PCDD/Fs in sediments of Central Vietnam coastal lagoons: in search of TCDD.

Samples from nine Central Vietnam coastal lagoons, together with three soils and sediments collected in two freshwater reservoirs of the Thua Thien-Hué province, were analysed for polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). Total concentrations are low, from 192 to 2912 pg g(-1) and depth profiles in Tam Giang-Cau Hai (TG-CH) sediment cores show only minor changes over time in PCDD/F input and composition. Octachloro dibenzo-p-dioxin (OCDD) is the prevailing congener (approximately 90%), indicating combustion as the main PCDD/F source to these coastal systems, whereas natural formation might be partly responsible for the presence at depth. 2,3,7,8-Tetrachloro dibenzo-p-dioxin (TCDD), largely sprayed together with Agent Orange over the study areas during the war (1961-1971), is absent or very low. This result supports the hypothesis of strong degradation soon after spraying. Multivariate statistical analyses account for the presence of local, short-range sources as observed in the northern part of the TG-CH lagoon.