Influence of the Ivankovo reservoir on the quality of the Volga water supply source for Moscow

The article analyzes the results of 1958-2004 regular long-term monitoring of water quality of the Ivankovo reservoir, the principal reservoir of the Volga water supply system for Moscow. The annual balances of ammonium ion, nitrates, phosphates, total iron, manganese, and organic matter (in terms of the permanganate oxid-ability index) were calculated for the observation period using data on water flow rates in the main tributaries of the reservoir, i.e. the Volga and Tvertsa rivers. Long-term fluctuations of the relative balances of chemicals in reservoirs are analyzed; the balances are calculated by the difference between the inflow and discharge of chemicals. The calculations show that under the slower river flow the concentrations and the flow of chemicals decrease in the Ivankovo reservoir downstream the Volga River section near Tver as a result of intra-reservoir processes. Significant statistical dependences of the relative balances of chemicals in the reservoir on the coefficient of its water exchange by the inflow were established, which show high correlation coefficients i.e. 0.67 for the ammonium ion, 0.69 for nitrates and iron, 0.71 for manganese, 0.75 for organic matter, and 0.78 for phosphates (at the 5% significance level). With a short period of water exchange in the Ivankovo reservoir, an increase in the nitrate forms of nitrogen due to the oxidation of the ammonium form does not compensate the influx of nitrates. As a result, there is a positive difference in the inflow and runoff of nitrates for the most of years under calculation. Relatively small decrease in the concentrations of ammonium nitrogen is due to its internal sources, and also wastewater from Tver, Konakovo and other localities. Specific features of organic matter transformation in the Ivankovo reservoir are revealed. The inflow of high-colored waters of the Orsha and Soz rivers draining the large Vasilyevsky Moh peat-deposit significantly smooth the decrease in organic matter content in the reservoir under the influence of intra-reservoir processes. Thus the average long-term values of permanganate oxidability for the Gorodnya section at the reservoir entrance and for the first ferry crossing section differ just slightly. Influence of waste water of the Tver city was evaluated by comparing the concentrations of pollutants upstream and downstream the municipal wastewater discharge point. It is shown that the degree of the influence depends on water amount of the year and significantly increases in the years with low runoff as a result of poorer dilution of pollutants by river water.

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