Assessment of anthropogenic pressure by nutrients and organic substances in the Sula river basin

Abstract

The paper focuses on a man caused pressure in the Sula River with organic substances and nutrients which are the main factors affecting the water status. The Sula River is a left tributary of the Dnipro River flows into the Sulynska Bay of the Kremenchuk Reservoir. The water resources of the Sula River are used for drinking water supply, melioration, fish farming, etc. The Sulynska Bay is a spawning area for fish and the place of their largest commercial concentration. For a long time, the bay has been experiencing frequent cases of a critical decrease in dissolved O2, resulting in fish kills. Materials and methods of the study. The Electronic Services Portal of the State Agency of Water Resources of Ukraine (e-services.davr.gov.ua) as of 2021 was used to assess the contribution of studied compounds from point sources. The population was calculated based on the Atlas of the Administrative and Territorial Arrangement of Ukraine (2021). The pressure of nutrients and organic substances is calculated based on the person’s excretion coefficients, their removal by treatment facilities (Behrendt, Huber, Kornmilch et al., 2000; Osadcha, Luzovitska, Ukhan et al., 2022). Results and discussion. The total pressure from point sources is the sum of the urban population's contributions and industrial enterprises. Pollution with organic substances and nutrients from point sources is mainly associated with the wastewater discharge from settlements. There are 1141 settlements in the Sula basin, including 7 ones with a population ≥ 10 ths. people (36%), and 17 settlements with 2-10 ths. people (12%). The pressure of organic substances from the population connected to sewer systems was 512.9 t•year-1 for biochemical oxygen demand (BOD5) and 726.7 t•year-1 for chemical oxygen demand (COD). In terms of nutrients, the flow to surface waters was 162.3 t•year-1 for N and 56.1 t•year-1 for P. The largest organic pollution is caused by wastewater from the Ichnia municipal facility, while the nutrient pressure was caused by the corresponding enterprise in Pryluky. Annually, 111.2 t•year-1 of organic substances in terms of BOD5 and 133.1 t•year-1 in terms of COD is discharged into the Sula River basin with industrial wastewater. The pressure of nutrients supplied by industrial wastewater was 18.4 t•year-1 of Total nitrogen (TN) and 1.8 t•year-1 of Total Phosphorus (TP). The largest polluter in the Sula basin is Linovytskyi Sugar Plant “Krasnyi”, whose wastewater discharged ~ 67% of nutrients and up to 96% of organic substances from the total amount. Diffuse sources are dominated by pollution from the population without access to sewerage networks. To a greater extent, this applies to small settlements of ≤ 2000 people. This pressure is 2304 t•year-1 for BOD5 and 3917 t•year-1 for COD. Additionally, 224.3 tons of nitrogen and 31.5 tons of phosphorus are discharged annually into the Sula basin. The role of other diffuse sources, such as agriculture, land use patterns, etc., has a much smaller impact on the formation of surface water quality in the Sula River basin. Conclusions. The ecological status of surface waters in the Sula River basin depends on the quantitative parameters of anthropogenic pressure, which, according to BOD5, is 2928 t•year-1, COD – 4777 t•year-1. For nitrogen and phosphorus compounds, the obtained indicators in the context of the year reach 405 t•year-1 and 89 t•year-1. The peculiarity of the Sula River basin is that the dominant share of organic substances and nitrogen comes from diffuse sources. The relative contribution of distributed sources for BOD5, COD, and TN is 79%, 82%, and 55%, respectively. At the same time, for TP, the main pressure is formed by point sources. A similar pattern is characteristic of other river basins in Ukraine and around the world and is related to the peculiarities of the P geochemical cycle. The contribution of diffuse sources is due to the total natural background and the population without access to sewerage networks. Among the point sources, 82 - 89% of the pressure is associated with wastewater discharge from municipal enterprises. The results obtained should serve as a basis for developing measures to overcome the impact of anthropogenic pressure.