Spatial and seasonal distribution of organochlorine pesticides in the sediments of the Yangtze Estuary.

The spatial and seasonal distribution of organochlorine pesticides (OCPs) in the sediments of the Yangtze Estuary was investigated. The total concentration of 19 OCPs (∑OCPs), including α-HCH, β-HCH, γ-HCH, δ-HCH, o, p'-DDE, p, p'-DDE, o, p'-DDD, p, p'-DDD, o, p'-DDT, p, p'-DDT, heptachlor, aldrin, heptachlor epoxide (Isomer B, Isomer A), endosulfan I, dieldrin, endrin, endosulfan II and methoxychlor, in the surface sediments ranged from 0.4 to 82.1 ng g(-1) d.w. The mean values of the ∑OCPs were 8.5 ± 8.3 ng g(-1) d.w., 26.7 ± 12.1 ng g(-1) d.w., 40.5 ± 18.2 ng g(-1) d.w., and 13.0 ± 11.2 ng g(-1) d.w. for August 2010, November 2010, February 2011 and May 2011, respectively. The highest concentration of OCPs was detected at the river-sea boundary because of the intense resuspension and redeposition of sediments in this area. The OCPs preferred to accumulate in the inner estuary, rather than in the offshore area in the dry season, whereas the opposite occurred in the flood season. This phenomenon was related to the seasonal changes of the Changjiang diluted water (CDW). OCPs in the offshore area were controlled by the ocean currents and the monsoon. There was a southward increase for the OCP deposition in the offshore area in the winter. This increase resulted from the influences of the southward Yellow Sea coastal current and the East China Sea coastal current, as well as the northwest wind in winter. The Three Gorges Dam had an impact on HCH composition by changing the sediment redistribution during the deposition process.