Study on the isotopic composition of rainwater in Long Khanh city, Southeast of the Mekong Delta region

Abstract

The ²H/¹H and ¹⁸O/¹⁶O isotope ratios in rainwater bring a lot of information about the fractionation of water molecules in the hydrosphere. The relationship between the isotope ratios of rainwater in an area characterised by the local meteoric water line, which is known as a reliable reference value for studies related to the identification of the recharge source of groundwater and climate change investigations. This study aims to establish a local meteoric water line in Long Khanh city (LK LMWL) in the period of 2020-2021, which is considered as a basis for research on the origin of groundwater in the area of Long Khanh city and subsequent studies on the origin of groundwater in the Southeast of the Mekong Delta region and the Dong Nai river basin. Results show that δ²H in rainwater ranges from -73.64 to 0.36 (‰ VSMOW) with an average value of -49.74‰ (n=19) and that figure of δ¹⁸O ranges from -10.91 to -1.59 (‰ VSMOW) with a mean of -7.68‰ (n=19). Due to the amount and specific meteorological conditions of the region, δ²H and δ¹⁸O in rainwater are enriched in the dry season but deplete in the rainy season. The LK LMWL follows a model of δ²H=(7.89±0.38)xδ¹⁸O + (10.28±2.93) (R²=0.96, n=19), which shows that the isotopic composition of δ¹⁸O in rainwater is more enriched than the isotopic composition of δ²H. The deuterium excess (d-excess) of rainwater in the region is found to be 10.28±2.93‰, which is comparable to those for the global scale of 10‰.