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indicate that a combined total of approximately 0 165 gallons of hydrocarbons may be remaining in the Site soil This estimate included 0 01 gallons of benzene, 0 007 gallons of toluene, 0 03 ' gallons of xylenes, 0 09 gallons of ethylbenzene, and 0 02 gallons of 1,2 DCA 3.3.2 Former Septic Leach Area Laboratoryanalyses of soil samples collected in the former septic leach area of the Mayor Mart site, ' indicated that the semivolatile organics 2-methylnaphthalene and naphthalene were detected at concentrations of 4 4 and 2 5 mg/kg, respectively, at a depth of 26 feet below ground surface (bgs) in boring SB-3 These materials were not detected in soil samples collected at a depth of 31 feet bgs Soil samples collected at 26 feet from SB-4 were not affected During the installation of monitoring well MW-4 by WESTON, it was reported that the well was drilled through the center of the area For worst case calculation, we have assumed that MW-4 is the center of the radius and the farther ' (and non-detect) soil boring, SB-4, delineates the exterior point of impacts A more realistic assumption is that soils affected form a conical shape around MW-4 ' Therefore, in order to develop a mass determination of the soil impacted by naptha compounds in the former septic leach area, the dimensions presented in Table 2B have been utilized Utilizing this ' conical volume, and as shown on Table 2B, the naphtha compound mass (naphthalene and 2- methylnaphthalene) in the former septic leach area is calculated to be 0 1615 gallons 3.4 WATER QUALITY IMPACT MODELING (SESOIL) Laboratory data from confirmation soil samples collected by WESTON from the base of the tank-pit excavation (WESTON, May 14, 1993) along with data from samples collected by SECOR from boring SB-1 (SECOR, September 30, 1994) indicate the presence of residual trace levels of BTEX compounds in Site soil Additionally, data collected from SECOR boring SB-3 (SECOR, September 30, 1994) in the vicinity of the former septic leach area indicated the presence of naphthalene and 2-methylnaphthalene in the soil column as a result of former septic waste discharge Due to the presence of these residual soil impacts, vadose zone chemical transport modeling was conducted to evaluate the possible future impacts to groundwater quality as a result of natural precipitation infiltration and/or a future rise in the groundwater level to historical levels Modeling was conducted using the vadose zone chemical transport model SESOIL, a seasonal soil compartment model (SESOIL, 1982) The SESOIL model is contained in a program package titled Graphical Exposure Modeling System (PCGEMS, 1986) This program was produced under contract to the Environmental Protection Agency (EPA) and has been used by the EPA Exposure Evaluation Division The PCGEMS package provides an expert system for estimation of modeling parameters and allows coupling of several types of models The SESOIL model estimates the rate ISECOR international Inc (SECOR®) CRC Env¢o=ental Risk Management Inc N0025-001-01 add 10 November 30 1994 i