The geochemical (alkaline earth) characteristics of groundwater in the Taiyuan Formation limestone aquifer in Huaibei coalfield

Deep groundwater often contains much geological information, after flowing through or remaining in the aquifer for long periods (Zouari et al. 2011; Ma et al. 2015). This information is very significant in relation to surrounding rock types, groundwater runoff conditions and hydrogeochemical processes. Many studies have focused on the hydrochemical characteristics of groundwater (Helstrup et al. 2007). Groundwater is of great significance for economic and societal development. With increasingly serious water shortages, deep groundwaters are becoming the main water supply for domestic, agricultural and industrial use, especially in the North China Plains.

The Huaibei coalfield lies on the Huaibei Plain, an important part of the North China Plain. With its rich coal reserves and good mining conditions, the coalfield is one of China's major coal sources. In the mining area, deep groundwaters are both a precious resource, subject to exploitation for urban use, and a threat to coal mine safety. Many studies, focusing on groundwater flow, hydrochemical evolution and water-rock interactions, have been conducted in relation to sustainable groundwater development and mining safety (Gui et al. 2011; Chen et al. 2014).

Previous studies show that major ion chemistry, particularly molar ion ratios, is useful in assessing solute sources and characterizing hydrogeochemical evolution in aquifers (Currell et al. 2011). The alkaline earth metals are the elements in group II A, including beryllium, magnesium, calcium, strontium and barium. Although they belong to the same group in the periodic table, they vary widely in abundance and in their behavior in solution. Although these alkaline earth metals are used widely for industrial applications, there are no major anthropogenic sources for them in groundwater (Minnesota Pollution Control Agency [MPCA] 1999). Thus, using these elements to trace groundwater circulation processes and hydrochemical evolution is feasible in theory.

In this study, a number of deep groundwater samples were collected for determination from the Taiyuan Formation limestone aquifer in the Huaibei coalfield, Anhui Province, China. The study's main objectives were: (1) to define the alkaline earth elements' characteristics in groundwater from the aquifer; (2) to constrain influencing factors, and the sources of ions and elements in the groundwater; and, (3) to understand the distribution of alkaline earth elements and groundwater flow.

Huaibei coalfield is in Northern Anhui Province, China, and incorporates 20 coal mines (Figure 1). The district's geological basement comprises Archean and Proterozoic metamorphic rocks, covered variously by strata from the late-Proterozoic to the Permian. The climate in the study area is marine-continental. In the period 2007 to 2017, the average July maximum and January minimum temperatures were 31.8 and −3.2 °C, respectively. Average annual precipitation and evaporation are about 867.0 and 832.4 mm, and more than 50% of the precipitation falls from June to September (Chen et al. 2014). The area is short of surface water, and deep groundwater is the main source for both industrial and domestic use.

As shown in Figure 1, the principal tectonic direction is to the north and east in the coalfield (Zheng et al. 2008). The coalfield is divided into northern and southern mining areas, with the boundary marked by the east-west striking Subei fault. The northern mining area can also be divided into the Suixiao and Zhahe areas, and the southern into the Suxian, Linhuan and Guoyang areas. The boundaries between the different areas are all faults or folds.

There are four aquifers in the Huaibei coalfield (Figure 2):

• the Quaternary aquifer (QA) comprises conglomerate, sand and clay;

• the coal-bearing sandstone aquifer (SA) consists of sandstones, siltstones and coal seams;

• the Taiyuan Formation limestone aquifer (TA), which is of Carboniferous age, comprises limestone; and,

• the Ordovician limestone aquifer (OA) consists of gray to dark-gray, thick-bedded limestone.

TA, which lies about 800 to 1,200 m below surface, was the main object of the study.

Twenty-nine groundwater samples were collected from the TA, each being put into a 2.5 L plastic bucket previously rinsed three times with the sample water. The location (longitude and latitude), temperature, pH, conductivity, and total dissolved solids (TDS) were all measured and recorded on site. Every fifth sample was taken in duplicate to ensure the accuracy of the determinations. All samples were analyzed for the major ions, plus beryllium, strontium and barium – the major ions were determined in the analytical laboratory of the Department of Coal Geology, Anhui Province, China. The analytical methods used were: K⁺ + Na⁺ by atomic absorption spectrometry, SO₄²⁻ and Cl⁻ by ion chromatography, Ca²⁺, Mg²⁺ by EDTA titration, alkalinity by acid-base titration.

Beryllium, strontium and barium were determined after pre-concentration by liquid-liquid extraction, and analyzed by inductively coupled plasma mass spectrometry using groundwater quality test method DZ/T0064.80-93 (Ministry of Environmental Protection 1993) by Wuhan Sample Solution Analytical Technology Co., Ltd. The Piper diagram and statistical analyses of the data were completed by software Aq.Qa and SPSS (version 19), respectively.

The major ion concentrations in the samples are presented in Table 1, and the hydrochemical characteristics shown in Figure 3. The ionic balances were checked with AqQa software. The groundwaters are alkaline, with pH values between 7.40 and 10.10. The TDS varied widely, with minimum and maximum reported concentrations of 123 and 5,520 mg/l. Within this, the Na⁺ + K⁺ concentration is generally the highest, followed by Ca²⁺ and then Mg²⁺. TDS concentrations also varied widely between the different coal mining areas. Typically the Zhahe area has the lowest TDS content, followed by Suxian, while the Linhuan area reported the highest. Groundwater TDS concentrations in samples from the Guobei and Suixiao areas typically fell between those of Suxian and Linhuan.

The Be, Sr and Ba concentrations in the groundwater samples are listed in Table 1. The Be concentrations vary considerably – from 0.03 to 0.0001 μg/l – and are lower than those of both Sr and Ba, and far below the Chinese drinking water standard (2 μg/l). Previous research suggests that the pH of the water is an important factor affecting the Be content, because an acidic environment favors Be migration, so its concentration in water bodies decreases sharply with rising pH (Vesely et al. 2002).

As shown in Table 1, the groundwater Sr concentration ranges from 13.08 to 18,168.5 μg/l. Groundwater Sr concentrations are not generally affected by human activity (MPCA 1999), variations often arising from the surrounding rock-types, runoff conditions and water-rock interaction time. Sr and Ca have similar ionic/atomic radii, and are often found isomorphically in minerals, especially in carbonate rocks where Sr is widespread. Thus, a positive correlation between groundwater Ca, Mg and Sr concentrations is expected.

Ba concentrations vary in the study are from 2.43 to 215.21 μg/l. Ba has a larger ionic radius than Sr and is not isomorphous with Ca. Other work has shown that the content of Ba in karst waters tends to increase with increasing Mg concentration (Ning et al. 2004).

In general, recharge, surrounding rock types and water-rock interactions influence hydrogeochemical characteristics. This is particularly true for groundwater flow rates and aquifer residence times, which affect the extent of water-rock interactions. Evaporite and carbonate dissolution, and silicate weathering are three common processes that contribute to the concentrations of solutes in groundwater. As shown in Figure 4(a) and 4(b), groundwater quality in the area is controlled largely by silicate weathering and carbonate dissolution (Gaillarde et al. 1999).

Previous studies have shown that the plot of Ca²⁺ + Mg²⁺ versus SO₄²⁻ + HCO₃⁻ would be close to a 1:1 line, if calcite, dolomite and gypsum dissolution were the dominant reactions in a system (Fisher & Mullican 1997). Ion exchange processes, however, tend to shift the points to the right because of the excess SO₄²⁻ + HCO₃⁻ (Fisher & Mullican 1997). Figure 4(c) shows that most groundwater samples from the study area plot near the 1:1 line, indicating that carbonate dissolution is dominant in the water-rock interactions. The small number of groundwater samples that plot to the right of the 1:1 line suggest that ion exchange might also be occurring.

Plots of Ca²⁺ versus Mg²⁺ are generally used to identify calcite and dolomite dissolution in groundwater. In Figure 4(d), most samples plot near the 1:1 line, with some samples above it. These features suggest that calcite dissolution is dominant in the hydrochemical process. This is consistent with regional geological conditions, the TA is mainly composed of limestone, with some dolomite in the upper and lower horizons.

As noted, the Be concentration varies widely, at low concentrations. In general, the pH and the presence of aluminum are the two main factors affecting the concentration of Be in groundwater (Gu et al. 1998). There is no mineral or other source containing aluminum in the district, which eliminates its potential effect on Be, so the main influence is expected to be pH. Figure 4(e) shows that the Be concentration has a negative relationship with pH, Be concentration decreasing sharply with increasing pH.

Because of its larger ionic radius, Ba cannot form isomorphic minerals with Ca, and the Ba concentration in limestone is low, so there is no obvious relationship between Ba and Ca in the groundwater. Ning et al. (2004) report that the Ba content in karst waters tends to increase with increasing Mg content. There is no obvious relationship between the concentrations of Ba and Mg in the study area and the phenomenon is not seen (Figure 4(f)).

In karst groundwaters, processes such as carbonate dissolution lead to the release of Ca, Mg and Sr, justifying the use of the Ca/Sr and Mg/Sr ratios (Negrel & Petelet-Giraud 2005). Figure 4(g) shows the strongly positive relationships between Ca/Sr and Mg/Sr in the groundwater samples, which suggests that Ca, Mg and Sr have the same sources. The correlations of 0.73 and 0.86, between Ca and Mg, respectively, and Sr support this view (Table 2). Thus, the concentration of Sr is expected to increase with increasing Ca content, and the strontium is mainly released from calcium containing minerals (Figure 4(h)).

Multivariate statistical analyses – e.g., cluster and correlation analysis – are commonly used to show inter-parameter relationships efficiently (Stetzenbach et al. 2015). Correlation analysis was applied in this study to indicate the characteristics and relationships between the major ions and other parameters. The results are presented in Table 2, which shows the positive correlation (0.79) between Cl⁻ and Na⁺ + K⁺, and between Ca²⁺ and SO₄²⁻ (0.88). Ca²⁺ and Mg²⁺ have an obvious positive correlation of 0.90. Sr shows positive relationships with Ca²⁺, Mg²⁺ and TDS, of 0.73, 0.86 and 0.80, respectively.

Because of water-rock interactions, the hydrochemical composition of groundwater often changes during circulation. In general, the major ion and TDS concentrations are often used to identify groundwater flow-lines and circulation, because the major ions are easily released into groundwater through water-rock interactions. Thus, the TDS should reflect the flow-lines and circulation. In soluble lithostratigraphy, low TDS concentrations in groundwater indicate weak water-rock interactions, often arising from short groundwater residence times in the aquifer. Groundwater often flows from areas with relatively lower TDS concentrations to areas in a hydrogeological unit where it is higher, and the TDS contour density reflects the degree of dissolution from the surrounding rock into the groundwater.

The TDS concentration contours for groundwater in the Huaibei coalfield are shown in Figure 5. The concentration increases from north to south, reflecting the groundwater flow largely from north to south. In the south, the groundwater flows from the both east and west into the middle. Taking account of this and other groundwater information for the aquifer (Gui & Chen 2007), it appears that the aquifer recharge area is in the north of the coalfield. Thus, the Zhahe and Suixiao areas are the main groundwater recharge zones, and Linhuan the main discharge area.

As noted, Sr concentrations increase along karst water flow paths, so that using them to identify groundwater circulation is feasible in theory. Sr also has a positive correlation with TDS – correlation 0.80 (Table 2). The Sr concentration contours for groundwater in the TA in the coalfield are shown in Figure 6. The groundwater flows from areas of lower to higher Sr concentration, and the contour density reflects the degree of dissolution of the surrounding rock by groundwater. The groundwater flow-paths between the north and south of the mining area are not obvious in Figure 6. In the south, groundwater flows are shown from both east and west into the middle, which is consistent with the deductions from the TDS contours.

Major ion chemistry, in particular molar ion ratios, is useful in defining the sources of solutes and the hydrogeochemical evolution of groundwaters. As noted, while Sr concentrations increase in karst water as water-rock interaction times increase, those of Ca and Mg rise to a lesser extent. Thus, the Sr/Ca and Sr/Mg ratios are ideal natural tracers, and are commonly used to determine groundwater flow conditions. In general, the ratios increase during flow between the recharge and discharge areas – being highest when groundwater residence time in the aquifer is longest. The two highest Sr/Ca ratios found in the study area are 340.1 and 116.6 in the Haizi and Yuanyi mines, respectively, suggesting that these groundwaters have been in the aquifer for the longest time, which is consistent with the indications the TDS and Sr contour maps.

All of this is supported by the fact that the groundwater is mainly of bicarbonate type in the Zhahe and Suixiao areas. However, in the Linhuan and Suxian areas the groundwaters are characterized by halide and sulfate, respectively. In other words, the groundwater type changes from bicarbonate to sulfate and halide between the recharge and discharge areas.

The major ion and alkaline earth metal concentrations were determined in 29 groundwater samples from the Taiyuan Formation limestone aquifer in Huaibei, China. Traditional graphic and correlation analyses were used to study the groundwater's hydrochemical characteristics, ion sources, influencing factors, hydrochemical distribution and groundwater flow. It was concluded that:

• (1)

  The groundwater is alkaline, with pH between 7.40 and 10.10. The TDS concentration varies from 123 to 5,520 mg/l, and the different areas have diverse hydrochemical characteristics. The concentrations of Be, Ba and Sr range from 0.0001 to 0.03 μg/l, 2.43 to 215.21 μg/l, and 13.08 to 18,168.5 μg/l, respectively.

• (2)

  Carbonate dissolution is the dominant water-rock interaction process, with ion exchange in some places. Groundwater pH is the main factor controlling the concentration of Be in the groundwater, with negative correlation between pH and Be. Ca, Mg and Sr have the same sources, as shown by the plots of Ca/Sr and Mg/Sr, and the correlation analysis. The Ba concentration in the groundwater varies widely, and the influencing factors need further study.

• (3)

  The TDS and Sr concentration contour diagrams could be used to identify groundwater flow-paths and circulation. The main groundwater recharge areas are the Zhahe and Suixiao districts, and Linhuan the discharge zone. The Sr/Mg and Sr/Ca ratios are highest where the aquifer residence time is longest, and the two highest values of these ratios are 340.1 and 116.6 in the Haizi and Yuanyi coal mines, respectively.

The study was supported by the Natural Science Foundation of Anhui Province (1708085QE125), the National Natural Science Foundation of China (41773100), the Postdoctoral Research Project of Anhui Province (2016B093) and Natural Science Projects of Colleges and Universities in Anhui Province (KJ2018A0450).