Bitter-Salt Lakes

Bitter-salt lakes can be divided by their origin into two large groups: continental and marine. The former, representing endorheic basins, owe their salinity to the leaching of surrounding rocks; the latter were formed with the direct participation of the sea, and the salt reserves in them are of marine origin. Transitional types can exist between purely continental and marine lakes.

The USSR occupies first place in the world in terms of the number and size of bitter-salt lakes. They number in the thousands and are located along the coast of the Black and Azov Seas, in the Volga basin, the Aral-Caspian lowland, Western Siberia, Transbaikalia, Turkestan, and other places. The widely known Odessa limans, Saki and Maynak lakes, and other bitter-salt bodies of water along the sea coast in the south of Ukraine and in Crimea are formations resulting from damming by a coastal ridge called a peresyp (dammed lakes).

Those of them which, by the shape of their bed, are a continuation of ravines or rivers and are replenished through them are called limans. Significant salinization occurs upon the loss of connection with the sea and due to the steppe character of the rivers feeding them. In the case of marine bays being separated from the sea by a peresyp, so-called bay lakes are formed, which include the Berdyansk, Sasyk-Sivash, and Maynak lakes. The replenishment of the salt mass in marine lakes can continue through the seepage of seawater through the peresyp.

A significant group of lakes with a salt mass of marine origin exists in the southern part of the Volga basin, in the Caspian region, and in the Aral-Caspian lowland. Huge lakes, called seas—the Caspian and Aral—separated from the world ocean due to dislocation processes and, upon the reduction of their area, left behind thousands of smaller drying-up bodies of water.

In closed basins lacking drainage, in areas with a dry, continental climate, continental bitter-salt lakes are formed. In some cases, the supply of salts occurs at the expense of their reserves in the strata of the earth; the lake basins themselves are formed by sinkholes resulting from the leaching of salt layers by waters. Such continental lakes include the Slavyansk lakes and, likely, Elton and Baskunchak.

The lakes of Western Siberia and Turkestan, which are very diverse in origin, size, and degree of salinity, have been very little studied. A feature of continental and, in particular, Siberian lakes should be considered their richness in sulfuric acid salts of sodium and magnesium, and sometimes also sodium bicarbonate. The salt-rich water of the lakes is called rapa (brine).

The degree of its salinity and the composition of the salt mass depend on a whole series of geological and climatic factors and can vary widely. With a decrease in the inflow of fresh or low-salinity waters and significant evaporation, salt precipitation can occur, and the body of water can transition into a self-depositing lake. Significant fluctuations in the concentration of brine in lakes, accompanied by the precipitation of calcium sulfate and its subsequent conversion by the vital activity of bacteria into calcium sulfide, contribute to a decrease in the sulfuric acid content in the brine.

As a result, in terms of the chemical composition of the salt mass, even lakes of marine origin often differ from seawater. The change in the composition of marine salts is significantly influenced by calcium carbonate, which continuously enters the body of water together with rainwater and spring water flowing down ravines. The following chemical reactions lead to the formation of magnesium and calcium chloride salts in the brine: 2 Ca(HCO₃)₂ + MgSO₄ = CaMg(CO₃)₂ + CaSO₄ + 2CO₂ + 2H₂O; 2 Ca(HCO₃)₂ + MgCl₂ = CaMg(CO₃)₂ + CaCl₂ + 2CO₂ + 2H₂O. Such a process of metamorphism of saline brines leads to the transformation of first-class brines, according to the classification of N. S. Kurnakov, containing CaSO₄, MgSO₄, Na₂SO₄, MgCl₂, NaCl, into second-class brines, containing CaSO₄, CaCl₂, MgCl₂, NaCl.

To characterize the degree of metamorphism, the coefficient K = MgSO₄ / MgCl₂ is used. For Crimean and Black Sea lakes with brine of a marine character, it varies from 0.4 to 0.8. Typical 1st-class lakes are: Khadzhibey and Sukhoi limans in the vicinity of Odessa, and the Berdyansk lakes; 2nd-class—Kuyalnik liman. Besides the two paths mentioned above, the desulfatization of brine can also proceed by virtue of the reaction of exchange adsorption between sodium and magnesium ions of the brine and calcium adsorbed in soil particles washed from the shores of the lake, according to the reaction: [2 Ca^~] + Na₂SO₄ + MgSO₄ = [2 Na⁺ + Mg] + 2 CaSO₄ (adsorbed).

Lakes of continental origin are distinguished by an extraordinary diversity in the chemical composition of dissolved salts. Lakes strongly enriched with sulfuric acid salts are known: in the North Caucasus—the Batalpashinsk and Tambukan lakes; in the Altai district—B. Marmyshanskoye, etc. The character of bottom sediments and the biochemical processes occurring in them can undoubtedly influence the composition of the brine. The intensity of biochemical processes depends to a great degree on the concentration of the brine. The saltier the lake, the poorer it usually is in organic matter. Bottom sediments are of mechanical and chemical origin. The former are formed by river deposits, spring and rainwater, erosion of shores, and also by dying organisms. The latter precipitate depending on changes in concentration and temperature and as a result of chemical reactions occurring in the body of water. On the bottom of the lakes, one finds deposits of mineral muds and precipitates of sulfuric and carbonic acid salts, magnesium and calcium, and sodium chloride and sulfate. The composition of the mud deposit is in a definite relationship with the composition of the brine. Bitter-salt lakes, which are not distinguished by their size, are short-lived. The silting up of their basins occurs in historical time and leads to the formation of salt marshes. Many of the lakes are used for the extraction of sodium chloride, Glauber's salt, and in some cases, magnesium salts and bromine. The brine and mineral muds of the following bitter-salt lakes of the USSR are widely known for their therapeutic properties: Saki and Maynak in Crimea, Kuyalnik, Khadzhibey, and Sukhoi limans in the vicinity of Odessa, Molochny liman, Berdyansk lakes, Khanskoye lake near Yeysk, Batalpashinsk and Tambukan in the North Caucasus, Tinak and Elton in the Astrakhan district, Slavyansk lakes in Ukraine, Manych lakes in the Don system, Iletskoye lake in the Ural system, and Gorkoye in the Chelyabinsk district. Among the Siberian lakes, it is necessary to note Karachinskoye, Yarovoye, Tagarskoye, Uchum, Shiro, Shunet, and others. Western Europe is poor in bitter-salt lakes. Numerous bitter-salt lakes are known in China.