In the past 20 years, comprehensive hydrogeochemical research on more than 200 thermal waters in rift zones on the east Asia continent including Baikal (Russia), Fengwei (China) and Hubsugul (Mongol) has been carried out by the authors. There are three end members of hydrochemical types: (1) SO₄-Na water, (2) Cl-SO₄-Na-(Ca) water, both of which have TDS values between 0.5 and 2.0 g/L, and (3) HCO₃-(CO₃)-Na water with TDS less than 0.5 g/L. All of them are formed in weakly alkaline environments. Their compositions are largely determined by water-rock interactions and local geothermal conditions. Besides, there are thermal waters of transitional types (SO₄-HCO₃-Na-(Ca) water, HCO₃-(Cl)- SO₄-Na-(Ca) water and SO₄-HCO₃ -Na-(Ca)water) whose compositions are controlled both by water -rock interactions and by mixing processes. Most of the HCO₃-(CO₃) - Na waters are alkaline (pH up to 9.4) with relatively high contents of CO 3 2 − https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203734049/8ccf6b8f-3ccf-4e19-b1d7-52ac3eb44ef1/content/eq213.tif"/> which intensifies the migration of fluorine leached from hostrocks. Intensive evaporation due to local arid climate is responsible for the occurrence of such alkaline waters.