The four types of water salinity (with respect to predominant anions) were distinguished. In 19 lakes, chlorine anions predominate, including 10 lakes with the high concentration of magnesium and/or magnesium bicarbonates, 6 lakes with only sodium cations, and 3 lakes with the high content of sodium bicarbonates. In 12 lakes, sulfates and chlorides predominate, including 7 lakes with the high content of magnesium and magnesium bicarbonates and 2 lakes with sodium bicarbonates. In 7 lakes, soda salinization is registered; bicarbonate anions are combined with sulfates in 3 lakes, chlorides in 3 lakes, and sulfates and chlorides in 1 lake. The concentration of salts in soda-saline lakes varies from 21 to 278 g/l; the highest values are in the southern part of the area (Tanatar 1, Zhivopisnoe, and Nikolaevskoe lakes). Often, the total concentration of salts and their chemical composition differ considerably in neighboring lakes. The chemistry of water salinity is controlled by the (i) composition of groundwater feeding a given lake, (ii) intensity of weathering processes in the rocks and soils within the lake catchment; and (iii) character of physicochemical and biological processes upon the precipitation and crystallization of salts in the bottom sediments. Different evaporites are synthesized: soda, gypsum, mirabilite, halite, etc.
In the salt crusts of sor solonchaks, sodium sulfates (thenardite, Na2SO4) and chlorides (halite, NaCl) predominate. Sodium carbonates and bicarbonates (trona, Na2CO3 x NaHCO3 x 2H2O and soda, Na2CO3 x 10 H2O) have been diagnosed in the salt crusts only around two lakes (Glauberovo and Petulhovskoe lakes). Calcite and dolomite have been identified both in the bottom sediments and in the crusts of coastal solonchaks; in the latter case, they are inherited from the parent material (former bottom sediments). Our data on the mineralogy of solid-phase components in the bottom sediments and sor solonchaks are in agreement with the general scheme of salt accumulation in salt lakes of the arid zone as described by N.M. Strakhov (1962).
The nature of alkalinity was studied by the method of Vorob'eva and Zamana (1994). In 12 lakes with different composition of salts and pH values, the three levels of total alkalinity were distinguished: (1) 2.2-16.7 meq/l, (2) 95-189 meq/l, and (3) 600-1505 meq/l. The pH of lake water rises from 7.6 to 10.3 with an increase in the total alkalinity. In the slightly alkaline lakes with sodium-chloride or sodium-magnesium-chloride composition of salts, the alkalinity is of organic nature with some participation of bicarbonates of alkali metals. In the soda lakes, the role of organic alkalinity decreases, whereas the role of carbonates and bicarbonates of alkali metals increases. In some of them, the presence of borates (borate alkalinity) was registered.
In general, the accumulation of salts in the lake ecosystems is related to the closed character of the lakes and arid climatic conditions. The borate alkalinity in soda lakes with the high salinity allows us to assume that these lakes develop in the areas with soda-saline groundwater. The areas with strongly saline groundwater have their own geography, which should be taken into account in the studies of modern salinization processes in soils around the salt lakes.
This study was supported by the Russian Foundation for Basic Research, project nos. 05-04-49098 and 04-04-48197.