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

JOURNAL OF WATER RESEARCH AND ENGINEERING Vol.1, No 1 ISSN: 3105-8973 19 DOI: 10.5281/zenodo.17528384 Received: 13 October 2025; Accepted: 1 November 2025 PRELIMINARY STUDY OF Δ²H AND Δ¹⁸O SIGNATURES IN THE SELBE RIVER BASIN DURING EARLY SPRING FLOW Purew-Erdene Erdemjargal1 , Dorjpalam Od, Chinzorig Sukhbaatar* 1Institute of Geography and Geoecology, Mongolia Academy of Science, Ulaanabaatar city, Mongolia E-mail: [email protected]1 Abstract: 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 24hour 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. Keywords: River, GNIP(Global Network of Isotopes in Precipitation), precipitation, Heavy isotope, Light isotope, Global Meteoric Water Line. I. INTRODUCTION An isotope refers to different forms (atoms) of a chemical element that have the same number of protons in the atomic nucleus but differ in the number of neutrons. They exhibit nearly identical chemical properties but differ in their physical properties. Isotopes are classified as stable isotopes or unstable (radioactive) isotopes. Stable isotopes do not decay (non-radioactive) or have extremely long half-lives, whereas radioactive isotopes have unstable nuclei and release energy through the process of decay [1]. Understanding the formation of runoff and its circulation processes is fundamental to water resource management, while the isotopic ratios in water provide a powerful tool for studying these mechanisms on a broader scale. Natural water molecules contain isotopes of hydrogen and oxygen (HD16O, H218O, HD18O), as well as radioactive isotopes such as tritium (³H), carbon14 (¹⁴C), chlorine-36 (³⁶Cl), and krypton-81 (⁸¹Kr). These isotopes carry valuable information about the global water cycle, groundwater origins and recharge sources, groundwater age, renewal rates, flow dynamics, and sources of water contamination [2]. The study of water isotopes emerged as a cutting-edge scientific technology in global research history starting in 1961. In Mongolia, the pioneering work was conducted by Soviet (former USSR) researcher Romanov V.V. and his team around the mid-1980s. They collected isotopic samples from precipitation, surface water, and groundwater in the eastern and central regions, determining that the age of groundwater near the Khalkh River area was approximately 1,000 years and confirming its recharge primarily from precipitation [3,4]. In line with the International Atomic Energy Agency (IAEA) developing and implementing methodologies to study water isotope composition as part of its efforts to promote peaceful uses of nuclear technology for development, Mongolia’s request to conduct research in arid and drought-prone regions, build human capacity, provide training, and apply isotope techniques to determine water movement, age, and recharge was accepted in 1988. Since then, the IAEA has been collaborating to improve water resource management in Mongolia. From 1988 onward, five IAEA Technical Cooperation projects and four RCA (Regional Cooperative Agreement for Asia and the Pacific) projects have been implemented in Mongolia to enhance the country's water resource management. JOURNAL OF WATER RESEARCH AND ENGINEERING Vol.1, No 1 ISSN: 3105-8973 20 Sustainable water resource management is of critical importance for maintaining ecosystem balance and socioeconomic development. The Selbe River serves as a major water source for Ulaanbaatar City, and studying its hydrological characteristics and isotopic composition helps to understand the water cycle mechanisms and recharge sources. The objective of this research is to determine the stable isotope (²H, ¹⁸O) signatures of the Selbe River and analyze the dynamics of its hydrological system. II. RESEARCH METHODS 1. Study area The Selbe River is located in the southern part of the Khentii Range and is located at an altitude of 1510-1620 meters above sea level. It covers a total area of 342.2 square kilometers. There are 7 species of trees and shrubs in the plant kingdom and 183 species of plants from 40 families in the river basin. The fauna includes hedgehogs, hares, squirrels, foxes, and gray wolves in small numbers, while wild boars, roe deer, elk, red deer, and lynx are seen in the stream and live temporarily. 2. Data and materials As part of the research, from April 28 to May 2, 2023, researchers from the Institute of Geography and Geoecology collected stable isotope samples from 10 surface water points and 40 groundwater points in the upper reaches of the Selbe River basin (Figure 1). A total of 42 isotope samples were also collected from 6 selected points at 4-hour intervals over a 24-hour period at the Selbe Restoration Project site. These samples were analyzed and processed as part of the study. The research used data from the Global Network of Isotopes of Precipitation (GNIP), an initiative of the IAEA and the World Meteorological Organization (WMO) in 1960. This network provides valuable information for water cycle, water resources management, climate change, and ecosystem studies, as well as for monitoring and studying the isotope composition of precipitation. The study of surface and groundwater isotopes (2H, 18O) in the Selbe River Basin used precipitation isotope data from the Ulaanbaatar City Precipitation Isotope Network for 1990-2001 and 2021-2022. Figure 1. Water stable isotope sampling points 3. Isotope sampling There are no special requirements for samples for 2HDeuterium and 18O-Oxygen determination, and samples were collected in high-density polyethylene plastic bottles with a capacity of approximately 50 ml and in glass bottles with screw caps. In field conditions, the sample bottles were not rinsed or cleaned using any chemical cleaners. After sampling, the water sample was placed 2/3 of the way up the bottle to prevent any expansion or freezing during transportation. The lid of each sample bottle was sealed with a plastic seal or double-sealed. The samples were stored at room temperature until delivery to the laboratory, and the lid was required to be tight enough to prevent evaporation and change in stable isotope composition. A total of 92 water samples were collected from April 28 to May 2, 2023. 4. Analyzing stable isotopes of water A total of 92 samples of surface and groundwater from the Selbe River were analyzed using the “LIMS for laser spectroscopy” program installed on a computer with 32MB of memory, with the sample size, location, instrument brand, and a brief description of the work being performed (Figure 4) entered into the Los Gatos 45EP model laser spectroscope instrument to obtain the results of the stable isotopes (2H, 18O) of the water. JOURNAL OF WATER RESEARCH AND ENGINEERING Vol.1, No 1 ISSN: 3105-8973 21 Figure 2. Preparing samples taken in the field for placement in the instrument's automatic sampler Figure 3. Stable isotope determination laser spectroscopy 5. Stable isotope processing of water Stable isotopes of water (2H, 18O) are nonradioactive and do not decay, so they only change as a result of the process of water changing from one state to another (evaporation and mixing). These isotopes can be used in a wide range of hydrological studies, and were used in this study to determine the source of water and whether it has been involved in the evaporation process. The amount of stable isotopes of oxygen, carbon, and hydrogen is not expressed in concentration, but is measured in parts per thousand (‰) or relative isotope ratios, and is denoted by delta (δ). 𝛿′𝐸 =[ 𝑅𝑠𝑖𝑚𝑝𝑙𝑒 𝑅𝑟𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒 −1]∗1000‰ (1) Where: i – atomic mass of the heavy isotope of the element (O or H) Rsample - ratio of the number of atoms of the heavy isotope (¹⁸O, ²H) to the light isotope (¹⁶O, ¹H) of element E in the sample Rreference – ratio of the number of atoms of the heavy isotope to the light isotope of the element in the reference material The reference for oxygen and hydrogen is the Vienna Standard Mean Ocean Water (VSMOW), where the values of δ¹⁸O and δ²H are defined as 0.00‰. The heavy isotope of hydrogen is commonly referred to as deuterium, and δ²H is sometimes denoted as δD. The isotopes (δ¹⁸O and δ²H) in atmospheric precipitation exhibit a linear relationship globally, known as the Global Meteoric Water Line (GMWL), which is expressed by the following formula: 𝛿2𝐻 =8𝛿18𝑂+10 (2) This correlation has been confirmed by numerous studies (Figure 4). Figure 4. The Global Meteoric Water Line (GMWL) - the global linear relationship of oxygen and hydrogen isotopes in water - serves as an isotopic geoindicator for studying water-rock interaction processes. Figure caption: 1Global Meteoric Water Line (GMWL), 2Moisture recycling, 3Isotopic exchange with H₂S, 4Isotopic exchange with CO₂, 5Water-rock interaction in geothermal systems, 6Mixed with andesitic water, 7Water affected by evaporation, 8Water mixed with seawater, 10Moisture condensation process, 11Paleowater. However, the regional precipitation isotope watershed is essential for interpreting stable isotope data from surface and groundwater in a given region. This stable isotope watershed is essential for studying the hydrological processes and circulation of the river or lake basin under study, their water sources, interactions between water and soil and rock, groundwater recharge, and the mixing of different types of water. The amount of stable isotopes in natural water allows us to generally determine the origin of the water, based on which type of trend is best represented. III. RESULT AND DISCUSSION 1. Isotope network values in global precipitation The International Atomic Energy Agency (IAEA) and the World Meteorological Organization (WMO) are jointly implementing the Global Network of Precipitation Isotopes (GNIP) worldwide. The network is based in Ulaanbaatar and collects monthly precipitation samples. JOURNAL OF WATER RESEARCH AND ENGINEERING Vol.1, No 1 ISSN: 3105-8973 22 The samples are sent to laboratories for analysis of stable isotopes (²H, ¹⁸O) in the water. This research provides basic information for climate change, water cycle, and hydrological studies. Figure 5. Stable isotope data for precipitation in Ulaanbaatar from the Global Network of Isotopes of Precipitation (GNIP) The precipitation isotope data for Ulaanbaatar city, as shown in Figure 5, clearly show seasonal effects, with the lowest or relatively light isotopes (18O -27.5 to -28.9‰, 2H -209 to -224.8‰) in November, December, and February. However, January is relatively higher than these months (18O -23.7‰, 2H -176.8‰). The heaviest isotopes are observed between June and September (18O -7.9 to -9.0‰, 2H -57.8 to -69.2‰). 2. The relationship between surface and groundwater As part of the Selbe River isotope study, 8 isotope samples were taken from the surface water of the upper and middle parts of the river basin, and 2 samples from the Belkh River on April 28, 2023. The results indicate that the source of the Selbe River water is likely to be recharged by precipitation in March and April. While the water isotopes were the lightest in the upper part of the river, the stable isotopes of the water become relatively heavier as it moves down the length of the river (Figure 6). This can be explained, firstly, by the effect of land elevation. The difference in elevation between the highest point (B-2) and the lowest point (B-13) is 154 m (Figure 1). Secondly, as the Selbe River water moves down the length of the river, it is more interconnected with groundwater and may be recharged to a certain extent by groundwater. This can be explained by the fact that the stable isotopes of the water become relatively heavier as it moves down the length of the river. In conclusion, based on the isotope analysis of Selbe River water taken on April 28, 2023, and groundwater taken on May 1-2, 2023, the Selbe River water may be fed by the yellow water of winter precipitation and the groundwater fed by the rain in the warm season. The isotope analysis of the Belkh River shows that the water in the river has a relatively heavy isotope composition that is different from that of the Selbe River (Figure 6). In other words, the water sample taken on April 28, 2023, coincides with the isotope composition of the warm season precipitation, which is due to the fact that the rainwater that fell in previous years is feeding the river through groundwater. 3. 24-hour stable isotope measurements at the project site As part of this study, 6 samples were collected at 4hour intervals over a 24-hour period on April 27 and 28, 2023, from a total of 7 locations along the length of the Selbe River from the Dambadarjaa Bridge to the confluence of the Selbe River and the Tuul River. A total of 42 isotope samples were collected and analyzed using an LGR-45EP laser spectrometer to produce stable water isotope results. The results of the water stable isotope measurements conducted at the Selbe River Restoration Project site show that the lightest isotopes were measured at point 1 near the Dambadarjaa Bridge and point 2 near the 6th station (18O -15.05 to -15.64‰, 2H -112.5 to -116.88‰). As the river goes down, the water stable isotopes become relatively heavier, reaching 18O 13.26 to -13.55‰ and 2H -102.93 to -100.43‰ at the point before the Selbe River flows into the Tuul River. This can be explained by two reasons as shown in Figure 7. 1. The effect of elevation; 2. It is mixed with groundwater or isotopic correlation. As can be seen from the figure below, the Selbe River water in the project area coincides with the precipitation isotopes of April and May, which may be due to the precipitation water of these months. The elevation of point 1 of the 24-hour measurement survey is 1350 m and the elevation of point 7 is 1271 m, or an elevation difference of about 80 m. This can be explained by the fact that the precipitation isotope becomes relatively lighter as the surface elevation increases. However, as shown in Figure 7, the most plausible explanation is that the Selbe River water is more likely to interact and mix with groundwater as it descends along its length. Point 1 is the lightest isotope measured near the Dambadarjaa Bridge and point 2 is the lightest isotope measured near the 6th station, while the groundwater in the Selbe River valley is relatively heavier than the surface water. This can be explained by the fact that the stable isotope of water becomes relatively heavier as it descends along the length of the river due to the interaction between surface water and groundwater. JOURNAL OF WATER RESEARCH AND ENGINEERING Vol.1, No 1 ISSN: 3105-8973 23 Figure 6. Stable isotopes of precipitation in Ulaanbaatar city and surface and groundwater in the Selbe River basin Figure 7. Global water lines, Ulaanbaatar city's meteorological water lines, groundwater and 24-hour isotope results JOURNAL OF WATER RESEARCH AND ENGINEERING Vol.1, No 1 ISSN: 3105-8973 24 IV. CONCLUSION As part of the isotope study being conducted as part of the Selbe Restoration Project, a total of 92 samples were collected and analyzed from surface and groundwater in the Selbe River basin from April 28 to May 2, 2023. The water of the Selbe River in the spring season may be fed by the yellow water fed by the precipitation in the winter and spring seasons and by the groundwater fed by the rainfall in the warm season. As the Selbe River water descends lengthwise, the water isotope shifts from light to heavy, approaching the isotope value of the groundwater, which is related to the increasing interaction of surface water with groundwater as it descends lengthwise. According to the results of the water samples taken from the Belkh River, the water of this river coincides with the isotope of the precipitation in the warm season, which is related to the fact that the rainfall in previous years feeds the river water through the groundwater. The result that the Belkh River has more groundwater fed may be related to the fact that the yellow water flow may not have occurred (or has already occurred) at the time of sampling. In the future, this research work can be carried out in detail by taking stable isotope and hydrochemical samples of water on a seasonal basis to understand the hydrological processes of surface and groundwater in the Selbe River basin, the water cycle in the basin, and their interrelationships, and by taking radioactive tritium samples to determine the age, movement, and nutrition of groundwater in the basin. ACKNOWLEDGEMENTS We would like to thank the scientists from the Department of Water Resources and Water Use at the Institute of Geography and Geoecology for their invaluable assistance in carrying out this research. REFERENCES [1] Э. Пүрэв-Эрдэнэ ба С. Чинзориг, Хөвсгөл нуур, түүний цутгал голын усны тогтвортой изотопын судалгаа, Улаанбаатар, 2024, pp. 109-120. [2] Л. Жанчивдорж , Б. 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