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Samples from nine Central Vietnam coastal lagoons, together with three soils and sediments collected in 24 two freshwater reservoirs of the Thua Thien-Hué province, were analysed for polychlorinated dibenzo-p- 25 dioxins and dibenzofurans (PCDD/Fs). Total concentrations are low, from 192 to 2912 pg g1 and depth 26 profiles in Tam Giang-Cau Hai (TG-CH) sediment cores show only minor changes over time in PCDD/F 27 input and composition. Octachloro dibenzo-p-dioxin (OCDD) is the prevailing congener (approximately 28 90%), indicating combustion as the main PCDD/F source to these coastal systems, whereas natural forma- 29 tion might be partly responsible for the presence at depth. 2,3,7,8-Tetrachloro dibenzo-p-dioxin (TCDD), 30 largely sprayed together with Agent Orange over the study areas during the war (1961–1971), is absent 31 or very low. This result supports the hypothesis of strong degradation soon after spraying. Multivariate 32 statistical analyses account for the presence of local, short-range sources as observed in the northern part 33 of the TG-CH lagoon

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Trace element concentrations have been measured in soil and sediment samples taken from the Thua Thien Hue (TT-H) Province and the Tam Giang-Cau Hai (TG-CH) Lagoon in 2002 and 2004. Results show that the lagoon is only slightly contaminated by elements such as Ag, Cd, Cr, Cu, Ni, Pb and Zn, whereas As, probably naturally enriched in this Asian region, reaches values above the lowest internationally accepted sediment quality guidelines. Concentrations in soils are ascribable to natural sources and distributions in the TG-CH Lagoon are mainly influenced by river inputs, with Ag and Cd undergoing estuarine desorption processes. However, concentration-depth profiles of most trace elements in sediments show a recent increasing trend that might be linked to the economic development of the area. The comparison of porosity and 210 Pb depth profiles (confirmed also by Cd and As) from repeated samplings of the same core locations in the TG-CH Lagoon seems to account for a loss of 5-10 cm of surficial sediment that took place during the time interval between the two samplings. It is hypothesized that extreme meteorological events (e.g. a major typhoon that hit the area in November 2003) could be responsible for such sediment displacement.

Sediment cores, obtained in 2004Sediment cores, obtained in -2005Sediment cores, obtained in and in 2008 from seven coastal lagoons of Central Vietnam (Lang Co, Nuoc Man, Nuoc Ngot, Thi Nai, O Loan, Thuy Trieu and Dam Nai), were analysed to assess changes in 210 Pb, porosity, grain size, and depth distributions of selected major and trace elements (Al, As, Ni, and Zn). The aim was to trace sedimentary processes in areas periodically subject to extreme events. The occurrence of lateral sedimentary inhomogeneities has been hypothesised to explain the differences observed at depth for some tracer profiles, whereas topmost levels present evidence that accounts for the loss of a sediment layer (from 2.0 to 7.5 cm thick) between the two samplings. These losses might be attributed to major typhoons that impacted the coasts of Central Vietnam during the study period.

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he system of coastal bodies of water in Vietnam consists of three main types as estuarine waters, lagoons and bays distinguished by the features of origin, evolution, dynamics factors, morphology, structure and ecosystem. In some cases, there exist of transitional waters in morphology between types, or this type is a secondary component of others. The closed degree, circulation, eroded – accumulated correlation, water salinity are the most important features of coastal bodies of water. The estuarine waters are divided into two kinds as delta and estuary. The deltas consists of fluvial- dominated (for examples, Red River Delta), wave – dominated (Thu Bon Deltas), and tide – dominated ones (Mekong Delta). The estuaries of which Bach Dang and Dong Nai are most typical locate on the coastal areas where the tide dynamics and eroded process are being dominated. Having 12 unites and total area of 458km2, the lagoons concentrate in coastal zone of the Vietnam Centre where generally there exists of strong wave, tide not large in range, and abundantly sandy sediments. By the closed degree, they can be divided into three kinds including nearby closed, very closed, and fully closed lagoons. The combination between enclosed degree of lagoons and local hydro-climate factors has formed three saline groups as brackish, brackish – haline, and haline lagoons. The coastal bays consists of 48 unites with the total of 4000 km2 and distinguished into bights/ shelters (called as Vũng by Vietnamese) when the area of waters is below 50 km2, and bays (called as Vịnh by Vietnamese) when the area of waters is from 50 km2 and larger. By the closed feature of waters, they divided into five degrees as very open, open, semi – closed, nearby closed, and very closed bays. From them, the most popular degrees are open 46%), then semi – closed bays (29%). Comparing with estuarine waters and lagoons, the coastal bays are rather stable in morphology and dynamical processes. They are different in geographical features by four coastal regions as North Vietnam, North of Centre Vietnam, South of Centre Vietnam, and South Islands Group.