The Chemical and Isotope Composition of Nitrogen Thermal Groundwaters of the Kuldur Spa (Jewish Autonomous Region, Russia)

The results of the hydrogeochemical study of poorly mineralized thermal groundwaters from the Kuldur Spa (Jewish Autonomous Region, Russia) are reported. These groundwaters are distinguished by their high temperature (73°С), low TDS (up to 0.38 g/L), and alkaline environment (pH 9). The dominant cation is sodium, and the dominant anion is hydrocarbonate. The waters are enriched in fluorine, silicon, aluminum, tungsten, molybdenum, and several other anionic elements. Detailed data on the chemical and mineral composition of water-hosting (intrusive) rocks are provided and the sources of the major brine components in the groundwaters are revealed. The study of the associated gases shows that the major component of the gaseous phase (N2) is atmogenic, while CH4 and CO2 are biogenic. Argon and oxygen are derived from air as well, whereas helium is predominantly of the radiogenic crustal origin. The isotope characteristics of oxygen and hydrogen in the aqueous phase indicate the undoubtedly meteoric genesis of the groundwater with an extended circulation period. The results suggest that the thermal groundwaters of the spa are formed via penetration of meteoric waters to a depth of 4 km and heating to 100 °C. The dissolved chemical elements that form the brine phase come via the dissolution of host rocks; however, since the groundwater circulates within the rocky massifs of poorly soluble granitoids, respectively, the TDS of the groundwater remains low.