Temporal Variation of Persistent Organochlorine Residues in Soils from Vietnam

A program to monitor the residual levels of some important groups of organochlorine compounds (DDTs, HCHs and PCBs) in soils from different locations in Vietnam has been carried out since 1990. So far, 480 soil samples have been collected and analyzed to understand contamination levels of the soils by these compounds. In general, concentrations of the analyzed compounds are in the following order of ∑DDTs > ∑PCBs > ∑HCHs. Concentrations of DDTs ranged from 0.02 ng g-1 to 1300.40 ng g-1; PCB concentrations ranged from 0.61 ng g-1 to 9960.00 ng g-1 whereas that of HCHs spanned from 0.05 ng g-1 to 71.00 ng g-1. Distributions of the chemicals varied by chemical and soil type. Higher concentrations of DDTs were recorded in agricultural soils, whereas higher levels of PCBs were found in soils from industrial areas. The concentrations of HCHs appeared not to vary much by soil type. Investigation of the ratios of DDT and HCH isomers in the soil samples indicated that there have been no recent inputs of these chemicals in the areas studied. Specific analysis of PCB isomers and congeners showed that six groups of PCBs (from tri-chlorinated to octa-chlorinated) were detected in the analysed samples. The mean percentage proportions of six selected PCB congeners followed the order of PCB138 > PCB 153 > PCB101 > PCB52 > PCB180 > PCB28. It is worth noting that the concentrations of DDTs and HCHs have gradually decreased over time from 1990 to the present. Concerning PCB levels in soil, an increasing trend over time can clearly be seen, which is further confirmed by high PCB residues detected in recently collected (2005-2007) soil samples.