1 

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

OFFICE OF AIR QUALITY PLANNING AND STANDARDS

HEALTH AND ENVIRONMENTAL IMPACTS DIVISION

RESEARCH TRIANGLE PARK, NC 27711

July 1, 2011 

TO: National Emission Standards for Hazardous Air Pollutants for Coal- and Oilfired Electric Utility Steam Generating Units docket 

FROM: Zachary Pekar (EPA OAQPS) 

SUBJECT: Clarification and Updating of Mercury Deposition Maps Provided in the 

Technical Support Document: National-Scale Mercury Risk Assessment 

Supporting the Appropriate and Necessary Finding for Coal- and Oil-Fired 

Electric Generating Units (Mercury Risk TSD)

Overview

During the Science Advisory Board (SAB) review of the Mercury Risk TSD held on June 15-

17th, questions were raised by Panel members regarding patterns of mercury deposition reflected 

in Figures 2-1 though 2-4 of the TSD. These figures were intended to show annual total mercury 

(Hg) deposition per unit area (in units of g/m2

) by watershed.1 However, the patterns reflected 

in the figures did not comport with generalized expectations given the Panel’s prior experience 

in studying patterns of total and Electric Generating Unit (U.S. EGU) – sourced Hg deposition 

within the U.S.. 

Questions raised by the Panel resulted in EPA staff reviewing the four figures in question 

subsequent to the SAB meeting, including the underlying data and spatial interpolation used in 

generating those figures. That review has resulted in our identifying that the original Figures 2-1 

through 2-4 actually displayed intermediate calculations that had not been adjusted by the 

waterbody-specific surface areas (i.e., these figures reflected an intermediate calculation and not 

the intended value of g mercury deposited per square-meter by watershed). This explains why 

the spatial patterns of Hg deposition presented in Figures 2-1 through 2-4 of the TSD did not 

match the expectations of Panel members and specifically why some of the larger watersheds 

had the highest deposition estimates. We have now updated Figures 2-1 though 2-4 (presented 

below) to correctly reflect total annual Hg deposition per square-meter by watershed. It is 

important to emphasize that because the risk estimates are based on proportional relationships 

between total and US EGU-sourced Hg deposition (i.e., application of a unitless ratio) at the 

watershed-level, they are not in any way affected by the choice of units reflected in the 

deposition maps.2

 However we felt it important to update the maps to reflect Hg deposition in 

terms of g/m2 by watershed to aid review and interpretation of spatial patterns reflected in those 

 

1 As discussed in Section 1.3 of the Mercury Risk TSD, Hydrologic Unit Code 12 scale watersheds (HUC12s) were 

used as the basis for generating risk estimates in the analysis. 

2 As noted in Section 1.3 of the Mercury Risk TSD, proportionality between changes in Hg deposition and changes 

in fish tissue methylmercury levels (which translate into risk) is used in projecting future risk for the 2016 scenario 

and is also used to apportion risk between total and US EGU-sourced Hg. This proportionality assumption (which is 

implemented using unitless ratios calculated using Hg deposition estimates) is based on Mercury Maps modeling. 
2 

maps. 

The remainder of this memo is organized as follows. First we provide additional clarification on 

the nature of the Hg deposition estimates provided in the original Figures 2-1 through 2-4. Then 

we provide updated Figures 2-1 through 2-4 based on Hg deposition normalized by unit surface 

area. 

Clarification of Hg Deposition Estimates Provided in Original Figures 2-1 through 2-4

Figure M-1 shows the underlying CMAQ deposition data from which the watershed-level values 

reflected in Figure 2-1 of the Mercury Risk TSD are derived. The CMAQ data are expressed in 

g/m2, with all grid cells being 12km squares. 

Figure M-1. Total mercury deposition (g/m2) by 12 km CMAQ grid cell (2005) 

To allocate these values to the watershed level, the following calculations were performed. For 

each watershed (i.e., HUC) that a CMAQ grid cell intersects, the amount of that grid cell (in 

terms of area) lying within the watershed was calculated and used to create a ratio used to 

apportion the total value for the grid cell into the watersheds that it intersects. The value for each 

watershed is then the sum of the apportioned values for all grid cells with which it intersects. 

That is, 

Watershedi deposition = ∑CMAQjdeposition * 

 j 

where j indicates all CMAQ grid cells that intersect watershed i and HUCij area is the amount of 

overlap between the watershed and grid cell. Applying this formula for all watersheds leads to 

HUC12ij area

CMAQjarea
3 

the map in Figure M-2, which matches Figure 2-1 in the Mercury Risk TSD. 

Presenting the maps reflecting the above calculation, however, ignores the differences in sizes of 

watersheds. Because larger watersheds intersect a larger number of grid cells, larger watersheds 

are apportioned more of the deposition. As a result, Figure M-2 most clearly shows the relative 

sizes of watersheds rather than the average deposition across the watersheds. 

Figure M2. Original mercury deposition map (2005) (without adjustment for watershed 

surface area) – corresponds to Figure 2-1 in Mercury Risk TSD 

When the values in Figure M-2 are normalized to account for the differences in watershed sizes, 

they appear as in Figure M-3, which shows the average deposition in g/m2 across watersheds. 
4 

Figure M-3. Total mercury deposition (g/m2

) by watershed (2005) 

It can be seen that after the watershed values are adjusted to account for differences in sizes, the 

distribution of values is consistent with what is seen in the underlying CMAQ data (i.e., the 

pattern reflected in Figure M-3 matches that in Figure M-1). Figure M-3 reflects the units 

intended for presentation in the Mercury Risk TSD (i.e., annual mercury deposition per squaremeter by watershed). The pattern of total annual mercury deposition reflected in Figure M-3 is 

likely more in-line with what SAB Panel members expected to see in Figure 2-1 of the Mercury 

Risk TSD. 

Updated Figures 2-1 through 2-4 for the Mercury Risk TSD

Updated versions of Figures 2-1 through 2-4 of the Mercury Risk TSD, presenting total and US 

EGU-sourced (for 2005 and 2016) annual deposition normalized by surface area (i.e., units of 

g/m2

 per watershed) are presented below as Figures M-4 through M-7. 
5 

Figure M-4. Total mercury deposition (g/m2

) by watershed (2005) (updates Figure 2-1 in 

the Mercury Risk TSD) 

Figure M-5. Total mercury deposition (g/m2

) by watershed (2016) (updates Figure 2-2 in 

the Mercury Risk TSD) 
6 

Figure M-6. U.S. EGU-attributable mercury deposition (g/m2

) by watershed (2005)

(updates Figure 2-3 in the Mercury Risk TSD) 

Figure M-7. U.S. EGU-attributable mercury deposition (g/m2) by watershed (2016)

(updates Figure 2-4 in the Mercury Risk TSD)