Abstract: In this paper, a new model WESC2D (Two-Dimensional Water Environmental Simulation Code) using an orthogonal curvilinear grid system is developed for the impact study of pollutants discharged into the lower reach of Ganjiang River. The model development is based on the eutrophication principle of the Water Quality Analysis Simulation Program (WASP) model and the Eulerian-Lagrangian alternating direction implicit (ELADI) scheme. The model convection terms are solved by the Eulerian-Lagrangian method based on the bilinear interpolation and sub-grid scale method. The simulation of WESC2D is compared to the analytic solution to the problem of simple concentration peak transport, and to the field measurement of dissolved oxygen from the lower reach of Ganjiang River. The dynamic relationship between the pollution loads and the water quality at sewage outlets along the reach is also examined. Results show that the simulation of WESC2D agrees well with the analytical solution at a substantially reduced numerical cost compared to that of using the general Eulerian-Lagrangian Method (ELM). The new model WESC2D provides an important tool for the study on the relationship between the pollution loads and the water quality in the lower reach of Ganjiang River.