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RISC User's Manual Version 4 0 <br /> I0,refu = water-filled porosity in the lens [cm3 water/cm3 soil] <br /> 811ew = total porosity in the lens [cm3 pores/cm3 soil] <br /> KH = Henry's Law Constant for chemical [(mg/I vapor)/(mg/l <br /> water)] <br /> 1 <br /> Note,the above porosity terms used by the diffusion equations should be total porosity <br /> ' rather than effective porosity The total effective diffusion coefficient for the vadose <br /> zone and lens combined is estimated as the depth-weighted average (Johnson and <br /> Ettinger, 1995) <br /> D — (hx + h,,,.� (A-13) <br /> eA I <br /> hlrnt + I[x <br /> D,ff lens Def i <br /> ' where <br /> Diff = depth-weighted average effective diffusion coefficient <br /> for the vadose zone in between the source and the soil <br /> surface [cm2/sl <br /> h„ = tluckness of the vadose zone above the source(minus the <br /> lens thickness) [cm] <br /> hie,,,. = thickness of the lens [cm] <br /> As a consequence of the modeling assumptions, one can actually account for more <br /> than one lens above the source zone, as long as each lens has the same material <br /> ' properties (total porosity, moisture content, etc ) if this is the case, then the user <br /> simply sets hie.. equal to the combined thickness of all the lenses and by is the <br /> remaining thickness of the vadose zone <br /> ' A.4.4 Overall Source Depletion Rate <br /> Combining equations A-8 through A-1 l yields <br /> dm 1 <br /> K D <br /> dt = -PM = — V 0. + L e� A-14 C. ( ) <br /> 1 d <br /> I <br /> A-90 <br /> 1 <br />