Abstract: Diffusion coefficients are usually taken as constants in solving the advection-diffusion equations and water quality models of environmental hydraulics. However, sometimes this is a poor simplification, such as in the case of pollutant mixing zones around the municipal sewer outfall in the city of Huangshaxi before the construction of the Three Gorges reservoir. We observed that the shapes of these mixing zones were much different from those that were predicted, under the assumption of constant diffusion coefficients, by using the standard curve equation (for the curved surface) of their outer boundaries or the relationship of their geometric characteristic parameters. In this study, focusing on the cases of point sources of equal intensity located at the sloped banks of rivers or reservoirs, we have solved a simplified three-dimensional advection-diffusion equation of pollutant concentration using non-uniform diffusion coefficients and analyzed the characteristics of concentration distribution. We also derived theoretical formulae of maximum length, maximum width, and maximum depth and their corresponding locations in the mixing zone as well as its surface area and underwater volume. In addition, this paper gives a standard curve equation for the outer boundaries of mixing zones and an analysis of the effects of spatial variation in diffusion coefficients on the shape of mixing zones, including the limitation of constant diffusion coefficient models, and develops a double cross-section method and isocratic line method to determine non-uniform diffusion coefficients and a calculation method for the maximum allowable pollutant load. Applications show that the non-uniform diffusion coefficient model and its calculations can better characterize the pollutant mixing zones around the sewer outfalls in Huangshaxi and the Fuling phosphate fertilizer plant.