Environmental assessment of chemical contamination from abandoned coal and mineral mines

Abstract

Abandoned mine lands (AMLs) are enduring sources of environmental pollution due to the release of toxic substances like metals, metalloids, organic and organometallic pollutants into the environment. These substances are transported through various processes such as acid mine drainage that aid the movement of pollutants across compartments and are driven by the pollutants' physicochemical properties, environmental conditions and the physical state of the abandoned mine. Chemicals released can be toxic, bioaccumulate, and persist; hence, highly polluted mine lands may pose significant ecological and human health risks. Worldwide, it is estimated that there are over a million abandoned mines. Nigeria is no exception, with over 1500 abandoned mines identified to date across many states, and so far, only a few have been reclaimed due to the high cost of restoration. This research focused on the environmental assessment of chemical contamination from abandoned coal and mineral mines in Nigeria. A systematic evidence map protocol was developed to evaluate current methods for assessing chemical contamination risks from abandoned coal and lead-zinc mines. The protocol was applied to review the evidence, which revealed that current AML risk assessment methods use contaminated land risk indices to evaluate chemical risk. Furthermore, the review identified evolving data-driven and probabilistic risk assessment methods, including logistic and linear regression models, which are emerging risk assessment approaches in AML chemical risk assessment. At the same time, Bayesian Kernel Machine Regression (BKMR), cumulative probability distribution analyses for HI, and Incremental Lifetime Cancer Risk (ILCR) have been reported for mixture risk assessment, and the study showed the relevance of bioindicator-based assessments for validating predictions from chemical-based assessments. A major aim of the project was to assess the chemical contamination and associated ecological and human health risks at coal and lead-zinc mines in Nigeria. In this phase of the research, environmental samples were collected from two abandoned lead-zinc mines at the Gimbi/Rikaya site in Plateau and the Abakaliki site in Ebonyi. The case study on abandoned coal mines was undertaken in Enugu, Nigeria. Site-specific contamination and ecological risk assessment at the Abakaliki and Gimbi/Rikaya sites revealed that both sites were heavily contaminated with Pb, Zn, and Cd, with higher concentrations at Abakaliki. Spatial distributions followed the pattern tailings > sediment > soil, with tailings as the primary contamination source. Sequential extraction confirmed that Pb, Cd, and Zn occur mainly in the exchangeable and reducible fractions, indicating high mobility and bioavailability, particularly in Abakaliki soils, where >34% of Pb occurred in mobile forms. In contrast, Pb mobility was lower at Gimbi, with only about 6% of F1 in the soil. Dissolved metals, Pb, Zn, Ni, Cu, and Mn in surface water at Abakaliki exceeded the environmental quality standards (EQS) and also the Biotic Ligand Model (BLM) bioavailability values. In contrast, only Zn exceeded EQS and bioavailability values at Gimbi. Ecological risk indices (Er, RI) and cumulative risk scores (CRS) indicated very high risk at Abakaliki and considerable risk at Gimbi, with Cd, Pb, and Zn as the dominant risk drivers. Chemical contamination and human health risk assessment (HHRA) at the Onyeama abandoned coal mine revealed the presence of both heavy metals and polycyclic aromatic hydrocarbons in soil, tailings, and water at varying concentrations. Fe, Mn, Zn, and Pb were the dominant metals of concern, reflecting inputs from historical mining residues and secondary weathering. The ΣBaPeq value (0.86 mg TEQ/kg) exceeded the Canadian Soil Quality Guideline (0.6 mg TEQ/kg), with BaP and DBA contributing substantially to the toxic load. The HHRA indicated ingestion and dermal contact as the dominant exposure pathways. Waterborne exposure contributed the highest non-carcinogenic and carcinogenic risk, with cumulative HI and CR values exceeding acceptable thresholds, particularly for children, highlighting their vulnerability. In contrast, soil and tailings contamination presented relatively lower cumulative HI and CCR. Overall, these findings confirm that abandoned Pb/Zn and coal mines pose both ecological and human health risks to inhabitants living on or near abandoned mines, pointing to the need for targeted management measures, including the containment of tailings, remediation of contaminated sediments, and monitoring of surface and groundwater pathways, to mitigate long-term ecological and human health impacts.