Water Quality of the Vychegda River under the Conditions of the Receipt of Treated Wastewater from the Pulp and Paper Industry

The article presents the results of 3-year measurements (2018–2020) of hydrophysical and hydrochemical indicators of the Vychegda River in places where effluent is discharged from the Mondi Syktyvkar JSC pulp and paper mill. At conditionally background stations and those exposed to wastewater, the water mineralization, content of ions of Ca²⁺, Mg²⁺, Cl^–, S\({\text{O}}_{4}^{{2 - }}\), N\({\text{O}}_{3}^{ - }\), heavy metals, petroleum products, anionic surfactants, and benz(a)pyrene were recorded in concentrations significantly lower than the MPC (MPC_FI), while suspended substances, COD, Р\({\text{O}}_{4}^{{3 - }}\), Fe, and Mn were in excess of the MPC_FI. Studies have shown that, in connection with the modernization of the treatment facilities of Mondi Syktyvkar JSC, there has been a noticeable improvement in the wastewater quality; the environmental load on the ecosystem of the Vychegda River at discharge points decreased. No significant excess over conditionally background values was detected at a high water level in the Vychegda River (2018–2019) for all studied hydrochemical indicators at the points of effluent discharge. Degradation of the water quality was recorded during the low-water period (2020), i.e., a decrease in the degree of dilution of effluent from treatment facilities by river waters. During this period, the wastewater-discharge points exhibited a significant excess over the conditionally background values of mass concentrations of suspended substances, Cl^–, S\({\text{O}}_{4}^{{2 - }}\), Р\({\text{O}}_{4}^{{3 - }}\), N\({\text{H}}_{4}^{ + }\), S_tot, K, Na, and phenol, as well as the chemical oxygen demand, permanganate value, turbidity, and electrical conductivity. In terms of environmental, sanitary, and toxicological indicators, the waters of all surveyed observation stations were assigned to a water-quality class 3 of (satisfactory purity).