  This work is a contribution to better understand the effect that domestic
sewage discharges may cause in a water body, specifically Igap\'o I Lake, in
Londrina, Paran\'a, Brazil. The simulation of the dynamics of pollutant
concentrations all over the water body is conducted by means of structured
discretization of the geometry of Igap\'o I Lake, together with the finite
differences and the finite elements methods. Firstly, the hydrodynamic flow
(without the pollutants), modeled by Navier-Stokes and pressure equations, is
numerically resolved by the finite differences method, and associated with the
fourth order Runge-Kutta procedure. After that, by using the hydrodynamic field
velocity, the flow of the reactive species (pollutants) is described through a
transport model, which considers advective and diffusive processes, as well as
through a reactions model, restricted to the carbon-nitrogen cycle. The
transport and reactions model is numerically resolved by the stabilized finite
elements method, by means of a semidiscrete formulation. A qualitative analysis
of the numerical simulations conducted in function of the diffusion coefficient
provided better understanding of the dynamics of the processes involved in the
flow of the reactive species, such as the dynamics of the nitrification
process, of the biochemical requirement of oxygen and of the level of oxygen
dissolved in the water body at Igap\'o I Lake.
