Overview of field monitoring for acid mine water system of the coal mine in Shandi river basin

Acid mine drainage (AMD) is widespread in coal mining areas such as southwest Shandong, Shanxi, Inner Mongolia, Yunnan and Guizhou. The pH value of acidic mine water often ranges between 2 and 5, with high sulfate, Hb, TDS(Total dissolved solids), Fe, Mn and others. The acidic characteristics of acid mine water make Hg, as, CD, Pb, Co, Ni and other trace elements dissolve in the coal seam, and hence accelerate the reaction speed of phenolic organics and increase the toxicological composition. These harmful substances will do severe harm to groundwater, surface water or soil. Shanxi Province, known as the "coal sea", is located in the middle reach of the Yellow River Basin. But the problem of coal resource depletion in Shanxi Province is gradually exposed. In 2010, the mined-out area covered about 20,000 square kilometers, accounting for one eighth of the land area of Shanxi Province. In the coming decades, a large number of acid mine water from closed coal mines in Shanxi Province will overflow and enter the surface water, and the groundwater will become a "permanent pollution source" exerting an extensive and profound impact. With two major water systems—the Yellow River Basin and the Haihe River Basin, Shanxi Province is known as the "water tower of north China Plain". Every year, nearly 5 billion cubic meters of surface water flows overseas, but the polluted surface water and groundwater resources may threaten the ecological environment of other river basins. Significant progress has been made in the study of the mechanism of acid mine water and environmental problems. However, most of the study areas are concentrated in Guizhou and other places in Southwest China, and relatively little research has been conducted in Shanxi, one of the semi-arid and semi humid areas, where the problem of potential acid drainage in coal mines is more serious. In this paper, the Shandi River Basin, a typical abandoned coal mine area in Yangquan City, Shanxi Province, is selected as the study area. Through hydrogeological survey, hydrogeological drilling, hydrogeological profile and other methods, the formation lithology and hydrogeological conditions of the Shandi river basin are described. It is concluded that due to the influence of coal mining, the supply, runoff and discharge conditions of surface water and groundwater in Shandi river basin have changed fundamentally, compared with natural conditions. The recharge mainly migrates vertically through the water-conducting fracture zone generated by destruction. The runoff mainly flows transversely through the tunnel and the water conducting-fracture zone. The drainage takes on two main types, pit drainage and spring overflow. The pH value of acid mine water in mountain basin is 2.47-7.28, averaging 4.17. Ca²⁺ value is 141.81-525.00 mg·L⁻¹, averaging 408.48 mg·L⁻¹. SO₄²⁻ value is 1,084.55-13,683.47 mg·L⁻¹, averaging167.82 mg·L⁻¹. The hydro chemical types are Na-SO₄, Ca·Mg-SO₄, Mg-SO₄ and Mg·Ca-SO₄. The cations are mainly Ca²⁺, Mg²⁺ and Na⁺, and the anions are mainly SO₄²⁻. The acidification reaction rate of FeS₂ in closed reduction environment is slow, but fast in open oxidation environment. The pH value of AMD at the main discharge outlet of Shandi river basin is 2.47, and the SO₄²⁻ value is 3,848.5mg·L⁻¹. The groundwater in this basin is mainly supplied by the infiltration of precipitation, surface water and coal mine drainage in the mined-out area, and finally discharges from Shandi village. Covering an area of 58.4 km², Shandi river basin is a complete and independent water circulation system where open-pit mining, in-situ mining and closed pit mining are integrated. In this circulation system, water in all kinds converges in the mined-out area, and then discharges. With unique hydrogeological conditions, the basin is a field monitoring and testing site specific for the production, migration and discharge of acid drainage in coal mines. At the same time, it is also a research site for prevention and treatment.