  The concentration of biochemical oxygen demand, BOD5, was studied in order to
evaluate the water quality of the Igap\'o I Lake, in Londrina, Paran\'a State,
Brazil. The simulation was conducted by means of the discretization in
curvilinear coordinates of the geometry of Igap\'o I Lake, together with finite
difference and finite element methods. The evaluation of the proposed numerical
model for water quality was performed by comparing the experimental values of
BOD5 with the numerical results. The evaluation of the model showed
quantitative results compatible with the actual behavior of Igap\'o I Lake in
relation to the simulated parameter. The qualitative analysis of the numerical
simulations provided a better understanding of the dynamics of the BOD5
concentration at Igap\'o I Lake, showing that such concentrations in the
central regions of the lake have values above those allowed by Brazilian law.
The results can help to guide choices by public officials, as: (i) improve the
identification mechanisms of pollutant emitters on Lake Igap\'o I, (ii)
contribute to the optimal treatment of the recovery of the polluted environment
and (iii) provide a better quality of life for the regulars of the lake as well
as for the residents living on the lakeside.
