SOURCES AND CONCENTRATION LEVELS OF HEAVY METALS IN SURFACE WATER AROUND ABANDONED COAL MINES IN NYABA CATCHMENT AREA, ENUGU STATE, NIGERIA

Abstract

This study assessed the sources, concentration levels, and health risks of heavy metals in surface waters around abandoned coal mines in the Nyaba catchment, Enugu State, Nigeria. Water samples were collected from Ekulu and Nyaba rivers at upstream, midstream, and downstream points during both dry and wet seasons. Eighteen samples from Ekulu River and sixteen from Nyaba River were collected at upstream, midstream, downstream, raw mine discharge, and control points during both wet (August 2023) and dry (January 2024) seasons. Standard Atomic Absorption Spectrophotometry (AAS) was used to analyze concentrations of As, Cd, Co, Fe, Hg, Mn, Ni, Pb, and Se. Data were subjected to descriptive statistics, Pearson correlation, and one-sample t-tests against international and national guideline values from WHO, USEPA, and FEPA/NESREA. The results indicate that both geogenic processes linked to Acid Mine Drainage (AMD) and anthropogenic activities contribute significantly to metal enrichment in the rivers. Correlation analysis suggests that Fe and Mn are strongly associated with AMD inputs from abandoned mine tunnels, while Cd, Pb, and As also reflect contributions from irrigation farming, sand dredging, laundry, and domestic waste disposal. Mean concentrations of Fe (1.181 mg/L in Ekulu dry season; 1.063 mg/L in Nyaba wet season) and Pb consistently exceeded permissible limits, while Cd and As also remained above safety thresholds across seasons. Seasonal variation showed reduced concentrations in Ekulu River during the wet season due to dilution, but increased contamination in Nyaba River during rainfall, reflecting enhanced leaching as well as intensified human activity. Health risk assessment indicated that Hazard Quotient (HQ) and Hazard Index (HI) values exceeded 1 for both adults and children, while Incremental Lifetime Cancer Risk (ILCR) values for As, Pb, and Cd were above 1 × 10⁻⁴, indicating unacceptable lifetime cancer risks. Children were particularly vulnerable, recording higher risk levels due to their greater sensitivity to metal exposure. The study concludes that Ekulu and Nyaba rivers are unsafe for direct domestic and agricultural use without treatment. It recommends urgent remediation of abandoned coal mines, continuous monitoring of water quality, provision of safe alternative water supplies, and public health interventions to protect exposed communities.