Human and Ecological Risk Assessment - Earthjustice

Appendix BWaste Management UnitsFor landfills, because the data were limited (8 sites), the model runs assumed that thepercent below grade ranged from 1 to 100 and was uniformly distributed. For each landfilliteration, a random value for percent below grade was picked and applied to the landfill depth todetermine depth below ground surface. This value was constrained to be no deeper than thewater table and was checked to see that EPACMTP groundwater mounding constraints were notviolated.For surface impoundments, more data were available (16 sites), with 8 sites beingconstructed entirely below grade and the remaining 8 sites ranging from 7.5 to 45 feet abovegrade. For each surface impoundment iteration, height above grade at these 15 sites wasrandomly sampled as an empirical distribution and applied to the overall surface impoundmentdepth to determine depth below ground surface.B.6 Calculation of WMU Depth and Imputation of Missing WMU DataThe EPRI survey includes information on the total area and total waste capacity of eachlandfill and surface impoundment included in the survey. To calculate average depth for eachWMU (a necessary EPACMTP model input), the total waste capacity was divided by the area.The resulting depths were then checked for reasonableness. For surface impoundments, onedepth (1 foot) was culled as being unrealistically low and one (700 feet) as too high. Two landfilldepths less than 2 feet and one depth greater than 350 feet were also removed from the database.In these cases the EPRI waste capacity data were culled and replaced using the regressionsdescribed below (i.e., WMU areas are considered more reliable than the capacity estimates in thesurvey data), and new capacities were estimated as described below.In addition, four landfills and six surface impoundments had neither area nor capacitydata in the EPRI survey. In these cases, the EIA facility locations were used to find the plantsand their WMUs on aerial photos from the Terraserver Web site (http://terraserver-usa.com/geographic.aspx), and a geographic information system (GIS) was used to measure the areas ofthe units in question. Capacities were then estimated as described below.To impute data for facilities missing either area or capacity data in the EPRI survey,linear regression equations were developed based on WMUs with both area and capacity data,one to predict area from capacity, and one to predict capacity from area. The final regressionequations are shown in Figures B-4 and B-5 for landfills and Figures B-6 and B-7 for surfaceimpoundments. In each case, a standard deviation around the regression line was also computedand used during source data file preparation to randomly vary the area or capacity from iterationto iteration within the bounds of the existing data set.April 2010–Draft EPA document. B-7