In this exercise you will implement a full advection/diffusion model of vertical transport through an abstract media. The media is discretized into boxes, and for simplicity the boxes are of the same size. Each box has a cross section area of 1 m2, and a height dx defined as the total length divided by the number of boxes. To further simplify the model, the retardation coefficient R and the diffusivity coefficient D are set explicitly. Normally you would calculate these using Kd, porosity and other media/substance specific parameters.