In Nam Dinh province, in the Red River delta plain in Northern Vietnam, groundwater in the shallow Holocene aquifer shows elevated total dissolved solids up to 35 km from the coastline, indicating a saltwater intrusion from the Gulf of Tonkin. High groundwater salinities have been encountered below and adjacent to the Red River in the deep Pleistocene aquifer. Since 1996, large-scale groundwater abstraction was initiated from the deep aquifer, and the observed elevated salinities now raise concerns about whether the groundwater abstraction is undertaken sustainably. We have conducted a study to obtain a fundamental understanding of the recharge mechanisms and salinization processes in the Nam Dinh province. A holistic approach with multiple methods including transient electromagnetic sounding and borehole logging, exploratory drilling, sampling and analyzing primary ion and stable isotope compositions of water and pore water, groundwater head monitoring, hydraulic experiments laboratory of clay layers, and groundwater modeling by using the SEAWAT code. Results reveal that saline river water is leached from the Red River and its distributaries into the shallow aquifers. The distribution and occurrence of salty pore water in the Holocene aquitard clay shows that meteoric water has not been flowing through these low permeable clay layers. Marine pore water has, however, been leached out of the Pleistocene clay. When this layer is present, it offers protection of the lower aquifer against high salinity water from above. Salinity as high as 80 % of oceanic water is observed in interstitial pore water of the transgressive Holocene clay. Saltwater is transported into the Pleistocene aquifer, where the Holocene clay is directly overlying the aquifer.
Sixteen samples of surface and groundwater from the most southern part of the Hanoi city (Vietnam) has been taken and analyzed for the soluble major ions, namely Na, and arsenic as well as molybdenum content along with isotopic composition of deuterium and oxygen (δ 2 H and δ 18 O), tritium activity, and δ 13 C in DIC. The stable isotopic composition of groundwater indicates the water in that area is recharged from the local meteoric and the Red River's water sources. The mean residence time of the groundwater should be before 60-ies of the past century as its 3 H activity ranged from LOD of the analytical technique (0.4 TU) to 2.1 TU only implying that As would not be related to its recent application. The concentrations of As in the water samples is weakly correlated with those of Fe 2+ (R 2 = 0.08) but it strongly did with the concentrations of bicarbonate (R 2 = 0.80). Thus, bicarbonate seems to liberate As adsorbed on hydrous ferric oxides (Hfo) into water through the displacement mechanism. The surface adsorption-desorption of As could be proven by a strong correlation between As and Mo concentrations (R 2 = 0.77). The bicarbonate formation in groundwater was thought to be, partly, due to the bio-mineralization of natural organic matter (NOM) by bacteria followed by the dissolution of calcite and/or dolomite present in the sediments. Evidence for the NOM bio-mineralization was found in the close relationship between δ 13 C and the concentration of bicarbonate in water. Therefore, the mobilization of As in groundwater of bicarbonate type in the southern part of Hanoi city seems to be not only due to the reductive dissolution of the Hfo but the bicarbonate displacement also should be considered.
<p>The transport of plastic in rivers is affected by a wide variety of factors, such as river discharge, wind drag, and tides. These dynamic processes include the travelling time or distance, retention time or location, and remobilization rate of plastic items, which can be quantified by using GPS-based trackers. However, these properties are still unknown in some specific locations, including river bifurcations where the changes in river discharge, flow velocity, and river morphology are significant. Here, we demonstrate the behaviour of plastic transport in a river bifurcation area not influenced by tides during flood season in the Red River system of Vietnam. While all trackers retained somewhere in the river after hours or days, we found that after 10.5 kilometers downstream of the bifurcation, 9 out of 10 trackers followed the main channel retained in the same approximately 6-kilometer-long river segment. Meanwhile, 50% of the 6 trackers that left the main channel to enter the tributary also retained in the same 2.5-kilometer-long river segment 9 kilometers downstream of the bifurcation. These findings are linked to the concept of rivers as plastic reservoirs, as none of the trackers that stranded on the riverbanks for several days was remobilized. Furthermore, the retention of trackers in the same area after leaving the bifurcation clearly indicates shared driving factors on plastic transport, which are likely the river discharge, wind direction or velocity, and river morphology. Our results underscore the need for future research on delineating exact accumulation zones of the plastics in riverbanks considering the effects of wind, river discharge, and river morphology.</p>
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.