Vertical Distribution of Organochlorine Pesticides in Farming Soils in Tianjin Area

The distribution of organochlorine pesticides in soil profiles in the farming soils of Tianjin area was studied.The results showed that the highest concentration for hexachlorocyclohexance can be found in vegetable plot.Concentrations in the kaoliang field are the next,followed by paddy soil and corn field.For DDTs,the highest concentration is found in vegetable fields,followed by kaoliang field.No significant differences can be found for paddy soil and corn field.The concentrations of organochlorine pesticides decreased with depth,including all compounds of organochlorine pesticides such as α－HCH,β－HCH,γ－HCH,δ－HCH,DDE, DDD and DDT.The physical and chemical features of organochlorine pesticides,contents of clay minerals,organic carbon and microorganisms are major factors influencing the vertical distribution of the pesticides in soil.The percentages of four isomers of the hexachlorocyclohexance are significantly different from those in original pesticides.This can be explained by the fact that degradation speeds of the four isomers are different and transformation of these isomers existed in the soil.In the surface soil of the sampling sites,the values of w(DDT)/w(DDTs) range from 0.033 to 0.188,far below 0.5.It shows that no new contamination of DDT occurs in the sampling areas.