In order to understand the isotopic characteristics of coal mine water in diverse aquifers, ten groundwater samples were collected from three aquifers – Quaternary (QA), Coal-bearing (CA), and Limestone (LA) – in Xutuan coal mine, Anhui Province, China. The geochemical characteristics of major ions and isotopes were determined, and the results showed that all of the groundwater samples are HCO3-Na·K or Cl-Na·K types. The concentrations of Na+ + K+ decreased in the order CA > QA > LA, whereas the content of Ca2+ and Mg2+ increased from CA to LA. Groundwater in LA is meteoric in origin, while that in QA is clearly influenced by surface water. The δ13Cdic and δ18Odic in groundwater samples from LA, QA and CA are influenced by the surrounding strata, CO2 and microorganism activity, respectively. The strontium concentration decreases in the sequence LA > CA > QA, but water samples from CA give the highest value of 87Sr/86Sr. Strontium is released by dissolution from the surrounding rock, and the 87Sr/86Sr ratio could be used to identify the groundwater source.

INTRODUCTION

In coal-mining regions, deep groundwater systems play important roles in water supply and coal mine exploitation. Many studies have been completed in coal-mining regions about groundwater quality, hydrochemical characteristics, water rock interactions and groundwater source identification (Kumar et al. 2009; Sun & Gui 2012; Chen et al. 2014; Gui 2014), based on the concentrations of major ions, trace elements and stable isotopes. All could be used as a basis for sustainable groundwater development. However, many different hydrogeological effects arising from aquifers, the host rocks and groundwater flow, determine the hydrochemistry. Hydrochemistry studies are necessary, especially concerning the groundwater's isotopic characteristics, in all coal-mining regions. Such work has not been done before in the Xutuan coal mine.

Major ion concentration ratios with trace elements – e.g., Ca2+ and Sr – in groundwater can present some useful relationships, as can the 87Sr/86Sr ratio. Previous studies have shown that strontium concentrations in groundwaters differ and that the strontium isotope ration can be used to trace the sources of strontium (Klaus et al. 2007). Likewise, the stable isotopes D and 18O also present different characteristics in different groundwater samples (Gui 2014). While studies covering both major ions and isotopes are limited in coal-mining regions, they can contribute to the understanding of hydrochemical characteristics, and the relationship between major ion concentrations and isotopes at depth.

In this study, ten groundwater samples were collected from aquifers in Xutuan coal mine. The major ions, Sr, D, 18O, 87Sr/86Sr, 13Cdic and 18Odic were all determined to enable discussion of the hydrochemical characteristics of the deep groundwater. The main targets were to (1) define the geochemical characteristics of groundwater collected from different aquifers, and (2) discuss the isotopic characteristics and their influencing factors in the groundwater samples.

MATERIALS AND METHODS

Xutuan coal mine is in northern Anhui Province, China (Figure 1). The climate is marine-continental, with an annual average temperature of 14.9 °C – the average monthly maximum and minimum temperatures in the period 2005 to 2015 are 31.8 °C and −3.2 °C, in July and January, respectively. Previous studies have shown that the groundwater system in the mine includes three principal aquifers: quaternary (QA), coal-bearing (CA) and limestone (LA), all three representing threats to the mine (Gui & Chen 2007). QA consists of yellow mudstone, sandstone and conglomerate, and is between 280 and 300 m bgl (below ground level). CA is characterized by mudstone, siltstone, sandstone and coal seams, and is between 300 and 700 m bgl. LA consists mainly of Taiyuan formation and Ordovician limestones.
Figure 1

The study area, northern Anhui Province, China.

Figure 1

The study area, northern Anhui Province, China.

Ten groundwater samples were collected from different aquifers in the mine, three from QA, four from CA, and three from LA. They were collected via drainage holes in the alleys, then filtered through 0.45 μm membranes and collected in polyethylene bottles cleaned using ‘trace element clean’ procedures. All samples were determined for the major ions, Sr, D, 18O, 87Sr/86Sr, 13Cdic and 18Odic.

The major ions – Ca, Mg, Na + K, Cl, SO4, and HCO3/CO3 – were determined at the Analytical Testing Center of the Department of Coal Geology of Anhui province, China. Isotopic compositions were determined at the National Engineering Research Center of Coal Mine Water Hazard Controlling, and δ18O and δD, 13Cdic and 18Odic data are reported with respect to Standard Mean Ocean Water (SMOW) and Pee Dee Belemnite (PDB), respectively.

RESULTS AND DISCUSSION

Major ion chemistry

The groundwater analytical results are given in Table 1. In general, groundwater pH was between 7.8 and 9.9 (mean 8.7), indicating that the groundwater is alkaline. All groundwaters are of the HCO3-Na·K and Cl-Na·K types, i.e., Na+ + K+, HCO3 and Cl are the principal ions.

Table 1

Major ions (mg/L), strontium (μg/L) and selected isotopic values/ratios for groundwater samples from Xutuan coal mine

No. K+ + Na+ Mg2+ Ca2+ Cl SO42− HCO3 CO32− pH Aquifer Sr 18Sr87/Sr86 13Cdic 18Odic 
X1 428.6 6.8 3.1 355.8 11.1 554.1 7.94 CA 1,976 −66.5 −9.08 0.714685 −6.71 −9.65 
X2 470.5 8.6 5.9 212.8 22.2 772.2 70.2 8.55 CA 1,975 −64.8 −9.25 0.71804 −5.77 −9.45 
X3 451.7 6.6 5.9 346.1 16.5 565.0 33.4 8.36 CA 1,674 −66.2 −9.05 0.715287 −7.34 −9.7 
X4 535.3 8.2 16.8 304.2 8.2 978.3 8.02 CA 2,136 −66.5 −8.91 0.716288 −13.24 −9.79 
X5 133.8 30.8 3.0 169.2 11.1 103.8 54.0 9.42 LA 3,049 −60.1 −8.08 0.711857 −9.62 −9.07 
X6 153.1 12.6 2.0 110.9 7.8 144.8 63.8 9.55 LA 2,249 −54 −8.36 0.712163 −10.56 −8.4 
X7 133.7 12.5 21.6 180.7 88.9 58.9 7.84 LA 2,568 −62.4 −9.31 0.711485 −16.5 −9.91 
X8 183.9 9.5 2.5 100.5 8.6 139.8 108.0 9.77 QA 1,896 −52.2 −6.96 0.712261 −9.9 −7.8 
X9 140.5 8.2 1.7 100.5 12.8 84.9 71.2 9.85 QA 1,753 −62.2 −8.99 0.713037 −1.9 −9.54 
X10 185.3 18.4 7.1 228.5 23.5 167.2 7.4 8.48 QA 1,121 −63.2 −8.87 0.71205 −7.67 −9.4 
No. K+ + Na+ Mg2+ Ca2+ Cl SO42− HCO3 CO32− pH Aquifer Sr 18Sr87/Sr86 13Cdic 18Odic 
X1 428.6 6.8 3.1 355.8 11.1 554.1 7.94 CA 1,976 −66.5 −9.08 0.714685 −6.71 −9.65 
X2 470.5 8.6 5.9 212.8 22.2 772.2 70.2 8.55 CA 1,975 −64.8 −9.25 0.71804 −5.77 −9.45 
X3 451.7 6.6 5.9 346.1 16.5 565.0 33.4 8.36 CA 1,674 −66.2 −9.05 0.715287 −7.34 −9.7 
X4 535.3 8.2 16.8 304.2 8.2 978.3 8.02 CA 2,136 −66.5 −8.91 0.716288 −13.24 −9.79 
X5 133.8 30.8 3.0 169.2 11.1 103.8 54.0 9.42 LA 3,049 −60.1 −8.08 0.711857 −9.62 −9.07 
X6 153.1 12.6 2.0 110.9 7.8 144.8 63.8 9.55 LA 2,249 −54 −8.36 0.712163 −10.56 −8.4 
X7 133.7 12.5 21.6 180.7 88.9 58.9 7.84 LA 2,568 −62.4 −9.31 0.711485 −16.5 −9.91 
X8 183.9 9.5 2.5 100.5 8.6 139.8 108.0 9.77 QA 1,896 −52.2 −6.96 0.712261 −9.9 −7.8 
X9 140.5 8.2 1.7 100.5 12.8 84.9 71.2 9.85 QA 1,753 −62.2 −8.99 0.713037 −1.9 −9.54 
X10 185.3 18.4 7.1 228.5 23.5 167.2 7.4 8.48 QA 1,121 −63.2 −8.87 0.71205 −7.67 −9.4 

To improve understanding of the groundwater's hydrogeochemical characteristics, the ionic concentrations were plotted on a Piper diagram (Figure 2). This shows that the alkali element content (K+ + Na+) exceeds that of the alkaline earth elements (Ca2+ + Mg2+). The diagram also shows that the soluble content of the CA groundwater consists mainly of Na+ + K+, Cl and HCO3, so that it could be described as of Cl·HCO3-Na·K type. The QA groundwater has higher concentrations of Ca2+ and Mg2+ than CA groundwater, although its anions were also dominated by Cl and HCO3. The concentrations of Ca2+ and Mg2+ were highest in the LA groundwater. In summary, the concentrations of Na+ + K+ decrease in the sequence: CA > QA > LA, whereas the Ca2+ and Mg2+ content increase from CA to LA.
Figure 2

Piper diagram, groundwater from Xutuan coal mine.

Figure 2

Piper diagram, groundwater from Xutuan coal mine.

D and 18O characteristics

The δD and δ18O groundwater results are shown in Figure 3. The δD and δ18O concentrations varied from −66.9 to −52.2‰, and −9.31 to −6.96‰, with average values of −62.27 and −8.71‰, respectively. Previous data are needed to obtain the most from the δD and δ18O groundwater determinations, so, the global meteoric water line (GMWL), local meteoric water line (LMWL) and local surface water line (LSWL) were gathered in relation to the δD and δ18O measurements:
  • GMWL was described by δD = 8*δ18O + 10.56, defined by Craig (1961);

  • LMWL was characterized as δD = 7.9*δ18O + 8.2, summarized from the data from stable isotopes (Zhang 1989); and,

  • LSWL is printed as the formula δD = 6.74*δ18O − 3.33 in Gui et al. (2005).

Figure 3

Relationship between δ18O and δD for groundwaters from Xutuan coal mine.

Figure 3

Relationship between δ18O and δD for groundwaters from Xutuan coal mine.

All lines and the isotopic data from the groundwater samples are plotted in Figure 3. The LSWL is below the LMWL and GMWL, indicating that the surface water was affected significantly by evaporation (Barth 2000).

The LA groundwater samples stand on the GMWL and LMWL, indicating that the groundwater source is meteoric (rainfall). The QA groundwater samples stand between the LMWL and LSWL, suggesting that QA groundwater is quite heavily influenced by surface water. The CA groundwater samples are below the LSWL, indicating that it has no contact with surface water but might be affected by some degree of evaporation.

13Cdic and 18Odic characteristics

Previous studies have shown that the δ13C of dissolved carbonate (DIC) in groundwater varies significantly, and the values of δ18Odic and δ13Cdic can be used to trace the evolution of dissolved carbonate in groundwater (Ian & Peter 1997). The groundwater δ18Odic and δ13Cdic results are given in Table 1, and the relationship between δ18Odic and δ13Cdic is shown in Figure 4. The concentrations of δ13Cdic and δ18Odic in the groundwater varied between −16.5 and −1.9‰, and −9.9 and −7.8‰, respectively, with averages of −8.92 and −9.27‰. The δ13Cdic values varied substantially, but those of δ18Odic only slightly. In detail, the CA δ18Odic concentrations are stable compared to those for LA and QA, while the δ13Cdic values varied significantly in the groundwaters from all three aquifers.
Figure 4

Groundwater δ13Cdic and δ18Odic in Xutuan coal mine.

Figure 4

Groundwater δ13Cdic and δ18Odic in Xutuan coal mine.

In general, the δ13Cdic and δ18Odic variability could be caused by such factors as surrounding rock, organic matter, micro-organic activity, etc, as the groundwater cycle in coal mining districts is complex. Figure 4 shows that the LA 13Cdic and 18Odic have a positive correlation, the 13Cdic increasing with 18Odic, indicating that carbonate dissolution is the principal factor affecting water type. The 13Cdic in QA are higher than in LA or CA, and the negative relationship between 13Cdic and 18Odic suggests that the main source of QA groundwater is meteoric, with strong influence of CO2. However, the CA 13Cdic decreases with depth, and the stable value of 18Odic could reflect the organic matter in the coal-bearing horizon. The micro-organic activity in CA causes the 13Cdic content to decrease downward. Thus, the groundwater δ13Cdic and δ18Odic from LA, QA and CA are influenced, respectively, by the surrounding rock, CO2 and micro-organic activity.

Sr and 87Sr/86Sr ratios

The Sr contents in groundwater are between 1,121.2 and 3,049.4 μg/l (average 2,124.3 μg/l) – see Table 1. LA waters have the highest Sr content at between 2,249 and 3,049 μg/l (average 2,262.3 μg/l). QA groundwaters, conversely, had the lowest Sr content with an average 1,590 μg/l. Obviously, the concentrations in CA fall between those of QA and LA, so that the Sr concentration sequence is LA > CA > QA (decreasing).

The 87Sr/86Sr ratios varied from 0.711485 to 0.718040 (average 0.713905) – see Table 1 – with groundwaters from different aquifers showing different ratios. The ratios in CA are highest, ranging from 0.714685 to 0.718040 (average 0.716075), whereas those for QA and LA are lower – average values 0.712449 and 0.711835, respectively.

Sr and Ca are alkaline earth elements with very similar geochemical behaviors. Previous studies have shown that the typical Sr concentration in water is approximately 1% that of Ca, and the two concentrations are strongly correlated (Nakano 2014). Thus, positive Ca:Sr concentration correlations are normal in groundwater studies. Figure 5(a) shows that a positive Ca:Sr correlation exists, indicating that the Sr was released into the groundwater by the weathering (dissolution) of Ca-containing minerals and not from other sources – i.e., the 87Sr/86Sr ratio reflects the geological environment closely.
Figure 5

Sr versus Ca and 87Sr/86Sr in groundwater collected from Xutuan coal mine.

Figure 5

Sr versus Ca and 87Sr/86Sr in groundwater collected from Xutuan coal mine.

Four stable strontium isotopes exist naturally – 84Sr, 86Sr, 87Sr and 88Sr. 87Sr is also generated by the radioactive decay of 87Rb. As that decay is extremely slow and strontium is found in rocks, the Rb-Sr method is widely used for geological dating. Natural 87Sr concentrations are stable over short time scales and groundwater with high Sr concentrations should, preferably, have relatively low 87Sr/86Sr ratios. In other words, the Sr concentration and the 87Sr/86Sr ratios should show a negative correlation, as seen in Figure 5(a).

CONCLUSIONS

Ten groundwater samples were collected from three aquifers in Xutuan coal mine, Anhui Province, China, for determination of major ions, strontium, and various isotopes (D, 18O, 13Cdic, 18Odic, 87Sr/86Sr). The hydrogeochemical characteristics and groundwater origins are discussed:

All groundwater samples are of HCO3-Na·K or Cl-Na·K types, in other words, Na+ + K+, HCO3 and Cl are the predominant ions in the groundwater. The concentrations of Na+ + K+ decrease in the sequence: CA > QA > LA, while the Ca2+ and Mg2+ concentrations increase from CA to LA.

The meteoric origin of groundwater in LA is confirmed by the D vs 18O plot – Figure 4 – while the QA groundwater is clearly influenced by surface water. The δ13Cdic and δ18Odic in LA, QA and CA groundwaters are influenced by the surrounding rock, CO2 and micro-organic activity, respectively.

The Sr concentrations in the groundwaters show their different characteristics, decreasing in the sequence LA > CA > QA.

ACKNOWLEDGEMENTS

The study was supported by the National Natural Science Foundation of China (41373095), the Natural Science Foundation of Anhui Province (1708085QE125), the Postdoctoral Research Project in Anhui Province (2016B093) and the opening scientific research platform in Suzhou University (2014YKF02).

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