Groundwater quality assessment near Abule-Egba dumpsite, Lagos State, Nigeria: A study of physicochemical, microbial load, and heavy metal contaminants

Groundwater contamination from the uncontrolled disposal of municipal solid waste (MSW) remains a critical public health and environmental issue in Nigeria. This study evaluated the impact of leachate migration on groundwater quality around the vicinity of the Abule-Egba dumpsite, Lagos, Nigeria, by conducting seasonal physicochemical, microbial, and heavy metal analyses. Water samples were collected from five boreholes along with leachate samples and assessed against World Health Organisation (WHO) and Nigerian Standard for Drinking Water Quality (NSDQW) drinking water standards. The groundwater and leachate samples indicated high levels of contamination. The chloride content varied from 372 mg/L to 1154 mg/L (BH 2 , wet season), which was well over 4.5 times the WHO acceptable level for drinking water, to iron that varied from 0.44 mg/L (wet) to 2.54 mg/L (dry), essentially nine times the threshold. The levels of lead were detected at 0.043 mg/L (dry) and four times the WHO's safe limit of 0.01 mg/L. Microbial analysis revealed Escherichia coli (E. coli) content at 33 CFU/100 mL in BH 3 (wet season) and 30,900 CFU/100 mL in leachate, indicating very high pathogen intrusion. Groundwater Quality Index (GWQI) ranged from 78.65 to 405.54 during the seasons, placing water from all the boreholes as not fit for consumption, with BH 2 in the worst condition of degradation. Seasonal patterns highlighted higher contaminant loads in the dry season, likely due to reduced dilution and concentration effects. The findings highlight the urgent need for improved leachate management, continuous groundwater monitoring, and community awareness to safeguard groundwater resources in the densely populated urban area. • Leachate showed E. coli up to 30,900 CFU/100 mL; borehole iron increased from 0.44 to 2.54 mg/L, exceeding WHO limit. • Iron spiked from 0.44 mg/L (wet season) to 2.54 mg/L (dry season) in some boreholes, exceeding WHO safe limit (0.3 mg/L). • Lead levels peaked at 0.043 mg/L, > 4x the WHO limit (0.01 mg/L). • GWQI rated all sampled boreholes as “unfit for consumption”, with values increasing from 210.68 to 405.54 in BH 2. • Nitrate levels in leachate remained high (7.2 mg/L wet, 7.05 mg/L dry) compared to the WHO limit of 3 mg/L.