Ecological and Health Risk Assessment of Total Petroleum Hydrocarbons and Metals in Water Samples from Bille Mangrove, Niger Delta, Nigeria

Petroleum exploitation in the Niger Delta has caused widespread contamination of mangrove ecosystems, yet studies that integrate total petroleum hydrocarbons (TPH) and metals in mangrove water are still very limited. This study presents the first dual-pollutant baseline assessment of TPH and five priority metals (Cd, Cr, Pb, Ni, Zn) in Bille mangrove water, a severely oil-impacted system supporting about 50,000 residents. Water samples were collected from six sites along a contamination gradient (flow station, pipeline passage, old bunkering site) and analyzed for TPH (C8–C40) and metals. All concentrations are reported in mg/L for direct comparability with World Health Organization (WHO) drinking-water guidelines and United States Environmental Protection Agency (USEPA) thresholds. TPH concentrations ranged from 0.18 to 57.66 mg/L, with Site 3 (pipeline passage) showing levels about 320-fold higher than reference sites and exceeding the WHO drinking-water guideline (0.05 mg/L) by up to 1153-fold. Cadmium (0.040–0.350 mg/L) and nickel (0.055–0.561 mg/L) exceeded WHO drinking-water guidelines (Cd 0.003 mg/L; Ni 0.07 mg/L) by 13–117- and up to 8-fold, respectively. Health risk assessment, using USEPA Risk Assessment Guidance for Superfund (RAGS) protocols, revealed a total cancer risk of 4.15 × 10−3 at Site 3, 41-fold above the USEPA acceptable threshold of 1 × 10−4, and extreme non-carcinogenic risk (Hazard Index = 20.03–25.51) at petroleum-infrastructure sites; cadmium contributed 86–88% of both carcinogenic and non-carcinogenic effects. Ecological risk indices classified Site 3 as extreme (Potential Ecological Risk Index = 722, against the Håkanson PERI = 600 “very-high-risk” threshold), mainly driven by cadmium (Er = 310–350) and nickel (Er = 140–150). Source apportionment using the Carbon Preference Index, enrichment factors, and strong TPH–metal correlations (r > 0.88, p < 0.01) clearly identified petroleum operations as the dominant contamination source. This work demonstrates the critical importance of integrated multi-pollutant assessments in petroleum-degraded mangrove water for guiding environmental protection and public-health interventions.