Human and Ecological Risk Assessment - Earthjustice

Section 4.0Risk Characterization4.4.3 Parameter Uncertainty and VariabilityParameter uncertainty occurs when (1) there is a lack of data about the values used in theequations, (2) the data that are available are not representative of the particular instance beingmodeled, or (3) parameter values have not been measured precisely or accurately because oflimitations in measurement technology. Random, or sample, errors are a common source ofparameter uncertainty that is especially critical for small sample sizes, as illustrated by the FBCwaste results discussed in Section 4.1.3.2. More difficult to recognize and address arenonrandom or systematic errors that can bias the analyses from sampling errors, faultyexperimental designs, or bad assumptions.Spatial and temporal variability in parameters used to model exposure account for thedistribution in the exposed population. For example, the rainfall or precipitation rates used tocalculate infiltration and recharge to groundwater are measured daily by the National WeatherService at many locations throughout the United States, and statistics about these parameters arewell documented. Although the distributions of these parameters may be well known, their actualvalues vary spatially and temporally and cannot be predicted exactly. Thus, the annual averageinfiltration rates used in the source model for a particular climate station provide information onaverage conditions appropriate for this analysis. Additionally, using data from multiple climatestations located throughout the United States can account for some, but not all, spatialvariability.4.4.3.1 Waste ConcentrationsThe CCW constituent database used to represent CCW total waste and waste leachateconcentrations is arguably the most important data set in terms of driving the risk assessmentresults. The constituent data are subject to two primary uncertainties beyond the normalsampling and analysis uncertainty associated with environmental measurements: (1) theappropriateness of the landfill leachate data used in the analysis and (2) high percentages ofnondetect analyses for some CCW constituents.Appropriateness of Leachate Data. The CCW leachate data were collected from avarying number of sites using a variety of methods. The available landfill data were largelyderived from the TCLP, a laboratory test designed to estimate leachate concentrations inmunicipal solid waste (MSW) landfills. The TCLP has been shown to both over- andunderpredict leachate concentrations for other waste disposal scenarios, so the use of the TCLPdata to represent CCW leachate is another source of uncertainty. However, as noted below, theTCLP data do appear to encompass the range of variability in CCW leachate concentrations thathave been measured in more recent studies.Surface impoundment leachate is represented by porewater measurements taken beneathactual impoundments, which should more closely represent the leachate seeping from the bottomof the impoundment than would bulk surface impoundment waste concentrations. The porewateris in direct contact with the waste, so these concentrations should typically be at least as great asconcentrations in the bulk surface impoundment. However, although these porewater dataarguably should better represent leachate concentrations, they are fewer in number than thelandfill data and therefore subject to uncertainty as to how representative they are of all CCWApril 2010–Draft EPA document. 4-46