Abstract: An analytical solution of contaminant diffusion through soil was presented based on the assumption of simplified non-linear adsorption isotherm.The moving boundary was introduced to indicate the notable change of retardation factor of clay with the increase of contaminant concentration level.The breakthrough curve obtained from the present solution was found to be between the two curves obtained from the analytical solution for one-dimensional diffusion considering no adsorption and linear adsorption.As for the present example,the breakthrough time of the contaminant obtained from the linear adsorption solution was 2.2 times greater than that obtained from the nonlinear adsorption solution presented here.The present method was relatively simple to apply and could be used for evaluating experimental results and verifying more complex numerical models.