The impact of climate changes on the aquatic system chemistry in the Kyiv Reservoir and the Desna river (Ukraine)

The Kyiv Reservoir and the Desna river are the main sources of drinking water supply for Kyiv, the capital of Ukraine. The impact of surface air temperature on the change of the aquatic system chemistry mentioned above water objects was studied based on long-term regular observations (1995–2018).

The findings are based on the analysis of daily air and water temperature, water pH, oxygen and carbon dioxide concentrations, water color index, dissolved organic substances (COD_Mn), Fe, Mn, and phytoplankton abundance (without identifying their species composition).

The winter period. Despite different hydraulic conditions in the Kyiv Reservoir and in the Desna river, the lack of ice cover due to an increase in winter air temperatures (December-February) led to significant improvement of the water oxygen regime in recent years. This fact, as well as the subsequent chain of changes in the water chemistry, contributed to the cheaper drinking water supply.

The ratio of the oxygen content change to the duration of the ice cover, determined by air temperature, was obtained. This allowed us to reconstruct years with observed hypoxia phenomena since 1850. Changes in the water chemistry, triggered by oxygen deficit, were described.

The summer period. The increase in summer air temperature led to a decrease in oxygen concentrations. It had the effect of slowing down the process of N-NH₄⁺ nitrification. Because of the high content of natural organic matters in these water bodies, dangerous hypoxia phenomena were reported in summertime. Such conditions contributed to the restoration of manganese to a mobile Mn²⁺.

All these features created additional difficulties at water treatment plants for supplying drinking water to residents of Kyiv.

In shallow waters, occupying almost half of the Kyiv Reservoir, an increase in water temperature during the summer period led to the intensification of the hydrobiological processes and “water blooming.” This is compounded by the high income of nitrogen and phosphorus from the point and diffuse sources.

The observed decrease in the Dnieper water flow does not allow the active use of releases from the Kyiv Reservoir. The only way to minimize negative consequences in the future is to prevent pollution and eliminate shallow water zones.

This is complicated by the fact that a significant amount of nitrogen compounds enters the Kyiv Reservoir with the water of its main tributary, Pripyat, which basin is highly marshy.

Dredging in the shallow areas will require further research because of the bedding of radionuclides of Chernobyl in the bottom sediments.