The study of sediment quality and ecological recovery in Victoria Harbour

Abstract

Victoria Harbour has received a substantial loading of pollutants from industrial and municipal wastewater since the 1950s. Contaminant inputs have declined drastically during the last two decades as a result of better source control and improved treatment processes. This study aims to examine the distribution, sources and geochemical behaviour of trace metals in Victoria Harbour sediments, in order to facilitate more effective management or restoration alternatives. Core and grab sediments were collected to study the concentrations of trace metals (Cu, Pb and Zn), Pb isotopic ratio, particle size and total organic content (TOC). Serious metals contamination was observed in the central part of Victoria Harbour. Proximity to urbanized and industrial areas, and harbour hydrodynamics were found to be the main factors controlling spatial distribution of metals in sediments. Sediment cores revealed a substantial increase of trace metals during the 1950s to 1980s, corresponding with the period of rapid urban and industrial development in Hong Kong. There was a major improvement of metals contamination from the late 1980s due to the relocation of industrial activities and set-up of the Chemical Waste Treatment Centre in the territory. Nevertheless, metals were still elevated in uppermost sediments, which may act as a secondary source of pollution. Anthropogenic sources of Pb were verified by a lower Pb isotopic composition in sediment samples, probably originating from leaded gasoline and other anthropogenic sources, such as industrial and ship repairing activities. The geochemical behaviour of metals (Fe, Mn, Cu, Pb and Zn) in surface sediments was investigated using a single extraction method with cold HC1 and a modified 5-step Tessier's sequential extraction technique. The harbour sediments, especially at the central part, were characterized by strongly reducing conditions and high amounts of acid volatile sulfide (AVS). Copper was predominantly associated with organic matter, carbonate/specifically adsorbed, and to a lesser extent sulfide phases; Zinc was mainly in the carbonate/specifically adsorbed, oxides phases, and possibly sulfide phases, while Pb was mainly found in the carbonate/specifically adsorbed phase in harbour sediments. Hence, the bioavailability order of trace metals was Pb > Zn > Cu. A major temporal change (October 2005 and April 2006) of redox conditions in sediments as a result of sediment resuspension was observed in the central part of the harbour. Sediments in October were characterized by less reducing conditions, which led to subsequent increases of Cu in the carbonate/specifically adsorbed phase and Zn in the oxide phase. Such changes of metal partitioning in harbour sediments may release metals to the water column and lead to a greater environmental risk to marine organisms.