Estimating risk assessment exposure point concentrations when the data arenot normal or lognormal

The U.S. Environmental Protection Agency (EPA) recommends the use of the on e-sided 95% upper confidence limit of the arithmetic mean based on either a normal or lognormal distribution for the contaminant (or exposure point) c oncentration term in the Superfund risk assessment process. When the data a re not normal or lognormal this recommended approach may overestimate the e xposure point concentration (EPC) and may lead to unecessary cleanup at a h azardous waste site. The EPA concentration term only seems to perform like alternative EPC methods when the data are well fit by a lognormal distribut ion. Several alternative methods for calculating the EPC are investigated a nd compared using soil data collected from three hazardous waste sites in M ontana, Utah, and Colorado. For data sets that are well fit by a lognormal distribution, values for the Chebychev inequality or the EPA concentration term may be appropriate EPCs. For data sets where the soil concentration da ta are well fit by gamma distributions, Wong's method may be used for calcu lating EPCs. The studentized bootstrap-t and Hall's bootstrap-t transformat ion are recommended for EPC calculation when all distribution fits are poor . If a data set is well fit by a distribution, parametric bootstrap may pro vide a suitable EPC.