Organochlorine pesticides contaminated surface soil as reemission source in the Haihe Plain, China

A large amount of organochlorine pesticides (OCPs) including hexachlorocyclohexane isomers (alpha-HCH, beta-HCH, gamma-HCH, and 6-HCH,sigmaHCH asthe total) and dichlorodiphenyltrichloroethane and metabolites (p,p'-DDT, p,p'-DDE, and p,p'-DDD, sigmaDDT as the total) have been applied over the Haihe Plain (an area of 300 000 km2) in Northern China. Even though the agricultural application of these OCPs was terminated more than a decade ago, their residues remain in the environment and continue to represent significant public health concern. In this study, more than 300 surface soil samples were collected from the Haihe Plain for measurement of these OCPs. The measured sigmaHCH and sigmaDDT residues were 3.9 +/- 26 and 64 +/- 260 ng/g as arithmetic means and standard deviations with median values of 0.38 and 6.5 ng/g, respectively. Although the levels were approximately 1 order of magnitude lower than those recorded in 1980s, it was estimated that there were 430 +/- 110 tons of sigmaHCH and 6100 +/- 760 tons of sigmaDDT in the surface soil of the area, respectively. The soils with high levels of OCP residuals were mostly distributed on the fringes of major cities, due to intensive farming and discharge from pesticide producers in the cities. The residuals of sigmaHCHs and sigmaDDTs were significantly correlated to soil organic matter content. Both alpha-HCH/beta-HCH and p,p'-DDT/p,p'-DDE ratios were log-normally distributed and negatively correlated to log(sigmaHCH) and log(sigmaDDT), respectively. Thus these ratios and correlations preclude certainty in distinguishing fresh application from historical usage. According to the total residuals and the distributions of alpha-HCH/beta-HCH and p,p'-DDT/p,p'-DDE ratios, it appears that significant recent input of either the OCPs is very unlikely. The calculated fugacities in soil and air provided quantitative evidence indicating a net and seasonally varied transport of sigmaHCH (0.31 ton/year) and sigmaDDT (1.9 ton/year) from soil to atmosphere in the study area. Such a surface-to-air transport suggested that after the ban, the surface soil had gradually converted from a major sink to an important emission source of OCPs and the reemission will continue for many years to come.