POLLUTION BY PERSISTENT TOXIC SUBSTANCES IN VIETNAM : A REVIEW OF TWO DECADE MONITORING STUDIES

Joint Research on Environmental Science and Technology for the Eart

Aromatic character of planar boron-based clusters revisited by ring current calculations

The planarity of small boron-based clusters is the result of an interplay between geometry, electron delocalization, covalent bonding and stability. These compounds contain two different bonding patterns involving both sigma and pi delocalized bonds, and up to now, their aromaticity has been assigned mainly using the classical (4N + 2) electron count for both types of electrons. In the present study, we reexplored the aromatic feature of different types of planar boron-based clusters making use of the ring current approach. B3(+/-), B-4(2-), B-5(+/-), B-6, B-7(-), B-8(2-), B-9(-), B-10(2-), B-11(-), B-12, B-13(+), B-14(2-) and B-16(2-) are characterized by magnetic responses to be doubly sigma and pi aromatic species in which the pi aromaticity can be predicted using the (4N + 2) electron count. The triply aromatic character of B-12 and B-13(+) is confirmed. The pi electrons of B-18(2-), B-19(-) and B-20(2-) obey the disk aromaticity rule with an electronic configuration of [1 sigma(2)1 pi(4)1 delta(4)2 sigma(2)] rather than the (4N + 2) count. The double aromaticity feature is observed for boron hydride cycles including B@B5H5+, Li7B5H5 and M@BnHnq clusters from both the (4N + 2) rule and ring current maps. The double pi and sigma aromaticity in carbon-boron planar cycles B7C-, B8C, B6C2, B9C-, B8C2 and B7C3- is in conflict with the Huckel electron count. This is also the case for the ions B11C5+/- whose ring current indicators suggest that they belong to the class of double aromaticity, in which the pi electrons obey the disk aromaticity characteristics. In many clusters, the classical electron count cannot be applied, and the magnetic responses of the electron density expressed in terms of the ring current provide us with a more consistent criterion for determining their aromatic character

ARSENIC POLLUTION IN TUBE WELL WATER AT HANOI SUBURB VILLAGES

Joint Research on Environmental Science and Technology for the Eart

Widespread contamination by persistent toxic substances in Vietnam and their implications on environmental quality and human health

Vietnam is a developing country located in the central part of the Southeast Asian tropical region. The country comprises the Red River and Mekong River Delta, inhabiting more than 30 million people, which is one of the most populous areas in the world. These deltas have become one of the most productive agricultural regions in Southeast Asia. Agrochemicals have been used extensively in the past and until very recently for agricultural purposes and malaria eradication program. The present paper provides a comprehensive overview of the environmental distribution, patterns and trends of contamination of toxic substances including persistent organochlorines (OCs) and endocrine active compounds (EACs) in different environmental compartments from Vietnam. Monitoring data reported during the 1990s demonstrated widespread and elevated contamination of DDTs in air, water, sediments and soils from in Vietnam. Recent studies in frame of the Asia-Pacific Mussel Watch Program have also revealed that fish, mussels and resident birds from Vietnam contained higher concentrations of DDTs as compared to other countries in region, suggesting the role of Vietnamese environment as a significant emission source of DDT in the Southeast Asian region. Subsequent surveys on coastal lines from north and middle part of Vietnam likewise demonstrated that contamination of some endocrine active compounds such as alkylphenols and phthalates are ubiquitous. In particular, relatively high concentrations of bis-phenol A were found in some locations in Red River delta, comparable or higher than those reported for several locations in developed nations in Western Europe and North America. A case study on seasonal variation of alkylphenols and phthalates in surface water of river delta and estuary of north and central Vietnam indicated the differences in distribution of these compounds between dry and rainy seasons. Higher concentrations of alkylphenols and phthalates were found in dry season in estuary; while the contrasting pattern was observed in the river delta, showing elevated residues in rainy season. This result suggests the different behavior of alkylphenols and phthalates in river delta and in coastal environment. The temperature dependence in tropical ecosystem and the influence of the specific local sources may be reasons for the observed results in the seasonal variations. To our knowledge, this is the first extensive study on the widespread contamination of EACs in Vietnam. Regarding the trends of contamination by OCs, preliminary survey conducted in Read River delta water and sediments indicated a rapid decline trend in water and a slow decrease in sediments during 1995-2001. From ecotoxicological and human health perspectives, concentrations of bis-phenol A and di (2-ethylhexyl) phthalates in surface water from some locations in Vietnam exceeded the guideline values for Ecotoxicological Effects and the Environmental Risk Limit, respectively, suggesting potential for toxic implications on aquatic wildlife. Human exposure to persistent organic pollutants indicated that DDT residues levels in human breast milk from both Hanoi and Hochiminh city were among the highest values reported for Asian developing countries as well as developed nations. Daily intakes estimating based on the exposure through sea-foods indicates that intakes of DDTs by Vietnamese populations were among the highest rank in Asia-Pacific countries, suggesting potential risk for human exposure to elevated DDT pollution. Future studies should be focused on the time trends of POPs and EACs in biota of Vietnam with view of predicting the future trend of contamination and to reveal new clues for understanding possible toxic impacts on aquatic organisms

Contamination by PCDDs, PCDFs and coplanar PCBs in open dumping sites in Vietnam-levels, patterns and toxic implications

Joint Research on Environmental Science and Technology for the Eart

PERSISTENT ORGANOCHLORINE RESIDUES IN HUMAN BREAST MILK FROM VIETNAM : CONTAMINATION, ACCUMULATION KINETICS AND RISK ASSESSMENT

Joint Research on Environmental Science and Technology for the Eart

Widespread contamination by persistent toxic substances in Vietnam and their implications on environmental quality and human health

Vietnam is a developing country located in the central part of the Southeast Asian tropical region. The country comprises the Red River and Mekong River Delta, inhabiting more than 30 million people, which is one of the most populous areas in the world. These deltas have become one of the most productive agricultural regions in Southeast Asia. Agrochemicals have been used extensively in the past and until very recently for agricultural purposes and malaria eradication program. The present paper provides a comprehensive overview of the environmental distribution, patterns and trends of contamination of toxic substances including persistent organochlorines (OCs) and endocrine active compounds (EACs) in different environmental compartments from Vietnam. Monitoring data reported during the 1990s demonstrated widespread and elevated contamination of DDTs in air, water, sediments and soils from in Vietnam. Recent studies in frame of the Asia-Pacific Mussel Watch Program have also revealed that fish, mussels and resident birds from Vietnam contained higher concentrations of DDTs as compared to other countries in region, suggesting the role of Vietnamese environment as a significant emission source of DDT in the Southeast Asian region. Subsequent surveys on coastal lines from north and middle part of Vietnam likewise demonstrated that contamination of some endocrine active compounds such as alkylphenols and phthalates are ubiquitous. In particular, relatively high concentrations of bis-phenol A were found in some locations in Red River delta, comparable or higher than those reported for several locations in developed nations in Western Europe and North America. A case study on seasonal variation of alkylphenols and phthalates in surface water of river delta and estuary of north and central Vietnam indicated the differences in distribution of these compounds between dry and rainy seasons. Higher concentrations of alkylphenols and phthalates were found in dry season in estuary; while the contrasting pattern was observed in the river delta, showing elevated residues in rainy season. This result suggests the different behavior of alkylphenols and phthalates in river delta and in coastal environment. The temperature dependence in tropical ecosystem and the influence of the specific local sources may be reasons for the observed results in the seasonal variations. To our knowledge, this is the first extensive study on the widespread contamination of EACs in Vietnam. Regarding the trends of contamination by OCs, preliminary survey conducted in Read River delta water and sediments indicated a rapid decline trend in water and a slow decrease in sediments during 1995-2001. From ecotoxicological and human health perspectives, concentrations of bis-phenol A and di (2-ethylhexyl) phthalates in surface water from some locations in Vietnam exceeded the guideline values for Ecotoxicological Effects and the Environmental Risk Limit, respectively, suggesting potential for toxic implications on aquatic wildlife. Human exposure to persistent organic pollutants indicated that DDT residues levels in human breast milk from both Hanoi and Hochiminh city were among the highest values reported for Asian developing countries as well as developed nations. Daily intakes estimating based on the exposure through sea-foods indicates that intakes of DDTs by Vietnamese populations were among the highest rank in Asia-Pacific countries, suggesting potential risk for human exposure to elevated DDT pollution. Future studies should be focused on the time trends of POPs and EACs in biota of Vietnam with view of predicting the future trend of contamination and to reveal new clues for understanding possible toxic impacts on aquatic organisms