Seasonal Variability and Risk Assessment of Heavy Metals in Shallow Groundwater of Selected Areas in Oyo Town, Southwestern Nigeria

Abstract

Groundwater remains the main source of potable water in many urban and peri-urban communities in Nigeria, particularly where municipal supply is inadequate. This study investigates the seasonal variability and potential health risks of heavy metals in shallow groundwater from selected areas of Oyo Town, Southwestern Nigeria. Forty groundwater samples (20 per season) were collected from hand-dug wells in both wet and dry seasons and analyzed for twelve heavy metals (Al, Cd, Fe, B, Ba, Co, Pb, Mn, Cr, Zn, Cu, Ni) using Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Data were assessed using descriptive statistics, ANOVA, Pearson correlation, and U.S. EPA health risk assessment models. Results revealed that aluminum (Al), lead (Pb), iron (Fe), manganese (Mn), and cadmium (Cd) exceeded WHO guideline values in multiple locations. Seven metals (Al, Cd, B, Ba, Mn, Cr, and Cu) showed statistically significant seasonal variations (p < 0.05): cadmium (Cd) and chromium (Cr) were elevated in the wet season, while aluminum (Al) and manganese (Mn) were higher in the dry season. Hazard Index (HI) values indicated greater non-carcinogenic risk in the wet season, with all locations exceeding the safety threshold (HI > 1) for both adults and children; children consistently demonstrated greater vulnerability. These findings underscore the urgent need for systematic groundwater monitoring, stricter regulation of waste disposal and agricultural practices, and the provision of alternative safe water sources, in order to safeguard public health and promote sustainable groundwater use in Oyo Town.