Mechanistic Understanding and Predictive Modelling of Microplastic Fate in Tropical River Systems: A Contribution from Nigerian Catchments

This dissertation advances the mechanistic understanding and predictive modelling of microplastic particle fate and transport in tropical river systems, making an original empirical and theoretical contribution using data from Nigerian catchment studies. The fate of microplastics in riverine environments is governed by processes including fragmentation, biofouling-induced density change, flocculation with organic matter, and hydrodynamically driven deposition and resuspension, but the parameterisation of these processes for tropical rivers with distinct hydraulic regimes, organic matter characteristics, and plastic composition profiles is absent from the literature. The dissertation integrates three complementary research phases: laboratory flume experiments quantifying the hydrodynamic settling and resuspension behaviour of environmentally relevant microplastic types under tropical river hydraulic conditions; field-based tracer studies using fluorescent microplastic analogues in the Osun and Ogun Rivers to constrain transport timescales and storage fractions in field conditions; and development and validation of a coupled hydrodynamic-fate model incorporating the mechanistically derived process equations. The model is validated against independent field data from the Cross River estuary. Original theoretical contributions include quantified biofilm-mediated density evolution models parameterised for tropical temperature and organic carbon conditions, a revised hydrodynamic transport algorithm that better represents particle hiding effects in heterogeneous river sediments, and a fragmentation kinetics model derived from field fragment size distribution data. The predictive model constitutes a tool for environmental impact assessment and emission scenario analysis for Nigerian river systems. Keywords: microplastics, fate modelling, tropical rivers, transport processes, Nigeria.