[H-TT16] Development and application of environmental traceability methods

[HTT16-P01] Oxygen and hydrogen isotopic signatures of surface and groundwater during a dry season in the Tonle Sap Lake basin

*Yumi Yoshioka¹, Takao Masumoto², Kumiko Tsujimoto³, Yuji Ito⁴ (1.Shimane University, 2.Akita Prefectural University, 3.Okayama Univerisity, 4.Kagoshima University)

Keywords:Stable isotope, Groundwater, Kinetic isotope effect , Lake water balance, Mekong River, Cambodia

Tonle Sap Lake located in Cambodia is the largest lake in South Asia. This lake has several inflow rivers and one outflow river. It is connected to the Mekong River via the outflow river and the Mekong river water inflows to the lake during wet seasons. Thus, the lake is characterized by key fluctuations of water levels and surface area. There is a need for better understanding of water supply to the lake from surface or under surface water. The contribution of groundwater is still unknown. Stable isotopes in water are often used to identify lake water balances. We therefore conducted a sampling campaign in February 2020 of a dry season. We collected water samples for four lake waters, nine river waters including inflow rivers, outflow river, and subbranch of the Mekong River, eight groundwater, and eleven commercial bottled water made in Cambodia and Thailand. Major dissolved ions, dissolved silica (DSi), and stable isotope ratios of oxygen and hydrogen (δ¹⁸O and δ²H) were analyzed. Isotopic composition of rain obtained in earlier studies were used for data analyses. The highest ratios were observed in the lake and were ranging from 0.0 to 2.2 ‰ for δ¹⁸O and -18 to -9 ‰ for δ²H. The isotopic ratios became higher in the order of subbranch of the Mekong River, the inflow rivers, and the outflow river. The isotopic composition of the groundwater varied widely: from -7.3 to -2.3 ‰ for δ¹⁸O and -52 to -21 ‰ for δ²H. Average isotopic composition of groundwater was consistent with the volume-weighted average isotopic compositions of rain. Isotope signatures of groundwater did not depend on the depths of the wells. The regression line was delineated on a dual-isotope plot by samples of the lake and outflow river, and it was defined as the local evaporated line (LEL). The slope of LEL was 5.0. In addition, the slopes of regression lines for inflow river and groundwater were 4.9 and 6.2, respectively. The flatter slopes than that of the LMWL indicated that surface and groundwater were more evaporative than the source water, and isotope compositions were enriched by the kinetic isotope effect during evaporation even in river water. Small isotopic differences (ratios and slopes) between groundwater and commercial bottled water were observed. The intersect of LEL and LMWL, which is the initial composition before isotopic enrichment process, was fall near samples of groundwater and the average isotopic compositions of rain. The DSi concentration is used as a residence time indicator in groundwater. The higher concentration in the lake (less than 20 mg/L) than in inflow rivers (less than 10 mg/L) was consistent with the contribution of groundwater to the lake. Water sampling survey with monthly interval has been implemented since May 2020. Further works are monitoring of isotopic fluctuation throughout dry and rainy seasons, and tracing water source of the lake.