Burden and depth distribution of organochlorine pesticides in the soil profiles of Yangtze River Delta Region, China: Implication for sources and vertical transportation

Soil is an important source of organochlorine pesticides (OCPs) to other environmental media and organisms. Large amounts of OCPs, such as hexachlorocyclohexane (HCH) and dichlorodiphenyltrichloroethane (DDT), have been intensively applied in the croplands of Yangtze River Delta (YRD) Region before being officially banned from use for agriculture in 1980s, China. Forty-four soil profiles which represent seven major soil types in YRD region were chosen and a total of 169 horizon samples were collected for analysis of HCH and DDT. The highest residual level of DDT (484.24 μg/kg d.w.) and HCH (17.93 μg/kg d.w.) was detected in cotton field and paddy field, respectively. It is estimated that approximately 580 tons of DDT is reserved in the soil depth of 100 cm at present, which is around two times higher than that of HCH. Unexpectedly, about 60% and 80% of the residual DDT and HCH on average are buried underlying layers within a depth of 15–100 cm respectively, imposing potential risks to shallow groundwater and underground ecosystem. Depth distribution of DDT and HCH compositions is presented fairly constant in the layers above the plough pan of paddy soil, implying that their downward migration is induced by bioturbation principally. While in the layers beneath the plough pan, dominance of more water-soluble compounds infer leaching of DDT and HCH by irrigation and rainfall. In dry land and upland, downward transportation mechanisms of DDT and HCH are shown as site-specific according to their depth distribution.