Sediment-toxic substance interaction modelling

Abstract

Water quality models with analytical solutions are developed to simulate the transport of toxic substances in different aquatic environments. The models take into account both the physical transport processes such as the transport by water flows and dispersion mixings, and the chemical processes such as the sorption kinetics between the toxic substances and the sediment. Analytical forms in this research are being derived because analytical solutions can act as fast predicting tools in relatively simple cases. They also offer fundamental insights into the contributions of particular physical or chemical parameters to the transport process. However, in order to solve the models analytically, the governing systems must be formulated in relatively simple forms, while still retaining sufficient physics and chemistry to be realistic. In accordance with the above requirements, a one dimensional, time varying governing system has been studied and solved analytically. The vertical axis z is chosen as the controlling dimension in the governing system in order to include the settling and the resuspension of sediment and toxic substances in the water column. An instantaneous equilibrium of sorption kinetics between dissolved and particulate toxic substances is assumed and a governing equation is written in terms of the total toxic substance concentration. The governing system is further studied by assuming two different flow conditions (constant and parabolic distribution of the turbulent mixing coefficient) and the equilibrium distribution of suspended sediment in the whole transport process. Three analytical water quality models with constant or specific coefficients in the governing equations are developed based on the governing system. The analysis of different parameters in the governing system is carried out to identify the parameter ranges under which the transport of toxic substances can be modeled by the developed models. Analysis is also conducted to identify the effects of particular parameters on toxic substance transports. Since the particulate toxic substances has an additional downward motion due to the particle settling. The overall concentration is smaller for cases that have a larger fraction of toxic substances in the particulate forms. Thus, a larger partition coefficient and settling velocity or a smaller mixing coefficient leads to lower concentrations. Finally, three cases studies are presented to demonstrate the applications of the developed models for toxic substance transports and also in other areas. It is found that the models are also useful for simulating the transport of sediment in the water column and of chemicals in soil layers.