SUMMARY

Nguyễn Thạc Cường, Hồ Minh Thọ, Đoàn Văn Cánh, Đặng Đức Nhận

Geochemistry and genesis of the geothermal water source in Vinh Phuong village, Nha Trang city of the Khanh Hoa province were studied by using traditional hydro-geological and advanced isotopic techniques. To interpret the origin, samples were taken from geothermal water and Son Trung stream located at a distance of 1500 m from the well of geothermal water during the dry (March) and rainy (August) seasons of years 2016-2017 to determine the chemical composition of this water as well as its isotopic composition (d^D and d¹⁸O) in water to have an insight into the geochemistry and genesis of the Vinh Phuong geothermal water source. Results of the hydro-geochemical analyses showed that the geothermal water has a mineral composition of Na-Ca-Cl type with a high mineralization (up to 6130 mg/l). It is a mineral water of silica type with a medium temperature of 58^oC at its resurgence. This implies that the Vinh Phuong geothermal water source has not been recharged from the local surface water. The isotopic signatures of the geothermal water showed that the genesis of the water is meteoric water. The recharge area was determined based on the gradient of d¹⁸O depletion by altitude.The age of the geothermal water was estimated based on the activity of carbon-14 in carbonate. The results obtained suggested that the geothermal water has been recharged from an area of the West, North-West of the Nha Trang city, at an altitude of 400 m above the sea level. The meteoric water infiltrated through fissures of tectonic rocks to thermal reservoir located at a depth of 3000 m below the ground surface. The temperature of the thermal reservoir was estimated based on the geothermometer dependence of the enthalpy and silica concentration in the hot deep water and in the cold surface water. Results showed that the temperature of the thermal reservoir variesranging from 101^oC to 129^oC. The hot deep water has a high pressure so that it could ascend to the surface through the fissures of the rocks in stratums. On the ways of ascending the hot water mixed with cold recharged water. The ratioof the cold water mixing with hot water at the resurgence was estimated to be 0.68.

This is, to our knowledge, the first time a thorough investigation into the geochemistry and genesis of valuable natural geothermal water resourcesbased on performance results of the project: Studying and guiding solutions to exploit, use reasonably and protect mineral water resources - hot water of Vietnam. The techniques applied in this study are advanced and this would be a new research area opened for young scientists in Vietnam.

Keywords: Isotopic composition in water; geothermometer; thermal Reservoir; Vinh Phuong geothermal water resource; Vietnam.