Abstract
Cohesive sediment deposits in the Three Gorges Reservoir, China, were used to investigate physical and geochemical properties, biofilm mass, and erosion and deposition characteristics. Biofilm cultivation was performed in a recirculating flume for three different periods (5, 10 and 15 days) under ambient temperature and with sufficient nutrients supply. Three groups of size-fractionated sediment were sequentially used, including 0–0.02 mm, 0.02–0.05 mm and 0.05–0.10 mm. Desired conditions for erosion and deposition were designed by managing high bed shear stress at the narrow part of upstream flume and low shear stress at the wide part of downstream flume. Biostabilization and transport characteristics of the biofilm coated sediment (bio-sediment) were strongly influenced by the cultivation period, and the results were compared with clean sediment. The bio-sediment was more resistant to erosion, and the mean shear stress was increased by factors of 2.65, 2.73 and 5.01 for sediment with 5, 10 and 15 days of biofilm growth compared with clean sediment, resulting in less sediment being eroded from the bed. Simultaneously, the settling velocity was smaller for bio-sediment due to higher organic content and porosity (i.e., lower density). Additionally, there was a smaller probability of deposition for sediment with a longer cultivation period after erosion, resulting in more retention time in aquatic systems. These results will benefit water management in natural rivers.
Highlights
In aquatic systems many contaminants of concern are associated with and transported by cohesive sediment [1]
Sediment is an assemblage of minerals, potentially resulting in complicated morphology and surface charge distribution [32, 33], which are beneficial for microorganism attachment and biofilm formation
The present study showed that biofilm growth on the sediment surface could enhance the biostabilization, significantly increasing the energy required for sediment erosion, and could affect sediment-associated contaminant transport
Summary
In aquatic systems many contaminants of concern are associated with and transported by cohesive sediment [1]. Biostabilization and transport of cohesive sediment are critical to the aquatic environment and bed morphology. Sediments in a trophic water column provide excellent substrata for microorganism colonization [2,3,4]. The secretion of metabolic products (e.g., extracellular polymeric substances, EPS) causes the formation and growth of biofilm on the sediment surfaces [5, 6]. Biofilm is a complex matrix of living microorganisms and their metabolic products. Sediment particles coated with biofilm are defined as bio-sediment to distinguish them from clean sediment
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