## Derive an equation representing diffusion from a small source into a reservoir

Does anyone have any suggestions on how to use the diffusion equation (Ficks Law) to model 3-dimensional diffusion from a point source into a reservoir? For example... two (cubical) reservoirs are connected at the center of one face by a straw. One reservoir has blue water, the other is clear. Once opened, blue water diffuses through the straw into the other reservoir and will (eventually) equilibrate between the two. How do I derive an equation for this from Fick's Law? I can calculate the flux through the straw (D*r^2*(c1-c2)/l), but I can't figure out how to account for the time required for the blue water entering the clear reservoir to equilibrate within the volume. Assume diffusion to be the only acting force (no flow, mixing) and that the relationship between volume and the diffusion coefficient is not negligible (i.e. the concentration of blue water at the center of the reservoir is not the same as the concentration at the opening of the straw). :/