Human and Ecological Risk Assessment - Earthjustice

Section 4.0Risk CharacterizationWith respect to the clay liners, the 2009 risk assessment used the assumption thatcompact clay liners were designed to have a hydraulic conductivity of 1×10 -7 cm/sec. This isconsistent with EPA’s Industrial D Guidance, which states that “clay liners should be at least2 feet thick and have a maximum hydraulic conductivity of 1×10 -7 cm/sec” (U.S. EPA, 2006d).However, clay liners designed to meet a 1×10 -7 cm/sec hydraulic conductivity could performdifferently in practice. In one liner study (Moo-Young et al., 2004), a small set of clay-linedlandfills were found to have field hydraulic conductivities ranging from 2×10 -9 to4.4×10 -8 cm/sec and a small set of surface impoundments were found to have field hydraulicconductivities ranging from 3×10 -6 to 3.2×10 -5 cm/sec. Thus, the assumption of clay linersperforming at 1×10 -7 cm/sec could lead to an under- or over-estimate of actual risks.Composite liners would also not be expected to perform consistently over 10,000 years aswas assumed in the model. Instead, the liner would eventually perform at the level of the claylayer once the synthetic layer had deteriorated. This simplification is likely to lead to anunderestimate of composite liner risks.4.4.3.3 Fate and Transport Model VariablesThe parameter values required to model contaminant fate and transport in groundwaterwere obtained from site-specific, regional, and national databases. Hydrogeologic environmentwas assigned to each site, based on geologic maps and soil conditions; where assignments wereuncertain, two or three settings might be used in the Monte Carlo analysis. Because aquiferproperties are highly variable and uncertain, reasonable sets of aquifer properties were selected,based on hydrogeologic environment, from a hydrogeologic database.Receptor Location (Drinking Water Wells). The sensitivity analysis (Section 4.3)showed that distance of a receptor from the contaminant source is an important influence onmedia concentration, especially for contaminants that strongly sorb to soil and aquifer materials.For the groundwater-to-drinking-water pathway, receptor location was represented as thedistance and position, relative to a contaminant plume, of residential drinking water wells fromthe WMU. Because no data were readily available on the distance of CCW disposal sites fromresidential wells, EPA used data from a survey of well distances from MSW landfills. Whetheror not this is an accurate representation of well distance for CCW landfills and surfaceimpoundment is an uncertainty in this analysis. EPA believes that the MSW well distancedistribution used is protective for CCW landfills and surface impoundments. See Appendix C,Section C.2, for more details.Location and Characteristics of Waterbodies. One aspect of the site configuration ofparticular relevance to the aquatic food chain modeling is the locations and characteristics of thewaterbodies. The size of the waterbodies (and the distance from the WMU) affects constituentconcentrations and loadings predicted for that waterbody. The distance from the WMU to thewaterbody was based on an empirical distribution of measurements, taken from actual CCWsites, of the distance from the edge of the WMU to the nearest stream or lake. The uncertaintyposed in this analysis is the sampling of this distribution as compared to a more certainmeasurement of the actual distance at each CCW site. Surface water variables, including flowand water quality parameters, were collected for the stream reach being modeled, or for a largerhydrologic region where data were not available for a particular reach.April 2010–Draft EPA document. 4-52