ИСТИНА

This paper presents geochemical data on the unique Khoja-Obi-Garm Spa (Tajikistan), located at the western part of Pamir-Alai mountain system. Thermal waters are characterized by high temperatures (57–93°C), slightly alkaline pH (7–8) and low TDS (< 0.5 g/l). These waters belong to Na–SO4–Cl–HCO3 or Na–Cl–SO4–HCO3 type with high content of silica (up to 140 mg/l), fluorine (up to 18.0 mg/l) and some specific microcomponents: Li, Al, Rb, As, Ga, Sr, Cs, Ba, V. High concentrations of radon (up to 814 Bq/l) were also detected. Detailed studying of the geological and hydrogeological settings of the spa together with thermodynamic simulations allowed us to reveal sources of chemical elements into these waters. Thermodynamic calculations indicated these groundwaters as supersaturated with clay minerals and low-temperature zeolites and undersaturated to carbonate and main allumosilicate minerals. The estimated subsurface temperatures of groundwater reservoir for these thermal waters vary from 140-156°C to 193-197 °C for silica and K/Na geothermometers consequently.