Seasonal variation, heavy metal pollution indices and health risk assessment in the Esa-Odo reservoir, Nigeria

This study investigated the spatial and temporal variations in heavy metal concentrations in the water of the Esa-Odo Reservoir, located in Osun State, Nigeria. Surface water samples were collected from three designated stations using an improvised water sampler between March 2018 and December 2019, covering two distinct seasons. The Atomic Absorption Spectrophotometric (AAS) method was employed to analyse the acidified water samples (two millilitres of concentrated HNO₃ per litre of water) for the selected heavy metals. Samples were collected bi-monthly, totaling thirty-six. The risk assessment utilized the hazard quotient and hazard index for non-carcinogenic risk, and the target cancer risk for carcinogenic risk. The findings indicated that zinc had the highest concentration, followed by iron and copper, with arsenic usually found in trace amounts. While spatial analysis showed no significant variation (p > 0.05) in heavy metal concentrations, seasonal variations were apparent, with notably higher concentrations during the wet season. Five of the analyzed heavy metals (Cr³⁺, Mn²⁺, Fe²⁺, Cu²⁺, and Zn²⁺) were within the NESREA and USEPA permissible limits. Cadmium and lead exceeded the recommended regulatory thresholds, indicating potential risks to human health and aquatic ecosystems. Although arsenic concentrations were below the WHO drinking water limit of 10 µg/L, they may still pose concern based on stricter NESREA/USEPA environmental quality criteria. Principal component analysis and cluster analysis identified correlations and grouping patterns among metals. The heavy metal pollution index (HMPI), calculated using a weighted index method, surpassed the critical threshold of 100 at all stations, with higher values recorded in the wet season (264.27) compared to the dry season (199.65). The hazard index (HI) values remained below the threshold of 1, suggesting a generally low health risk. Furthermore, the Esa-Odo Reservoir showed a low carcinogenic risk for both adults and children based on the target cancer risk (TCR) assessments for heavy metals from ingestion and dermal contact. The study linked increased heavy metal concentrations in the reservoir to agricultural runoff, artisanal mining, and other environmental disturbances surrounding the catchment area. The observed seasonal variations highlight the dynamics of pollution, emphasizing the need for ongoing monitoring and management strategies to mitigate pollution impacts and preserve water quality.