Analysis - The Institute for Southern Studies

Step 2. Determine Potentially Affected Populations of Groundwater Drinkers
With information on the universe of facilities, WMUs, and liners nearby groundwater-drinking populations were assigned. To accomplish this,
EPA first assigned latitude and longitude coordinates to the 495 sites based on its 2007 eGRID database. Only a few sites were not in eGRID,
and it is assumed that these sites were constructed since the 2007 eGRID data collection. Once latitude and longitude data were assigned, EPA
used GIS data to ascertain the location of private groundwater wells within a one-mile radius from the latitude and longitude coordinates, and
then the number of individuals drinking from those wells. 109,110 The data divide populations into adults (18 and older) and children, the same
two populations examined in the 2009 risk assessment. Once these data were attached to specific sites, they were aggregated based on the
liner/WMU categories above.
Aggregated data were then scaled to account for the missing population information and population growth. First, the data was scaled up to
account for the missing population data in the sites not identified in eGRID. There were 5 unlined landfills, 1 clay-lined landfill, and 7
composite-lined landfills that had onsite disposal but no eGRID data from which to determine the population. All surface impoundments had
the necessary eGRID data. To account for these individuals, EPA made the assumption that these plants had populations similar to the plants
EPA had data for, since EPA had no data to suggest otherwise. 111 Thus, the population was scaled up by a scaling factor equal to the total
number of plants divided by the number of plants for which EPA had population data as follows:
1) Unlined Landfills = 76/71 (~1.07)
2) Clay-Lined Landfills = 28/27 (~1.04)
3) Composite-Lined Landfills = 150/143 (~1.05)
The populations were then scaled up to current population levels based on Census data, resulting in a scaling factor of 1.093.
Once these preliminary population estimates were produced, it was also necessary to account for the size of the waste management unit. In the
2009 risk assessment, WMUs were assumed to be square as a requirement of the model. Using the same assumption here, the actual 1-mile
radius around the square area of a WMU could be estimated by scaling the population density of the original 1-mile radius up to the area 1-mile
around a square WMU of average size. This led to scaling factors of 1.81 and 2.56 for landfills and surface impoundments, respectively.
Further discussion of this area-based scaling can be found in Appendix K2.
In addition to accounting for the increased area in the 1-mile radius, EPA assumed that half of the receptors would be up-gradient and half
would be down-gradient of the WMU. For the purposes of this assessment, populations were assumed to be equally distributed within the
109 This data was developed through the use of the 2000 census block data in combination with the 1990 census drinking water source data. For a further discussion of
population development, see Appendix K3 of this RIA.
110 Municipal water systems using groundwater often rely on deeper aquifers, in which case they would be less susceptible to contamination from CCR releases. Therefore,
these systems were not included in the 2009 Risk Assessment or in this analysis. However, exposure through this pathway is possible, which means that these population
estimates could underestimate the population that is exposed to these wastes.
111 EPA does not account for CCR disposed off-site as in the Gambrills MD and Chesapeake VA damage cases.
113