[The peculiar hydrogeochemical features of the hydrosphere in the regions of salt-dome tectonics (as exemplified by the giant salt domes of the Peri-Caspian Lowland megabasin)].

The Peri-Caspian Lowland megabasin is a vast region of ancient salt accumulation that occurred during the Permian period. The territory over which the salt domes were formed gave rise to and accumulated huge resources of subterranean brackish and salted waters within the hypergenesis zone that also included brine and mineral mud lakes containing chemical elements of great therapeutic value. Nowadays, the subterranean waters (leaches, pools, and springs) provide the sources of the mineral-rich materials of balneological significance, such as sulfide, bromine, and boron-containing mineral waters the ion and mineral composition of which together with the high degree of mineralization makes them highly valuable for medicinal applications. Both leaches and peloids are extensively used in chemical industry and cosmetology. Brine lakes of the Peri-Caspian Depression are hypersaline lake water bodies with waters of the chlorine-magnesium type having marine origin. These lakes are fed by ground waters (especially those coming from the salt springs) in the combination with atmospheric precipitation. The chemical composition of brine and peloids of lakes Inder and Baskunchak is characterized by the high degree of mineralization with sodium chloride the concentration of which amounts to 264 и 325 g/dm3 respectively. The natural brine of the lake Inder contains the following biologically active components: Br - 460 mg/dm3 and H3BO3 - 100 mg/dm3. The territories surrounding the lakes Inder, Baskunchak, and El'ton are the sources of salt-saturated sulfide-silty muds characterized by a high degree of mineralization (250-306 mg/dm3) due to the presence of 0,15-0,50% of FeS. In addition, they contain biologically active bromine and borates, besides hydrogen sulphide. The ground-water discharge sites at the territories around lakes Inder and Baskunchak are located largely in the vicinity of the Permian sulphate-halogen deposits. The springs bring mostly water with the enhanced content of sodium chloride and the high degree of mineralization ranging from 20-30 g/dm3 to 150 g/dm3. They can be considered to be the analogues of waters of the Usol'skiy, Staroruskiy, ands Vologodskiy types and are highly suitable for the application for the purpose of balneotherapy.