Theoretical Development and Experimental Validation of a Reactive Transport Model for Contaminant Fate in Mangrove Sediments of the Niger Delta

Mangrove wetland sediments in the Niger Delta, South South Nigeria, function as biogeochemical reactors that simultaneously accumulate, transform, and release hydrocarbon contaminants, heavy metals, and nutrients, yet predictive models integrating these coupled reactive transport processes under Niger Delta-specific conditions are absent from the scientific literature. This study develops and experimentally validates a two-dimensional reactive transport model for hydrocarbon and heavy metal fate in mangrove sediment porewater, incorporating advection-diffusion-reaction equations with sub-models for sulphate reduction, methanogenesis, iron reduction, organic matter mineralisation, heavy metal speciation, and sorption kinetics parameterised from Niger Delta mangrove sediment data. Model code was developed in MATLAB using a finite difference discretisation scheme. Field experiments for model validation were conducted at contaminated and uncontaminated mangrove sites in Bonny and Andoni Local Government Areas of Rivers State, including sediment coring, porewater sampling by Rhizon samplers, solid phase characterisation, and in-situ gas flux measurements over 18 months. Model predictions of porewater hydrocarbon concentrations, sulphate profiles, methane flux, and iron distributions were validated against measured data, achieving mean absolute percentage errors below 18% for all state variables. Sensitivity analysis identified organic carbon content and bioturbation rate as the dominant parameters controlling contaminant fate. Scenario modelling predicted that mangrove restoration through replanting would reduce hydrocarbon mobility by 41% within 10 years through enhanced microbial attenuation and organic matter binding. This constitutes the first mechanistic model for contaminant reactive transport parameterised with Niger Delta mangrove field data. Keywords: reactive transport modelling, mangrove sediment, Niger Delta, hydrocarbon fate, heavy metals