We present a mathematical model for a quantitative estimation of the damage to aquatic life resulting from a pollutant discharge into aqueous environment. With the use of the Lagrangian description of fluid motion, we introduce a set of hydrophysical parameters on the basis of which hydrobiologists can estimate the damage. The computation of these parameters is illustrated by the example of a problem of a pollutant spreading in a canal. The problem is solved numerically on a deformable Lagrangian grid. To ensure computational stability a special grid reconstruction procedure with the subsequent interpolation of the parameters computed is used. An original interpolation technique is proposed which ensures the preservation of the most important hydrophysical quantities.