Two Lagrangian models are developed for the accurate simulation of advection-diffusion transport in unsteady open channel flows. The first one is based on a second-order partial differential equation (PDE) for pollutant concentration (model I), and the second one is a first-order system with diffusive flux as another primary variable (model II). To solve the two models, the meshless smoothed particle hydrodynamics (SPH) method is employed. To enforce the inlet and outlet boundary conditions, an extrapolation scheme based on cubic spline is used. Numerical results are presented for tracer distributions with boundary layer in a uniform flow and are compared with analytical solutions. It is demonstrated that both the two models can accurately solve the advection-diffusion transport problems, for which numerical diffusion and disperse oscillation are often observed in most Eulerian schemes. Therefore, the Lagrangian particle models are efficient and accurate tools to predict advection dominated transport for water quality in river systems and for transport with a boundary layer.