The age, distribution, and geochemical characteristics of groundwater in the Ordovician limestone aquifer in the Huaibei coalfield, China

Deep groundwater usually contains a wealth of geological information, as it flows through, or remains in, aquifers for long periods of time (Zouari et al. 2011; Ma et al. 2015). As such, geochemical indicators are often used to answer questions about hydrogeological conditions, groundwater evolution, and water-rock interactions (Price & Swart 2006; Currell et al. 2016). In addition, the deep groundwater, which is renewed slowly and generally unpolluted, serves as a major storage zone for water resources. In mining areas, deep groundwater is not only a precious resource that is exploited for agricultural, urban, and industrial uses, but is also a threat to the safety of coal mine exploitation. Many studies have examined the hydrochemical evolution of groundwater, groundwater runoff, and hydrogeochemical processes in recent decades (Gui et al. 2011; Sun et al. 2011; Chen & Gui 2015), and these studies have provided useful information to support efficient groundwater development schemes and have helped to reduce both coal mining hazards and groundwater quality deterioration.

Previous studies have frequently reported information about the hydrochemical characteristics, water-rock interactions, and hydrogeochemical processes (Chen et al. 2013; 2014), but not about the age, renewal, and evolution of deep groundwater. Radiogenic isotopes, such as ¹⁴C and ³H, can be used to determine the age, evolution, and flow of deep groundwater. In this study, we collected samples of deep groundwater from the Huaibei coalfield, Anhui Province, China. The main objectives of the studies were (1) to define the geochemical characteristics of the groundwater in the Ordovician limestone aquifer (OA); (2) to constrain the age and evolution of deep groundwater, and (3) to understand the spatial variations in hydrochemistry and recharge of deep groundwater.

The Huaibei coalfield extends over most of the North China Plain and has significant coal resources. This area has a marine-continental climate, an annual average temperature of 14.7 °C, and average monthly maximum and minimum temperatures of 31.8 °C and −3.2 °C in July and January, respectively, measured for the period from 2005 to 2015. The average annual precipitation and evaporation are about 867.0 mm and 832.4 mm, respectively, and more than 50% of the total precipitation falls in the period from June to September (Chen et al. 2014). Surface water is scarce in the study area, so deep groundwater is the main source of water for industrial and domestic uses.

We collected 22 groundwater samples from the OA in the Huaibei coalfield. Every fifth sample was collected in duplicate so the analyses could be checked for accuracy. The samples were put into 2.5 L plastic barrels that had been rinsed 3 times with the groundwater, and the sampling location (longitude and latitude), temperature, pH, conductivity, and total dissolved solids (TDS) were all recorded at each site. Groundwater samples were collected from drainage holes in alleys, and were filtered through a 0.45-μm membrane into sterilized polyethylene bottles that had been cleaned following standard trace element cleaning procedures. All samples were analyzed for major ions and hydrogen and oxygen isotopes. Major ions were determined in the analytical laboratory of the Department of Coal Geology, Anhui Province, China. The K⁺ + Na⁺, and Cl⁻, Ca²⁺ and Mg²⁺, and alkaline concentrations were determined by atomic absorption spectrometry, ion chromatography, EDTA titration, and acid-based titration, respectively. The isotopic compositions were determined in the laboratory of the National Engineering Research Center for the Control of Coal Mine Water Hazards. The isotopic data are reported relative to Standard Mean Ocean Water (SMOW), and the overall precisions of δ¹⁸O and δD were 0.2 and 2‰, respectively. The ¹⁴C concentrations in groundwater were tested with an ultra-low background liquid scintillation spectrometer (Quantulus 1200) in the Karst Geological Resource Environment Testing Center of the Land and Resources Ministry P.R.C.

The groundwater types and hydrochemical characteristics were different for the various mining areas. The concentrations of Ca²⁺ and Mg²⁺ were higher in the Zhahe and Suixiao mining areas than in the other mining areas, and the groundwaters were the HCO₃-Ca·Mg type. In the other coal mining areas, the Na⁺ + K⁺ contents were higher, and the groundwaters were mainly the Cl·SO₄-Na type. Of the five coal mining areas, the TDS contents were highest in the Linhuan mining area, where the average value was 1,084 mg/l, and lowest in the Suixiao mining area, where the average value was 183 mg/l. The TDS contents in the Guobei, Taoyuan, and Zhahe mining areas were ranked second, third, and fourth, with average values of 663, 598, and 420 mg/l, respectively.

Additional background data are needed to generate further information about the isotopic characteristics of groundwater. We used the global meteoric water line (GMWL), the local meteoric water line (LMWL), and the local surface water line (LSWL) for δD and δ¹⁸O as references. The GMWL was described by Craig (1961) as δD = 8*δ¹⁸O + 10.56, the LMWL was summarized as δD = 7.9*δ¹⁸O + 8.2 by Zhang (1989), and the LSWL was defined by Gui et al. (2005) as δD = 6.74*δ¹⁸O − 3.33. All the lines and isotopic signatures of the groundwater samples are shown in Figure 3. The plots of groundwater samples from the OA are distributed on the LSWL. The slope of distribution line is close to the LMWL, indicating that the groundwater was recharged by rainfall and surface water.

The groundwater age is an important index of the renewal ability of the groundwater, and radioactive isotopes, such as ¹⁴C and ³H, are generally used to establish the age of the groundwater. The younger groundwater is more rapidly renewed than the older groundwater. Previous studies showed that ¹⁴C concentrations in water, which are affected by the dissolution of CO₂ in the atmosphere, could be used to determine the water age. Thus, the ¹⁴C method can only be used for groundwater, and the upper and lower limits of dating range from 35,000a to 40,000a and from 500a to 1,000a, respectively (Aggarwal et al. 2014). To do this analysis, about 4 g of solid carbonate should be precipitated out of the groundwater, and then the carbonate should be used to determine the ¹⁴C by the liquid scintillation counting method. This means that, for this study area, each groundwater sample should have been about 60 liters. Because of limitations in the sampling conditions, we determined the ¹⁴C contents on only 3 such samples collected from the OA.

The ¹⁴C age of the groundwater in OA ranged from 2,660a to 10,040a, and the groundwater ages in the OA varied widely in the Huaibei coalfield. The groundwater was oldest in the Renlou coal mine in the Linhuan coal-mining district, which suggests that groundwater renewal and runoff were slow in that area. The groundwater was youngest in the Yangzhuang coal mine in the Zhahe coal-mining district, which indicates that the groundwater in this mine was renewed rapidly, and was mainly recharged by atmospheric precipitation. The OA is therefore recharged in the Zhahe coal-mining district.

Changes in the hydrochemical composition of groundwater that occur during the circulation process can be deduced from water-rock interactions. The major ions, which are present at high concentrations in the surrounding rocks and are very reactive, are released into groundwater easily. As a comprehensive index of the solids concentrations in groundwater, TDS can reflect the groundwater runoff and circulation. In general, a low TDS value in groundwater indicates that the water supply is sufficient, and that the water-rock interactions are weak because of the short residence time in the aquifer. In a hydrogeological unit, groundwater flows from areas where the TDS is lower to where the TDS is higher. The intensity of the TDS contours reflect the degree of dissolution between groundwater and the surrounding rocks.

The conventional chemical characteristics of the groundwater also provide evidence to support this view. In the Zhahe and Suixiao coal-mining districts, the groundwater is mainly the bicarbonate type, while the groundwaters in the Linhuan and Suxian coal-mining districts are mainly characterized by halide and sulfate, respectively. While there are clear differences in the distribution of groundwater hydrochemical characteristics, the chemical type changes from bicarbonate in the recharge area to sulfate and halide in the drainage area.

We determined the major ion, ¹⁴C, and hydrogen and oxygen isotope concentrations of 22 groundwater samples, collected from the OA in the Huaibei coalfield, China. We used traditional graphic analysis to examine the hydrochemistry characteristics, and groundwater runoff and age.

Our results showed that all the groundwater samples were alkaline, with pH values between 7.10 and 10.80. The range of the TDS contents was from 119 to 2,443 mg/l and the average was 680.6 mg/l. The groundwater from the different coal-mining districts had different characteristics. The concentrations of ⁠, Ca²⁺, and Mg²⁺ were high, and the TDS values were relatively low in groundwater from the Zhahe and Suixiao coal-mining districts. Groundwater from the Linhuan and Suxian coal-mining districts had high contents of Cl⁻ and ⁠, and higher TDS concentrations than the Zhahe and Suixiao coal-mining districts.

The δD and δ¹⁸O concentrations in groundwater varied from −64.32‰ to −42.76‰, and from −8.62 to −5.40‰, with mean values of −56.38‰ and −7.62‰, respectively. The δD and δ¹⁸O plots indicate that the groundwater is recharged by rainfall or surface water. The negative correlation between TDS and the isotopes (δD and δ¹⁸O) indicates that the hydrogen and oxygen concentrations in groundwater were influenced by the water-rock interactions.

The groundwater age in the OA, calculated from the ¹⁴C concentrations in groundwater, varied from 2,660a to 10,040a. The oldest and youngest groundwaters were collected from the Renlou and Yangzhuang coal mines, respectively. When this information was combined with the TDS contour diagrams, we found that the groundwater in the OA mainly flowed from the north to the south, with recharge mainly in the Zhahe and Suixiao coal-mining districts and discharge mainly in the Linhuan coal-mining district. The chemical type of groundwater changed from bicarbonate in the recharge area to sulfate and halide in the drainage area.

The study was supported by the National Natural Science Foundation of China (41373095), the Natural Science Foundation of Anhui Province (1708085QE125) and the Postdoctoral Research Project of Anhui Province (2016B093).