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As China's main energy source, coal production and consumption account for 72% and 62%, respectively, of primary energy. The total output of raw coal was 3.41 billion tonnes (t) in 2016, down 9.0% from the same period a year earlier, and about 90% of coal comes from underground mines. In recent years, many coal mines have been closed because of the depletion of coal resources, resource integration, and coal mine production safety. Consequently, groundwater levels in these mines rise rapidly after the mine drainage systems stop working. Moreover, other aquifers are polluted with mine water through mining-induced fractures, faults, and poorly sealed drill holes. In northern China, the Ordovician or Cambrian aquifer, which is the main coal seam floor, is an important water source [1]. This water resource is at risk of being polluted when the groundwater above becomes polluted. For example, due to the influence of the coal mine water in Zibo mining area of Shandong Province, and Yangquan mining area in Shanxi Province, the karst groundwater has been polluted in different degrees [2-3]. Investigations show that around 40% of total groundwater in northern China

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