### Appendix 8 Dispersion Model Receptor Revisions and Additions

### Dispersion Model Receptor Revisions and Additions for the Ferroalloys Source Category

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
di spersion model receptors. The census block centroids are generally good surrogate s for where
people live within a census block

. A census block generally encompasses about  0 people or  0 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

an y 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 very 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 add ed dispersion model receptors

ot her 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 i n 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 this source category, 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 the Ferroalloys Source Category

Centroid Revisions
Block ID NEI ID New
Latitude New

Longitude Comment

391670203001029 39.36404

- 81.53392 Not representive of population

540539548022127 38.952101

- 81.922325 On plant property

540539548022183 38.944595

- 81.921297 Not representive of population

39167NEI11660 39.384305

- 81.509796 Additional receptor

39167NEI11660 39.386712

- 81.517476 Additional receptor

39167NEI11660 39.373718

- 81.532765 Additional receptor

39167NEI11660 39.36754

- 81.535303 Additional receptor

54053NEIWV053FELMAN 38.948407

- 81.923071 Additional receptor

54053NEIWV053FELMAN 38.960334

- 81.939735 Additional receptor