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' RISC User's Manual Version 4 0 <br /> w , <br /> • The size of the source in the vadose zone is "mapped" onto the water table and <br /> becomes the footprint of the groundwater source size if the receptor well is <br /> close to the source region,it may be more conservative to use a smaller source <br /> ' size and a larger source concentration than to average the concentrations <br /> across a large source region More concentrated(but smaller)sources in the <br /> groundwater yield higher concentrations along the centerline of the plume <br /> ' when the receptor point is close to the source region <br /> C �1 <br /> RISC's vadose zone transport model uses a "compartmental" approach, in other <br /> words, different models are used to describe the source zone, the vadose zone above <br /> the source, and the vadose zone between the source and groundwater <br /> ' The source zone is described as being a well-mixed finite source that depletes with <br /> time, while the vadose zone above the source is treated as being one-dimensional and <br /> ' • at pseudo-steady state <br /> ' Beneath the source zone, solute transport of leachate occurs via advection and <br /> dispersion, and mass loss may occur through a first-order degradation reaction The <br /> one-dimensional transport equation (including adsorption)in this zone is given by <br /> z <br /> R ?�d^t = Dx Wiz% —v " —PC',, (A-1) <br /> ' dx <br /> where <br /> Cµ — <br /> dissolved phase concentration of chemical [mg/L] <br /> Ds = dispersion coefficient in the unsaturated zone [cm2/di <br /> ' v = seepage velocity(or interstitial velocity)(cm/d] <br /> u = first-order decay coefficient for chemical [1/d] <br /> ' x = distance below the source(measured positively <br /> downward) [cm] <br /> t = time [d] <br /> • <br /> 1 <br /> A-4 <br /> 1 <br />