The paper presents a new method of calculating the mixing length of pollutants, based on the coefficient of the degree of concentrations levelling. Two models have been used: of Bielski and of Boczar of advective transport of pollutants with transverse dispersion. A method of determining the coefficient a,, in Bielski's formula for rectangular or other concentration distribution of pollutants in the initial cross-section has been given. The differences which will occur when determining using any of these methods the mixing length and the concentration distribution on the width of water-course are analysed. It has been noticed, that the differences are due to the different method used when deriving the formula (9) and (11). It has been found, that in order to shorten the mixing length the pollutants should be disposed, as far as possible, near the river axis and also in such a way as to make the formed zone of inflow of pollutants, of the width 2L, as large as possible.