Geochemistry of nitric thermal springs from Khoja-Obi-Garm thermal area (Pamir-Tien Shan region)

This paper presents data on the composition of the low-enthalpy thermal waters from the unique Khoja-Obi-Garm field located at the central part of the southern slope of the Gissar Range, in the gorge of the Khoja- Obi-Garm River, at an altitude of 1740-1960 meters. In this place, the eponymous SPA-center was built. Groundwaters from springs and boreholes of Khoja-Obi-Garm thermal area have temperatures from 57 to 93 °C, high pH (up to 8), and low TDS (less than 0.5 g/l). These waters belong to Na–SO 4 –HCO 3 type with high content of H 2 SiO 3 (~140 mg/l), F (up to 18 mg/l), and Rn (up to 814 Bq/l). The specific feature of these thermal waters is a predominance of N 2 in the gas phase (up to 83-98 vol.%), while other gases (O 2 , CO 2 , Ar, Кr, Хe, Нe, and Ne) are nonessential. Gas content at Khoja-Obi-Garm spa waters is usually not high, does not exceed 30 ml/l. We prove that N2 and O 2 are atmospheric gases, but CO 2 is biogenic gas (δ 13 C TIC -30‰ – -22 ‰). The estimated subsurface temperature is 193-197 °C based on the silica and K/Na geothermometer. Thermodynamic speciations indicate that these groundwaters are supersaturated with clay minerals and lowtemperature zeolites and undersaturated with carbonate and aluminosilicate minerals.