ASSESSMENT OF RIVER WATER QUALITY IN THE CITY BY HYDROCHEMICAL INDICES (THE OKHTA RIVER, ST. PETERSBURG)

Abstract

The Okhta River water quality was assessed in the period 2016–2020 using the component- wise assessment method and three hydrochemical indices (Water Pollution Index (WPI), Specific Combinatorial Water Pollution Index (SCWPI), and Canadian Council of Ministers of the Environment Water Quality Index (CCME WQI)). The component-wise assessment demonstrated that the pH was neutral or slightly alkaline, and water hardness was low. Dissolved oxygen deficit was observed at most stations during the entire research period. The content of dissolved oxygen declined downstream along the river. At the river mouth, the oxygen situation is slightly better due to the inflow of the Neva River water. The BOD5 exceeded the MPC at all sampling stations, suggesting the Okhta River water was polluted with readily oxidizable organic matter. The iron content in the water exceeded the MPC manifold. An elevated content of various forms of nitrogen was also revealed. Over the entire observation period, increased concentrations of oil products were constantly recorded (exceeding MPC more than 50‑fold in different years). The Okhta River water was characterized as “dirty” and “extremely dirty”, and the water quality was “poor” according to the calculated values of the indices – WPI, SCWPI and CCME WQI. The methodology of the little-known in Russia CCME WQI is considered separately. It was compared with the WPI and SCWPI. The relationship between the indices is clarified. The use of CCME WQI for surface water quality assessment along with SCWPI is recommended.