A systematic review of heavy metal contamination in groundwater of oil-producing regions in Nigeria: A comprehensive review of sources, fate, transport, and human health implications

The Niger Delta is one of the world's most high-profile oil-producing regions, yet its shallow aquifers, which supply drinking water to over 20 million people, are constantly contaminated by heavy metals from both geogenic processes and decades of petroleum exploration activities. This review systematically examines the region's hydrogeological setting, natural and operational sources of metals, and the occurrence, and toxicological significance of priority metals such as lead (Pb), cadmium (Cd), chromium (Cr), nickel (Ni), arsenic (As), mercury (Hg), iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), barium (Ba), and vanadium (V). The review further analyzes transport pathways and hydrogeochemical controls such as pH, redox, salinity, and dissolved organic carbon that controls metal movement in the Benin Formation and Delta basin aquifers. Case studies from Ogoniland, Warri, Nembe, and other oil-bearing communities are used to evaluate the longitudinal, latitudinal, temporal contamination patterns. Human health risk assessments are appraised considerably, revealing widespread non-carcinogenic and carcinogenic risks, particularly for Pb, Cd, and As, while epidemiological evidence of elevated blood lead and urinary cadmium is also highlighted. The work also reviewed the current state of analytical monitoring, pollution indices, multivariate source apportionment, and Nigeria's regulatory framework, including the roles of National Environmental Standards and Regulations Enforcement Agency (NESREA) and Nigerian Upstream Petroleum Regulatory Commission (NUPRC). Remediation technologies such as conventional pump-and-treat to emerging phytoremediation and biochar-based approaches, are reviewed for their applicability in the region's hydrogeochemical environment. Finally, the work identified knowledge gaps concerning mixed contaminant interactions, microbial modulation of metal mobility, the legacy of artisanal refining and orphan wells, and the compounding effects of climate change and saltwater interference, and a way forward for a sustainable groundwater management in the Niger Delta.