Human and Ecological Risk Assessment - Earthjustice

Section 4.0Risk Characterization90th Percentile Risk Results by CCW Type: SurfaceImpoundments, Groundwater-to-Surface-Water Pathway (continued)Chemical b90th Percentile HQ or Cancer Risk Value aUnlined UnitsClay-Lined UnitsComposite-LinedUnitsSelenium VI 1 0.8 7E-10aValues are HQs for all chemicals except arsenic; arsenic values are cancer risk. Zero resultsindicate that contaminant infiltration rates were too small for the contaminant plume to reachthe receptor during the 10,000 year period of the analysis.b Note that only the chemicals with adequate data that were identified in the screening analysis asneeding further assessment (see Section 3.2.4) were modeled.Table 4-11. 50th Percentile Risk Results by CCW Type: SurfaceImpoundments, Groundwater-to-Surface-Water PathwayChemical b50th Percentile HQ or Cancer Risk Value aUnlined UnitsConventional CCW – 44 surface impoundmentsCancerClay-Lined UnitsComposite–LinedUnitsArsenic III 4E-08 6E-10 0Arsenic V 7E-09 2E-11 0NoncancerCadmium 6E-04 6E-06 0Selenium IV 5E-05 1E-11 0Selenium VI 0.02 3E-04 0Codisposed CCW and Coal Refuse – 72 surface impoundmentsCancerArsenic III 6E-08 1E-08 0Arsenic V 3E-08 2E-09 0NoncancerCadmium 1E-03 2E-04 0Selenium IV 8E-05 4E-07 0Selenium VI 3E-03 8E-04 0aValues are HQs for all chemicals except arsenic; arsenic values are cancer risk. Zero resultsindicate that contaminant infiltration rates were too small for the contaminant plume to reachthe receptor during the 10,000 year period of the analysis.b Note that only the chemicals with adequate data that were identified in the screening analysisas needing further assessment (see Section 3.2.4) were modeled.As with the groundwater-to-drinking-water pathway analysis, the absence of risk fromcomposite-lined units indicates that the liners modeled in this analysis are effective at preventingcontaminants from reaching the surface waterbodies of interest. One should keep in mind that allsurface water results are calculated assuming that constituents are being added to thewaterbodies only via groundwater. However, for surface impoundment operation, effluent isconstantly being discharged directly into that same waterbody. These discharges are regulatedApril 2010–Draft EPA document. 4-16