Dispersion Model Receptor Revisions and Additions for the Phosphoric Acid 

Manufacturing and Phosphate Fertilizer Production Source Categories

To estimate ambient concentrations for evaluating long-term exposures, the HEM-3 

model uses the geographic centroids of census blocks (currently utilizing the 2010 Census) as 

dispersion model receptors. The census block centroids are generally good surrogates for where 

people live within a census block. A census block generally encompasses about 40 people or 10-

15 households. However, in cases where a block centroid is located on industrial property, or 

where a census block is large and the centroid less likely to be representative of the block’s 

residential locations, the block centroid may not be an appropriate surrogate.

Census block centroids that are on facility property can sometimes be identified by their 

proximity to emission sources. In cases where a census block centroid was within 300 meters of 

any emission source, we viewed aerial images of the facility to determine whether the block 

centroid was likely located on facility property. The selection of the 300-meter distance reflects a 

compromise between too few and too many blocks identified as being potentially on facility 

property. Distances smaller than 300 meters would identify only block centroids near the 

emission sources and could exclude some block centroids that are still within facility boundaries, 

particularly for large facilities. Distances significantly larger than 300 meters would identify 

many block centroids that are outside facility boundaries, particularly for small facilities. Where 

we confirmed a block centroid on facility property, we moved the block centroid to a location 

that best represents the residential locations in the block.

In addition, census block centroids for blocks with large areas may not be representative 

of residential locations. Risk estimates based on such centroids can be understated if there are 

residences nearer to a facility than the centroid, and overstated if the residences are farther from 

the facility than the centroid. To avoid understating the maximum individual risk associated 

with a facility, in some cases we relocated block centroids, or added dispersion model receptors 

other than the block centroid. We examined aerial images of all large census blocks within one

kilometer of any emission source. Experience from previous risks characterizations show that in 

most cases the MIR is generally located within 1 km of the facility boundary. If the block 

centroid did not represent the residential locations, we relocated it to better represent them. If 

residential locations could not be represented by a single receptor (that is, the residences were

spread out over the block), we added additional receptors for residences nearer to the facility 

than the centroid. 

For these source categories, the table below contains each census block for which we 

changed the centroid location because it was on facility property or was otherwise not 

representative of the residential locations in the block. The table also contains the locations of 

additional receptors that were included to represent residential locations nearer to the facility 

than the block centroid.
Revised Census Block Centroid Locations and Additional Receptors 

for Phosphate Fertilizer Production and Phosphoric Acid Manufacturing 

Centroid Revisions

Additional Receptors

Source Category NEI ID

HEM Census 

Block Latitude Longitude Note

Phosphoric Acid and Fertilizer 

Produuction 16029NEIIDR029000 160299602002082 42.692691 -111.551232

Change centroid to a 

population area

Phosphoric Acid and Fertilizer 

Produuction 12057NEIFL0570008 120570136041053

27.865795 -82.380258

Change centroid to a 

population area

Phosphoric Acid and Fertilizer 

Produuction 12057NEIRFL050059 120570139032000 27.846196 -82.084055

Change centroid to a 

population area

Phosphoric Acid and Fertilizer 

Produuction 12105NEIFL12286 121050148043024 27.932274 -81.893307

Change centroid to a 

population area

Phosphoric Acid 

Manufacturing 22047NEI6147

220479532003048 30.21858 -91.09597

Change centroid to a 

population area

Phosphoric Acid and Fertilizer 

Produuction 37013NEI45571 370139308001152 35.356689 -76.734247

Change centroid to a 

population area

Phosphoric Acid and Fertilizer 

Produuction 37013NEI45571 370139308001023 35.338224 -76.860086

Change centroid to a 

population area

Phosphoric Acid and Fertilizer 

Produuction 12047NEIFLT#4056 120479601001112 30.476025 -82.860864

Change centroid to a 

population area

Phosphoric Acid and Fertilizer 

Produuction 12057NEIFL0570008 120570136041073 27.869574 -82.395876

Make Zero Population

Recommended Corr.

Source Category NEI ID

HEM Census 

Block Latitude Longitude Note

Phosphoric Acid and Fertilizer 

Produuction 12101NEIFL0570005

28.172136 -82.153143 Add Receptor

Phosphoric Acid and Fertilizer 

Produuction 12057NEIFL0570008 27.865253 -82.38053

Add Receptor

Phosphoric Acid and Fertilizer 

Produuction 12057NEIRFL050059

27.84420 -82.07470 Add Receptor

Phosphoric Acid and Fertilizer 

Produuction 12057NEIRFL050059

27.827050 -82.087423 Add Receptor

Phosphoric Acid 

Manufacturing 22047NEI6147

30.218330 -91.083472 Add Receptor

Recommended Corr.