## Appendix 7

Dispersion Model Receptor Revisions and AdditionsDispersion Model Receptor Revisions and Additions for the P rimary Aluminum Reduction Plant Source Categor y

To estimate ambient concentrations for evaluating long

- term exposures , t he HEM

- 3 model uses the geographic centroids of census blocks ( currently utilizing the 20 1 2 Census) as dispersion model receptors . The census block centroids are generally good surrogate s for where people live within a census block

. A census block generally encompasses abo ut 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 sur rogate. 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 ae rial 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 prope rty. 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 residenti al 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 relocate d block centroid s , or add ed 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 f r om 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 residen tial locations , we relocated it to better represent the m . If residen tial locations could not be represented by a single receptor (that is, the residences were spread out over the block), we add ed additional receptors for residences nearer to the facility than the centroid

. For th ese source categories , t he 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.Additional Receptors for P rimary Aluminum Re duction Plants

A dditional Receptors