Pollution flushing from an idealized tidal embayment has been extensively studied both analytically and numerically. The analytical tidal prism model of Barber [1] results in a simple and elegant set of equations giving the pollution concentration at the nodes in the flushing cycle. It is limited in its ability to handle the bathymetry of real embayments, complex pollution loading functions and return flow conditions. The MSB model is a numerical approach written in Stella[2], (Stella is a software product and trademark of High Performance Systems, Inc. Hanover, USA). This model is not limited by bathymetry or in its ability to handle variable pollution loading functions. In this paper we extend the MSB model to include cascading basins. The model is developed using the concept of coupled chemical reactors, a well-known chemical engineering problem involving chemical productions. As an illustration, the issue of return pollutant flow to a single embayment is studied. The results are used to calibrate a return flow parameter that is often used to overcome the difficulty of dealing with this problem in analytical frameworks. ...