  This work proposes a mathematical model on the water quality for Igap\'o I
Lake, located in Londrina, Paran\'a. A qualitative analysis on the pollution of
the lake is carried out through numerical simulations which provide
informations for the understanding of the processes involved. For the
hydrodynamic flow of the lake, a horizontal two-dimensional model is
considered. The hydrodynamic velocity field is simulated by the Navier-Stokes
and pressure equations, supposing an incompressible newtonian fluid. This
hydrodynamic velocity field is inserted in the reactive-transport model. The
reactive part of this model considers only the carbon-nitrogen cycle, described
by the QUAL2E model. A qualitative and quantitative analysis of the numerical
simulations conducted in function of the deficit of dissolved oxygen, of the
biochemical oxygen demand and of the load of pollutants provided a better
understanding of the hydrology and of the water quality in the Igap\'o I Lake.
