Purpose: To develop a methodology for determining a rational relationship between the water quality of water bodies and the quality of wastewater discharged into them with existing investments in the reconstruction of industrial wastewater treatment systems of several water users within one water basin. Methods: Analytical solution of the problem of determining the optimal purity of water in the river in the conditions of implementation of water protection measures with limited financial resources and their rational distribution among water users. Results: Relative financial resources for water protection measures in the Russian Federation are insufficient and at the same time have been declining over the past 30 years. Despite this, the maximum permissible concentrations of pollutants in wastewater discharged into water bodies and wastewater disposal systems in the Russian Federation are often very strict, sometimes even tougher than for drinking water, which leads to unjustified investment costs. It is also noted that at present, market relations in the field of nature management, including in the field of water use, have not yet been fully formed in the Russian Federation. The main difficulties of implementing these relations in this country are outlined. With the help of the proposed methodology, a reasonable relationship between the water quality of a watercourse and the quality of wastewater discharged into them, as well as a rational distribution of financing between water users, is determined on a concrete example. It is shown that the economically justified requirements for effluents discharged into a water body are background concentrations of pollutants in the water of this water body. Practical significance: The practical feasibility of the proposed methodology, which can be recommended for practical application, is demonstrated using an example.