Historical distribution of DDT residues in pond sediments in an intensive agricultural watershed in the Yangtze-Huaihe region, China

Purpose The extensive use of chemical pesticides on farmlands during the last several decades in China has led to a rapid deterioration of environmental water quality in recent years. The aims of this study were to: reconstruct the history of pesticide residues, determine the input load and residual load of dichlorodiphenyltrichloroethane (DDT) pesticides, and assess the risk of pesticide residues to aquatic ecosystem and human health. Materials and methods Caesium-137 was used to date sediment cores collected from ponds representing four land use types of an agricultural watershed with high-yield grain production and characterized by multipond systems in the Yangtze-Huaihe region of China. These ponds were selected to establish the historic pattern of DDT pesticide residues. Results and discussion (1) The mean total concentration of DDT residues including p, p'-dichlorodiphenyldichloroethylene (p,p'-DDE), p,p'-dichlorodiphenyldichloroethane (p,p'-DDD), o,p'-DDT, and p, p'-DDT in sediment in the watershed was 82 mu g kg(-1), ranging from under the detection limit to 457 mu g kg(-1), which was mostly contributed by p, p'-DDE (57 mu g kg(-1) on average). Spatially, total concentrations of DDT residues in farmland pond sediment were the highest, reaching as high as 457 mu g kg(-1). Temporally, an inflection point appeared in the 1970s, prior to which DDT contents increased with time, after which concentrations showed a decreasing trend. (2) In total, 323 kg DDT pesticide was applied to the Liuchahe Watershed since 1955. The total retention of four DDT residues in the multipond system was 14 kg (similar to 4% of the input), and most DDT pesticide was degraded to p,p'-DDE. (3) More than 80% of sediment DDT residues exceeded their interim freshwater sediment quality guidelines, and the percentages of DDT,DDD, and DDE residues exceeding probable effect limit (PEL) values were 57, 29, and 70%, respectively, which indicated a moderate to high ecological risk of DDT in this watershed. Conclusions Our results clearly reveal that the extensive use of pesticides has resulted in significant pesticide residual pollution in this watershed, which could severely deteriorate water quality and threaten aquatic ecosystem and human health in the watershed and, thus, remain a cause for concern.