A numerical framework for modelling sediment and chemical constituents transport in the Lower Athabasca River

Abstract

The transport of fine sediments and associated chemical constituents originating from potential anthropogenic and natural sources is becoming an issue of increasing importance in the Lower Athabasca River (LAR) ecosystem in northern Alberta, Canada. This study aims to (1) establish an integrated numerical modelling framework to investigate the transport of fine cohesive sediments and associated chemical constituents during both ice-covered and open-water periods and (2) apply the modelling framework to investigate the state and temporal/spatial variation in sediment and selected chemical constituents within the LAR. One-dimensional hydrodynamic and transport models, combined with a river ice model, are used to predict the flow characteristics, transport of sediments and a selection of three metals and three polycyclic aromatic hydrocarbons (PAHs) within a ∼200 km reach of the LAR, both in open-water and ice-covered conditions. The models are validated using available field measurements and are applied to investigate the state and variation of sediment and chemicals for a baseline period as well as to assess the effect of various hypothetical pollution scenarios. The model simulations successfully reproduce the hydrodynamics and sediment transport patterns as well as the state and variation of selected metal and PAH constituents. The results generally show that the concentration of chemical constituents in the bed sediment is the major factor in determining the state and variation of their concentration in the water column, and the high-flow season is the critical period for the transport of sediment and chemicals in the system. The scenario simulation results indicate that increases in the concentrations of chemical constituents in tributary streams have to be orders of magnitude higher to have a noticeable effect on their corresponding water column concentration in the LAR. Those effects are also found to be higher only within the immediate vicinity of the tributary confluences and gradually diminish with distance downstream of the confluences. The numerical modelling framework developed in this study provides a tool for investigation and understanding of the state and temporal/spatial variation of sediment and associated chemical constituents within cold region rivers such as the LAR. By conducting additional scenario-based studies (such as future climate and chemicals loading), the models can be used to identify possible future states of sediment and water quality constituents in the LAR ecosystem.