Flocculation Potential of Estuarine Particles: The Importance of Environmental Factors and of the Spatial and Seasonal Variability of Suspended Particulate Matter |
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| Authors: | Romaric Verney Robert Lafite Jean-Claude Brun-Cottan |
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| Institution: | 1.IFREMER, DYNECO-PHYSED,Plouzane Cedex,France;2.UMR CNRS 6143 M2C,University of Rouen,Mont Saint Aignan Cedex,France;3.UMR CNRS 6143 M2C,University of Caen,Caen,France |
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| Abstract: | Estuarine systems are complex environments where seasonal and spatial variations occur in concentrations of suspended particulate
matter, in primary constituents, and in organic matter content. This study investigated in the laboratory the flocculation
potential of estuarine-suspended particulate matter throughout the year in order to better identify the controlling factors
and their hierarchy. Kinetic experiments were performed in the lab with a “video in lab” device, based on a jar test technique,
using suspended sediments sampled every 2 months over a 14-month period at three stations in the Seine estuary (France). These
sampling stations are representative of (1) the upper estuary, dominated by freshwater, and (2) the middle estuary, characterized
by a strong salinity gradient and the presence of an estuarine turbidity maximum. Experiments were performed at a constant
low turbulent shear stress characteristic of slack water periods (i.e., a Kolmogorov microscale >1,000 μm). Flocculation processes
were estimated using three parameters: flocculation efficiency, flocculation speed, and flocculation time. Results showed
that the flocculation that occurred at the three stations was mainly influenced by the concentration of the suspended particulate
matter: maximum floc size was observed for concentrations above 0.1 g l−1 while no flocculation was observed for concentrations below 0.004 g l−1. Diatom blooms strongly enhanced flocculation speed and, to a lesser extent, flocculation efficiency. During this period,
the maximum flocculation speed of 6 μm min−1 corresponded to a flocculation time of less than 20 min. Salinity did not appear to automatically enhance flocculation, which
depended on the constituents of suspended sediments and on the content and concentration of organic matter. Examination of
the variability of 2D fractal dimension during flocculation experiments revealed restructuring of flocs during aggregation.
This was observed as a rapid decrease in the floc fractal dimension from 2 to 1.4 during the first minutes of the flocculation
stage, followed by a slight increase up to 1.8. Deflocculation experiments enabled determination of the influence of turbulent
structures on flocculation processes and confirmed that turbulent intensity is one of the main determining factors of maximum
floc size. |
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