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RISC User's Manual Version 4 0 phase concentration that applies over the entire source volume and is constant for the duration of the release, i e the length of the pulse The way in which the transport equation defined in B-1 is solved, however, requires a mass loading rate as the source term and not a concentration The RISC code calculates the mass loading rate as follows M = A V C�„ rI0001) (B,9) Il m JI where M = mass loading rate [mg/day] A = cross-sectional area of the source perpendicular to groundwater flow(=y*z dimensions) [m2] V = groundwater flux(Darcy velocity) [m/day] Cg„ = concentration in groundwater at the source [mg/1] . B 4.1 Using the GroundwaterModelWith a Source Containing Non , ` "Mobile Residual Hydrocarbons.,` There are several ways to use RISC to model the plume emanating from a residual source The first option is to use the dissolved phase groundwater model with a constant source term This approach is described below The second option is to use the saturated soil model described in Appendix C The saturated soil model is appropriate if the volume of the source and the concentrations of the chemicals in the source are known The saturated soil model uses a depleting source term and therefore accounts for mass balance The dissolved phase groundwater model can be used with some conservative assumptions to predict concentrations downgradzent of a source that has residual hydrocarbon present This model is appropriate when the volume of and/or concentrations in the source are not known This is a fairly common situation when the product has reached the water table as a separate phase Residual non-mobile hydrocarbon is not free to move on top of the groundwater as a separate phase The groundwater model cannot simulate the movement of free phase residual, rather it • B-8