Impacts of the Liujiang coal mine on groundwater quality based on hierarchical cluster analysis

During the process of coal mining, the mine water inrush or draining can change the regional groundwater flow pattern to different degree and lead to the change in groundwater quality. Based on the Hierarchical cluster analysis (HCA) and the ratio coefficient of the main ions, the impacts of the Liujiang coal mining activities on groundwater quality is studied. The groundwater can be divided into three types (A, B and C) by using HCA, and the predominant cations among the three types are calcium and magnesium. The prevailing anions are different. Sulfate is dominating in water type A occurring along the coal-bearing strata in the center of the study area. The main hydrochemical types are SO₄·HCO₃—Ca, SO₄·HCO₃—Ca(Mg), SO₄·HCO₃—Ca(Mg/Na) and SO₄·HCO₃—Ca·Na(Mg). The water type B is governed by bicarbonate which is dominated by HCO₃—Ca, HCO₃—Ca(Mg) and HCO₃·SO₄—Ca(Mg), and allocates in the west and east of the Liujiang basin. Groundwater of type C concentrates in the south of the study area and is controlled by nitrate, and hydrochemical types are NO₃·HCO₃—Ca, NO₃·SO₄—Ca·Mg(Na) and NO₃·Cl—Ca. Water type A is characterized by high sulfate concentration due to the oxidation of sulfide minerals in the coal-bearing strata during exploration. The acid water produced during this process can promote the dissolution of carbonate rocks, and elevate the sulfate concentration in groundwater. There are 13% of groundwater samples exceeding the third class of Quality Standard for Groundwater (GB/T 14848—93). The ion ratios analysis and Q mode cluster analysis demonstrate that the major source of sulfate in groundwater is the oxidation of sulfide in the coal-bearing strata instead of dissolution of gypsum or domestic swage/agriculture utilization. High concentration of nitrate due to domestic swage/agriculture utilization and accumulation along the groundwater flow (from north to south) can be an oxidant thus promotes the oxidation of sulfide minerals once it gets into the closed aquifer. Other human activities can accelerated the groundwater degradation caused by coal mining activities.