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 centreline 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.