Quality of water from subsidence area: a case study in the Luling coal mine, northern Anhui Province, China

Coal mining has greatly contributed to the development of the economy and society in China with the social-economic development, because more than 50% of the primary energy was contributed by coal during the recent years. However, more than 90% of the coal production comes from underground (Yao et al. 2010), and a series of geological and environmental problems have been induced (e.g. contamination of water, soil and air, ground subsidence and other engineering geological hazard) (Li et al. 2007), which have severely affected the life of human, ecological environment and the development of regional economy in mining areas (Yang & Liu 2012).

Among those problems, subsidence related to coal mining is a common environmental geologic hazard, which can not only destroy the construction and vegetation, but also have an influence on the surface water and groundwater systems that may seriously deteriorate the ecological environment of mining areas (Yin 1997). And now, it has been considered to be the most serious one among the environmental problems related to coal mining: the increasing of the area of the subsidence is up to 130 km² per year and the subsidence land area has raised up to 700,000 km² by the end of 2006 (Meng et al. 2009).

Water shortage in China is serious, especially in coal mining areas. Previous studies revealed that more than 71% of the coal mining areas in China were lacking of water and 40% of them were serious (Gui et al. 2011). And therefore, the large amount of water in the subsidence area might be a good choice for solving the issue except for the water from underground (Gui et al. 2009). And therefore, the restoration of the subsidence area is becoming more and more important and attracted large number of studies. These studies suggested that the main techniques related to restoration include improvement of soil, vegetation, and applications of soil animals and micro-organisms (Liu & Lu 2009).

However, the subsidence area is a system can be affected by multi factors, including natural and anthropogenic (e.g. water rock interaction, precipitation, evaporation and waste dispose). Therefore, before the application of the water in the subsidence area, the quality evaluation, as well as the source of the chemical constitutes in the water should be firstly understood, because these studies will determine whether the water can be used or, how to use.

In this study, a total of forty-two water samples have been collected from the subsidence water area in the Luling coal mine, a representative coal mine in northern Anhui Province, China, and their major ion concentrations have been measured for evaluating the quality of the water (drinking and irrigation) and identifying the main mechanisms controlling the chemical variations of the water. The study can provide information for the usage of the water and the management of the subsidence area.

The main coal seams in the coal mine are 8th and 10th. After coal mining, eight subsidence areas have been formed before 2002, including 8₁, 8₂, 8₃, 8₄, 8₆, 8₈, 10₁ and 10₂ (Wang et al. 2002). And the area has been increased up to 1,000 acres today. Most of the subsidence depth is higher than 7 m, and the water depth is near 3 m (Figure 1).

A total of forty two water samples have been collected from the 8₁, 8₂, 8₃, 8₄ subsidence water areas (12, 13, 10 and 7 samples from pools I, II, III and IV in the Figure 1, respectively). Concentrations of eight kinds of major ions (Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, SO₄^2–, and CO₃²⁻) and total dissolved solids (TDS) have been analyzed. The analytical methods are as follows: Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻ and were analyzed by ion chromatography, whereas and CO₃²⁻ were analyzed by acid–base titration in the Engineering and Technological Research Center of Coal Exploration, Anhui Province, China.

Comparatively, samples from the pool I have the lowest mean concentrations of K⁺ (3.03 mg/l), Ca²⁺ (14.2 mg/l) and Mg²⁺ (13.1 mg/l) but highest mean concentration of (113 mg/l), samples from the pool II have the lowest mean concentrations of Na⁺ (116 mg/l), Cl⁻ (44.7 mg/l), (74.9 mg/l) and (365 mg/l). However, samples from the pool III have the highest mean concentrations of Na⁺ (149 mg/l), Ca²⁺ (24.8 mg/l), Mg²⁺ (20.6 mg/l), Cl⁻ (54.2 mg/l) and (571 mg/l), whereas samples from the pool IV have the highest mean concentrations of K⁺ (5.84 mg/l).

Classification of hydro-chemical types for groundwater is important because of the dominant anion species of water change systematically from ⁠, to Cl⁻ as groundwater flows from the recharge zone to the discharge zone (e.g. Toth 1999). However, it is also important for surface water because it can provide information for determine the main mechanism controlling the water chemistry as evaporation controlling tends to obtain higher concentrations of and Cl⁻ relative to ⁠.

Classification of water in this study is based on the concentration of cations and anions by using Aquachem and Piper diagram, and the result is shown in Figure 2. The result indicates that all of the water samples are classified to be Na–HCO₃ types, suggesting that evaporation is not important for the chemistry of these water samples. Moreover, slight differences can be found in Figure 2 that samples from pool I have relative higher concentrations of relative to other samples, which might be an indication of higher contribution of evaporation in pool I relative to other pools.

Based on results, the quality of the water for drinking can be classified to be five classes (excellent <50, good 50–100, poor 100–200, very poor 200–300 and unsuitable >300). The WQI for the water from the subsidence area in this study have WQI range from 29.4 to 39.3 (mean = 35.2), suggesting that these water are excellent for drinking when considering about only their major ion concentrations. However, the water from the four pools have different water qualities, samples from pool II have the lowest WQI (mean = 30.5), whereas samples from the pool III have the highest WQI (mean = 38.9).

There are several parameters have been applied for quality evaluation of irrigation, including sodium absorption ratio (SAR), percentage sodium (% Na) and permeability index (PI), residual sodium carbonate (RSC), Kelly's ratio and magnesium ratio. In this study, the most popular applied parameters (SAR and RSC) have been chosen for quality evaluation for irrigation.

The index used is the Sodium Adsorption Ratio (SAR) that expresses the relative activity of sodium ions in the exchange reactions with the soil. This ration measures the relative concentration of sodium to calcium and Magnesium. SAR is an important parameter for determining the suitability of groundwater for irrigation. Excess sodium concentration can reduce the soil permeability and soil structure (Todd 1995). Irrigation using water with high sodium adsorption ratio may require soil amendments to prevent long-term damage to the soil (Michael et al. 2008). SAR is a measure estimated by 2 × Na⁺/(Ca²⁺ + Mg²⁺) (in meq/l). The calculated values of SAR in this study ranges from 4.37 to 7.07 (mean = 5.54), and all of the samples were within permissible limit (<10).

RSC exists in irrigation water when the carbonate (CO₃) plus bicarbonate (HCO₃) content exceeds the calcium (Ca²⁺) plus magnesium (Mg²⁺) content of the water. An excess value of RSC in water leads to an increase in the adsorption of sodium in soil (Eaton 1950). The results of this include direct toxicity to crops, excess soil salinity (EC) and associated poor plant performance, and where appreciable clay or silt is present in the soil, loss of soil structure and associated decrease in soil permeability. RSC is a measure employed by calculating -(Ca²⁺ + Mg²⁺) (Ragunath 1987). RSC value <1.25 meq/l indicates good water quality. If the value of RSC is between 1.25 and 2.5 meq/l, the water is slightly suitable while a value >2.5 meq/l the water is considered as unsuitable for irrigation. RSC values in this study range from 4.66 to 8.99 meq/l (mean = 6.02 meq/l) and suggesting that all of the water samples cannot be used for irrigation purpose.

Such results indicate that the water in this study have suitable Na⁺ concentrations relative to Ca²⁺ and Mg²⁺, however, they have higher concentrations of and relative to Ca²⁺ and Mg²⁺. The former will not lead to the decreasing of infiltration and permeability of the soil, whereas the latter can drastically reduce its infiltration capacity. And therefore, Ca²⁺ and Mg²⁺ should be added before the application of the water for irrigation, because it can balance additional and in the water.

For getting more information about the source of major ions in the water, statistical analysis, including factor analysis and EPA Unmix model have been applied, which have long been used for quantifying the source of major ions in the groundwater (Sun 2015). With eigenvalue higher than one after varimax rotation, two factors have been obtained (Table 2). As can be seen from the table, factor one has high positive loadings of Mg²⁺, Ca²⁺ and ⁠, which accounts for 62.0% of the total variance, whereas factor two has high loadings of Na⁺, Cl⁻ and ⁠, which accounts for 27.0% of the total variance. According to the discussions above, as well as the relationships between major ions during dissolution or weathering of minerals, factor one and two can be explained to be carbonate and evaporate factors, respectively.

Based on the major ion concentrations of water samples from the subsidence area in the Luling coal mine, northern Anhui Province, China, the following conclusions have been obtained:

1. Water samples from different subsidence pools have different concentrations of major ions, whereas all of them are classified to be Na-HCO₃ type;

2. All of the samples have WQI range from 29.4 to 39.3 (mean = 35.2), suggesting that they are excellent for drinking when considering about only their major ion concentrations;

3. SAR and RSC values for the samples are 4.37 to 7.07 (mean = 5.54) and 4.66 to 8.99 (mean = 6.02), respectively, lower than the permissible limit of SAR (<10) but higher than the permissible limit of RSC (<2.5), respectively, which is due to the higher concentrations of and relative to Ca²⁺ and Mg²⁺;

4. Gibbs diagrams and correlations between Na⁺ normalized Ca²⁺, Mg²⁺ and suggest, as well as statistical analysis indicate that different kinds and extents of water rock interactions are responsible for the chemical variations of water samples in this study.

This work was financially supported by National Natural Science Foundation of China (41302274), the Foundation of Scholarship Leaders in Suzhou University (2014XJXS05) and the Foundation of Scientific Platform in Suzhou University (2014YKF05).