The waters of Lake Baikal are formed owing to the tributary waters and precipitation over its surface.
The precipitation waters are very low-mineralized (the total number of ions is about 10 mg/l), hydro-carbon-calcium. Their yearly output is only about 13% of the total yearly inflow into the lake (Afanasiev, 1960). The rest is brought by the tributaries of the Baikal. In its chemical balance the precipitation waters quota is still smaller. The total number of ions hardly reaches 1.4%. Therefore, the chemical composition of the Baikal water is practically influenced by the average chemical composition of the tributary waters.
Formation of the water chemical composition and hydrochemical conditions of the rivers of the lake basin is going on mostly among weakly lixivated igneous or metamorphed Archaic and Proterozoic rocks. It influences their belonging to the group of calcium of hydro-carbon water class.
The most important component of the Baikal tributary waters anion composition is the hydro-carbon ion. In the cations the main component is calcium.
The waters of mineral and thermal springs which are situated near Lake Baikal and in other places of its basin are specific. As a rule the waters of these springs belong to the group of sodium of the sulphate class. In some cases, e.g. in the Kotelnikovsky spring which is situated on the north-western shore of the Baikal, the first place is taken by silicon, up to 112 mg/l.
In spite of the common character of the geochemical image the rivers, tributaries of Lake Baikal, are very different in the degree of the water mineralization and sometimes in their chemical composition and hydrochemical conditions. This may be explained by differences in the geological composition and the topsoil of some regions of the lake basin, their climatic peculiarities and first of all differences in the amount of precipitation and their yearly distribution, and also partly in the difference of the underwater feeding role in the total water inflow.
According to the degree of water mineralization the tributaries of Lake Baikal are divided into 5 groups.
1. With very low water mineralization. Such territories are mostly in the southern and northern ends of the Baikal. Their basins are located in the mountain taiga with podzol mountain-taiga soils characteristic of it. The supply of these rivers is mainly superficial, mostly with snow. In winter they usually freeze or have extremely insignificant water expenditure. The total mineralization (the number of ions) in the waters of the mentioned group does not exceed 50 mg/l, it is mostly within 20-40 mg/l.
2. With water mineralization of 50-100 mg/l. These are also mountain rivers collecting their waters very close to Lake Baikal, in the mountain ranges adjacent to the lake. The Upper Angara and Turka rivers are exceptions the basins of which are outside the mountain ranges.
3. With water mineralization from 100 to 200 mg/l. These are the Selenga, Barguzin rivers and some smaller lake tributaries.
The waters of the Selenga river are formed in the mountainous landscapes of Northern Mongolia and in the steppe, forest-steppe and partly taiga landscapes of the Western Transbaikal land. Throughout a bigger part of its flow the river preserves its steppe character. In its feeding the main role is played by rain waters causing great summer floods. But thanks to the peculiarities of the basin geological composition which is formed by crystalline rocks, good irrigation of the soils and insignificant underground supply, the Selenga river is low-mineralized.
The Barguzin river collects its waters partly in the mountainous and taiga regions, partly in the forest-steppe and steppe regions, in the area of spreading of ancient crystalline rocks which also provides low mineralization of its water. Out of other Baikal tributaries with water mineralization of 100-200 mg/l such rivers can be mentioned the basins of which are connected with carbonate rocks spreading.
4. With water mineralization of 200-300 mg/l. They are interspersed in different parts of the Baikal coast and timed to the area of carbonates bedding. These rivers are the Malaya Buguldeika, the Khaluri, the Ilga and others. The group under question is not big in number. It plays the subordinate role in the total chemical inflow into the lake.
5. The Bolshaya Buguldeika river, the only tributary mineralized over 300 mg/l. Its basin is located in the area of the carbonate rocks development. Besides, some streams of Olkhon island can be referred to this group, the supply of which is evidently connected with more mineralized deep waters.
Gas conditions of the Baikal tributaries is characterized by oxygen content close to the normal saturation, by low content of free carbon dioxide in the iceless period and by significant decrease of the oxygen content and increased content of CO₂ by the end of the winter under-ice period.
Only in some rivers with rapid current having non-freezing sections (unfrozen patches or exhalations), the gas condition is favourable for hydrobionts living in them throughout the year.
The water pH values usually change in accordance with seasonal fluctuations of the free CO₂. In general they are mostly within neutral-alkaline area, 7.2-7.8.
According to the character of seasonal changes in the content of some components and general mineralization the tributaries of the Baikal are grouped into five types (Votintsev, Glazunov, Tolmacheva, 1965).
The first type of hydrochemical condition is characterized by sharp decrease of the water mineralization during the spring flood and preservation of the minimum values throughout the warm season. It corresponds to the Siberian type of hydrochemical river conditions according to O.A. Alekin’s classification. The rivers of this type are the Sarma, the Solntsepad, the Shinanda, the Davsha, the Khara-Murin, the Utulik and many others.
The second type of the hydrochemical conditions is also characterized by the water mineralization decrease up to the year minimum in spring, increase in the periods of the summer boundaries, the following decrease during the summer and autumn flood and slow increase throughout the rest of the year. The rivers of this type are the Selenga, the Chikoi, the Khilok, the Dzhida, the Muren, the Turka and others. It is clear that the hydrochemical type of the Selenga river is fully determined by the type of its main tributaries hydrochemical condition.
The third type of the hydrochemical condition is characterized by sharp short-term decrease of the water mineralization during spring flood with the following fast and considerable increase and comparative permanency in the other months of the year. O.A. Alekin names this type West European one. The Baikal tributaries of this type are the Krestovka and the Khargino rivers.
The fourth type is characterized by rather fast and considerable decrease of the water mineralization in spring with the following slow increase of it throughout spring and summer. This type is most considerably expressed in the rivers of Goloustnaya, Snezhnaya and Polovinnaya, it can be seen in the rivers of Tyya, Tompuda and Barguzin. This new type of the river hydrochemical conditions is named the Baikal one.
The fifth type of hydrochemical conditions is known for one river only, for the Rel. It is characterized by a considerable decrease of the general water mineralization from the early spring (March) until September when it comes to the year minimum. From May until June a small increase of mineralization is observed which coincides with the maximum of river water expenditure. This type is named the condensation one.
The difference of changes in the water mineralization values of the Baikal basin rivers within a year does not exceed 1.5-2.5 in most cases.
The characteristic of the Baikal basin waters in hydrochemical facies is very interesting. The overwhelming majority of tributaries including the main ones, are referred to HCO₃-Ca-SiO₂ or to HCO₃-SiO₂-Ca hydrochemical facies. These facies are characteristic of the temperate climatic zones rivers in the mountainous regions and make up a special mountain (vertical) zone.
The waters of the Baikal 70 tributaries out of the total number of the researched ones can be referred to other hydrofacies.
The yearly spreading of the chemical inflow can be characterized by the following data. Throughout the open water period an average of 88% of the yearly number of ions and 84% of silicon dioxide go into the Baikal. The winter inflow is correspondingly 12 and 16% of the year values. For iron the winter inflow is still lower, only 5.5% of the year total. For organic substances it is 5.1%. It is worth mentioning that the winter water inflow of the Baikal tributaries makes up 12.9% of the year total.
The formation of the chemical composition of Lake Baikal water is the result of a number of complicated processes.
The Baikal waters can be referred to low-mineralized soft waters of the hydrocarbonate class, the group of calcium of the first type according to O.A. Alekin’s classification. The waters of the lake belong to HCO₃-Ca-SO₄ hydrochemical facies. In this way the specific character of the Baikal lake waters formation processes is displayed.
It is extremely interesting that in the waters of Lake Baikal the compounds of ammonium and nitrite nitrogen are absent, at least down to the depths of 500 m.
The content of silicon increases with the depth going up an average from 1.07 mg/l on the surface to 2.5 mg/l at the maximum depths of the lake.
The waters of the Baikal contain iron, not more than 0.06 mg/l, usually 0.02-0.03 mg/l.
The content of manganese in the Baikal waters is still lower, 0.0012-0.0023 mg/l.
Saturation of the lake waters with oxygen causes the development of the organic life at all the depths including the maximum ones. At the same time the waters rich in dissolved oxygen have negative meaning, too: they cause great iron corrosion. This should be taken into consideration when building different engineering constructions permanently in contact with water.
A number of hydrochemical components go into Lake Baikal in bigger quantities than outflow from the lake. In Lake Baikal up to 32% of sulphates, 47% of chlorides, 30% of magnesium, more than 70% of organic substances and silicon, as well as almost all the iron coming with the waters feeding the lake stay in the Baikal every year. On the whole up to 1,890 thousand tons of mineral and organic substances are accumulated in the Baikal.
It has been already mentioned that coming into the Baikal many chemical components are transformed and under the influence of these or those processes are taken out of the total circulation in the water masses.
The research has shown that a number of substances fall out and are buried in the ground sediments. In particular 495 thousand tons of silicon dioxide become ground sediments every year.
Under the influence of dissolved oxygen oxidation of ferric salts goes on. The formed hydro-oxide falls out to the bottom after coagulation. Partly iron is transformed into the ground sediments in the form of phosphate iron binding up the extra phosphates.