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Description of GIS analyses conducted to support the national‐scale risk assessment for US Energy

Generating Units (US EGUs)

March, 2011  

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

pekar.zachary@epa.gov)

We have docketed the shapefiles used as the basis for the mercury risk assessment. This document

provides detail on how these shapefiles were used (including specific queries and functions applied

within GIS) to generate the subsets of data used in the risk assessment. For the populations modeled for

risk, we implemented the following step‐wise procedure in using GIS to support risk modeling:

Step 1) Identify set of watersheds (hydrologic unit codes – HUCs) with fish tissue methylmercury (MeHg)

sampling data that are less likely to be impacted by mercury loading from non‐USEGU sources: Intersect

the “Full_fish_tissue_MeHg_data_by_HUC” shapefile with (a) the “mineplant‐locations‐listed‐as‐gold‐

commodity” shapefile and (b) the “TRI‐2008‐nonEGU‐Hgsources” shapefile to identify those watersheds

(HUCs) which might have significant non‐EGU mercury loading. This subset of HUCs is removed to

produce a subset of HUCs which are less likely to experience significant non‐EGU mercury loading. Note,

this step is done once and then this dataset is used for all subsequent queries for the different fisher

populations modeled (i.e., this step does not have to be repeated).

Step 2) Link the fish tissue MeHg data at the HUC level from Step 1 with CMAQ air quality mercury

deposition modeling results (also at the HUC level): Here we intersect the results of step 1 (i.e., the set of

HUCs with fish tissue MeHg data that excludes HUCs with potential non‐USEGU mercury loading) with

the shapefiles containing CMAQ air quality mercury deposition modeling data presented at the HUC‐

level. There are two sets of CMAQ modeling results (one for the 2005 scenario – “2005_Hg_deposition”

‐  and one for the 2016 scenario – “2016_Hg_deposition”). Within each of these shapefiles, there are

fields for total mercury deposition (HG_base) and USEGU deposition (Hg_DIFF). By linking the fish tissue

data HUC shapefile with each of the air modeling results shapefiles, we now have a single shapefile that

includes (for the subset of HUCs with fish tissue data), both the fish tissue MeHg data and the mercury

deposition data.  As with Step 1 above, Step 2 is also completed once (for the 2005 and 2016 air

modeling results) and then those shapefiles  are used in modeling risk for each of the fisher populations

assessed in the analysis.    

Step 3) Identify US Census tracts with “source populations” for each of the fisher populations we are

modeling: Use the “US_Census_tract‐level_with_vietnamese_and_laotian_counts” shapefile to identify

US Census tracts with “source populations” for the specific fishing population being modeled (source

populations are described in the Mercury Risk TSD). For example, for black fishers in the Southeast, we

would identify those tracts in the Southeastern states that had at least 25 black individuals living below

the poverty line.   

Step 4) Identify which of the HUCs with fish tissue MeHg data are associated with the specific fisher

population being modeled (i.e., which HUCs will be modeled for subsistence fisher risk for that particular
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population): Intersect the output of step 3 above with the output of step 2 (i.e., intersect the shapefile

with US Census tracts containing source populations for the fishers being modeled with shapefile

containing the set of HUCs with MeHg fish tissue data and merged mercury deposition modeling

estimates). This merged dataset identifies those HUCs (watersheds) with MeHg fish tissue data that are

associated with US Census tracts containing the source population for the fisher population being

modeled. This subset of HUCs would be the set assessed for subsistence‐level risk for the specific fisher

population under consideration.  

We then extracted the HUC‐level data generated in Step 4 and imported it into the risk spreadsheet

used in the analysis (i.e., within “subsistence‐mercury‐risk.xlsx”) .  In developing the risk spreadsheet,

we did incorporate the ability to read in (using the VLOOKUP function) updated mercury deposition

estimates at the HUC‐level to increase the flexibility of the risk spreadsheets to look at different

deposition scenarios. This functionality is reflected in the version of the spreadsheet that is docketed

(see the “notes” tab in the excel spreadsheet).    

Note, that the above 4‐step procedure is used for modeling 6 of the 7 populations assessed in this

analysis. However, for the Tribal population near the Great Lakes (primarily Ojibwe and Chippewa), we

use the ceded territories shapefile (all_tribal_ceded_territory_Ojibwe) to identify HUCs that could be

subjected to Tribal subsistence fishing (rather than using US Census tracts containing the Tribal

populations as a means for identifying source populations).