Groundwater contamination and health risk assessment across an urban gradient: Case study of Onitcha metropolis, south-eastern Nigeria

Urban population growth in south-Eastern Nigeria and associated proliferation of built-up areas predispose groundwater to contaminants of anthropogenic origin. This study compared the groundwater quality across an urban gradient, in Onitsha, a notable urbanized region and beehive of commercial activity in southeastern Nigeria. Groundwater samples (n = 120) from sixty boreholes tapping into aquifers spanning major settlement clusters at upper Iweka, Mgbuka and Nkpor, were sampled over 5-months from August to December 2019. Faecal coliforms, water-soluble metals, and nutrients (nitrates and phosphates) were estimated in groundwater samples. Noncarcinogenic risks of heavy metals in ground water samples was estimated for Average daily dose (ADD) and Total Hazard Quotient (THQ) using both deterministic and probabilistic approaches. Concentrations of mercury and heavy metals (Fe, Pb, and Cd) estimated in water samples revealed a general exceedance of World Health Organization (2009) standards for drinking water. However, significantly higher Hg, Pb and faecal coliform levels (0.122 ± 0.03 mg/L, 0.115 ± 0.014 mg/L, 2.3 × 106 ± 300 MPN respectively) in groundwater of the more built-up areas of upper Iweka highlight increased chances of metals and pathogens impacts on water quality with increased urbanization. High incidence of nutrients (nitrates: 258.40 ± 62.1 and phosphorus: 33.05 ± 4.06 mg/L) in groundwater of less urbanized areas suggests impacts of fertilizer applications from greater agricultural activity. Probabilistic estimates for heavy metals in ground water from Onitsha via ingestion route, showed that Upper Iweka had the highest HQ (3.262) followed by Mgbuka (2.1233) and Nkpor (2.0187). On the other hand, estimates showed that Mgbuka had the highest HQ (0.0241) followed by Nkpor (0.016) and Upper Iweka (0.014) for dermal exposure routes. Strategic mitigation measures including urban planning and infrastructural upgrade are recommended to protect the water resources, prevent further groundwater contamination and reduce population health risks emanating from poor drinking water sources.