Scenario Uncertainty

Sources of scenario uncertainty include the assumptions and modeling decisions that are made to represent an exposure scenario. Because this risk assessment attempted to characterize current conditions by estimating risks from actual CCW disposal sites across the country, it was subject to less scenario uncertainty than risk assessments that rely on hypothetical conceptual models. However, certain aspects of the scenario are uncertain.

CCW Management Unit Data. The landfills and surface impoundments modeled in this risk assessment were placed, sized, and lined according to data from the 1995 EPRI survey (EPRI, 1997). New data collected by EPA and DOE since this risk assessment was conducted (U.S. DOE, 2006) indicate that liners are much more prevalent in WMUs constructed or

may be lower for future CCW disposal facilities (although most of the unlined WMUs have been closed with wastes remaining in the units).

Liner-related questions are especially important because liner configurations greatly influence infiltration rates, one of the most sensitive parameters in the risk assessment. In terms of risks through groundwater pathways, this risk assessment has shown that liners, in particular composite (combined clay and synthetic) liners, can limit risks through subsurface exposure pathway, and the DOE/EPA survey shows that liners are more prevalent in newly constructed WMUs and WMU expansions. Although the DOE/EPA survey does not shed light on how many unlined facilities are still operating today, it does indicate that more units are lined today than were 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 different hypothetical liner scenarios for each facility, such an approach was outside the original scope of this risk assessment, which was to evaluate current CCW management activities, not

hypothetical management scenarios. Furthermore, this approach likely would not have changed the general conclusion of the risk assessment that composite lined landfills pose less risk than clay lined landfills and that unlined landfills pose the greatest risk.

Direct Discharge of CCW Impoundments into Surface Water. Because this risk assessment addressed CCW disposal under RCRA, it did not include risks from the direct discharge of wastes into waterbodies, which are regulated under the Clean Water Act. Although not relevant for the management of RCRA waste disposal, EPA recognizes that CCW surface impoundment effluent may pose additional risks.

Effect of the 10,000-Year Timeframe for Groundwater. The risk assessment assumed that contaminant concentrations in the leachate remain constant throughout the 10,000-year modeling timeframe, although leaching may or may not persist for 10,000 years, depending on model inputs. The waste concentration model input was assumed to be a portion of the total waste concentration available to be leached, and it was assumed that 100% of the constituent in the waste could leach from the landfill. The nonlinear fate and transport solution used for metallic constituents in the unsaturated zone module of EPACMTP is based on the assumption that 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 time might be more realistic, the simplified leaching profile used by the model does not lead to a poorer estimate of risk associated with groundwater exposures. The adoption of a simplified leaching profile to support a non-linear sorption approach in the unsaturated zone offered a greater benefit and defensibility to the overall approach than assuming linear partitioning and a depleting leachate profile would have.

Receptor Populations Evaluated. The human receptors evaluated for the CCW risk analysis were a family with children residing near the CCW disposal facility, drinking from a private well screened in a surficial aquifer or eating fish caught from a nearby stream or lake impacted by CCW leachate. Additionally, except for a 15-day vacation, it was assumed that adults and children were exposed daily and that the private well was the only source of drinking water. Although it is possible for other types of individuals to be exposed, the use of the resident adult and child as protective of other receptors and pathways is a high-end, simplifying

assumption of the analysis. The lack of information to define and model actual exposure conditions also introduces uncertainty into this assessment, but EPA believes that the national distribution of exposure factors used is appropriate for a national assessment.

In addition, not all possible exposure pathways were evaluated. For example, the risk assessment did not consider potential indirect exposure to humans through game species that may have been exposed to surface impoundment waste (e.g., deer drinking surface impoundment water). This represents a potential uncertainty in the analysis.

Additive Risks Across Pathways. The human receptors evaluated in the CCW risk assessment were assumed not to consume both contaminated fish and drinking water from the same waterbody because untreated surface water is not considered potable water (municipal water treatment facilities were assumed to reduce contaminant levels prior to consumption). EPA also did not consider the potential cumulative exposure from contaminated fish and groundwater in the CCW risk assessment, because the exposures are likely to occur over different timeframes (because of differences in transit time of the contaminant plume to wells versus surface

waterbodies) and may involve different receptors (because a resident near a CCW surface impoundment or landfill and exposed via groundwater may not be a recreational fisher).

Although this could potentially miss some higher exposures for a maximally exposed individual, analysis of the individual pathway results does not indicate that adding such risks would change the conclusions of this risk assessment in terms of the constituents exceeding the risk criteria. Also, risks were high enough for single chemicals for human exposure pathways (notably arsenic) that this would not change the basic conclusion of the risk assessment that there are potentially significant risks to human health from CCW disposal in landfills and surface impoundments.

Co-Occurrence of Ecological Receptors and Constituents. As a simplification for national-scale analyses in the absence of site-based data, co-occurrence of the ecological receptors and the constituents of concern is typically assumed. However, the prior probability that a receptor will be found in waterbodies affected by constituent releases from CCW WMUs is not known, nor is it known whether a receptor will forage for food in contaminated areas or if those areas do, in fact, support the type of habitat needed by the receptor. Although the

assumption of co-occurrence was necessary for this analysis, relatively few field studies are available to demonstrate the relationship between adverse ecological effects and constituent releases from CCW as it is currently managed.

Ecosystems and Receptors at Risk. One challenge in conducting a predictive ecological risk assessment intended to reflect risks at a national scale is representing all of the receptors and ecosystems at risk. In Wastes from the Combustion of Coal by Electric Utility Power Plants -

Report to Congress (U.S. EPA, 1988b), the authors pointed out that plants or animals of concern

were located within a 5-km radius of the CCW WMUs at 12 to 32 percent of the sites. Although these figures are of limited spatial resolution, they suggest the possibility that threatened and endangered species or critical habitats may be at risk from CCW constituents. Examples of other critical assessment endpoints not evaluated in this analysis include the following:

ƒ Managed Lands: Because protected lands play a critical role in preserving plant and animal species, managed areas in the United States represent well-recognized ecological

values. Managed lands refer to a variety of lands designated by the federal government as worthy of protection, including National Wildlife Refuges, National Forests, Wilderness areas, and National Recreation areas.

ƒ 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 recognized as serving critical ecological functions (e.g., maintenance of water quality). The U.S. Fish and Wildlife Service estimates that approximately 45 percent of the Nation’s threatened and 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 the selection of ecological receptors (e.g., amphibians, waterfowl) and the construction of appropriate food webs.

ƒ Threatened and Endangered Species: For most ecological risk assessments of chemical stressors, available data on toxicity and biological uptake are sufficient to support the evaluation of effects on representative species populations or generalized communities (e.g., the aquatic community). However, despite their obvious value, threatened and endangered species are frequently excluded from the analytical framework for national rulemakings. The assessment of threatened and endangered species requires a site- specific approach in which locations, habitats, and species of concern are identified and characterized with respect to the spatial scale of constituent releases.

Although these classes of receptors and potential ecological hazards are not explicitly considered in the analysis, conditions represented by simulations in the upper end of the risk distribution (higher risk scenarios) should reasonably characterize many situations with such sensitive species or habitats.

Impact on Groundwater as a Resource. The risk assessment did not explicitly consider potential impacts on the availability of groundwater in the future (e.g., contaminated

groundwater becoming unsuitable for consumption), but the results do clearly indicate that there can be a reduction in resource availability if CCW is improperly disposed. However, the scope of the risk assessment was to evaluate human health and ecological effects associated with current waste disposal practices and conditions, and a quantitative evaluation of potential future

reductions in groundwater availability as a consequence of CCW disposal practices was not conducted as part of this analysis.