Human and Ecological Risk Assessment - Earthjustice

Section 4.0Risk Characterizationarsenic and selenium were originally based on 100% arsenic III and selenium VI, which is ahigh-end assumption (i.e., arsenic III has higher risks than arsenic V and selenium VI has higherrisks than selenium IV). Although arsenic is generally thought to occur in the +3 form inleachate, there is evidence from damage cases at CCW disposal sites that suggests that arsenic IIIis converted to arsenic V during subsurface transport at some sites (see, for example, U.S. EPA,2000, 2003e; Lang and Schlictmann, 2004; Zillmer and Fauble, 2004). To address theuncertainty of running the model with 100% arsenic III and selenium VI, the models were alsorun assuming 100% arsenic V and selenium IV. The results from the two species should bracketthe results expected given some mixing of oxidation states.Bioavailability of Constituents to Ecological Receptors. For the purposes of thisanalysis, the model assumed that all forms of a constituent were equally bioavailable toecological receptors, and therefore, the actual exposures that may occur in the field tend to beoverestimated, thus making this a high-end assumption. Both the chemical form and theenvironmental conditions influence bioavailability and ultimately the expression of adverseeffects. For example, as discussed above, the form of arsenic has been shown to profoundlyinfluence mobility and toxicity.Compaction of Landfilled Waste. The source model did not consider potentialcompaction of CCW waste over time. Such compaction could decrease the hydraulicconductivity and the associated water infiltration. However, no readily available data wereidentified to support an analysis of the influence of CCW compaction on infiltration rates. Thecurrent approach would tend to overestimate infiltration rates compared to a model that wouldadjust the hydraulic conductivity over time due to compaction. EPA believes this is anappropriately conservative assumption given the lack of the information needed to accuratelymodel the effects of waste compaction.Landfills Assumed to be Above Water Table. The landfill source model andEPACMTP assume that the source is above the water table. However, some actual CCWdisposal units do extend below the water table. Because waste intersecting the saturated zonemay increase groundwater concentrations, the approach may underestimate risk in some cases.However, including this effect would strengthen a general conclusion of the analysis thatpotentially unacceptable risks exist in some cases with unlined and clay lined CCW landfills.Indirect Ecological Effects. Indirect ecological effects (e.g., depletion of food resources)were not considered in the analysis. For any given facility, the spatial scale of potentialcontamination would affect a very small proportion of the home range for typical species;determining impacts on food supply and habitat quality with regard to the landscape and overallhealth of the animals is not currently possible in a national-level assessment (and difficult tounderstand or estimate in the majority of site-specific assessments). In addition, many speciesare opportunistic feeders and will seek other areas if food sources decline, regardless of thesource of the stress to the food supply. For these reasons, EPA does not believe that it is possibleto consider indirect ecological effects in a national risk assessment like CCW.Aquifer pH. As explained in Section 3.4, aquifer pH was used to select the metalsorption coefficients that were in turn used to calculate retardation coefficients for groundwatertransport of the CCW constituents. To estimate pH in an aquifer impacted by CCW leachate, theApril 2010–Draft EPA document. 4-43