Ian G. Droppo

Polycyclic aromatic hydrocarbons (PAHs) originating from natural sources, and potentially from the Athabasca Oil Sands development, are of concern for the Athabasca River and Lake Athabasca delta ecosystems. In order to model the transport of fine sediments (and associated PAHs), it is important to describe the sediment dynamics within the river system. Flocs possess different settling characteristics compared to individual particles. A key aspect in modelling floc settling behaviour is the mathematical linkage of the floc density to floc size. In this paper, a rotating annular flume is used to determine the settling characteristics of Muskeg River (a tributary of the Athabasca River) sediments under different shear conditions. Simulations of the settling and flocculation behaviour of these sediments were used to calibrate a density vs. floc size model. A relationship of the parameters relating floc size and density with the fractal dimension F shows that as diameter increases flocs become weaker. Recommendations for the practical application of the model are further formulated in this paper. The deposition tests offer a quantitative measure of the relative amount of sediment that is likely to be transported through the river for given flow conditions.

Using an annular flume, erosion characteristics of five diverse sediment types (stormwater pond, contaminated lacustrine, fluvial, aquaculture waste and industrial grade kaolin sediment), each with different physical and biological characteristics, were examined for relative erosion resistance and factors contributing to bed sediment stability. Suspended flocs represent primary building blocks of bed sediment with mass settling being independent of suspended solid concentration. Biofilm growth period, depositing floc structure and composition, nutrient supply and sediment properties all played a complex role in dictating the stability of the sediments. The river, lake and stormwater pond sediments were the most resistant to erosion relative to the high nutrient and organic content aquaculture sediment and kaolin. Biofilms developed to varying degrees on all sediments with the greatest growth occurring with the aquaculture sediment and the least with kaolin. While electrochemical properties will provide some attraction and stabilising forces, with no measurable consolidation evident for the examined sediments, it is suggested that active biofilm development was the dominant factor controlling bed stability and erosion potential. Differences in biological mediation of strength between sediments were partially attributed to the structural differences within the biofilms and integration of the extracellular polymeric fibril matrix within the sediment pores.

The transport and fate of indicator E. coli and Salmonella are shown to be highly influenced by their relationship with flocculated suspended and bed sediment particles. Flocs were found to dominate the suspended sediment load and have the effect of increasing the downward flux of the sediment to the river bed. Bacteria counts were consistently higher within sediment compartments (suspended and bed) than for the water alone, with the bed sediment found to represent a possible reservoir of pathogens for subsequent remobilization and transport to potentially high risk areas. The mechanism of microbial attachment and entrapment within the sediment was strongly linked to the EPS fibrils secreted by the biological consortium of the aquatic system. It is suggested that the sediment/pathogen relationship should be of concern to public health officials because of its potential effects on pathogen source fate and effect with implications on public health risk assessment. Current standard sampling strategies, however, are based on an assumption that bacteria are entirely planktonic and do not account for the potentially significant concentration of bacteria from the sediment compartments. The lack of understanding around pathogen/sediment associations may lead to an inaccurate estimate of public health risk, and, as such, possible modification of sampling strategies to reflect this association may be warranted.

An assessment of bed sediment stability was carried out over biostabilized and nonbiostabilized cohesive sediment within a laboratory wave flume. Biofilms of 5, 9, and 15 d were grown over kaolinite, and changes in bed shear stress were applied through increasing wave height. Results show that the stability of the bed was highly related to the structural integrity of the overlying and integrated biofilm. Biofilms with substantial algal growth and lifting, due to decay and gas production, were weaker than those in contact with the bed. Generally, biostabilized beds of the same age failed at similar shear stresses (although the mechanism of failure varied), and the 9‐d biofilms exhibited the greatest strength. Changes in the microbial diversity and dominant species with time, as measured with phospholipids fatty acid analysis (PLFA) and denaturing gradient gel electrophoresis (DGGE), are believed to have influenced the observed variations in bed strength. Young biofilms were dominated by bacterial species, whereas older biofilms were dominated more by algae (particularly cyanobacteria). Fungal species also played a large role in the biofilm development and structure. Laser confocal microscopy and environmental scanning electron microscopy were used to show structural differences between biofilms and changes with wave energy. Biofilm development and bio‐armoring of underlying cohesive sediments are successional processes, with new layers of biofilm integrating into the sediment beneath older decaying biofilm layers. As such, there is a continuous temporal oscillation in bed sediment stability depending on the stage of biofilm development and decay.