Environmental consequences of petroleum drilling in the Niger Delta: a systematic review of heavy metal contamination in soil, water, and sediments

Environmental degradation from petroleum drilling poses a persistent threat to ecosystem integrity in Nigeria’s Niger Delta. This systematic review synthesises four decades of evidence (1980–2025) on heavy-metal contamination of soil, water, sediment, and biota associated with petroleum exploration and production. Following PRISMA 2020 guidelines, major scientific databases were searched using predefined keywords. After screening and eligibility assessment, 104 peer-reviewed studies were included, while 39 review articles and non-quantitative studies were excluded. Extracted data included study location, environmental medium, heavy metals, analytical methods, and environmental outcomes. Research output has increased substantially, with Rivers, Bayelsa, and Delta States receiving the greatest attention. Across the reviewed studies, heavy-metal concentrations frequently exceeded international guideline values. Reported maximum concentrations in soils and sediments exceeded 100 mg kg^⁻1 for Pb and Zn, 50 mg kg^⁻1 for Cr and Ni, and 1 mg kg^⁻1 for Cd. In surface water and groundwater, dissolved Pb and Cd often exceeded World Health Organisation drinking-water limits. Soil and sediment were identified as the primary sinks for contamination. Findings from the review demonstrate widespread and persistent heavy-metal pollution linked to petroleum activities, highlighting the need for strengthened environmental monitoring, improved waste management, and sustainable petroleum governance to protect ecosystems.