The isotopic composition (δ2H and δ18O) and unstable parameters (temperature, Eh, pH) of the natural waters in this area were obtained to clarify the features of the formation of the thermal water. The nitrogen thermal (72 °C) and relatively cold (20–25 °C) groundwater as well as surface water was studied in 2022. The isotopic composition of the surface water in the Kuldur River varies from δ18O≈−13.30‰ and δ2H≈−95.8‰ to δ18O≈−12.53‰ and δ2H≈−90.5‰ and fits on the approximation line δ2H = 7.14 × δ18O–1.2 (R2 = 0.91), which is close to the Local Meteoric Water Line (LMWL δ2H = 7.24 × δ18O + 0.5). Thermal groundwater has the isotope composition from δ18O≈−13.83‰ and δ2H≈−100.6‰ to δ18O≈−12.29‰ and δ2H≈−95.1‰ and it is approximated by equation δ2H = 3.23 × δ18O–56.2 (R2 = 0.84). The initial isotopic composition of the recharge water is about δ18O≈−14.1‰ and δ2H≈−102‰, which was found as intersection of the approximation line and LMWL. A comparison of the water temperature and its isotopic composition shows that thermal waters are most likely the result of a mixture of three components. The first end-member is the modern recharge water (it is obviously from tritium content), the second end-member is thermal fluid, and the third one is thawed permafrost, as the authors suggest. This assumption is supported by water from one of the observation boreholes noticeably shifts from the main area of the isotopic composition of the thermal water towards the isotopic enrichment. Apparently, this deviation is attributed to the isotopic fractionation during the groundwater freezing, when aquifer was turned into permafrost in last ice age, which is now thawing under the climate influence. The contribution of meltwater cannot yet possible to assess. Data on the tritium content and the isotopic composition of the dissolved noble gases makes it possible to estimate characteristics of the modern recharge water. Calculation gives an age of the modern component about 55 years and initial tritium concentrations of about 98 TU, which is fully consistent with field observations.
