Theoretical Development and Experimental Validation of a Reactive Transport Model for Contaminant Migration in Lateritic Unsaturated Zones of South West Nigeria

Contaminant migration through lateritic unsaturated zones in South West Nigeria is governed by complex reactive transport processes including non-linear adsorption, redox-controlled dissolution, and heterogeneous preferential flow, yet theoretical models calibrated for lateritic tropical soil conditions are absent from the published literature. This study develops and experimentally validates a reactive transport model for contaminant migration in lateritic unsaturated zones representative of South West Nigeria. The theoretical model development addresses three novel aspects: a non-linear kinetic adsorption formulation for heavy metals on iron-oxide-rich lateritic soils (calibrated from batch and column adsorption experiments), a preferential flow component using the dual-permeability concept adapted for lateritic macro-pore structure, and a redox module accounting for iron reduction under saturated zones following wetting events. The model is implemented in a modified HYDRUS-2D framework. Laboratory column experiments using repacked and undisturbed laterite cores from Ogun State characterise the transport parameters. Field validation is conducted at an industrially contaminated laterite site in Lagos State, where 3-year breakthrough monitoring data are available. Model prediction of lead and chromium breakthrough shows Nash-Sutcliffe efficiency above 0.85. Sensitivity analysis identifies iron-oxide content and macro-pore connectivity as the dominant transport controlling parameters. The theoretical framework provides an original contribution to reactive transport science in tropical lateritic systems with implications for contaminated land assessment in South West Nigeria.

Keywords: reactive transport modelling, lateritic soils, contaminant migration, unsaturated zone, South West Nigeria