Multiple Particle Types for Modeling Particle - Associated Contaminant Transport: Towards a Particle Based Transport Model for Hydrophobic Organic Chemicals in Aquatic Systems
This research is designed to investigate the potential need to incorporate multiple particle types, rather than a single particle type, to accurately model particle associated hydrophobic contaminant transport in aquatic systems. A significant component is the introduction of a particle based modeling approach that is able to incorporate multiple particle properties such as size, density, settling rate, organic carbon fraction, and equilibrium partitioning coefficient. The two main hypotheses driving this research are: (1) more than one particle type must be considered when modeling contaminant transport, where particle type is defined in terms of the above properties; and (2) the particle based method is a better alternative for incorporating multiple particle types in the model, compared with standard mass concentration based methods. In order to evaluate these two hypotheses, calculations for sediment and contaminant concentrations were performed for a single mixed reactor and for multiple reactors in series using both the particle concentration based and standard mass concentration based methods. Equilibrium partitioning is assumed, and particles do not interact with each other, although the particle based approach provides a convenient framework for incorporating such interactions, as well as particle source tracking. The single mixed reactor provides a simple, convenient system that allows for analytical solutions of the mass balance equations to check numerical solutions, and in which the impact of using multiple particle types may be demonstrated in isolation from other possibly complicating factors. Calculations for multiple reactors further demonstrate the differences found between models based on single and multiple particle types, and serve as a first test of the model in a spatially resolved system. Application of the results is also discussed within the context of building a three-dimensional particle and contaminant transport model, where the multiple particle approach and chemical partitioning calculations would be combined with a particle tracking model. Results from this research show the degree to which calculated sediment and contaminant concentrations may differ when a single (average) particle is used in sediment and contaminant transport calculations, compared with results obtained using multiple particle types. The magnitude of this difference depends on the actual distributions of particle properties, but in general it is found that using five to ten particle types is usually sufficient. Experience gained through preliminary development of a three-dimensional model using the particle based method also shows the ease with which multiple particle types with different properties may be incorporated, and suggests this is a more convenient approach than the typical control volume based calculations, which would require solving a mass balance equation for every particle type considered.
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