Geochemical characteristics of contaminated surficial sediments in Halifax Harbour: impact of waste discharge

Abstract

Analyses of 274 surface sediment samples collected from Halifax inlet between 1986 and 1990 permit assessment of the chemical environmental quality of this major east coast Canadian harbour. Contamination of the marine sediments is due to discharge of untreated sewage and industrial waste directly into the harbour, and leaching of solid waste deposits on land. Concentrations of total Cu, Pb, Zn, Hg, and Cd in the uppermost 2 cm of sediments on the sea floor ranks Halifax Harbour among the most contaminated marine areas in the industrialized world. Statistical factor analysis has been used to identify five types of sediments based on sedimentological and geochemical characteristics. The dominant factor type accounts for 41% of the chemical variance and represents sediments that are highly contaminated with metals and organics derived from sewage effluents and waste deposits. More than 50% of the total Cu, Zn, and Pb in the sediments is potentially reactive as reducible or oxidizable metal.A simple two-layer estuarine circulation model has been used to assess the significance of dissolved metal sources and sinks in the harbour. The contribution to sediment contamination from dissolved metal inputs is relatively minor, with more than 88% of the potentially reactive Cu, Zn, Pb, and Hg being directly associated with particulate deposition. However, the model suggests that high proportions of the dissolved annual inputs of Zn (40% of 5.5 t), Pb (100% of 1.3 t), Hg (100% of 0.02 t), and Mn (89% of 19.6 t) are taken up by the bottom sediments.Future environmental quality management of Halifax Harbour should take into consideration the geochemical stability of sediments. New sewage treatment facilities may reduce the loading of contaminants, but some remobilization of metals from sediments may reduce long-term improvements in contaminant levels in the water column.