This paper, the second of a series, presents the results of a numerical study of the advection–diffusion water mixing process between the Havre aux Basques lagoon and the Gulf of St. Lawrence, after the proposed reopening of the lagoon. In this study, the reopening scheme of the inlet, which has been closed in 1957, is analyzed by using a horizontal two-dimensional numerical model. The transport model is based on the Douglas–Wang finite element formulation for a space discretization. The approximation is quadratic, using six-node triangular elements. The semi-implicit Crank–Nicholson scheme is used for a time discretization. The results show that after reopening the lagoon, mixing may take between 5 and 22 days for a diffusion coefficient considered constant throughout the region and varying from 5 to 500 m2/s.Key words: lagoon, Havre aux Basques, advection–diffusion, mixing, numerical model, finite element, Douglas–Wang.