Trace metals and rare earth elements in sediments along the Muanda River estuary and coastal ocean continuum in an oil exploitation area

This study evaluated the distribution and ecological risks of trace metals and rare earth elements (REEs) in surface sediments along the river–estuary–coastal continuum of Muanda, Democratic Republic of the Congo (DRC), a region under strong environmental pressure by oil exploitation. Fifteen sediment samples were analyzed by ICP-MS, AAS, and DMA. Sediments were mainly sandy, with variable organic matter (1.75–10.30%) and CaCO3 (0.26–9.14%) contents, which significantly influenced metal retention. Elevated concentrations of Zn (up to 523.6 mg·kg⁻1), Pb (311.2 mg·kg⁻1), Cr (253.6 mg·kg⁻1), and Cu (165 mg·kg⁻1) exceeded international Sediment Quality Guidelines (SQGs) and Probable Effect Levels (PELs) on aquatic life, especially near industrialized estuarine sites. Geoaccumulation Index (Igeo) and Enrichment Factor (EF) indices confirmed moderate to severe anthropogenic pollution, while contamination and ecological risk indices (CF, CD, Eri, RI) indicated localized moderate ecological risk, despite overall low cumulative risk due to sedimentary buffering. Total REEs ranged from 3.28 to 6.72 mg·kg⁻1, markedly lower than shale standards, with slight negative Ce (0.91–0.93) and Eu (0.70–1.17) anomalies, reflecting primarily natural geochemical control. Statistical correlations revealed that organic matter and CaCO3 acted as key scavengers, enhancing retention of Cu, Zn, Pb, and Hg. Spearman rank correlation analysis confirmed oil industry-related discharges as the dominant source of trace metal enrichment. Spearman correlations highlighted the role of organic matter (OM) and calcium carbonate (CaCO3) as major binding phases for metals, while PCA confirmed their clustering with REEs in fine sediments and distinguished metals from anthropogenic sources (V, Ni, Cu, Pb, Zn, As, Cd) from metals of geogenic origin. Petroleum exploitation was identified as the main source of contamination, with additional contributions from agriculture, industrial effluents, and maritime activities.