Source Apportionment and Toxicological Risk of Potentially Toxic Elements in River Osun Ecosystem Within Ilesha Gold Belt

Technical Summary: Source Apportionment and Toxicological Risk of Potentially Toxic Elements in River Osun Ecosystem Within Ilesha Gold Belt

Problem Statement
The study addresses the environmental and public health risks associated with potentially toxic elements (PTEs) in the River Osun ecosystem, specifically within the Ilesha gold belt of Osun State, Nigeria. Gold mining activities, particularly artisanal operations, are identified as significant sources of heavy metal contamination released into water and sediment through industrial processes, smelting, and improper waste disposal. The research focuses on characterizing the physico-chemical parameters and metal concentrations in these matrices to evaluate ecological risks and potential human health hazards arising from long-term exposure, specifically ingestion.

Methodology
The research employed a multi-faceted approach involving field sampling, laboratory analysis, and statistical modeling:

• Sampling: Water and sediment samples were collected in September 2024 from four distinct locations within the gold belt: two sites near Ora (ORS1 and ORS2), one at Oke-Ibode (OKS), and one at Osogbo (OSS). Sediment was collected from a depth of 0–25 cm using an Ekman grab.
• Physicochemical Analysis: In-situ measurements were taken for pH, dissolved oxygen, total dissolved solids, electrical conductivity, and total hardness using a multi-parameter probe.
• Sample Preparation and Digestion: Water samples were digested with concentrated nitric acid (HNO₃) and hydrochloric acid (HCl) at 90°C. Sediment samples were air-dried, sieved (2mm), and extracted using a mixture of HCl, HNO₃, and phosphoric acid (H₃PO₄).
• Instrumental Analysis: Heavy metal concentrations were quantified using Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) with an Agilent 720-ES system.
• Statistical and Risk Assessment:
  • Source Apportionment: Principal Component Analysis (PCA) with varimax rotation was utilized to identify patterns, group elements with similar geochemical trends, and distinguish between natural geological processes and anthropogenic inputs.
  • Ecological Risk: The Potential Ecological Risk Index (ERI) was calculated based on contamination factors and toxic response factors (Hakanson, 1980).
  • Human Health Risk: A non-carcinogenic risk assessment was conducted using United States Environmental Protection Agency (USEPA, 2011) models. This involved calculating the Chronic Daily Intake (CDI) and the Hazard Quotient (HQ) for adult exposure via ingestion.

Key Results

• Metal Concentrations:
  • Water: Significant variations were observed across sites. ORW1 exhibited notably higher concentrations of Potassium (2.59 mg/L), Calcium (3.59 mg/L), Barium (4.59 mg/L), and Vanadium (5.59 mg/L) compared to other sites. Iron (Fe) and Manganese (Mn) levels also varied, with ORW1 showing the highest Fe concentration (1.30 mg/L).
  • Sediment: The ORS1 site displayed the highest mean concentrations for several elements, including Sodium (4.92 mg/kg), Magnesium (7.48 mg/kg), and Aluminum (1077.82 mg/kg). ORS2 showed elevated levels of Silver (8.88 mg/kg) and Cadmium (0.38 mg/kg).
• Source Apportionment (PCA): Three principal components were identified.
  • Component 1 accounted for the majority of variance, grouping elements like Na, Mg, K, Ba, V, Fe, Ni, Cu, Al, Se, Th, and U, suggesting a common geochemical or mineralogical origin.
  • Component 2 highlighted variations in transition metals and calcium-rich compounds (Ca, Mn, Fe, Co, Pb, Cd).
  • Component 3 was characterized by Cr, Ag, Cd, and Th, indicating specific trace metal distributions potentially linked to industrial or environmental factors. Silver (Ag) was identified as an outlier, behaving differently from the clustered elements.
• Ecological Risk: The Ecological Risk Index (ERI) indicated that Cadmium (Cd) and Lead (Pb) posed significant ecological risks, particularly in sediment samples.
• Human Health Risk:
  • The Hazard Quotient (HQ) analysis revealed that Cadmium (Cd), Lead (Pb), and Nickel (Ni) posed significant risks through long-term ingestion.
  • Specific HQ values for sediment ingestion exceeded unity for several elements, including Cobalt (Co: 1.789), Nickel (Ni: 1.897), Copper (Cu: 1.765), Zinc (Zn: 1.876), and Uranium (U: 1.548), indicating potential non-carcinogenic health risks for adults exposed to these sediments.

Significance and Claims
The study establishes that heavy metal concentrations in the water and sediment of the Ilesha gold belt exhibit varying levels of contamination, with specific sites (notably ORS1 and ORS2) showing signs of significant to extreme contamination. The authors conclude that the observed elemental variations are driven by a combination of mining operations and natural geological processes.

The primary significance of the work lies in its identification of specific toxicological risks. The findings suggest that regulatory measures are necessary in regions where mining activities occur to protect public health. The paper explicitly calls for the continuous monitoring of heavy metal concentrations to mitigate environmental hazards and preserve water quality. Furthermore, it advocates for the adoption of sustainable mining methods and appropriate waste management strategies to reduce contamination levels associated with the region's gold mining activities. The study does not propose new experimental protocols or future applications beyond these regulatory and management recommendations.