Sediment - Organic matter interaction in coastal area of Rhine- ROFI\ from Laboratory to in situ

Abstract:

Sediment - Organic matter interaction in coastal area of Rhine- ROFI
from Laboratory to in situ
Z.Safar, C. Chassagne, J.D. Pietrzak

SPM settling velocity is an important parameter for large scale sediment dynamic models. The settling velocity however, depends on the SPM particle properties such as size, effective density and shape which are determined by the composition of a SPM particle (floc). Therefore analyzing the floc composition is important to classify them and predict the settling velocity. Studies have shown that the settling velocity was not directly correlated with the size and floc effective density and shape were deterministic parameters linked to the size and settling velocity [Maggi2009,Khelifa2006,Manning,Soulsby2013]. To classify the flocs based on their composition, it is necessary to understand the interactions between sediment and organic matter.

To analyze these interactions, flocculation experiments were conducted with natural organic matter (EPS), dead algae (freeze dried algae) and living algae. In addition salt flocculation and salt-organic matter combination were also performed to analyze the impact of salt on these interactions to mimic the natural saline water conditions. For these experiments cleaned clay for laboratory purposes was

From these experiments it was found that EPS and Freeze dried Algae (FDA) does not flocculate with clay due to electrostatic repulsion between the both negatively charged particles. Addition of salts which are positively charged, however mediated the flocculation of the clay with FDA and EPS.

The living algae showed a bi-modal distribution confirming its elongated shape in agreement with other studies that have shown the phytoplankton effects on laser diffraction measurement [Karp-Boss2007] After addition of clay, the PSD turned to monomodal which means that the particles shape turned to spherical as the algae flocculated with clay.\\

These results can be taken as reference to analyze the in situ particles size distributions measured by LISST 100X. Multimodal distribution can refer to presence of elongated particles, i.e bare algae species, or flocculated to some extend of coiling. In addition LISST-HOLO results confirm the presence of these algae species in the water column during the neap tide with calm weather conditions.\\