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RISKPRO'S SESOIL for Windows User's Guide <br /> 3.5.3. Volatilization/Diffusion <br /> In SESOIL, volatilization/diffusion includes movement of the pollutant from the soil surface to <br /> the atmosphere,and from lower soil layers to upper ones Note that vapor phase diffusion in <br /> SESOIL operates in the upward direction only The rate of diffusion for a chemical is ' <br /> determined by the properties of the chemical, the soil properties, and environmental conditions <br /> The volatilization/diffusion model in SESOIL is based on the model of Farmer et al (1980) and ' <br /> Millington and Quirk (1961) and is a'discretized version of Fick's first law over space, assuming <br /> r vapor phase diffusion as the rate controlling process That is, the same equation-is used for <br /> volatilization to the atmosphere as is used for diffusion from lower layers to upper ones The <br /> vapor phase diffusion flux through the soil 1, (µg/cm2/s) is described as <br /> 10 <br /> ^, n <br /> 3 <br /> J - -D f° dC� (13) <br /> ° ° dZ 1 <br /> where- <br /> D, = the vapor diffusion coefficient-of the compound in air (cm2/s) <br /> c„ = comes from Eq (7) and f and f, are as defined previously in Section 3 5 1 <br /> The volatilization algorithm in the original version of SESOIL allowed pollutant in the second <br /> (or lower) layer to volatilize directly to the atmosphere. This algorithm was modified by Hetrick i <br /> -- -et al= (1989)=The-pollutant-can volatilize directly to-the-atmosphere from the surface layer, but - <br /> if the chemical is in the second or lower layer, and the concentration in that layer is greater than <br /> W the layer alioveii, Cheri the—chemical-will diffuse into-the upper layer re <br /> -rather than volatilize <br /> directly into the atmosphere. <br /> An option the user has in the volatilization algorithm is to "turn off' the calculation by use of <br /> an input index parameter (for each layer) For example, if the index`is set to 0 0 for each layer, <br /> w the pollutant would not be allowed to diffuse upward or volatilize to the atmosphere, only <br /> downward movement of the pollutant with the soil moisture would occur. Also, if data are <br /> available, this index parameter can be vaned to dalibrate calculations to the measurements <br /> 3.5.4 Sorption: Adsorption/Desorption and Cation Exchange <br /> SESOIL includes two partitioning processes for movement of pollutants from soil moisture or ' <br /> soil air to soil solids the sorption process and the canon exchange mechanism. <br /> Page 24 ' <br />