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MEMO: Description of docketed GIS analyses , fish tissue Hg database and Excel-based risk 

assessment spreadsheet modeling conducted to support the revised national-scale mercury risk 

assessment

December, 2011 

(point of contact: Dr. Zachary Pekar, USEPA, Office of Air Quality Planning and Standards, e-mail: 

pekar.zachary@epa.gov)

This memo briefly outlines the GIS-related analyses completed in support of the revised national-scale 

mercury risk assessment and how information generated through those analyses was incorporated into 

the revised risk assessment spreadsheet. The revised risk assessment spreadsheet is included in the 

docket with the final rule as “Revised Risk Assessment Model-2016.xlsx”. This memo also describes the 

excel files comprising the fish tissue Hg database used in the risk assessment, which have been docketed 

as “2011 mercury fish tissue dataset” with the final rule. This memo is not intended to provide a 

rigorous overview of the risk assessment approach, which is provided in the “Revised Technical Support 

Document: National-Scale Assessment of Mercury Risk to Populations with High Consumption of Selfcaught Freshwater Fish In Support of the Appropriate and Necessary Finding for Coal- and Oil-Fired 

Electric Generating Units” (Revised Mercury Risk TSD). 

A number of GIS-related shapefiles were docketed with Mercury Risk TSD accompanying the proposed 

rule and were used without additional modification in the revised risk assessment (docket id: EPA-HQOAR-2009-0234-3079). For that reason, only those GIS shapefiles which have been updated as part of 

the revised risk assessment have been docketed with the final rule, including updated shapefiles of the 

watershed-level interpolation of mercury deposition for the 2005 and 2016 scenarios. In addition, we 

have also docketed the revised fish tissue mercury database used in the risk assessment. The updated 

fish tissue Hg data were incorporated into the analysis as Excel files and used directly in the risk 

calculation spreadsheet (rather than being incorporated into the GIS analysis). And for this reason, we 

have docketed the full fish tissue mercury dataset (see below), but have not docketed any shapefiles 

related to the fish tissue Hg data, since they were not used in the risk assessment. 

The following procedure was used to generate risk estimates for the revised national-scale mercury risk 

assessment:

Step 1) Develop an updated (augmented) fish tissue Hg dataset for use in the risk assessment: The 

procedure used to derive the 2011 mercury fish tissue dataset is fully documented in the Revised 

Mercury Risk TSD, section 1.4.2). Figures 1-3 through 1-6 illustrate how the various underlying fish tissue 

Hg datasets were filtered and integrated to produce the final 2011 mercury fish tissue HUC-level dataset 

used to derive the 75th percentile fish tissue Hg estimates. Each of the underlying datasets identified in 

these figures used in deriving the final integrated dataset (e.g., USGS States, USGS Others, NRSA) have 

been docketed as separate Excel files under the title “2011 mercury fish tissue dataset”. 

Step 2) Generate HUC-level fish tissue Hg 75th percentile values (using the fish tissue Hg dataset 

described in Step 1) and incorporate those estimates into the “Revised Risk Assessment Model-
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2016.xlsx” spreadsheet: The procedure used for this is described in detail in the Revised Mercury Risk 

TSD. The screening of watersheds to exclude non-air Hg sources was accomplished as part of the 

process of developing the fish tissue database and will not be discussed further here. The 75th percentile 

fish tissue watershed-level data (i.e., 3,141 watershed-level fish tissue Hg concentrations) are 

incorporated as a separate sheet in the Revised Risk Assessment Model-2016.xlsx which has been 

docketed (see <MeHg fish tissue combined> worksheet within “Revised Risk Assessment Model2016.xlsx”).

Step 3) Incorporate the watershed-level CMAQ-derived total and U.S. EGU-related Hg deposition (for 

the 2005 and 2016 scenarios) into the “Revised Risk Assessment Model-2016.xlsx” spreadsheet: We

have corrected the spatial interpolation of the gridded modeling results to the watersheds, using the 

CMAQ modeling conducted in support of the Mercury Risk TSD accompanying the proposed rule. These

corrected watershed-level Hg deposition concentrations are docketed as shapefiles and have been 

incorporated into the “Revised Risk Assessment Model-2016.xlsx” spreadsheet as a separate worksheet 

(presenting watershed-level Hg deposition values - see <2005 Hg dep> and <2016 Hg dep> worksheets).

Step 4) Identify watersheds where each of the female subsistence fish consumer scenarios are active:

We used shapefiles from the March analysis to identify which watersheds potentially have fishing 

activity by each of the female subsistence fish consuming populations we were considering in the 

analysis (and consequently, those shapefiles are not redocketed here). However, for this analysis, rather 

than doing the intersection of watersheds with potential activity by specific subsistence scenarios and 

those with mercury fish tissue Hg levels in GIS, we implemented this step within the “Revised Risk 

Assessment Model-2016.xlsx” spreadsheet. Specifically, we had a sheet identifying which tracts had 

potential activity for a given female subsistence fish consuming scenario (see <Scenario location by 

HUC> worksheet) and then we assessed the intersection within the “Revised Risk Assessment Model2016.xlsx” spreadsheet. For the revised mercury risk assessment, the typical female subsistence fish 

consumer scenario was evaluated for all watersheds with fish tissue Hg data (i.e., we did not filter out 

any of these watersheds based on application of census data for this scenario). 

Additional detail on the layout of the Excel risk assessment spreadsheet can be found on the “NOTES” 

sheet incorporated as part of the spreadsheet.