Human and Ecological Risk Assessment - Earthjustice

Section 3.0AnalysisNoncancer risk was characterized through the use of HQs, which are generated bydividing an ADD (see Section 3.8.3) for ingestion pathways by the corresponding RfD. 13 An HQestablishes whether a particular individual has experienced exposure above a threshold for aspecific health effect. Therefore, unlike cancer risk estimates, HQs are not probabilitystatements. Rather, the RfD represents an estimate (with uncertainty spanning perhaps an orderof magnitude) of a daily oral exposure to the human population (including sensitive subgroups)that is likely to be without an appreciable risk of deleterious effects during a lifetime. It can bederived from a no observed adverse exposure level (NOAEL), lowest observed adverse exposurelevel (LOAEL), or benchmark dose, with uncertainty factors generally applied to reflectlimitations of the data used. Equation 3-5 shows the calculation for the ingestion HQ. Thiscalculation was completed for each pathway considered (i.e., drinking water ingestion and fishconsumption).ADDiHQi = (3-5)RfDwhereADD i = average daily dose for ingestion pathway i (mg/kg-d)i = pathway indexRfD = reference dose (mg/kg-d).The risk results address risk from exposure via the groundwater-to-drinking-water andgroundwater-to-surface-water pathway separately. This is appropriate because the residentconsuming contaminated groundwater may not be the recreational fisher who is consumingcontaminated fish. Also, the arrival time of the contaminant plume to the stream and the humanreceptor may not be the same for a particular iteration. 14 However, a resident may consume fishcaught from a nearby stream or lake and contaminated drinking water if the travel times aresimilar, so that possibility should be considered as an uncertainty in this analysis (see Section4.4.1).For each receptor type, lifetime excess cancer risk estimates for arsenic were calculatedseparately for the drinking water and fish consumption pathways.3.9.2 Ecological Risk EstimationThe full-scale analysis addressed two routes of exposure for ecological receptors: directcontact with contaminated media and ingestion of contaminated food items. HQs were calculatedusing chemical-specific media concentrations assumed to be protective of ecological receptors ofconcern through either exposure route (CSCLs). As described in Section 3.1.2, these ecologicalbenchmarks were developed for representative organisms and communities in eachenvironmental medium of concern.13 HQs calculated for lead in drinking water were based on the drinking water action level (0.015 mg/L); leadexposures from fish ingestion are reported as an ADD.14 Stream distance and well distance were sampled independently in the Monte Carlo analysis.April 2010–Draft EPA document. 3-46