[Spatial distribution and risk assessment of insecticides in surface soil from a rapidly urbanizing region]

To examine the distribution patterns of organic contaminants in rapidly urbanizing regions, the levels and spatial distributions of 19 overlooked insecticides, i. e., phenyl-pyrazole class (fipronil), chlordane, endosulfan, nonachlor, hexachlorobenzene, heptachlor, dieldrin, aldrin, endrin, methoxychlor and their metabolites, were examined in 229 soil samples collected from the Pearl River Delta (PRD) and surrounding areas. The results indicated that higher insecticide levels distributed in the central PRD, while lower levels congested in the surrounding areas. The similar spatial patterns between the levels of insecticides and economic prosperity or population density demonstrated that social-economic factors may have dictated the spatial patterns of insecticides. In addition, the changing of land-use types during urbanization processes, e.g., historical plowlands have been converted into residential landscapes, resulted in high concentrations of banned insecticides in metropolis of the central PRD. Source diagnostics indicated that new inputs of technical chlordane products existed in the PRD and surrounding areas. Fipronil was degraded into fipronil sulfone and fipronil sulfide in most soil samples because of its low half-life in soil. Finally, a risk assessment of 19 insecticides in soil for human health suggested that six samples collected from the major administrative districts with dense population had potential cancer or non-cancer risk to human health. Therefore, these overlooked insecticides should be concerned in future environmental research.