Chemical and isotopic groundwater composition analyses in the south-eastern Transbaikal case study region allowed three geochemical types of waters and fifteen subtypes to be identified: bisiallite (CaHCO3), soda (NaHCO3) and salt (NaHCO3ClSO4). All three types differed significantly with respect to TDS content, pH, composition of chemical elements and heavy water isotopes. The transition from the first to the subsequent types was accompanied by CO32, HCO3, SO42, Cl and Na+ accumulation, whereas a weaker dependence was exhibited for Mg2+ and K+, and no relationship with Ca2+ was found. Water was found to be gradually enriched in heavy isotopes and some trace elements (e.g. F, Mo, Br, B and Li). Findings showed that the groundwaterrock system is one of an equilibrium-disequilibrium. From the extent of the deeper horizons to the surface water, groundwater in the study region continuously dissolves minerals that are in disequilibrium with the groundwater and simultaneously forms new minerals. Moreover, over time, not only does the composition of the solution change, but the type of secondary minerals and the ratio between chemical elements also changes because some elements precipitate out of solution, while others continue to accumulate. The stage of soda-type groundwater formation was found to be at an intermediate level between the formation of fresh bisiallite (CaHCO3) water of different cationic composition and salt (NaHCO3ClSO4) water with mixed anionic composition.