Human and Ecological Risk Assessment - Earthjustice

Section 4.0Risk Characterizationchemical and the percent of site-averaged values 11 that are composed entirely of nondetectmeasurements. Although some constituents have a large number of nondetect values, many ofthose could still be modeled (substituting half the detection limit for nondetect values). Wherethere are detections for a chemical, the specific substitute value used for nondetect values doesnot affect the upper percentile risks, because the upper percentile risks are associated with thehigher, detectable source concentrations in the distribution rather than the lower sourceconcentrations associated with nondetect values. Values for nondetects will be in the lowerpercentiles whether they are half the detection limit or some other value.Table 4-30. Proportion of Nondetect Analyses for Modeled CCW ConstituentsChemical a Measurements SitesNumber % nondetects Number % with all nondetectsLandfillsAluminum 397 18% 61 5%Antimony 496 50% 66 41%Arsenic 1,182 49% 128 20%Barium 1,225 11% 126 5%Boron 930 8% 83 2%Cadmium 1,237 50% 124 31%Cobalt 559 56% 52 19%Lead 1,109 60% 125 30%Mercury 974 91% 101 58%Molybdenum 373 24% 58 10%Nitrate/Nitrite 141 48% 20 15%Selenium 1,227 49% 131 17%Thallium 402 60% 40 45%Surface ImpoundmentsAluminum 158 10% 16 6%Antimony 11 100% 2 100%Arsenic 155 16% 16 6%Barium 161 14% 16 13%Boron 164 7% 171 6%Cadmium 164 68% 16 50%Cobalt 49 59% 4 50%Lead 138 78% 14 36%Mercury 1 100% 1 100%Molybdenum 161 37% 17 24%Nitrate/Nitrite 267 59% 14 7%Selenium 140 33% 15 20%(continued)11As explained in Appendix A, the CCW risk assessment used site-averaged constituent concentrations. That is,an average value was used when there were multiple measurements for a chemical at a particular site.April 2010–Draft EPA document. 4-49