A two-dimensional vertical (2-DV) model has been set up to simulate the flow and concentration field of a merged plume emitted from a multiport diffuser. In our application, two quadtree grids were employed; one for the flow field, and the other for concentration of the solute. The quadtree grids allow refinement for hydrodynamic modelling at the proximity of the port outlet, and concentration-at the interface between the plume and ambient water. In the test with stagnant ambient water, the computed velocity and concentration widths of plume expand at a slower rate compared to those calculated from analytical formulae; the difference between simulation result and formulae for centre line velocity and for concentration are 16% and 7%, respectively. In the test with uniform ambient crossflow, the simulated trajectory is higher than that computed with DKHW, and the simulated dilution ratios are lower than those of CorMix and DKHW.