Human and Ecological Risk Assessment - Earthjustice

Section 4.0Risk Characterizationmay be lower for future CCW disposal facilities (although most of the unlined WMUs have beenclosed with wastes remaining in the units).Liner-related questions are especially important because liner configurations greatlyinfluence infiltration rates, one of the most sensitive parameters in the risk assessment. In termsof risks through groundwater pathways, this risk assessment has shown that liners, in particularcomposite (combined clay and synthetic) liners, can limit risks through subsurface exposurepathway, and the DOE/EPA survey shows that liners are more prevalent in newly constructedWMUs and WMU expansions. Although the DOE/EPA survey does not shed light on how manyunlined facilities are still operating today, it does indicate that more units are lined today thanwere in the 1995 EPRI survey data set on which this risk assessment was based.Although it would have been possible to address this uncertainty by evaluating differenthypothetical liner scenarios for each facility, such an approach was outside the original scope ofthis risk assessment, which was to evaluate current CCW management activities, nothypothetical management scenarios. Furthermore, this approach likely would not have changedthe general conclusion of the risk assessment that composite lined landfills pose less risk thanclay lined landfills and that unlined landfills pose the greatest risk.Direct Discharge of CCW Impoundments into Surface Water. Because this riskassessment addressed CCW disposal under RCRA, it did not include risks from the directdischarge of wastes into waterbodies, which are regulated under the Clean Water Act. Althoughnot relevant for the management of RCRA waste disposal, EPA recognizes that CCW surfaceimpoundment effluent may pose additional risks.Effect of the 10,000-Year Timeframe for Groundwater. The risk assessment assumedthat contaminant concentrations in the leachate remain constant throughout the 10,000-yearmodeling timeframe, although leaching may or may not persist for 10,000 years, depending onmodel inputs. The waste concentration model input was assumed to be a portion of the totalwaste concentration available to be leached, and it was assumed that 100% of the constituent inthe waste could leach from the landfill. The nonlinear fate and transport solution used formetallic constituents in the unsaturated zone module of EPACMTP is based on the assumptionthat the leachate concentration released from the waste management unit is constant over time(see Section 3.3.5.3 of U.S. EPA, 2003b). Although a leaching profile that changes over timemight be more realistic, the simplified leaching profile used by the model does not lead to apoorer estimate of risk associated with groundwater exposures. The adoption of a simplifiedleaching profile to support a non-linear sorption approach in the unsaturated zone offered agreater benefit and defensibility to the overall approach than assuming linear partitioning and adepleting leachate profile would have.Receptor Populations Evaluated. The human receptors evaluated for the CCW riskanalysis were a family with children residing near the CCW disposal facility, drinking from aprivate well screened in a surficial aquifer or eating fish caught from a nearby stream or lakeimpacted by CCW leachate. Additionally, except for a 15-day vacation, it was assumed thatadults and children were exposed daily and that the private well was the only source of drinkingwater. Although it is possible for other types of individuals to be exposed, the use of the residentadult and child as protective of other receptors and pathways is a high-end, simplifyingApril 2010–Draft EPA document. 4-39