Tracing sources of ammonium in reducing groundwater in a well field in Hanoi (Vietnam) by means of stable nitrogen isotope (δ15N) values.

Abstract:The origin and chemistry of the groundwater in the middle Pleistocene (qp 2 3 ), lower Pleistocene (qp 1 ), upper Pliocene (n 2 2 ), lower Pliocene (n 2 1 ), and the Miocene (n 1 3 ) in the Mekong river delta (MKRD) were investigated by using isotopic and geochemical techniques. The origin of the groundwater was evaluated based on the composition of the water stable isotopes (d 2 H and d 18 O) in the local precipitation, in water from the rivers system, and in the groundwater samples. The hydraulic interaction between the surface water and the groundwater as well as between the aquifers was assessed by a statistical treatment for the mean and standard deviation of the d 18 O signature and based on the 14 C-ages of the water samples taken from different aquifers. The salinization of groundwater in the deep aquifers was investigated using the d 18 O signature combined with the geochemical composition of the water samples. It was revealed that the groundwater in the deep aquifers in the MKRD could be divided into two groups. The first group is fresh and represents the regional precipitation with the long traveling time ranging from older than 100 years to older than 40 ka BP (kilo years Before Present). The second group is the regional precipitation that is recharged from the remote areas mixed with the seawater. Statistical treatment with the mean d 18 O using the Mann-Whitney test showed that the water from the Mekong river system did not or very weakly recharged the deep aquifers. The groundwater in the deep aquifers in that region was likely to be connate, so that fresh groundwater resource in the region seemed to be limited. The chemistry of the groundwater in the study region is controlled by the incongruent dissolution of the Mg-calcite as well as sulfate and iron oxy-hydroxide reductions by organic matters presented in the aquifer sediment. The groundwater in the deep aquifers in the MKRD from some locations was saline, but the salinity in most aquifers was thought to result from the migration of saline water entrapped in the marine sediment pores to the fresh water in the aquifers. Meanwhile, in other locations, the salinity was suggested to result from the salt intrusion due to the over-abstraction rate, as it was evident from the d 18 O vs. [Cl -] relationship, or due to the up conning of saline water from the deeper aquifers to the upper ones. Particular measures must be developed for the better management of groundwater in the MKRD to ensure a sustainable resource of freshwater being supplied to the local population in future.

The behaviour and water-sediment distribution of particle-reactive Polonium-210 in the marine environment with high concentration of total suspended particulates (TSP) and dissolved organic matters (DOC) along the coast of the Tonkin Gulf (North Vietnam) were investigated. It was revealed that the water-sediment distribution coefficient, Kd(s), of 210Po varied from 2.39x103 to 1.9x104 (L kg-1) and from 7x103to 2.5x105 (L kg-1), respectively, in the rainy and dry season. This implies that in the aquatic environment 210Po tends to be of high affinity to suspended particulates. The 210Po Kd(s) was positively correlated with salinity in both rainy and dry seasons. With DOC the Kd(s) was positively correlated in the rainy season, but in the dry season the relationship tended to be reverse. This behaviour of 210Po in the coastal region was explained by the variation of pH of seawater and by the complexation of the isotope with DOC. The Kd(s) found in this study was in an order lower compared to that reported by Malaysian researchers for the Thailand Gulf. The most important source of 210Po was suggested to be from in-situ generation by the decay of its grand-parent 210Pb which mainly derived from the atmospheric fall-out.

Strawberry samples were collected from Dalat and Mocchau regions and extracted by a cryogenic vacuum method. Stable isotope compositions δ2H và δ18O in the extracted water were then analysed by a Liquid Water Isotope Analyzer (LWIA-24D). The evaluated results showed that, at the statistical significance level α=0.05, the δ2H and δ18O values obtained at Mocchau and Dalat samples are clearly different (p<0.001). In detail, the mean value of δ2H for strawberries grown in Mocchau (-42.83‰) was more enriched than that in Dalat (-53.99‰). In contrast, the value of δ18O for strawberries grown in Mocchau (-9.25‰) is more depleted than in Dalat (-5.67‰). In addition, the δ2H and δ18O values in each region are strongly correlated and can be distinguished based on the position of the correlation line relative to the global meteoric water line.

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‰.