PRELIMINARY STUDY OF Δ²H AND Δ¹⁸O SIGNATURES IN THE SELBE RIVER BASIN DURING EARLY SPRING FLOW

This research study aimed to determine the stable isotope (²H, ¹⁸O) composition of surface and groundwater in the Selbe River basin. A total of 50 samples were collected from surface and groundwater sources in the basin on April 27, 28, May 1, and 2, 2023, and analyzed. Additionally, on April 27 and 28, sampling was conducted every 4 hours over a 24-hour period (6 sampling rounds in total), collecting 42 water samples from 7 locations along the Selbe River, from the Dambadarjaa Bridge to its confluence with the Tuul River. The samples were analyzed using the LGR-45EP laser spectroscopy instrument at the Institute of Geography and Geoecology. The study utilized data from the GNIP (Global Network of Isotopes in Precipitation) station in Ulaanbaatar as baseline information. According to GNIP data, the isotopic composition of precipitation exhibits significant seasonal variation, with the lightest values (δ¹⁸O: -27.5 to -28.9‰; δ²H: -209 to -224.8‰) observed in November, December, and February, while relatively heavier values (δ¹⁸O: -23.7‰; δ²H: -176.8‰) were recorded in January. The heaviest isotopic values (δ¹⁸O: -7.9 to -9.0‰; δ²H: -57.8 to -69.2‰) were registered between June and September. The isotopic analysis results of the Selbe River surface water samples collected during the study period suggest that the water source during that time was likely primarily derived from March–April precipitation. Additionally, the isotopic composition was lighter in the upper reaches of the river but became progressively heavier downstream, indicating an influence from factors such as elevation and groundwater contributions. For further research on the water cycle processes in the Selbe River basin, it is necessary to conduct seasonal stable isotope and hydrochemical sampling, as well as tritium analysis to determine the age, recharge, and movement of groundwater.