SITE INFORMATION AND CORRESPONDENCE_2001-2005

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Lincoln Center, Stockton • 0 w Revised Draft Feasibility Study - 2 - February 13, 2003 The soil RAO is to reduce the mass of Hazardous Substances in soil and soil vapor to concentrations that (1)would result in an indoor-air-inhalation risk of less than or equal to 10-5 excess lifetime cancer risk of a hypothetical worker and(2)result in a concentration of Hazardous Substances in groundwater at concentrations less than or equal to MCLS. Use of the hypothetical worker exposure scenario can only be used in conjunction with a selected remedy that includes institutional controls such as deed restrictions to assure no future residences, schools or hospitals are constructed on the site. We note that the"point of departure"in the NCP is an excess cancer risk level of 10-6. Use of the 10-5 excess cancer risk level must be justified. Lastly,under Board requirements, resulting concentrations in groundwater must be as low as technically and economically feasible. In no case, can resulting concentrations be greater than MCLS. The RAOs should be revised to incorporate these concepts. 3. Section 2.3.1 entitled"RAOs Based on Indoor-Air Inhalation" again improperly uses a worker based exposure scenario. The FS should include the details and assumptions used in the calculation of allowable air concentrations and the vapor-transport modeling. The sensitivity of the model to a reasonable range of values for each input parameter into the model should be presented and discussed. 4. Section 2.3.2 entitled"RAOs Based on Groundwater" indicates that analytical modeling was performed using EPA's Soil Screening Guidance. We do not currently accept the use of soil screening levels developed using that guidance. RAOs should be developed using site specific parameters, input into a vadose zone contaminant transport model such as VLEACH. RAOs should be based on leachate concentrations, as predicted by the model, without inclusion of dilution of leachate by the groundwater. 5. Section 2.4 entitled"Remedial Action Objectives for Groundwater" again improperly uses a worker based exposure scenario. Also,reference is made to MCLs as the RAO when, under Board requirements, resulting concentrations in groundwater must be as low as technically and economically feasible. Equations, assumptions, etc. used in the back-calculated health- based RAO should be provided. 6. On pages 103 and 109, under the paragraphs entitled"Reduction of Total Mass of Hazardous Substances",the document states that the RAO for vadose-zone soils is 150 ug/1 for PCE in soil gas. Please see our comments above regarding the development of an appropriate RAO for vadose-zone soils. 7. The"Recommended Remedial Plan"is described in Section 6 of the document. The recommended is Alternative 4 that involves the following activities: • vadose-zone source reduction through continued operation of the Soil Vapor extraction and treatment system, • continued operation of the groundwater extraction and treatment system, • A-zone groundwater source reduction, and