adam zonshine

Hey. I am trying to run a variably saturated, mass transport model. The goal is to recreate glacial / interglacial groundwater simulations of saltwater intrusion. however, the model will always end up stuck in a loop where an area gets excessively concentrated areas (different areas in the transition zone each time), where I end up with a tiny area that has impossible concentrations, as well as negative concentrations at the landward side. I have tried to make a smaller mesh, and to make the time series of the sea level rise and fall in constant steps to reduce computation costs. I guess my question is, if anyone has prior experience with this type of issues, how to make a transient state model not to loop due to mass transport issues, and how to make such a model run for thousands of years, without it actually taking thousands of years. Adam