Spatial Distribution and Source Apportionment of Heavy Metals in Urban Soils of Lagos Metropolitan Area, South West Nigeria

This study investigates the spatial distribution and source apportionment of heavy metals in urban soils across Lagos metropolitan area, located in Nigeria's South West geopolitical zone. Urban soil heavy metal contamination is a significant public health concern in Lagos, where industrial activities, dense road traffic, informal waste management, and historical land use combine to introduce multiple metal contaminants into the soil environment. Using a systematic grid-based sampling design with 100 surface soil samples collected at a 2 by 2 km grid across 10 administrative local government areas, soils were analysed for cadmium, lead, chromium, nickel, copper, zinc, arsenic, and mercury by ICP-MS following aqua regia digestion. Spatial distribution was mapped by kriging interpolation in ArcGIS. Source apportionment was conducted using positive matrix factorisation (PMF) analysis and enrichment factor calculation against geochemical background values derived from rural reference sites. Geo-accumulation indices and ecological risk indices were calculated. Findings revealed that lead and cadmium showed the greatest spatial enrichment, with geo-accumulation indices above 3 (heavily polluted) in industrial corridor zones of Apapa, Amuwo-Odofin, and Oshodi-Isolo. PMF analysis identified traffic emissions, industrial activities, and e-waste dismantling as three distinct source factors contributing 34, 29, and 22 percent of total metal loading respectively. Ecological risk was highest for cadmium, with risk indices exceeding 200 in 18 percent of sampling locations. The study provides an evidence base for priority contamination remediation zones in Lagos and recommends source-targeted regulatory interventions for traffic and industrial metal emissions. Keywords: urban soils, heavy metals, source apportionment, Lagos, spatial distribution.