Human and Ecological Risk Assessment - Earthjustice

Section 4.0Risk Characterizationand aluminum showed 90th percentile risks above the ecological risk criterion, with boronshowing an HQ over 2,000 for the unlined units, and other HQs ranging from 3 to 22 for unlinedunits. The 50th percentile results are all well below an HQ of 1 for landfills and only exceed anHQ of 1 for boron in unlined surface impoundments (HQ = 7).As with other pathways and receptors, the difference in the number and magnitude ofecological HQs that exceed the risk criterion between landfills and surface impoundments islikely the result of (1) higher CCW constituent concentrations in surface impoundmentporewater and (2) the greater flux of contaminants to groundwater predicted during the activelife of the surface impoundment. As discussed in Section 4.1, the higher infiltration rates forsurface impoundments result from a higher hydraulic head in the impoundment and a higherproportion of unlined surface impoundments than landfills in the 1995 EPRI survey data used forthis risk assessment.Table 4-21. Summary of 90th Percentile Full-Scale CCW Ecological Risk Results:Groundwater-to-Surface-Water Pathway, Aquatic Receptors aChemicalLandfillsUnlined Units90th Percentile Ecological HQClay-LinedUnitsComposite-Lined UnitsExposurePathwayReceptorBoron 281 78 0.07 direct contact aquatic biotaLead 8 0.4 2E-06 ingestion river otterSelenium (VI) 2 0.7 3E-04 direct contact aquatic biotaArsenic (V) 2 0.1 4E-08 direct contact aquatic biotaBarium 2 0.2 0 direct contact aquatic biotaCadmium 0.5 0.1 3E-05 direct contact aquatic biotaAluminum 0.01 0.003 1E-07 direct contact aquatic biotaSurface ImpoundmentsBoron 2,375 854 257 direct contact aquatic biotaLead 22 7 2 ingestion river otterArsenic (V) 13 4 5 direct contact aquatic biotaSelenium (VI) 12 4 1 direct contact aquatic biotaCobalt 6 3 5 direct contact aquatic biotaBarium 3 1 0.8 direct contact aquatic biotaCadmium 1 0.7 0.4 direct contact aquatic biotaAluminum 0.03 0.01 0.008 direct contact aquatic biotaaZero results indicate that contaminant infiltration rates were too small for the contaminant plume toreach the receptor during the 10,000 year period of the analysis.April 2010–Draft EPA document. 4-29