IN-SITU OBSERVATION OF SUSPENDED SOLID AGGREGATES IN RIVERS

Suspended solids are a major pathway of the biogeochemical fate of man y contaminants in aquatic systems. Aggregation processes of particles are poorly reported in rivers but they are likely to exist at low flow , mostly because of sticky polysaccharides produced by living organism s. These processes affect suspended particle transport by changing par ticle sizes and densities and may also limit exchanges of matter betwe en solid and dissolved phases. The main difficulty in flee studies is aggregate fragility, which requires the use of specialized in situ tec hniques to analyze aggregated suspended solids. We describe two in sit u methods for observing suspended particles which do not need heavy fi eld equipment. The first one is based on the filtration of a thin wate r layer directly in the natural flow through a membrane which can subs equently be observed by microscope. The second one is based on in situ video snapshots of suspended solids by an endoscope. The video camera linked to the microscope or the endoscope supplies images which are a utomatically analyzed by image processing to give size distributions. Procedures and validation for both methods are described and results c ompared with a standard method. The filtration method has been used to trace suspended solids from sewer overflows in the Seine River downst ream of Paris. Freshwater flocs are described and a discussion of the fate of aggregates is presented.