Spatio-Seasonal Distribution, Source Apportionment, and Human Health Risk Assessment of Heavy Metals in Apodu Reservoir, Kwara State, Nigeria

Abstract

Tropical freshwater reservoirs are vulnerable to heavy metal contamination from geogenic weathering and diffuse anthropogenic inputs, yet integrated assessments combining seasonal, spatial, source, and health-risk dimensions remain scarce in Nigeria. This study quantified six heavy metals (As, Cd, Cr, Fe, Mn, Pb) in surface water from Apodu Reservoir, Kwara State, Nigeria. Sixty-one samples were collected across wet (May 2024) and dry (February 2025) seasons from inflow, transitional, and lacustrine zones. Concentrations were determined by atomic absorption spectrometry and analysed using paired t-tests, Moran’s I, Getis-Ord General G, principal component analysis (PCA), and USEPA health risk indices. All concentrations fell below WHO and NSDWQ drinking-water guideline limits. Wet-season Fe and Mn were significantly higher (p < 0.001 after Bonferroni correction), consistent with reductive dissolution of Fe/Mn oxyhydroxides and catchment flushing. No significant spatial autocorrelation was detected (p > 0.05), indicating broadly uniform, non-clustered distributions. PCA tentatively indicated mixed geogenic and diffuse anthropogenic controls, with no single dominant source resolvable. Non-carcinogenic risk was minimal (HI < 1) for both age groups. Arsenic alone exceeded the 1 × 10⁻⁴ carcinogenic threshold (ILCR = 1.155 × 10⁻⁴, children; 1.135 × 10⁻⁴, adults), while Cr, Cd, and Pb remained within the acceptable 10⁻⁶–10⁻⁴ range. Combined ΣILCR reached 1.996 × 10⁻⁴ (children) and 1.891 × 10⁻⁴ (adults) under conservative upper-bound assumptions, exceeding the threshold cumulatively. These findings establish a quantitative baseline for Apodu Reservoir and a transferable framework for tropical reservoirs across sub-Saharan Africa facing similar diffuse inputs and censored trace-metal data.