Using the data of long-term studies of the chemical composition of groundwater of the Tersinskoye field and the results of field observations, the prevalence of chemical elements was studied, the processes of formation of natural water composition were determined, the degree of equilibrium of water with minerals of the host rocks was investigated and the formation of alkaline carbonate-calcium geochemical type of water was substantiated. Acidulous waters are valuable hydro-mineral resources that have a therapeutic effect on the human body. Research on the peculiarities of chemical composition of the acidulous water deposit known as the Tersin deposit, which is unique for the Kuzbass Region, has been carried out for over fifty years, of formation of water composition and ways of entering and enriching the water with carbon dioxide remain unstudied. Previous research mainly focused on hydro-geological aspects of study of the water, and until now no unified complex vision of the Tersin mineral water formation has been offered. On the basis of long-term studies of the chemical composition of Tersin groundwater and the results of field observations, the prevalence of chemical elements was studied, the processes of formation of natural water composition were determined, the degree of balance of water with minerals of surrounding rocks was investigated and the formation of alkaline-carbonic acid geochemical type of water was substantiated.