Abstract: Waste from local zinc smelting and lead and zinc milling plants using traditional techniques have severely polluted rivers with heavy metals in the Northwest of Guizhou.Studies on geochemical fractions of heavy metals in the sediment in the Hou River bed reveal that geochemical fractions of heavy metals differed sharply not only between sample sites,but also between elements,which is possibly related to difference in affinity between heavy metals and components of the sediment,contents of clay minerals,organic matter and Fe-Mn oxides in different river sections,and intensity of human activities(discharge of domestic sewage,wastewater from ore milling,slag,etc.).The carbonatite background of the river beds may also be a contributor to release of heavy metals in waste from mining.Assessment of their ecological risks shows that the potential ecological hazards of heavy metals in the sediment are serious.The main factor causing ecological hazards is lead,and then followed by cadmium,copper,zinc.

Key words: heavy metal pollution, chemical fractionation, ecological risk assessment, sediment, carbonatite-rich mine area