact rates of mercury 

methylation (e.g., sulfur deposition to waterbodies, pH and eutrification for those watebodies) 

have also likely not remained constant over the past 1-2 decades for most watersheds. The fact 

that many of these factors related to methylation in fish have not remained constant does 

introduce uncertainty into the application of the MMaps approach. However, we believe that the 
79

MMaps approach for apportioning fish tissue mercury levels is still appropriate to use, 

particularly if we are not attempting to characterize the temporal profile for apportionment and 

instead, can assume that sufficient time has passed for near steady state conditions to be 

reached.48

CMAQ modeling

Uncertainty related to the application of the MMaps model in the context of this risk 

assessment is further discussed in Appendix F.

The Community Multi-scale Air Quality (CMAQ) model v4.7.1 (www.cmaq-model.org) is a 

state of the science three-dimensional Eularian “one-atmosphere” photochemical transport model 

used to estimate air quality (Byun and Schere 2005; Appel, Gilliland et al. 2007; Appel, Bhave et 

al. 2008). CMAQ simulates the formation and fate of photochemical oxidants, ozone, primary 

and secondary PM concentrations, and air toxics over regional and urban spatial scales for given 

input sets of meteorological conditions and emissions. Mercury oxidation pathways are 

represented for both the gas and aqueous phases in addition to aqueous phase reduction reactions 

(Bullock and Brehme 2002). The emissions data used in the base year and future reference and 

future emissions adjustment case are based on the 2005 v4.1 platform. Emissions are processed 

to photochemical model inputs with the SMOKE emissions modeling system (Houyoux et al., 

2000). The 2016 reference case is intended to represent the emissions associated with growth and 

controls in that year projected from the 2005 simulation year. Other North American emissions 

of criteria and toxic pollutants (including mercury) are based on a 2005 Canadian inventory and 

1999 Mexican inventory. Global emissions of criteria and toxic pollutants (including mercury) 

are included in the modeling system through boundary condition inflow. The lateral boundary 

and initial species concentrations are provided by a three-dimensional global atmospheric 

chemistry model, the GEOS-CHEM model (standard version 7-04-11). 

 48 We recognize that these predictions of U.S. EGU apportionment of fish tissue Hg levels may not be realized for 

several years or even decades, and, for that reason, the current risk from Hg from EGUs may be considerably higher 

because EGU mercury emissions were substantially higher prior to 2005. Furthermore, we recognize that these 

MMaps-based apportionments of fish tissue Hg levels assume that conditions (in terms of patterns of mercury 

deposition and factors related to methylation such as sulfate deposition) also hold relatively constant for years to 

decades such that near steady state conditions in the fish tissue mercury concentrations are realized.
80

F. Variability and Uncertainty

Table F-1. Key sources of variability associated with the analysis and degree to which they are reflected in the design of the 

analysis 

Source of variability Description Degree to which source is reflected in design of risk analysis

Variation in the 

patterns of total and 

U.S. EGU-attributable 

mercury deposition 

across the U.S.

Patterns of annual deposition of mercury 

including total (all source) and estimates of 

the U.S. EGU fraction (by watershed) 

displays considerable spatial variability 

across the U.S. based on the results of 12 km 

grid cell CMAQ modeling (see Section 2.3) 

By extrapolating CMAQ grid cell results at the more spatially refined HUC12 

watershed level, we retain the greater degree of spatial resolution in characterizing 

mercury deposition obtained through the use of the 12k CMAQ grid cell simulations. 

Variation in the 

patterns of mercury fish 

tissue levels across the 

U.S. 

Mercury fish tissue measurements can 

display considerable spatial variability 

across watersheds.

We have fish tissue measurements for roughly 4% of the watersheds in the U.S. (i.e., 

2,461 watersheds with measured values out of 88,000 based on data from 2000-

2009). While our measured fish tissue levels do generally provide some degree of 

coverage for areas with elevated mercury deposition (in terms of both total and U.S. 

EGU-attributable), this coverage is limited and there are a large number of 

watersheds with high total and U.S. EGU-attributable mercury deposition for which 

we do not have fish tissue measurements (see Section 2.5). 

Variable response of 

mercury fish tissue 

levels to changes in 

patterns of mercury 

deposition (MMaps 

approach)

The impact of changes in mercury deposition 

on mercury concentrations in fish within a 

given watershed can vary greatly depending 

on a number of factors (e.g., role of 

watershed in loading to waterbody, 

methylation potential of the waterbody, rates 

of sulfate deposition, nature of aquatic biotic 

foodweb including mix of upper-level 

trophic fish etc). Not only do these factors 

contribute to variation in the degree to which 

fish tissue mercury levels will respond to 

changes in mercury deposition, they also 

affect the temporal profile of that response.

Variation in the methylation potential across waterbodies is minimized as a factor in 

our analysis to a certain extent given our use of the MMaps approach combined with 

measured mercury fish tissue levels. Specifically, variation in methylation potential 

should be implicitly reflected in the measured fish tissue levels used in the analysis 

(i.e., if watershed “a” has a much greater methylation potential than watershed “b”, 

then measured fish tissue levels for “a” should be higher reflecting that difference in 

methylation potential). In other words, we are applying the MMaps proportionality 

assumption to measured fish tissue levels that should reflect underlying differences in 

methylation potential across waterbodies. Furthermore, because we are not predicting 

temporal trends in fish tissue levels and instead consider a future point in time (once 

near-steady state conditions are reached), variation in the temporal profile of changes 

in fish tissue levels related to differences in methylation potential of different

watersheds is also minimized as a factor in the analysis. 

Variation in the types 

of subsistence fisher 

populations active in 

different regions 

Studies reviewed in developing the approach 

for this analysis suggests that there can be 

considerable variation in the nature of highend self-caught fisher populations across 

regions of the country. This variation reflects 

ethnic and cultural practices and can also be 

Surveys of near-subsistence and subsistence fishing populations allow us to clearly 

define this type of activity for specific areas covered by those surveys (e.g., Hispanic, 

Vietnamese and Laotian fishing activity in specific regions of California, high-end 

fishing activity by blacks and whites in South Carolina and Tribal activity near the 

Great Lakes). However, available studies on this type of high-end fishing activity at 

inland freshwater bodies do not provide comprehensive coverage for all regions in 
81

Source of variability Description Degree to which source is reflected in design of risk analysis

driven be socio-economic status (SES) 

related factors (e.g., poverty, degrees of 

income). In addition, we would expect that 

access to freshwater fishing locations would 

also play a role.

the U.S. Therefore, we did extrapolate coverage from areas reflected in these studies 

to other portions of the U.S. There is uncertainty associated with this process. As 

discussed in Section 1.3 a lack of comprehensive survey information on nearsubsistence and subsistence fishing activity for all regions in the U.S. is a limitation 

of the analysis. 

For a given fisher 

population, variability 

in fishing activity (e.g., 

species harvested, 

fishing activity focused 

at one or more 

waterbodies)

High-end fisher populations could display 

considerable variability both in terms of the 

degree to which they frequent specific 

waterbodies or watersheds and the degree to 

which they target specific types of fish (or at 

least sizes of fish). Both of these factors can 

impact estimates of exposure. If a fisher 

population distributes their activity across a 

range of waterbodies and harvests a variety 

of fish species (and sizes) than the 

distribution of exposure and risk across that 

population will be smaller compared with a 

population that focuses activity at individual 

waterbodies and tends to focus on larger fish 

and/or higher trophic level fish (which will 

tend to have higher mercury concentrations, 

other factors equal).

We do not have comprehensive information characterizing the nature of high-end 

fisher behavior in terms of the factors listed here (i.e., degree to which fishing 

activity targets individual waterbodies and fish species). Given that this analysis was 

aimed at assessing whether a public health hazard exists due to U.S. EGU mercury 

emissions we did consider the situation in which high-end fishers would engage in 

fishing activity that could result in somewhat higher exposure and risk, other factors 

equal . Specifically, we modeled exposure and risk assuming that (a) fishing activity 

is focused within a given watershed (i.e., at waterbodies within that watershed) and 

(b) that the fishers modeled would tend to favor somewhat larger fish given that they 

are engaged in subsistence activity and therefore supplementing their diet with fish 

(recall that we used the 75th percentile fish tissue value in exposure and risk

simulations – see Section 1.3). Note, that if a portion of a fisher population actually 

distributes their activity between watersheds and/or consumes a mixture of fish 

species and sizes (reflecting a fish tissue level closer to the median or mean for a 

watershed), then risks would be lower than those estimated here. 

Table F-2. Key sources of uncertainty associated with the analysis, the nature of their potential impact on risk estimates, and 

degree to which they are characterized 

Source of 

uncertainty Description 

Nature of potential impact on the exposure and 

risk estimates

Degree to which the potential impact of the 

source of uncertainty is characterized as 

part of the analysis

Factors relating to 

the estimation of 

mercury 

deposition over

watersheds using 

the CMAQ model 

(e.g, estimating 

Emissions are quantified from 

anthropogenic and natural 

sources, but re-emission of 

historical emissions (pre-2005) 

are not well characterized by 

the modeling system.

Generally all of the sources of uncertainty reflect the 

fact that mercury deposition estimated over specific 

watersheds may be over or under-estimated.

The one source of uncertainty for which we can 

differentiate the potential direction of impact (i.e., 

bias) is mercury wet deposition where we believe that 

The analysis did not include any specific 

quantitative analyses aimed at characterizing 

uncertainty associated with these three sources 

of uncertainty, with the exception of qualitative 

consideration for the potential seasonallydifferentiated bias in wet deposition discussed 

here.
82

Source of 

uncertainty Description 

Nature of potential impact on the exposure and 

risk estimates

Degree to which the potential impact of the 

source of uncertainty is characterized as 

part of the analysis

mercury 

emissions from 

U.S. EGUs, 

chemistry 

associated with 

mercury fate and 

transport, 

prediction of wet 

and dry 

deposition, and 

global inflow of 

mercury into the 

U.S.)

The complete set of mercury 

oxidation and reduction 

reactions has not been 

identified by the scientific 

community.

Uncertainty in a wide variety 

of model inputs (e.g., 

emissions, meteorology, global 

inflow to the modeling 

domain, and chemistry) 

impacts estimates of mercury 

wet and dry deposition. 

There is considerable 

uncertainty in the global 

emissions inventory for 

mercury and given the long 

residence time of elemental 

mercury it is possible that 

inflow into the modeling 

domain may reflect 

deficiencies in the global 

emissions inventory.

estimates may be slightly low in the fall and slightly 

high in the winter. This assessment of potential bias 

is based on comparisons of weekly estimates 

(generated by CMAQ) against measurements of wet 

mercury deposition collected as part of the Mercury 

Deposition Network, which operates under the 

National Atmospheric Deposition Program 

(http://nadp.sws.uiuc.edu/MDN/).

Characterizing 

subsistence 

fishing activity 

within areas of 

high U.S. EGU

mercury 

deposition

There is uncertainty associated 

with predicting high-end 

fishing activity at specific 

watershed and at watersheds 

located within specific regions. 

Furthermore, there is 

uncertainty associated with 

characterizing the nature of 

that fishing activity (including 

the frequency of activity at 

different waterbodies, types of 

fish targeted).

If subsistence fishing activity is assumed at a given 

watershed or at group of watersheds within a region 

and in reality, factors preclude that type of fishing 

activity (e.g., lack of ready access, poor fishing stock 

etc), then those risk estimates are not representative 

and actual risk (assessed over the set of watersheds 

modeled) would be reduced, since these point 

estimates would be removed. Similarly, if high-end 

fishing activity by a given fisher or fishing family 

tends to be distributed across watersheds and not 

focused at a single watershed, as assumed here, then 

the upper-end risks estimated across the watersheds 

will be reduced, since watersheds with the highest 

We did not explore these sources of 

uncertainty. Given that the focus of this 

analysis is on assessing risk for populations 

likely to experience the highest reasonable U.S. 

EGU-attributable risk, we concluded that 

because it is reasonable to assume that some 

fraction of high-end fishing populations could 

focus their activity on a single watershed (and 

could favor larger fish), we would model this 

behavior in our analysis. If these assumptions 

are relaxed, then risk will be reduced 

(specifically for the watersheds with the highest 

U.S. EGU-attributable risk). However, we did 
83

Source of 

uncertainty Description 

Nature of potential impact on the exposure and 

risk estimates

Degree to which the potential impact of the 

source of uncertainty is characterized as 

part of the analysis

U.S. EGU-attributable risks will be combined with 

lesser-impacted watersheds in generating risk 

estimates (and not assessed independently for risk 

assuming activity focused specifically on them). 

Similarly, for the types of fish taken, if high-end 

fishers do not favor larger fish, then risk could be 

overstated by our use of the 75th percentile fish. 

not explicitly model these alternative 

behavioral profiles, because of our goal of 

capturing reasonable estimates of high-end risk. 

Application of 

MMaps 

assumption in 

generating 

estimates of the 

U.S. EGUattributable 

fraction of risk 

Uncertainty associated with 

Applying the MMaps 

assumption in the context of 

this analysis results from a 

number of factors: (a) fish 

tissue measurement data used 

in the analysis may still reflect 

earlier historical patterns of 

mercury deposition (from the 

1990’s), (b) watersheds display 

substantially different 

methylation potentials, 

resulting in differences in the 

impact of a unit change in 

mercury deposition on fish 

tissue levels, (c) factors related 

to methylation in watersheds 

(e.g., sulfate deposition, pH, 

eutrafication) have not 

remained constant over time, 

resulting in variation in the 

methylation potential of 

watersheds over time, (d) 

despite efforts to exclude 

watersheds with substantial 

non-air sources of mercury 

loading, some watersheds with 

substantial non-air impacts 

may have been retained in the 

analysis, and (e) the potential 

If air deposition patterns from the 1990s are reflected 

in some of the mercury fish tissue measurements we 

are using the implications can vary depending on the 

nature of that difference. If fish tissue levels for a 

watershed still reflect higher 1990 deposition values, 

then we may overstate U.S. EGU-attributable risk, 

since in reality we would expect the underlying fish 

tissue levels to decrease as the impact of those earlier 

higher deposition values dissipates. Conversely, if 

total deposition remains the same, but only the source 

distribution has changed since the 1990’s, then the 

effect on our risk assessment may not be that 

significant, since we are making projections based on 

the current source-mix (or future source-mix) 

assuming near steady-state assumptions are reached 

given those specific source mixes. 

Differences in methylation potential across 

watersheds (as discussed in table C-1), are likely to be 

reflected in the underlying fish tissue levels 

themselves. Therefore, these methylation differences 

are likely not to have a substantial impact on our 

analysis (note, that the time sequence of changes in 

fish tissue levels will depend on differences in 

methylation potentials, but we are not attempting to 

predict these temporal profiles). 

The potential that we may have failed to exclude 

watersheds with significant non-air mercury loadings 

could introduce high-bias into our estimates of U.S. 

As part of the sensitivity analyses completed 

for the analysis, we did consider the issue of 

potentially having included watersheds with 

substantial non-air impacts. Although as 

described in Appendix E, we did exclude 

watersheds with active gold mines and/or nonEGU anthropogenic sources of Hg release 

meeting specified criteria, there still is the 

potential that we may included some 

watersheds that should have been excluded. 

Specifically, this concern exists in LA, SC, 

MN, and ME, where there are either broader 

concerns over non-air mercury sources (e.g., 

taconite mining in MN and gold mining in SC) 

or where there appear to be relatively higher 

mercury fish tissue levels in the absence of 

elevated mercury deposition (again raising the 

concern over non-air deposition sources). To 

examine the potential impact of locations with 

elevated non-air mercury sources, we 

completed two sensitivity analyses including: 

(a) an analysis of risks when watersheds falling 

in these four states are excluded and (b) an 

analysis for risk only for the subset of 2,366 

watersheds falling in the upper 25th percentile 

with regard to total mercury deposition (i.e., 

those watersheds having relatively elevated 

mercury deposition) (see Section 2.7 for 

additional detail on these sensitivity analyses). 

As part of our sensitivity analyses, we also 
84

Source of 

uncertainty Description 

Nature of potential impact on the exposure and 

risk estimates

Degree to which the potential impact of the 

source of uncertainty is characterized as 

part of the analysis

that the MMaps approach is 

more applicable in the context 

of stationary waterbodies 

(lakes and ponds) than flowing 

waterbodies (streams and 

rivers).

EGU-attributable risk, since we would overstate the 

role of U.S. EGUs in contributing to risk, by 

overlooking the other non-air sources. 

If the MMaps approach is more effective at linking 

areal Hg deposition to Hg fish tissue levels for 

stationary waterbodies, then our application of the 

MMaps approach to both stationary and flowing 

could result in a distribution of watershed-level risks 

that could be incorrect. 

examined the issue of applying MMaps to 

stationary and flowing waterbodies. 

Specifically, we ran the model for a lakes-only 

simulation and compared these results with the 

baseline run including both lakes and rivers 

(see Section 2.7).

Estimating IQ 

loss for children 

born to high-fish 

consuming 

mothers modeled 

in the analysis

There are a number of sources 

of uncertainty associated with 

modeling IQ loss in this 

analysis: (a) IQ may not fully 

capture the most sensitivity 

cognitive endpoints associated 

with mercury exposure , (b) 

potential confounding from 

long-chained polyunsaturated 

fatty acids (LCPFAs) found in 

fish, and (c) including potential 

outliers from the 

epidemiological datasets used 

in deriving the IQ loss 

functions. 

IQ may not represent the most sensitive cognitive 

endpoint for mercury exposure (Axelrad et al., 2007 -

see Section 1.2). In addition deficits in some 

categories of cognitive functioning are not captured 

by IQ. Together, these sources of uncertainty suggest 

that we could be under-estimating the extent of 

cognitive impacts associated with mercury exposure 

(when we focus on modeling the IQ loss endpoint 

alone). The potentially beneficial effects of LCPFAs 

(found in fish) on neurological development can 

confound the effects of mercury by potentially 

masking those adverse effects. This would result in 

IQ loss slopes that are biased low, since the IQ loss 

they are representing is counteracted to some extent 

by the LCPFAs exposure. Regarding outliers, when 

an outlier datapoint from the Seychelles study was 

included in the integrated derivation of the IQ loss 

slope factor, the factor was reduced by 25 percent 

(from -0.18 IQ points per unit ppm hair mercury, to -

0.125). If in reality, this outlier actually reflects the 

true response for a subset of the populations, then 

risks (as modeled) could be biased high specifically 

for this subpopulation. 

Because we do not have readily available data 

to support quantitative analyses of the first two 

sources of uncertainty (IQ loss not capturing all 

of the cognitive effects and potential 

confounding by LCPFAs), we could only 

address these factors qualitatively. We note, 

that in both cases, the potential effect on the 

risk assessment would be to potentially downbias our estimate of cognitive endpoint-related 

risk for children. In the case of excluding the 

outlier from the Seychelles study, we note that 

the effect (given the linear nature of the IQ loss 

slope) would be to simply result in a 25% 

reduction in risk, if we were to include the 

outlier in derivation of the slope function (i.e., a 

formal rerun of the model with this alternative 

slope is not required – we can just consider this 

magnitude of impact on the primary risk 

estimates we generate for the analysis). 
85

G. Potential for Deposition “Hotspots” in Areas Near U.S. EGUs

EPA also evaluated the potential for “hot spot” deposition near U.S. EGU emission 

sources on a national scale, based on the CMAQ modeled Hg deposition for 2005 and 2016. We 

calculated the excess deposition within 50 km of U.S. EGU sources by first calculating the 

average U.S. EGU attributable Hg deposition within a 500 km radius around the U.S. EGU 

source. This deposition represents the likely regional contribution around the EGU. We then 

calculated the average U.S. EGU attributable Hg deposition within 50 km of the U.S. EGUs to 

characterize local deposition plus regional deposition near the EGU. Excess local deposition is 

then the 50 km radius average deposition minus the 500 km radius average 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. 

The maps in Figures 1 and 2 were generated by applying an averaging kernel to the 12km 

EGU attributable mercury deposition estimates from CMAQ. Averaging kernels assign a mean 

value to each grid based on the averages of all neighboring grids within a predefined window. In 

this case, window or kernel sizes were 50 km and 500 km-radiuses. Then 50km-radius average 

values were subtracted from 500km-radius averages to create the final hot spot image.

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 around 120 percent of the 

average deposition, while for the top 10 percent of Hg emitting U.S. EGU, local deposition was 

around 3.5 times the regional average. By 2016, the absolute excess deposition falls, however, 

the local excess still remains around 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.
86

Figure G-1. Excess Local Deposition in 2005
87

Figure G-2. Excess Local Deposition in 2016 (Base Case)
88

Table G-1. Excess local deposition of hg based on 2005 CMAQ modeled Hg deposition

50km-Radius-Average Excess Local Deposition values 

(μg/m2)

Mean Across EGUs (percent of regional average deposition)

2005 2016

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

(672 sites) 1.65 (119%) 0.36 (93%)

Top ten percent U.S. EGU in Hg 

emissions

(67 sites)

4.89 (352%) 1.18 (302%)
89

United States

Environmental Protection

Agency

Office of Air Quality Planning and Standards

Health and Environmental Impacts Division 

Research Triangle Park, NC

EPA-452/D-11-002

March 2011