Towards a Unified Theory of Contaminant Attenuation in Nigerian Laterite Vadose Zones: Implications for Groundwater Protection

This dissertation develops a unified theoretical treatment of contaminant attenuation processes in Nigerian laterite vadose zones, integrating physical, chemical, and biological attenuation mechanisms into a predictive framework applicable to groundwater protection assessment for diverse contaminant classes. Laterite soils cover a large proportion of southern Nigeria's land surface and form the unsaturated zone through which infiltrating water and dissolved contaminants must pass before reaching groundwater. Despite their widespread occurrence as the primary soil type overlying Nigerian aquifers, the specific physical and geochemical properties of Nigerian laterites relevant to contaminant fate, including their iron and aluminium oxyhydroxide mineralogy, aggregate structure, and preferential flow network geometry, have not been systematically characterised in a manner useful for engineering protection assessment. The dissertation conducts an extensive experimental characterisation programme across twelve representative laterite soil profiles sampled from across the South West, South East, and North Central zones, determining hydraulic properties, sorption isotherms for selected contaminant classes including heavy metals, nitrate, petroleum hydrocarbons, and pharmaceuticals, and biodegradation rate constants through laboratory microcosm incubation. The theoretical contribution integrates the experimental data into a dual-domain transport model that accounts for both matrix flow and preferential macropore flow as competing pathways with distinct attenuation efficiencies. The model is validated against field tracer and contaminant breakthrough data from three instrumented laterite vadose zone study sites. The unified framework enables site-specific groundwater protection distance and time calculations for diverse land use and contamination scenarios. Keywords: contaminant attenuation, laterite vadose zone, groundwater protection, Nigeria, transport modelling.