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1 <br /> RISC User's Manual Version 4 0 <br /> The source depletion term, Q, is described in equations A-19 and A-23 At a "long <br /> distance" below the source the concentration gradient is always zero <br /> axw t) 0 <br /> (A-4) <br /> The solution for the advection-dispersion equation (A-1) with the above boundary <br /> conditions is given by van Genuchten and Alves (1982) <br /> C,,(x,t) = Coe B(x,t) (A-5) <br /> ' where <br /> I (v -w)x Rx -�-wt 1 (v+ w)x Rx-+- wt <br /> ' 13(x,t) � —exp erfc + —exp ]ef, <br /> 2 2Dx 121D.Rt 2 2D= 2 DxRt <br /> and <br /> W= v 1+ 4DX [µ— RPI (A-7) <br /> v <br /> where <br /> C„(x,t} = dissolved phase concentration of chemical at distance x <br /> (in cm)below the source and time(in days) [mglL] <br /> — source zone depletion coefficient(loss term)defined in <br /> equations A-19 and A-23 [-] <br /> In order to solve the above set of equations, the first-order source decay rate must be <br /> ' determined <br /> A-6 <br />