Hydrogeochemical features of thermal water of South Trungbo
The paper presents the results of studying hydrogeological features, chemical and isotope composition of thermal waters in central Vietnam strongly affected by intrusive magmatism. It is established that in the study area low- and high-temperature groundwaters are developed with the temperatures varying in the range from 30 to 85 °C. Total hydrothermal mineralization ranges from 0.05 to 10.05 g/dm<sup>3</sup>. The presence of two levels of underground water circulation is suggested differing in temperature and chemical composition. There is a regular change in the composition of the hydrothermal springs in the direction from the mineralized chloride sodium, including those with an increased content of calcium to fresh sodium bicarbonate. The ratio of δ<sup>18</sup>O and δ<sup>2</sup>H isotopes suggests that meteoric water is the basis of the water component. In the coastal areas isotopic shift of ocean water is observed which is also confirmed by the geochemical data. Key factors in the formation of chemical composition of thermal waters of South Trungbo are their genetic type, interaction processes in the system «water-rock-gas-organic substance» and their equilibrium-non-equilibrium state.