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. w <br /> SNITH <br /> Molecular Weight <br /> 106.16 grams/mole - 1,2-Xylene (page 505 of ref. 4) <br /> 106.17 grams/mole - 1,3-Xylene (page 516 of ref. 4) <br /> 106.17 grams/mole - 1,4-Xylene (page 526 of ref. 4) <br /> average = 106.17 grams/mole <br /> Diffusion Coefficient in air <br /> Xylene's molecular diffusion coefficient in air of 0.43 m2/day (0.050 cm2/sec) is <br /> reported in the caption to Figure 15.2.11 of reference no. 3. Air diffusion plays a <br /> minor transportation role compared to infiltration transport. <br /> Organic carbon Partitioning Coefficients, Koc <br /> Reference no, 3, Table 15.2.5, page 15.2.5 lists a Koc of 830 L/kg for o-Xylene (1,2- <br /> Xylene). Values for the other xylene isomers are not given in reference no. 3 but <br /> reference no. 2 reports 692 cc/g for all three isomers. The more conservative value of <br /> 692 cc/g is specified. <br /> Biodegradation <br /> According to the Handbook of Hydrology (Reference no. 3), the rate of biodegradation is <br /> determined by the number and type of microorganisms present, the toxicity of the parent <br /> compound or its daughter products to the microorganism population, the water content and <br /> temperature of the soil, the presence of electron acceptors and the oxidation-reduction <br /> potential, the soil pH, the availability of other nutrients for microbial metabolism, the water <br /> solubility of the chemical and possibly other factors. Biodegradation in SESOIL is modeled <br /> mathematically as a simple first order decay function for both the liquid phase and the solid <br /> phase. SESOIL does not attempt.to model the extremely complex process of biodegradation. <br /> The chemical specific decay rates for both the liquid phase and the solid phase applicable to <br /> the given site are input. <br /> Biodegradation rates are rarely measured for the liquid phase and the solid phase. The liquid <br /> phase and the solid phase biodegradation are generally lumped together and the over-all <br /> biodegradation rate is reported. Biodegradation rates for benzene, toluene and xylenes are <br /> calculated from conservative biodegradation half-life's for natural sites reported in Table <br /> 15.2.8 of reference no. 3. A biodegradation rate for ethylbenzene in soil and groundwater was <br /> located in Table 9.9 of reference no. 6. <br /> Upon approval of PHS/EHD, Smith Environmental will incorporate the above parameters into the <br /> SESOIL program and subsequently prepare a report including a summary of the models <br /> conclusions. <br /> N:\150114\model\param.doc 5 <br />