Ho Tay (West Lake) in Hanoi, Vietnam receives wastewater from the city center and the surrounding residential areas, which can cause both eutrophication and enrichment of toxic heavy metals in the lake ecosystem. The aim of this study is to evaluate the recent trends of metal contamination in this lake from their spatial (horizontal and vertical) distributions in the lake sediments. Sediment cores with up to 70 cm in depth were sampled from four locations in the lake and analyzed for heavy metals (Cd, Cr, Cu, Mn, Ni, Pb and Zn) including a metalloid (As) and total organic carbon (TOC). High concentrations of the metals (except for Mn) and TOC have accumulated in sediment at the site where an inlet of sewage from the city center was located nearby. Increasing trends of the metal contents in the sediment profile toward the surface at the sites distant from the sewage inlets imply that the loads of these metals into this lake have been continuously increasing. In addition, Pb isotopic ratios in sediment profile could be used as an indicator of anthropogenic Pb pollution in the lake.
Keywords: heavy metals, lake sediment, spatial distribution
INTRODUCTIONMunicipal wastewater is not only the pollution source of organic matter and nutrients, but also that of heavy metals to water environment. For example, in Lake Teganuma, one of the most hypereutrophic lakes in Japan, high concentrations of heavy metals, especially Zn, had accumulated in the surface sediment, and the accumulations were most evident near the inlets of two main inflow rivers with urbanized watersheds . Elevated concentrations of toxic metals in natural water could have adverse effects not only on living organisms in the ecosystem but also on people ingesting fishery products. In addition, heavy metals in sediment could leach out and then contaminate the surrounding environment if the contaminated sediment is dredged and oxidized without control.