The work was performed in framework of the State program for the improvement of the competitiveness of Kazan (Volga) Federal University among the world's leading research and education centers and subsidies unveiled to Kazan Federal University to perform public tasks in the field of scientific research. In the current methodological recommendations "Guide for assessing the risk to public health under the influence of chemicals that pollute the environment," P 2.1.10.1920-04 there is regulated the determination of quantitative and/or qualitative characteristics of the harmful effects to human health from exposure to environmental factors. We proposed to complement the methodological approaches presented in P 2.1.10.1920-04, with the estimation of the probability of pollutants input in the body with drinking water which is the greater, the higher the order of the excess of the actual concentrations of the substances in comparison with background concentrations. In the paper there is proposed a method of calculation of the probability of exceeding the actual concentrations of metal cations above the background in samples of drinking water consumed by the population, which were selected at the end points of consumption in houses and apartments, to accommodate the passage of secondary pollution ofwater pipelines and distributing paths. Research was performed on the example of Kazan, divided into zones. The calculation of probabilities was made with the use of Bayes' theorem.