Human and Ecological Risk Assessment - Earthjustice

Section 4.0Risk Characterizationfatal to humans 13 leads EPA to conclude that omitting the thallium results from this report mightcause thallium’s existence in coal combustion residues to be overlooked during the riskmanagement decision making, and thus EPA has chosen to retain those modeling results in thisreport.ConstituentTable 4-31. RfD Uncertainty Factors for and Benchmark Confidence for CCWConstituents with HQs Over 1RfD(mg/kg-day)SourceUncertaintyFactorBenchmarkConfidenceHighestCCWHQCCW Scenario forHighest HQAntimony 4.0E-04 IRIS 1,000 low 3 GW-DW, FBC wastes,clay-lined landfillsBoron 2.0E-01 IRIS 66 high 7 GW-DW, ConventionalCCW, unlined SIsCadmium 5.0E-04 IRIS 10 high 9 GW-DW, CodisposedCCW, unlined SIsCobalt 3.0E-04 PPRTV 1,000 low 500 GW-DW, CodisposedCCW, unlined SIsMolybdenum 5.0E-03 IRIS 30 medium 8 GW-DW, ConventionalCCW, unlined SIsSelenium 5.0E-03 IRIS 3 high 3 GW-SW, ConventionalCCW, unlined SIsThallium 8.0E-05 IRIS 3,000 low 4 GW-DW, FBC wastes,clay-lined landfillsMost health benchmarks used in this analysis were from IRIS. Human health benchmarksin IRIS have been subjected to rigorous internal and external reviews and represent Agency-wideconsensus human health risk information. However, some benchmarks in IRIS are quite dated.Provisional human health benchmarks derived by the Superfund Technical Support Center havebeen peer reviewed and are used where there is no IRIS value.Chemical-specific health benchmarks were used for all constituents assessed in theanalyses. However, the RfD for fluoride was based on fluorine; the RfDs for mercuric chlorideand methyl mercury were used as surrogates for elemental mercury from food, soil, and wateringestion, and fish ingestion, respectively; and the RfD for thallium was based on thalliumchloride. The use of these surrogate data is not thought to have introduced any significantuncertainty. Human health benchmarks are not age-specific, and therefore, were applied to bothchild and adult receptors, thereby introducing some uncertainty.EPA used the drinking water MCL for lead to estimate risks from drinking waterexposure. The IEUBK model may better quantify risk for a young child exposed to lead;therefore, use of the MCL may introduce some uncertainty. However, risks from lead exposurewere relatively low, well below the risk criterion for landfills and at or slightly above the riskcriterion for surface impoundments, and did not drive the risk assessment conclusions.13 “Temporary hair loss, vomiting, and diarrhea can also occur and death may result after exposure to large amountsof thallium for short periods. Thallium can be fatal from a dose as low as 1 gram.” (ATSDR, 1992)April 2010–Draft EPA document. 4-56