The cyclicity in the changes in the chemical composition of the water source of the Angara River (Baikal Stock) in 2017-2018 in comparison with the last 20 years of data.

The relevance of the research is due to the increasing need for drinking water in the world, the shortage of which is experienced in many countries. The main aim is the study of the chemical composition of freshwater of the Angara River (the drain of Lake Baikal) and the characteristics of its changes. Methods of research: water was sampled at a distance of 1.5-2 m from the shore, from a depth of 0.4-0.5 m, and at the source water intake with the help of a pump. Samples were taken in clean plastic bottles of 1 L in size to determine SO2, Cl, HCO, NO, HNO, SiO, F, PO, NH, O, E, E, Ca, Mg, Na, K, pH, and temperature. The analyses were accomplished according to standard techniques. It is shown that minor changes in the total content of ions in the water of the Angara River source occur daily, monthly, and depend on a season. Graphically, this is expressed by the curve (broken) line, which manifested the increase, maximum, decrease, minimum, and eventually an endless cycle of repeating the sequence of monthly and annual concentrations of the elements. It was found that the average values of the main ions in the water and the total mineralization of the water source of Angara River and Lake Baikal correspond to their average annual values over the past 20 years. The reason for the constant chemical composition of Baikal and Angara waters may be not only the possibility of already proven chemical and biological self-purification but also the additional penetration of deep waters as a result of upwelling and earthquakes on Lake Baikal. It is concluded that it is necessary to continue further geochemical and microbiological monitoring of the Baikal aquatic ecosystem to assess its composition and quality.