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Mr David DeWitt Page 4 November 2 , 1993 Unocal Corporation in Table 1. The approximate locations of the water wells are shown on the attached Well Location Map. Domestic wells are located at sites 91, #41 #5 , #6 , and #7 , and monitoring wells are located at sites #2 , #3 , and #8 . Based on a consistent southern gradient at the subject Unocal site, sites #6 and #7 are located downgradient of Unocal. Sites #1, #3 , #4 , and #5 are located either upgradient or cross-gradient of the sub3ect Unocal site. The two downgradient sites (#6 and #70) are located approximately 1, 500 feet and 2 , 000 feet from the Unocal site, respectively. It is important to note that ground water samples collected from the most downgradient well (MW8) at the Unocal site have consistently indicated non-detectable concentrations of TPH as gasoline and BTEx since the inception of sampling on January 3 , 1992 (over one hydrologic cycle) . Based on the consistent southerly ground water flow direction, the distance to the nearest potential receptor wells, and the consistent non-detectable concentrations of ground water contaminants in well MW3 , it does not appear likely that any of the residual ground water contamina- tion at the subject Unocal site could impact the identified wells FATE AND TRANSPORT COMPUTER MODEL - SESOIL iThe SESOIL model 15 one module of a larger software package called PCGEMS . PCGEMS was developed for the U S . Environmental Protection Agency (EPA) by the General Sciences Corporation. This software was developed for the EPA to provide an information management tool for use in exposure assessment studies The results of the SESOIL model can be used in the determination of whether ground water is threatened, and to what extent, by a given source of contamination. The State of California Water Resources Control Board (CWRCB) Leaking Underground Fuel Tank (LUFT) Field Manual recommends the application of the SESOIL model. Appendix E of the California LUFT manual describes in detail how the SESOIL model is used for general risk appraisal. KEI conducted three simulations utilizing the SESOIL computer package. Simulation #1 was conducted with a number of extremely conservative "worst case" assumptions for a simulation period of 10 years. Simulation #2 was conducted with the same simulation period of 10 years; however, assumptions were utilized that were more realistic and representative of the actual parameters encountered in the field. Simulation #3 was conducted using the same assump- tions as in Simulation #2, except that the simulation period was extended from 10 to 20 years. . The results of the three SESOIL computer simulations indicate that no significant impact to the ground water can be expected from the source of contamination at the sub)ect Unocal site. Of the three