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David Stavarek, R.G. - June 29, 1998 Page 5 8. Revise the statement: "Exposure point concentrations for direct contact exposure routes (i.e., incidental ingestion of soil and dermal contact) will be based on either the 95%UCL [of the mean] or the maximum detected concentration, whichever is lower." (page 18). This is conceptually wrong. If a reasonable number of samples are taken(n>_20),then by definition, the maximum value of a distribution(i.e.,the 100th percentile) is always greater than any estimate of central tendency. If the sample size is limited,then the maximum concentration should be used as a conservative value to account for uncertainty, which is the approach required by PEA Guidance. In other words, the decision on which value of concentration will be used should depend on the number of samples analyzed for(i.e., some sort of"weight of evidence"), and not on which result is smaller. 9. The final sentence in Section 5.0 (page 20)reads: "Typically, criteria used for selecting COPCs for HRAs are: frequency of detection, concentrations, aerial distribution in site media, and potential for human exposure(i.e., environmental behavior considerations." Comments: (a) Replace are with include. The first two criteria listed are only a partial list of the considerations in selecting COPCs; (b) Distribution of the contaminant in the site in question and potential for human exposure, are NOT considerations for selecting COPCs. The fundamental criteria for selection of COPCs are analytical chemistry and data quality. Please omit this part of the statement. SECTION 6.0 Exp OSUREASSESSMENT 1. The residential exposure scenario should specify a child exposure scenario. 2. Four models for environmental movement of contaminants are used in this Workplan: transport leaching from soil to groundwater (SESOIL model), dispersion and migration in groundwater(VHS model), volatilization into indoor air, and fugitive dust emissions. As these models are outside my expertise, I have asked professionals from the Department of Toxic Substances Control to review of the models proposed and the results obtained. 3. This Section in general describes that"relative insignificant" exposure pathways will not be evaluated as an exposure pathway of concern. The relative contribution of each contaminant and exposure pathway is only estimated following the risk characterization. Often, a single very potent carcinogen in a given exposure pathway a priori considered "non-important" may drive the overall cancer risk in a risk assessment. OEHHA discourages the elimination