Shallow water simulation enables the study of problems such as dam break, river, canal and coastal hydrodynamics, as well as the transport of inert substances, such as pollutants, on a fluid. This article describes a GPU efficient and cost-effective CUDA implementation of a finite volume numerical scheme for solving pollutant transport problems in bidimensional domains. The fluid is modeled by 2D shallow water equations, while the transport of pollutant is modeled by a transport equation. The 2D domain is discretized using a first order finite volume scheme. The evaluation using a realistic problem shows that the implementation makes a good usage of the computational resources, being very efficient for real-life complex simulations. The speedup reached allowed us to complete a simulation in 2 hours in contrast with the 239 hours (10 days) required by a sequential execution in a standard CPU.