Human and Ecological Risk Assessment - Earthjustice

Section 4.0Risk Characterizationvalues. Managed lands refer to a variety of lands designated by the federal government asworthy of protection, including National Wildlife Refuges, National Forests, Wildernessareas, and National Recreation areas.• Critical Habitats: Although critical habitats may be defined in a number of ways (e.g.,presence of threatened species, decreasing habitat area), wetlands are widely recognizedas serving critical ecological functions (e.g., maintenance of water quality). The U.S. Fishand Wildlife Service estimates that approximately 45 percent of the Nation’s threatenedand endangered species directly depend on aquatic and wetland habitats. Consequently,impacts of chemical stressors on wetland habitats may have high ecological (and societal)significance. The presence of critical habitats such as wetlands is also used to inform theselection of ecological receptors (e.g., amphibians, waterfowl) and the construction ofappropriate food webs.• Threatened and Endangered Species: For most ecological risk assessments of chemicalstressors, available data on toxicity and biological uptake are sufficient to support theevaluation of effects on representative species populations or generalized communities(e.g., the aquatic community). However, despite their obvious value, threatened andendangered species are frequently excluded from the analytical framework for nationalrulemakings. The assessment of threatened and endangered species requires a sitespecificapproach in which locations, habitats, and species of concern are identified andcharacterized with respect to the spatial scale of constituent releases.Although these classes of receptors and potential ecological hazards are not explicitly consideredin the analysis, conditions represented by simulations in the upper end of the risk distribution(higher risk scenarios) should reasonably characterize many situations with such sensitivespecies or habitats.Impact on Groundwater as a Resource. The risk assessment did not explicitly considerpotential impacts on the availability of groundwater in the future (e.g., contaminatedgroundwater becoming unsuitable for consumption), but the results do clearly indicate that therecan be a reduction in resource availability if CCW is improperly disposed. However, the scope ofthe risk assessment was to evaluate human health and ecological effects associated with currentwaste disposal practices and conditions, and a quantitative evaluation of potential futurereductions in groundwater availability as a consequence of CCW disposal practices was notconducted as part of this analysis.4.4.2 Model UncertaintyModel uncertainty is associated with all models used in a risk assessment because modelsand their mathematical expressions are simplifications of reality that are used to approximatereal-world conditions and processes and their relationships. Computer models are simplificationsof reality, requiring exclusion of some variables that influence predictions but that cannot beincluded in models either because of their complexity or because data are lacking on a particularparameter. Models do not include all parameters or equations necessary to express realitybecause of the inherent complexity of the natural environment and the lack of sufficient data todescribe the natural environment. Because this was a probabilistic assessment that predictedApril 2010–Draft EPA document. 4-41