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Wendy L. Cohen - 2- • 29 September 1998 7 <br /> 5. The risk assessment is to include an assessment of the current and future risks of the off-site <br /> plume of groundwater. <br /> 6. Section 5.4 of the work plan contains misleading information. Any chemical found at the site is a <br /> COPC. All COPCs are run through the entire risk assessment. Chemicals of Concern (CDCs) are <br /> identified through the risk assessment as the risk"drivers", i.e., those chemicals for which some <br /> action must be taken in order to reduce the overall health risk at the site. Section 5.4 references a <br /> 5 March 1997 memo in which I identified chemicals in groundwater which exceeded water quality <br /> criteria. This memo was not intended to identify COPCs or CDCs. <br /> 7. Unocal proposes to utilize two models to determine saturated zone transport and the impact of <br /> chemicals leaching from the soil into the groundwater. Board staff have expertise in using and <br /> reviewing these types of models. To prevent the need to make major modifications of the final <br /> risk assessment, an interim submittal should be a description of each of the models used, the <br /> default assumptions, the input conditions, and the results. Once Board staff review and approve <br /> the results, Unocal can use these models in the risk assessment. <br /> 8. Section 6.2.1 states that local groundwater will not be evaluated as a domestic water supply. <br /> However, the Basin Plan states all sources of groundwater are to be considered suitable for <br /> domestic supply, and cleanup numbers are to reflect this potential use. Consequently, the risk <br /> assessment needs to assume that the groundwater is used as drinking water. <br /> 9. Unocal proposes to calculate cleanup levels based on the probabilistic HRA results only. Because <br /> the probabilistic method is so new,I would also like to see target cleanup levels based on the <br /> deterministic HRA results. It is acceptable to assume that a total carcinogenic risk of less than <br /> 10"5 will not pose a health risk. <br /> 10. At our meeting of 3 September 1998, Unocal had questions about how the Board determines <br /> cleanup levels. This information is provided in the Basin Plan, and may be summarized as <br /> follows: Contaminants are to be cleaned up to background levels unless the Discharger shows that <br /> it is technically or economically infeasible to do so. In that case, the cleanup level will range from <br /> background to the lower of the human health risk value or the most appropriate water quality <br /> criterion. <br /> WSW <br />