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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