**1. Introduction**

Coal is a complex system, which contains most elements in the periodic table. The origin of the coal was organic matter containing virtually every element in the periodic table, mainly carbon, but also trace elements. The elements with relative higher content in the coal and host rock, such as iron (Fe) and aluminum (Al), which usually take 1–20% of the rock, respectively, and sodium (Na), potassium (K), calcium (Ca), magnesium (Mg), which are usually in the range of 0.01–10% of the rock, respectively. The trace elements refer to the elements at the 10–10,000 ppm levels in coal, rocks, and soil, etc. A variety of chemicals are associated with coal that is either found in the coal or in the rock layers that lie above and beneath the seams of coal [1]. Some of the trace elements are of great health concern. For example, lead (Pb) accounts

for most of the cases of pediatric heavy metal poisoning and makes it difficult for children to learn, pay attention, and succeed in school. Mercury (Hg) exposure puts newborns at risk of neurological deficits and increased cardiovascular risk in adults. Arsenic (As) could cause heavy metal poisoning in adults and does not leave the body once it enters.

Coal mining has caused global environmental concern due to mainly two reasons—first, the coal and host rock contains multiple kinds of toxic trace elements, some of which are of great environmental and health issues, most of them (As, Cd, Co, Cr, Cu, Mn, Ni, Pb, Se, Sn V, and Zn) are associated with inorganic matter [2, 3]; second, the trace elements may be released through combustion and water-rock interaction [3–9].

The coal mine water, containing toxic trace elements, has influenced the water quality of both the groundwater and surface water in China. To control the contamination of trace elements, a lot of efforts have been making in both research and management. According to the Chinese national standard GB/T 19223-2015, the coal mine water is defined as bursting water, infiltrating water from surface water, and working produced water, during coal mining activity. The water is classified into acid (pH < 6.0), neutral (6.0–9.0) and alkaline (pH > 9), low- (<1000 mg/L), medium- (1000–6000 mg/L), and high-mineralized water (>6000 mg/L), and low- (<50 mg/L), medium- (50–500 mg/L), and high-suspended (>500 mg/L) coal mine water, regarding pH value, total dissolved solids, and suspended matter, respectively. Trace elements released from the coal and rock may contaminate surface and groundwater, including selenium (Se), As, Pb, fluorine (F), Hg, etc., leading to some different unique characteristics of the coal mine water. However, the releasing patterns are relatively similar among the coal mine waters. In the coal-bearing seam, the primitive environment is H-rich and reductive, where some reductive minerals are stable, such as pyrite, chalcopyrite, and sphalerite. While the coal and rock seam contact with air, the Eh value of the surrounding environment is elevated, and the minerals are oxidized [10, 11]. Through this process, the pH value may be reduced, accompanying the release of metal elements into the water, and high concentrations of metal trace elements in the water [12–14]. However, the neutral and alkaline mine water is also common, because of the dissolution of alkaline minerals, such as calcite and dolomite. The net effect of which determines the pH value of coal mine water produces a high mineralization value [12, 15].

Besides the water parameters, the occurrence of trace elements also influences its migration [16–19]. Main minerals in coal include quartz, clay, sulfur-contained minerals, and a lesser number of feldspars and carbonates [20, 21]. As, Cr, Pb, Hg, Mo, Zn, and Sb were found to be enriched in coal compared with continent crust [22–25], while compared to coal, host rock and gangue rejected on the land of coal can release up to 10 times toxic elements into water [2, 26–28].

The migration behavior of trace elements is controlled by two factors, the trace element occurrence and the surrounding environment. However, migration patterns and mechanism of trace elements into a surrounding water body are complex and different depending on the investigating sites. Traditional methods to investigate this process are based on geochemical surveys and testing. The information and pattern behind the data matrix are hard to identify. Along with the development of machine learning, multivariate analytical technology has been applied in some different areas of the geochemical research, the fourth paradigm for the research is becoming a more and more powerful tool to find a solution among the mass data. The multivariate analysis has been used to study the water characteristics [29], source [30, 31],

groundwater pathway [32–34], etc. By using the method of multivariate technology, it is possible to disclose the leaching mechanism from the view of trace element occurrence and leaching behavior.
