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Mr. Victor Izzo August 28, 1994 Page 3 Comment IV.- Section 5.1 Transport Media, Paragraph 1; The report states, "Based on the nature and extent of impact, the chemicals of concern appear to have migrated in ground water from the site towards the south and southwest. " Another alternative to this south to southwest trend is that the sewer line acted as a. conduit for the TCE contamination, while the contamination originally leaked from the sewer line itself. Since the plume and sewer line location coincide, and the city sewer plant ponds are located to the southh causing a constant high head and probably a northerly ground water gradient, rather than to the south, the leakage from the sewer line may have caused this southerly extension of this plume. This scenario should be considered when evaluating transport mechanism and TCE release points. Response: In all previous reports to the RWQCB between 1988 and 1991, maps provided by the San Joaquin County Flood Control and Water Conservation District were provided were included showing current ground water gradients, compared to information from a 1967 California Department of Water Resources Bulletin (No. 146, July 1967). The 1967 data clearly shows a southwestward ground water gradient in the vicinity of Ripon, parallel to the Stanislaus River, while,more recent data from 1988, 1989, 1990 and 1991 show a northerly gradient toward Stockton. Thus there is documented data that a southerly gradient existed during TCE use at the plant. Further, as discussed in Appendix F of the RI in connection with the ground water modeling, the sewer ponds (in particular the eastern pond) tended to recharge any TCE-containing wastes not lost to evaporation. This mechanism also accounts for the presence of TCE impacts to the south and west of the facility. Comment V.• Section. 5.4 Chemical Migration; Though we generally agree with the statements made inthis section, the nature and extent of soil contamination has not been adequately investigated. Because of this, the inference made of minimal vadose zone impacts can not be substantiated. Also the statement, "...ongoing soil vapor extraction has greatly reduced organic vapor concentration. " has little supporting data. The above conclusions need to be supported by data. Response: See the response to Comment III, in regard to adequate investigation of the vadose zone. The RWQCB's statement that little data is presented to support the reduction of organic vapor concentration ignores the data presented in Appendix D of the RI, which is a compilation of all the vapor readings from the initiation of the vapor extraction systems. These readings clearly show (as also presented in the Vacuum Extraction Systems Status Report) the significant mass of chemicals which have