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Vapor Transport From Groundwater <br /> Vapor diffusion through the vadose zone is assumed to be steady-state The <br /> source concentration in groundwater underneath the building can be specified <br /> by the user(a constant concentration) or it can be estimated using one of the <br /> groundwater models(transient source concentration) <br /> Biodegradation of the chemical vapors is not considered This is appropriate <br /> for chemicals that do not degrade readily,that travel very short diffusion <br /> distances,and/or for screening level evaluations BTEX constituents can have <br /> very high degradation rates(higher than in groundwater)under certain <br /> conditions <br /> • If measured soil gas concentrations have been taken between the groundwater <br /> and the source, it would be more appropriate to use one of the soil vapor <br /> models with a soil gas source term The groundwater volatilization model <br /> estimates the soil gas concentration using Henry's Law and then estimates the <br /> diffusion rate through the capillary fringe After that point,this model is <br /> identical to the Johnson and Ettinger model from a soil gas source(Appendix <br /> D)when building under-pressurization is not considered(set to zero) <br /> A£;. xx W 7� v Ha �e F° v -• 9 ` a k � � 4 s 4 `3 . . �W 4.: E tenF T�� � ey` <br /> r t ` -rte � 9•N Y4+�si 6 rN ._ .. % .xis.� .. . �.� . ' <br /> yE.3, TRANSPORT�'EQUATION AND PRO ESSES E <br /> Steady-state one-dimensional diffusive vapor transport over a length, d, can be <br /> described by <br /> F — Dei.(CC,f} (E-1) <br /> Ld <br /> where <br /> F = volatile emission rate of the chemical constituent being <br /> modeled Lg/cm2/s] <br /> Dell = effective diffusion coefficient{cm2/s] <br /> Cvs = vapor phase concentration dust above the water table in <br /> the capillary fringe [g/cm3] <br /> C,f = vapor phase concentration in the soil at the building <br /> foundation[g/cm3] <br /> Ld = diffusion path length(distance from the water table to <br /> the foundation) [cm] <br /> • <br /> E-3 <br />