The study presents a model of distribution of toxic agents in flat aquatic regions such as lakes, lagoons, and coastal seas, two sources of pollution are modeled, one from the waste of industrial plants and another one, a product of the runoff phenomenon of the agricultural industry. Methodologically, a mathematical model based on the advection-diffusion equation is used, which allows knowing the concentration of the contaminant at all times in time for the given region. The problem system is described in its variational formulation in order to approximate its solution by the finite element method. The parameters used in the simulations are taken from secondary sources, in order to compare results and deferences in the same scenarios included in other models.