The discrepancy between numerical solution and analytical solution

Dear Sir of Madam,

When I run the simple 1D advection-dispersion model with Kd adsorption, I found there is a difference between the numerical solution and analytical solution. When x = 0, the C0/C = 1.0 in analytical solution, while C0/C < 1.0 in numerical solution. I am confused by this discrepancy. I would appreciate it if anybody could explain it for me. Thank you!