Chemical composition of salt lakes in East Transbaikalia (Russia)

Abstract

We have developed a new concept on the formation of diverse geochemical types of salt lakes, in particular for those in the Transbaikal region. This concept explains the nature of their different chemical composition under the close landscape-climatic and geological-geochemical conditions. The extensive hydrogeochemical field data and thermodynamic calculations demonstrate that in addition to evaporation, interactions with rocks play a substantial role in the formation of the composition of these salt lakes. Such processes are most widespread in soda lakes, which is confirmed by the highest pH values (9.0–10.7). These lakes provide evidence of the interaction of the lake waters with aluminosilicates; their hydrolysis provides a pH increase and consequently a high concentration of carbonate ions. These high pH and HCO3− and CO32− contents in turn lead to the maximum precipitation of Ca, Mg and Fe carbonates. Soda lakes also contain high chloride ion concentrations, since a small portion of those ions evaporate under the lake conditions. If evaporation continues and the amount of carbonate ions in the lake does not increase, or increases slowly due to binding with carbonate minerals, the chlorine content becomes higher, the pH lowers to less than 9.0, and the lakes become chloride. Another fundamental process in these lake environments is sulphate reduction, which results in the production of hydrogen sulphide, fundamentally changing the geochemical environment from oxidizing to reducing. The presence of an oxidizing environment and sulphides in rocks gives additional sources for sulphates, which ensures the formation of sulphate-type lakes.