Isotopic characteristics of D, ¹⁸O, ¹³C_dic, ¹⁸O_dic, ⁸⁷Sr/⁸⁶Sr and their application in coal mine water: a case study

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, ¹³C_dic and ¹⁸O_dic 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.

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, ¹³C_dic and ¹⁸O_dic.

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, ¹³C_dic and ¹⁸O_dic data are reported with respect to Standard Mean Ocean Water (SMOW) and Pee Dee Belemnite (PDB), respectively.

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.

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.

In general, the δ¹³C_dic and δ¹⁸O_dic 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 ¹³C_dic and ¹⁸O_dic have a positive correlation, the ¹³C_dic increasing with ¹⁸O_dic, indicating that carbonate dissolution is the principal factor affecting water type. The ¹³C_dic in QA are higher than in LA or CA, and the negative relationship between ¹³C_dic and ¹⁸O_dic suggests that the main source of QA groundwater is meteoric, with strong influence of CO₂. However, the CA ¹³C_dic decreases with depth, and the stable value of ¹⁸O_dic could reflect the organic matter in the coal-bearing horizon. The micro-organic activity in CA causes the ¹³C_dic content to decrease downward. Thus, the groundwater δ¹³C_dic and δ¹⁸O_dic from LA, QA and CA are influenced, respectively, by the surrounding rock, CO₂ and micro-organic activity.

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.

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).

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, ¹³C_dic, ¹⁸O_dic, 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 δ¹³C_dic and δ¹⁸O_dic 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.

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).