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.

The aim of the study was to characterize a variety of microstructure development-levels and geotechnical property sequences of the late Pleistocene–Holocene deposits in the Mekong River delta (MRD), and the paper furthermore discusses the influences of delta formation mechanisms on them. The survey associated the geotechnical engineering and the sedimentary geology of the late Pleistocene–Holocene deposits at five sites and also undifferentiated Pleistocene sediments. A cross-section which was rebuilt in the delta progradation-direction and between the Mekong and Bassac rivers represents the stratigraphy. Each sedimentary unit was formed under a different delta formation mechanism and revealed a typical geotechnical property sequence. The mechanical behaviors of the sediment succession in the tide-dominated delta with significant fluvial-activity and material source tend to be more cohesionless soils and strengths than those in the tide- and wave-dominated delta and even the coast. The particular tendency of the mechanical behavior of the deposit succession can be reasonably estimated from the delta formation mechanism. The characteristics of the clay minerals from the Mekong River produced the argillaceous soil which does not have extremely high plasticity. The microstructure development-levels are low to very high indicating how to choose hydraulic conductivity value, k, for estimating overconsolidation ratio, OCR, by the piezocone penetration tests (CPTU). The OCR of sediments in the delta types strangely change with depth but none less than 1. The post-depositional processes significantly influenced the microstructure development, particularly the dehydrating and oxidizing processes.

Arsenic (As) contamination in groundwater has been recognized in the Mekong River Delta (MRD) since 2001, especially in Dong Thap and An Giang provinces. The investigations were carried out during 2008-2009 and from 2012 to present. The investigations of As distribution in groundwater and sediments were based upon the field, and laboratory analyses using field kit (Hironaka, 1998) and AOAC laboratory-Vietnamese standard 6626-2000 (Vilas). Results show that the As concentration ranging from 15 to 1,650 µg/l is found in 520 private tubewells at depth between 15 and 90 m, while being rare or very low at depth between 200 and 300 m. High As content is usually found in 15-70m tubewells in which approximately 70-80% of tubewells having arsenic levels of >500 µg/l are located at some areas in Dong Thap and An Giang provinces in the upper MRD plain. Bore core drilling records show a probable relationship between As concentration in groundwater and sedimentary facies in the MRD. The highest As content is found along the Mekong River valleys in the Late Pleistocene and Holocene aquifers.

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