Characteristics and evolution of hydrochemical compositions in groundwater under mining-induced disturbance: a case study

Deep groundwater plays a key role at present. Especially, in the coal mine district, groundwater system is not only the mainly water supply for domestic, agricultural and industrial use, but also influencing the coal mine exploration safely. Therefore, the studies focused on the groundwater quality, ions source, hydrochemical characters and statistical analysis are rich (Kumar et al. 2009; Chen et al. 2013; Ramkumar et al. 2013), what based on the test of chemical composition of groundwater. However, research related the hydrochemical evolution of groundwater is limited, for the scarcity historical data. Moreover, the evolution characteristics of hydrochemistry in deep groundwater are important for understanding the impact of human activities, what could be as a basis to forecast the change regulation of hydrochemistry characters of groundwater.

The purpose of this study is to discuss the hydrochemical characteristics of deep groundwater and the evolutionary character of hydrochemistry, what based on the historical data about deep groundwater collected from Qinan coal mine, Anhui province, China. The results will be used as a basis for understanding the evolution of hydrochemistry of groundwater, making sustainable groundwater development schemes and identification the groundwater source.

Qinan coal mine, belonging to Huaibei mining area, is located at northern Anhui Province, China, which constituted by about thirty couples mines (Figure 1). The climate in the area belongs to marine-continental climate, with an annual average temperature of 14.9 °C.

Previous research showed Qinan coal mine mainly includes three aquifers: the quaternary aquifers (QA), coal bearing aquifer (CA) and limestone aquifer (LA), all the three aquifers have threat to the coal mine exploration (Gui & Chen 2007). The QA is constituted by yellow mudstone, sandstone and conglomerate, with a depth ranging from 280 to 300 m. The CA is characterized by mudstone, siltstone and sandstone, with a depth between 300 and 700 m. LAs are mainly composed of limestone with clastic rocks, which belong to Taiyuan formation and Ordovician.

Geological engineer must establishment the hydrologic parameter for the coal mine safely working, which contain the water level, water quantity and water chemistry information. Thus, the historical hydrochemical data of groundwater samples collected from three aquifers (QA, CA, LA) are possible. In total 66 groundwater samples data were gathered from Qidong coal mine, and the groundwater samples are as flows: 7 samples from QA, 32 samples from CA and 27 samples from LA. All the 26 samples had been analyzed for major ions.

The historical chemistry data of deep groundwater, distributing in 15 years between 1997 and 2011, were collected from Qinan coal mine. To reveal the hydrochemical characteristics of groundwater, the chemistry data of deep groundwater were plotted on a piper diagram (Figure 2), some information could present. In general, the concentrations of are low for all groundwater samples, whereas the contents of Cl⁻ and are high with diversity degrees for groundwater. The cations are characterized by higher concentration of Na⁺ + K⁺, with the lower value of content of Ca²⁺ and Mg²⁺.

In detail, the groundwater samples collected from three different aquifers are presented diverse features. The groundwater samples from SA were mainly constituted by Na⁺ + K⁺, Cl⁻ and ⁠, the groundwater could by described the Cl·HCO₃-Na·K type. However, the QA groundwater have higher concentrations of Ca²⁺ and Mg²⁺, comparing with the groundwater from CA, the anions were dominated by Cl⁻ and ⁠, what are similar with the CA. The concentrations of Ca²⁺ and Mg²⁺ were higher in the groundwater from LA. In brief, the concentrations of Na⁺ + K⁺ were decreasing as follows: LA < QA < CA, whereas the content of Ca²⁺ and Mg²⁺ were increasing from CA to LA in opposite.

For understand the hydrochemical evolution characters of groundwater in Qinan coal mine from 1997 to 2011 effectively, the historical data were divided into four periods according to the scale of production of Qinan cola mine, and the four periods are Pre-2001, 2001–2004, 2005–2008 and 2009–2011, respectively. The chemical data of groundwater in each page are presented in Table 1 and Figure 3.

From Table 1 and Figure 3, a series views could be concluded, that concentrations of Na⁺ + K⁺ and Mg²⁺ in QA are increased from 1997 to 2011, with the average value from 363.68 mg/kg to 581.3 mg/kg and 3.4 mg/kg to 67.63 mg/kg, respectively. However, the content of Ca²⁺ varied little, ranging from 93.39 to 100.11 mg/kg. In the anion, concentrations of Cl⁻ and decreased, while the content of increased, with the highest value 528.91 mg/kg in the period 2005–2008.

In the CA groundwater, the content of Na⁺ + K⁺ varied from 294.81 mg/kg to 393.11 mg/kg, while the Ca²⁺ and Mg²⁺ showed increase trend, with the concentrations increased from 13.8 mg/kg to 24.57 mg/kg and 6.03 mg/kg to 22.72 mg/kg, respectively. The anions are presented by the decreasing typical in Cl⁻ and and increasing trend in and ⁠.

All the cations concentrations of groundwater in LA are decreasing from 1997 to 2011, while the Na⁺ + K⁺ have the highest value in the 2002–2004 period. The anions, in LA groundwater presented the similar characteristics as the cation, otherwise the concentrations of were increased first, then decreased.

In general, the chemical compositions of groundwater collected from three aquifer were varied obviously. If the first period Pre-2001 could be as a natural standard, for the coal mine just exploitation, all the groundwater aquifers are influenced by coal mine exploitation clearly. The increased concentration of Na⁺ + K⁺ in QA could be caused by the enhance liquidity according to the coal mine exploitation, otherwise in the CA the Na⁺ + K⁺ decreased. The groundwater in LA is characterized by rich concentration in Ca²⁺ and Mg²⁺; however, the concentration of Ca²⁺ and Mg²⁺ declines along with the coal mining. All the phenomenon could reflects that water-rock interaction between groundwater and surrounding rock are sufficient in the beginning of coal mine exploitation, with the aquifer relative static hysteresis. And all the groundwater samples were characterized by the typical ions released from surround rock. However, with the coal mining exploitation, the more open environment of groundwater result in decreasing of typical ions in groundwater, while the other ion concentration ascension.

Previous studied showed multivariate statistical analysis was an efficient way to display complex relationships among many variables and their roles (Chen et al. 2013). To reveal the relation between the parameter, the principal component analysis (PCA) were obtained in the study, the result are present in Table 2.

The rotated PCA loading are present in Table 2, what could be usually used for obtaining the detailed statistical information, three principal components were emerged, with more than 82.9% of cumulative variance. The parameter of Ca²⁺, Mg²⁺ and are all have high value in PC1, with the value 0.93, 0.92 and 0.73, especially the values of Ca²⁺and Mg²⁺ are very higher, what could be revealed the factor is the dissolution of limestone, dolomite and gypsum dissolution.

PC2 accounts for 24.15% of total variance with the eigenvalue 1.69, what suggested the PC2 could be as the carbonation process or desulfurizing process, for the high loading for Cl⁻ and ⁠, the value were 0.94 and 0.88. PC3 account 14.55% of variance with the eigenvalue 1.02, indicated the weathering process of feldspar minerals weathering by the carbonate acid, for the Na⁺ + K⁺ and have high value.

To reveal the hydrochemical evolution process of groundwater collected from Sunan cola mine, all the samples factor score values were plotted on the score diagram (Figure 4), in which the diverse shapes and colors represent the different aquifers and periods, respectively.

It can be seen from Figure 4, the groundwater in QA are influenced by three factors. The score values of three factors are all be zero before 2001 years, while PC1 and PC2, PC1 and PC3 plays a key role in the 2001–2004 and 2005–2011 periods, what indicated dissolution of limestone and dolomite are obviously in the periods 2001–2004, however, weathering of feldspar are important hydrochemical process after 2005 years. The groundwater in CA does not showed regular clearly, what reflected that so many factors influenced the hydrochemistry composition in CA, such as carbonate mineral, FeS₂, cation exchange adsorption effect and so on. The groundwater in LA are influenced by PC2 and PC1 clearly at the beginning, but the two factors influencing decreased obviously along with the coal mine exploitation, especially the effect of carbonation.

Sixty-six historical chemical data of groundwater samples were collected from 1997 to 2011 in Qinan cola mine, Anhui Province, China, the hydrochemical characteristics, evolution of hydrochemistry and statistical analysis were carried out, and a series conclusion could be obtained:

All the groundwater are characterized by higher concentration of Na⁺ + K⁺, with the lower value of content of Ca²⁺ and Mg²⁺. The concentrations of are low for all groundwater samples, whereas the contents of Cl⁻ and are high with diversity degrees. The concentrations of Na⁺ + K⁺ were decreasing as follows: LA < QA < CA, whereas the content of Ca²⁺ and Mg²⁺ were increasing from CA to LA in opposite.

The concentrations of Na⁺ + K⁺, Mg²⁺ and in QA are increased from 1997 to 2011, while the Cl⁻ and decreased clearly. The groundwater in CA are characterized by the increased concentration of Ca²⁺, Mg²⁺, and ⁠. Whereas the content of Ca²⁺ and Mg²⁺ are decreased obviously in LA groundwater, with the concentrations of decreased finally.

Three principle component factors could be extracted through statistical approach, PC1 was affirmed the dissolution of limestone, dolomite and gypsum dissolution. PC2 could be as the carbonation process or desulfurizing process, while PC3 indicated the weathering process of feldspar minerals weathering by the carbonate acid.

The study was supported by the National Nature Science Foundation of China (41373095), the Program for Innovative Research Team in Suzhou University (2013kjtd01) and the opening scientific research platform in Suzhou University (2013YKF01).