Suspended particulate concentrations were determined by centrifugation of water for four sampling periods at different seasonal conditions throughout the St. Lawrence River between May 1985 and June 1987. Average concentrations of 0.9 mg l-1 were found to exit Lake Ontario with a nearly linear increase to 10 mg l-1 at Quebec City. In 1987, raw and centrifuged water samples were also sequentially filtered through a set of membrane filters with pore sizes of 8.0, 0.4 and 0.2 μm. For most raw water samples, filtration resulted in a lower total particulate retention than centrifugation. Exceptions were water samples with high levels of humic substances, especially those from the Ottawa River tributary. The divergence of filtration and centrifugation results is thought to reflect alterations of the suspended particles by the mechanical and gravitational forces acting on them during centrifugation. These effects may also produce changes in the contaminant partitioning between water and the particulates, especially in samples where such contaminants are not evenly distributed in the particulates. Our data indicate that the concentration of polychlorinated biphenyls (PCB) in particulates of the size 0.2-0.4 μm is up to 10 times higher than in the larger particles found in St. Lawrence River water. The presence of high levels of humic substances in comparatively soft surface water will therefore lower the centrifuge efficiencies for the recovery of fine suspended particles. As a consequence, hydrophobic contaminants, such as PCB, which tend to adsorb onto clay and organic particulates, may only incompletely be recovered with these particles and remain in the centrifugate from which they can be determined by extraction with suitable solvents. The determination of contaminant partitioning factors between aquatic compartments will also be influenced by these effects. © 1990.
