The Impact of Pesticides - Academy Publish

are DDT and HCH products used by humans in the study areas. Besides, thedeposition of these compounds by atmosphere transportation from other regions isalso remarkable. The second trend is a decrease of the concentrations of selectedOCPs when the dissipation of study compounds took place. Dissipation comprisestwo main types of processes: transport processes, such as volatilisation, leaching,plant uptake, runoff or erosion that transfer substances to different environmentalcompartments; and transformation processes such as microbial degradation,hydrolysis and photolysis that produce transformation products. These processes ledto a decrease of their concentration during the time. Therefore, the significantdecreasing trend of ΣDDT and ΣHCH reflects that the dissipation of thesecompounds took place for a long time, whereas the new contaminative sources arelimited. It is possible that their use for crop protection and vector control for publichealth purposes took place during the period from 1992 to 1998, and then wasdiscontinued from 1998 to 2006. According to Thao et al. (1993), the analyses ofcollected soil samples from Hanoi and Bacninh in 1992 showed that p,p’-DDT andα-HCH are predominant in comparison with the other DDT metabolites and HCHisomers, respectively. It clearly indicates their new inputs in study areas in 1992.This could relate to the DDT and HCH history of use. After the announcements bythe Ministry of Agriculture and Rural Development 1994, their use was banned.However, there are likely illegal supplies of DDT and HCH in use in 1998. DDTand HCH can be brought to Vietnam by illegal way from China. In many case,farmers and other pesticide users ignored the risks of using these products. Anothercontaminative source could be the products which have been stored in stocks inother provinces before proper disposal. The poor control and unregulated storagecan lead to their release into the soil environment. But after 6-8 years of the ban, thissituation seems to be under control. Composition analyses of soil samples in 2006show that there is no recent input in the study areas.The decrease of ΣHCH during the period from 1992 to 2006 is faster than ΣDDT.Between 1992 and 2006, ΣDDT and ΣHCH concentrations in soil from Hanoidecreased by factor 0.21 and 0.14, respectively (Table 11). It can be explained bythe relatively shorter environmental half-lives of HCH in soil, lower log K ow andhigher vapour pressure than that of DDT. Besides, HCH may have been used to alesser extent as compared to DDT.Table 11. Factors of declining trends of ΣDDT and ΣHCHLocationRatios of mean concentration over the sampling yearsA. Hanoi1992/1995 1992/1998 1992/2001 1992/2005 1992/2006ΣDDT 0.68 0.45 0.39 0.22 0.21ΣHCH 0.72 0.49 0.31 0.15 0.14B. BacninhΣDDT 0.91 0.56 0.45 0.25 0.24ΣHCH 0.70 0.49 0.34 0.15 0.14AcademyPublish.org - The Impact of Pesticides315