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1 ' RISC User's Manual Version 4 0 In the RISC model, the dissolved phase concentration, C..., is checked for solubility ' limitations and therefore is calculated differently depending on whether or not residual phase hydrocarbon (NAPL) is present (see Sections A 4 5 through A 4 7) (Unitl et ' al , 1992,assumes that the source always contains immiscible phase hydrocarbons) ' A 4.2 Volatilization Losses (incorporating Lens) The diffusive vapor loss is estimated using Fick's Law, assuming a linear concentration drop from the source area to the soli surface Source zone volatilization losses are calculated using a pseudo-steady state vapor flux model based on Fick's Law In this approach, the steady-state solution for vapor transport is ' coupled with a source zone concentration that is changing with time It is assumed that vapor transport reaches steady conditions much faster than the rate at which the t source zone concentration changes with time It is also assumed that vapor concentrations at ground surface are much less than those at the source zone The mass depletion rate of the source due to volatilization(the volatile losses)is calculated from tJ = D C,, (86400s l A-10) J, �� L, d I ' where A, = rate of mass depletion per area due to volatilization Def = effective diffusion coefficient in soil [cm2/s] Cy, = vapor-phase concentration of chemical in the source area ' [g chemical/cm3 vapor] Ld = diffusion path length [cm] ' Ap Equation A-10 is Fick's Law applied to the concentration gradient from the source to ' the ground surface The concentration of chemical in vapor at the soil surface is assumed to be equal to zero (or very small compared to the source vapor phase concentration) The diffusion path length is assumed to be equal to the distance from the soil surface to the center of the source 1 A-S 1