Geochemical evaluation of trace metals and rare-earth elements in soils and sediments from a legacy copper mining site in Kongo Central, Democratic Republic of the Congo

Abstract This study assessed trace metals, metalloids, and rare earth elements (REEs) in soils and sediments around an abandoned artisanal copper mine in Madimba Territory, Kongo Central Province (DR Congo). Trace metals and REEs in soils and sediments were quantified following complete acid digestion and analysis by ICP-MS, using collision-cell technology to minimize spectral interferences. Total mercury was determined independently using a DMA-80 direct analyzer, which measures Hg via thermal decomposition, catalytic reduction, gold amalgamation, and CV-AAS detection without chemical digestion. The Enrichment Factor (EF), Geoaccumulation Index (Igeo), and Potential Ecological Risk Index (RI) were used to evaluate contamination levels and ecological risks. The EF and Igeo results highlighted anthropogenic inputs, explicitly confirming that historical mining activities were the main source of Cu, Pb, As, and Hg contamination. Sediments showed high concentrations of Cu (591.75 mg kg⁻¹ dw), Pb (179.54 mg kg⁻¹ dw), As (8.57 mg kg⁻¹ dw), and Hg (0.36 mg kg⁻¹ dw), largely exceeding international guidelines on sediment quality. Furthermore, the EF and Igeo values indicated extreme enrichment and strong pollution from anthropogenic sources mainly linked to historical mining activities. The total ecological risk (RI = 287.8–2655.7) classified the area as from considerable to very high ecological risk, with Hg and Cu as the main contributors to environmental contamination. The sum of REEs (4.08–72.92 mg kg⁻¹ dw) showed minor enrichment but displayed distinct Ce, Eu, and Gd anomalies. Negative Ce–Eu anomalies reflected redox-controlled fractionation, whereas positive Gd anomalies suggested interactions with organic matter, promoting metal co-mobility and sediment accumulation. The Principal Component Analysis (PCA) was applied to integrate physicochemical parameters, trace metals, and REEs. The first two principal components explained 73.1% of the total variance. PC1 correlated strongly with mining-related metals (Cu, Pb, Zn, Co) and ΣREE, highlighting anthropogenic contamination and ore-derived enrichment. PC2 was associated with organic matter, CaCO₃, and sediment texture, reflecting geochemical controls of soil and sediment composition. PCA effectively distinguished contaminated sites from less-impacted areas and elucidated relationships between REEs, heavy metals, and sediment properties. Overall, Cu, Pb, As, and Hg emerged as the key pollutants of concern, posing significant ecological threats. The results underscore the urgent need for remediation and long-term environmental monitoring to protect local ecosystems and communities.