Abstract: A total of 19 surface sediments and 5 sediment cores were collected at typical sites in Lake Caohai, located in Guizhou Province. The spatiotemporal distribution and sources of seven heavy metals (Cr, Ni, Cd, Co, Pb, Zn and Sb) in the sediments were investigated, and the potential ecological risks of those heavy metals were assessed. The results showed that the concentrations of Cd, Pb, Zn and Sb in the surface sediments were relatively higher in the north, central and southwest areas of the lake; these pollutants mainly resulted from the mineral exploitation in the catchment. The concentrations of Cr, Ni and Co were relatively higher in the most west area, mainly due to urban sewage discharge (westernmost areas). Based on the geoaccumulation index (I_geo), Cd and Sb were obviously enriched in the sediments of Lake Caohai. The potential ecological risk of Cd was higher in the lake sediments, especially in the sediments from the central part of the lake. The profiles of the variations of heavy metals contents in the sediment cores corresponded well to the history of the mineral exploitation in the catchment of Lake Caohai. Although the mineral exploitation was ended, its remaining problems are still the main cause of heavy metal pollution in Caohai Lake. Therefore, it is necessary to adopt reasonable methods to strictly control the heavy metal pollution and to establish suitable measures to effectively treat the heavy metal pollution, recovering the ecological environment of Lake Caohai.

Key words: dicyandiamide, nitrogen fertilizer, nitrous oxide, wheat and maize rotation system.