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Along the Mekong River (MR), arsenic contamination in the groundwater has been recognized since the 2000s. Due to arsenic contamination in shallow aquifers, the drinking water has been prevented from being contaminated with arsenic by installing wells, 200 m to 400 m deep. However, it has been known that groundwater was contaminated by not only arsenic but also other trace elements. The need to reveal characteristics on the distribution of chemical composition by horizontally and vertically is increasing for purposes of appropriate development and management of groundwater. This study summarized the results of surveys for arsenic since 2008 and surveys for dissolved ions and trace elements since 2015, and described characteristics on distribution for chemical composition at the regional areas along the MR and Bassac (Hậu) River (BR). Groundwater samples were collected from existing wells in 8 provinces and 1 city. The authors used Piper and Stiff diagram that reflect the chemical composition of groundwater to respond to varying horizontal and vertical distributions. The Principal Component Analysis (PCA) was also carried out to reveal the correspondences of dissolved ions and trace elements. Variety concentrations and distributions of ions and trace elements show the obvious distinction between upper (200 m deep) aquifers broadly divided by geologic age. A wide variety of ion compositions and trace elements was found in the upper aquifer, and concentration of some trace elements in groundwater was high and exceed the World Health Organization (WHO) drinking water guideline value. On the other hand, groundwater in the lower aquifer is simple and rarely contaminated by trace elements. However, the arsenic concentration ranging from 0.01–0.065 mg/L slightly exceeding the WHO guideline value was found at the lower aquifer in the downstream areas of the Mekong River Delta. These results on the distribution analysis provide the insight that deep groundwater is not necessarily safe by over-pumping from deep aquifer and drilling new wells in the downstream areas.

The Eastern coastal area of the Mekong River Delta has distinctly ecological zones and a potentiality for development of agriculture, forestry and fishery according to their ecological characteristics. The farmer life is mainly depending on exploiting agriculture, aquaculture and there are many supporting the development of social-economics in delta. However, the recent years, farming rice, vegetables, crops, fruit trees and raising shrimp, hard clam as well have heavily affected by drought and salt intrusion due to climate change. This has leaded to decrease the agricultural, fishery yield and the farmer’s income, therefore, the farmer’s life in the Eastern coastal area faces to the difficulties. The previous farming models are necessary to alter and this is a significant solution to minimize the risk and to cope the climate change such as farming system changes, to sustain agricultural production or transformation to alternative systems and combination between the rotational rice-shrimp-crops farming models following the season. Thus, the study proposes the farming models that can be suitable for the Eastern coastal locals of Mekong river delta. On the basis of document synthesis of natural characteristics, morpho-sedimentology, soil, vegatation cover, innundation, salinity, tidal water level indicating ecology and traditional farming, the study has given a general ecological model for the Eastern coastal area (Mekong Delta) to adapt to the climate change context. The general farming model including five models are clam farming, mangrove forest- shrimp farming, vegetables and fruit trees farming, rotational farming of rice, and rice- vegetables, and Rotation farming rice- shrimp, fruit- shrimp and fishes.

The model of clam seed nursery in the coastal area of Tra Vinh province was carried out from June 2019 to February 2020 in the coastal dune and tidal flat in Thanh Dat Cooperative. The clam seed source is the natural seed harvested in the coast of Tra Vinh province with the goal of exploiting and using local resources to provide clam seeds for the cooperative clam farming. The model consists of two phases: the first phase of nursing is in ponds on the coastal dune and the second phase of nursing is on the tidal flat in natural condition. It indicates that during the first phase, the input of model used 50 kg clam grade I (150,000 ind./ kg) was released into the nursing ponds, the nursing pond with oxygen supplementation is a good facility resulting high survival rate of 62.8% in comparison to 43.5% in case of pond without oxygen supplementation. In October 2019, total 285 kg of grade II clams (15,000 ind./ kg) was obtained and released on 6,000 m2 clam ground in Thanh Dat Cooperative. In February 2020, total 2.33 million clam individuals were harvested, it is about 3,880 kg clam seed with the size of 600 ind./ kg. The model of clam seed nursing in the tidal flat was completed with clam seed survival rate of 54.5%. All of clam seed is continued culturing to the commercial clam production on the same ground of Thanh Dat Cooperative.

Salt water intrusion in the Mekong River, Vietnam, was studied using a towing type echo-sounder (Model SC-3) and water quality meter (TPM CLOROTEC) observation instruments during spring tide of the low flow season in May, 2005. Observations were taken at two stations located at points 26 and 8km from the river mouth. Measurements were taken at hourly intervals for 24 hours during one week, using several instruments set on the river bottom. A joint towing observation system using echo-sounder and water quality meters has been proved its usefulness for analyzing the time-space behavior of salt intrusion in the Mekong River. Precise measurements of tide-related phenomena including water level, bottom currents and water quality (temperature, salinity, turbidity and chlorophyll-a) at two stations revealed detailed movements of the saline water-mass.

Salt water intrusion in the Mekong River has been studied using a newly-developed acoustic reflection profiling system (Model SC-3). This system determines the spatial distribution of the halocline by measurements of water quality, especially of salinity. This system was made by improvement of a 200kHz precision echo sounder. In the intermediate flow seasons of Dec., 2000 and Jan., 2002, salt water was present only downstream of the mound at 8km in ebb tide, but progressed upstream beyond the mound at 14km toward flood tide. A typical wedge-like profile was obtained at the observation toward flood tide on 28 Dec., 2000. Time series echo-sounding profiles obtained from ebb tide toward rising tide on 2 and 4 Jan., 2002 at the river mouth station is in accordance with each salinity profile. In the low flow season of May 2001, salt water reached as far upstream as the mound at 28km near flood tide, and possibly entered the pool at 33km. The echo-sounding salinity profiles and the surface salinity data reveal the presence of strongly vertically-mixed water in the entire area invaded by salt water, although partly stratified structures may be formed during slack tide. The present echo-sounding system has proven to be very effective in unraveling the time-space behavior of salt water intrusion in the Mekong River.

The boring core KM-11, located at the center of Lake Kamo, Sado Island, provided a good opportunity to examine the Late Pleistocene and Holocene diatom biofacies. Fossil diatom assemblages in the core as well as sedimentary properties were useful for examining historical changes of Holocene sedimentary environment and relative sea level. Based on ecological groups, diatoms of this site are classified into 14 ecological groups. They are marine planktonic, marine epiphytic, brackish planktonic, marine/brackish epipsammic, marine/brackish epipelic, marine/brackish aerophilous, marine/ brackish epiphytic, brackish/fresh tychoplanktonic, brackish/fresh epiphytic, brackish/fresh planktonic, brackish/fresh aerophilous, fresh water epiphytic, fresh water planktonic and fresh water epipelic diatom groups. The stratigraphical change of these groups is divided into seven diatom divisions. Each diatom division is closely related to the evolution of sedimentation caused by Holocene sea level and geomorphological changes. In the Late Pleistocene, fluvial sediments were deposited in the site, then overlain by estuary sediments at the beginning of the Holocene transgression. Following the maximum Holocene transgression in 5,000 yr.BP, two small regressions occurred alternatively with two small transgressions respectively. After 1,800 yr.BP. fresh/brackish lacustrine sediments were deposited and marine/brackish lagoonal sediments have been deposited again since 1902 due to the construction of an artificial water way.

Based on ecological spectra, diatoms of the Fukushima-gata well core are grouped into five diatom assemblages, namely marine, marine-brackish, brackish, fresh-brackish and fresh water diatom assemblages. The change of these assemblages is divided into six diatom divisions in ascending order; (1) Fresh water diatom division 1 (FD1) is characterized by a fresh water diatom assemblage.(2) Transitional division 1 (Tr1) is characterized by the alternation of marine and fresh water diatom assemblages. (3) Marine diatom division 1 (MD1) is dominated by a marine diatom assemblage reflecting Holocene transgression. (4) Transitional division 2 (Tr2) which composes of marine and fresh water diatom assemblages indicates a regression after the culmination of the maximum Holocene transgression. (5) Marine diatom division 2 (MD2) indicating a small transgression is dominated by marine diatoms. (6) Fresh water diatom division 2 (FD2) is dominated by fresh water diatoms. It is probably that each diatom division closely related to the development of sedimentation was caused by Holocene sea level and geomorphological changes.

Sand samples from the mouths of the Red and Mekong Rivers were analyzed to determine the provenance and exhumation history of their source regions. U‐Pb dating of detrital zircon grains shows that the main sources comprise crust formed within the Yangtze Craton and during the Triassic Indosinian Orogeny. Indosinian grains in the Mekong are younger (210–240 Ma) than those in the Red River (230–290 Ma), suggesting preferential erosion of the Qiangtang Block of Tibet into the Mekong. The Red River has a higher proportion of 700–800 Ma grains originally derived from the Yangtze Craton. 40Ar/39Ar dating of muscovite grains demonstrates that rocks cooled during the Indosinian Orogeny are dominant in both rivers, although the Mekong also shows a grain population cooling at 150–200 Ma that is not seen in the Red River and which is probably of original Qiangtang Block origin. Conversely, the Red River contains a significant mica population (350–500 Ma) eroded from the Yangtze Craton. High‐grade metamorphic rocks exposed in the Cenozoic shear zones of southeast Tibet‐Yunnan are minority sources to the rivers. However, apatite and zircon fission track ages show evidence for the dominant sources, especially in the Red River, only being exhumed through the shallowest 5–3 km of the crust since ∼25 Ma. The thermochronology data are consistent with erosion of recycled sediment from the inverted Simao and Chuxiong Basins, from gorges that incise the eastern flank of the plateau. Average Neogene exhumation rates are 104–191 m/Myr in the Red River basin, which is within error of the 178 ± 35 m/Myr estimated from Pleistocene sediment volumes. Sparse fission track data from the Mekong River support the Ar‐Ar and U‐Pb ages in favoring tectonically driven rock uplift and gorge incision as the dominant control on erosion, with precipitation being an important secondary influence.