Study on the characteristics and causes of groundwater pollution in coal gangue dumps

The stock of coal gangue in China is large, and it still increases rapidly, posing a high risk on the environment. However, the effect of the self-ignition degree of coal gangue on the release of leachates pollutant is unclear, and the information on the natural attenuation of these leachate pollutants is limited. The coal gangue with different self-ignition degrees and the contaminated groundwater were collected from a typical coal gangue dump site in Taiyuan City, Shanxi Province. The composition and concentration of heavy metals, inorganic salts and organic compounds in coal gangue and groundwater were investigated, and the sources of groundwater pollution were explored based on statistical analysis. The results showed that the gangue extracts and groundwater flowing from the mountainside below the gangue dump were not contaminated by Cd, Pb, As, and Zn, though they had been lightly polluted by Cr and Ni. The concentrations of SO₄ ²⁻, Fe, and Mn were high in the extracts, with SO₄ ²⁻ concentration of 5 982 mg/L and Fe and Mn concentrations of 1 081 and 19 times higher than the groundwater Ⅳ quality standard specified in Quality Standard for Ground (GB/T 14848-93), respectively. According to the pollution source analysis, it could be obtained that Na⁺, K⁺, Cl⁻ and NO₃ ⁻ in groundwater near the gangue dump sites mainly originated from soil and aquifer medium, while SO₄ ²⁻, Fe, Mn, Cd, Zn, As, Cr, and Ni were mainly released from gangue leaching. The pollution release of the coal gangue with different self-ignition degrees was as follows: burning gangue > burned gangue > fresh gangue. The self-ignition of coal gangue contributed to the release of pollution. Natural attenuation of heavy metal significantly occurred during the migration process of gangue leachates. However, the concentration of SO₄ ²⁻, Ni, and Mn still had certain risks, exceeding Class Ⅳ groundwater standard specified in GB/T 14848-93.