Flocculation Potential of Estuarine Particles: The Importance of Environmental Factors and of the Spatial and Seasonal Variability of Suspended Particulate Matter

Abstract

International audienceEstuarine 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. Keywords Flocculatio

Estuarine Turbidity Maxima and Variations of Aggregate Parameters in the Cam-Nam Trieu Estuary, North Vietnam, in Early Wet Season

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“…The experiments were carried out in two main steps: The first step is to apply the highest tested shear stress of 60 N/m 2 to break down any potential macroflocs in suspension as the initial state; the second step, which is the main testing part, is to run the agitator at a series of rotational speeds to generate the desired turbulent shear stresses, ranging from 0.57 N/m 2 to 8.55 N/m 2 for a duration of 120 min as suggested by (Mikes et al 2004;Verney et al 2009). This is a significant time period for flocculation to occur in natural water bodies (Le Hir et al 2001), where the average floc size no longer changes after this time.…”

“…Floc size was obtained using the contrast between the dark background and the white silhouettes of the floc. The surface equivalent diameter d was calculated by converted particle area ( ) into equivalent circular diameter (Flory et al 2004;Mikes et al 2004;Verney et al 2009) as:…”