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Diffusion Coefficient in air �-' Xylene's molecular diffusion coefficient in air of 0.43 m2/day (0.050 cm2/sec) is reported in the caption to Figure 15.2.11 of reference no. 3. Air diffusion plays a minor transportation role compared to infiltration transport. Organic Carbon Partitioning Coefficients, Koc Reference no, 3, Table 15.2.5, page 15.2.5 lists a Koc of 830 L/kg for o-Xylene (1,2- Xylene). Values for the other xylene isomers are not given in reference no. 3 but reference no. 2 reports 692 cc/g for all three isomers. The more conservative value of 692 cc/g is specified. According to the Handbook of Hydrology (1993), the rate of biodegradation is determined by the number and type of microorganisms present, the toxicity of the parent compound or its daughter products to the microorganism population, the water content and temperature of the soil, the presence of electron acceptors and the oxidation-reduction potential, the soil pH, the availability of other nutrients for microbial metabolism, the water solubility of the chemical and possibly other factors. Biodegradation in SESOIL is modeled mathematically as a simple first order decay function for both the liquid phase and the solid phase. SESOIL does not attempt to model the extremely complex process of biodegradation. The chemical specific decay rates for both the liquid phase and the solid phase applicable to the given site are input. w Biodegradation rates are rarely measured for the liquid phase and the solid phase. The liquid phase and the solid phase biodegradation are generally lumped together and the over-all biodegradation rate is reported. Biodegradation rates for benzene, toluene and xylenes are calculated from conservative biodegradation half-life's for natural sites reported in Table 15.2.8 of reference no. 3. A biodegradation rate for ethylbenzene in soil and groundwater was located in Table 9.9 of reference no. 6. 5.4 Results of Analysis 5.4.1 SESOIL Output Files Using the input parameters discussed in Section 5.3, four output runs for benzene, toluene, ethyl benzene, and xylenes were generated to assess the potential for volatilization and leaching of aromatic +- constituents under a conservative model scenario. Model input parameters for these runs are shown on the output files, as well as monthly and annual summaries of the output. The model for benzene, the most soluble of the aromatic hydrocarbons, was run for a period of ten years. The complete output file for this model run is contained in Appendix A. The models for toluene, ethylbenzene, and xylenes were each run for 99 years and the output files are contained in Appendices B, C, and D, respectively. Due to the size of these output files these appendices only contain the input parameter summary and the annual summaries for years 1, 5, 10, 25, 50, and 99. r w:194368VepwU\sesoil.dm 10 SMTH