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of vertical chemical transport and transformation in the soil column in terms of mass and concentration distributions among the soil, water and air phases in the unsaturated soil zone The ' soil column is defined as a compartment which extends from the soil surface through the unsaturated zone to the groundwater table Model output includes concentrations of source chemicals In vadose zone soil moisture at varying depths within the model layers Soil moisture is derived from rainfall ' recharge at the surface as it infiltrates through the soil column 3.4.1 Modeling Assumptions Residual BTEX compounds and 1,2 dichloroethane (1,2 DCA) present in the former tank pit area were modeled based on a conceptual soil column consisting of 62 feet of silty clay underlain by 8 feet of silty sand representing the groundwater zone The chemical distribution within the model is based on the results of soil sampling discussed above The input chemical distribution for BTEX and 1,2-DCA extends from the base of the former tank pit excavation (approximately 32 feet below grade)to the top of the sandy material at 62 feet and includes the detections at 66 feet below grade ' as presented in Table 3 An average source concentration of 9 9 ppb benzene between 32 and 66 feet below grade was assumed in the model ' Residual naphthalene compounds present in soil in the former septic leach area were detected at a depth of 26 feet below grade Due to a greater mobility than naphthalene, 2-methylnaphthalene was used as the indicator compound for modeling the two naphtha compounds detected The conceptual SESOIL model for the septic leach area is identical to that for the former tank pit area, with the exception that the presence of 2-methylnaphthalene was assumed to be distributed throughout the upper 26 feet of the soil column only Table 3 presents a summary of important chemical input parameters used in the SESOIL model runs Detailed printouts of all model input parameters are included as Appendix D 3.4.2 Modeling Results The results of SESOIL modeling indicate that dissolved concentrations of modeled compounds in soil moisture would be present at levels below the primary MCL for each compound with the exception of benzene and 1,2 DCA Benzene modeling results based on an average source concentration of 9 9 ppb from 32 to 66 feet indicate a soil moisture concentration at the groundwater interface of approximately 2 4 ppb The modeling results for 1,2 DCA in the same soil column based on an average source of 33 ppb from 32 to 66 ft bgs indicates that soil moisture concentrations could be derived at concentrations of 33 ppb at the groundwater interface Table 4 presents a summary of modeling results for each compound In reviewing these results, it must be taken into consideration that the derived soil moisture concentrations occurring at the groundwater interface will be much higher than the resulting dissolved concentration in groundwater This fact is primarily the result of dissolution and dispersion phenomenon within the fully saturated groundwater zone In comparing these modeling results with current detections in groundwater, it SECOR Intematzonal Inc (SECOR®) CRC Environmental Risk Management7c 11 N0026-001-01 add November 30 1994