AN EVALUATION OF FRESH WATER SEDIMENTS CONTAMINATION: THE LACHINE CANAL SEDIMENTS CASE, MONTRÉAL, CANADA. PART II

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Abstract

Over the last century, discharge of industrial effluents and municipal wastewater have contaminated the Lachine Canal sediments. This study investigated the associations between heavy metals and natural sediment constituents. X-ray Diffraction, Transmission Electron Microscopy and geochemical analysis revealed that the sediments consisted mainly of silt and clay size fractions composed of: feldspar, illite, kaolinite, chlorite, calcite and dolomite as well as minor amounts of Fe minerals. Organic matter and amorphous metal oxides were also identified. Each of these constituents bound heavy metals at varying degrees as assessed by a sequential chemical extraction (SCE) protocol. The associations with each geochemical phase were (in order of decreasing significance): 1) residual phase, 2) oxide phase, 3) carbonate phase, 4) organic phase and 5) exchangeable phase. According to the cation exchange capacity (CEC), carbonate and oxide content measurements, the heavy metals occupied a minor fraction of the total capacity of the sediments to retain metals by these mechanisms. The SCE results revealed that the partition patterns varied with pH. The phases associated to the carbonate and the exchangeable phases were the most sensitive to a change in pH with the residual phase being almost unchanged. The Zn and Cd were sensitive to release when the conditions drifted to acidic conditions. The partition patterns for various grain size fractions (<53 μm, 53–75 μm, 75–175 μm and >175 μm) revealed that no particular fraction accumulated a certain heavy metal. Finally, from protocols, techniques and results interpretation in this study, various engineering applications such as the technical choice of clean-up scenarios, screening of remediation techniques and the development of remediation quality criteria were proposed within the framework of the management of contaminated sediments.

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