Abstract

Samples of river water and groundwater from Quaternary (QA), sandstone (SA), Taiyuan formation (TA), and Ordovician limestone (OA) aquifers in the Suxian coal-mining district, Anhui Province, China were collected. Their physicochemical properties, and major ion and isotopic compositions were determined. The samples were alkaline, with pH values exceeding 8, and the total dissolved solids concentrations depended on the water source. The δD and δ18O contents were highest in the river water samples and lowest in the SA groundwater. The isotopic characteristics of the QA and OA groundwaters suggest recharge by rainfall and surface water. The isotopic characteristics of river water were controlled mainly by evaporation. Water–rock interactions, the flow rate, and the main water sources were the most important influences on groundwaters in QA, OA, and TA, but the properties of SA groundwater were controlled by static reserves. Two discriminant functions, explaining more than 98.2% of the total variances, indicated that QA, TA, and OA were hydraulically connected. Three groundwater sources were identified as end-members, and a conceptual model was established to calculate water mixing ratios.

Highlights

  • This paper systematically discusses the chemical characteristics of elements and isotopes of groundwater in multi aquifers in coal mine area, and establishes a mixed water source identification model by using statistical method. The research results have certain significance for the establishment of groundwater chemistry and water source identification model.

INTRODUCTION

Groundwater systems serving as major water resources are an important part of the earth's surface environment. Groundwater that flows through aquifers or remains in them for long periods contains abundant geological information. Geochemical indicators can therefore be effective tools for solving hydrological problems.

In mining areas, groundwater is exploited intensively for agricultural, urban, and industrial uses (Zouari et al. 2011; Ma et al. 2015), but also threatens the safety of coal mining activities. In the past few decades, incidents involving rapid groundwater inflows have occurred frequently as coal mining activities have gone progressively deeper below the surface in China. Thus, studies of the hydrochemistry and dynamics of groundwater are important, as they can improve understanding of regional hydrogeological conditions and water–rock interactions in coalfields.

Many studies have already examined runoff control, hydrologic processes, and the chemical and isotopic characteristics of groundwater (Gui et al. 2011; Sun et al. 2011; Chen & Gui 2015). However, few studies have examined the factors influencing the geochemical properties of groundwater in multiple aquifers. Since the concentrations of environmentally stable isotopes (D, 18O) in groundwater change little with water–rock interactions at low temperatures, stable isotopes can be used to bridge this knowledge gap (Molla et al. 2007; Huang & Chen 2012). Groundwaters in diverse aquifers usually have hydraulic connections in coal mining districts, and both their hydrogeochemistry and stable isotope ratios can be used to characterize groundwater recharge processes and circulation mechanisms, and examine the mixing ratios of groundwaters in different aquifers (Chen et al. 2014).

Many previous studies have focused on the hydrogeochemical characteristics of groundwater from single aquifers, and overlooked the differences between the diverse aquifers in Huaibei coal mine (Gui 2014). Very few studies have used hydrogeochemical and stable isotope characteristics to examine interactive relationships between groundwaters from different aquifers and/or trace groundwater flow. For this study, surface- and groundwater samples were collected and analyzed in July 2015 in the Suxian coal-mining district in Anhui Province, China. The main objectives were (1) to define the geochemical characteristics of, and relationships between, groundwaters from different aquifers, (2) to understand the factors controlling the hydrochemical processes, and (3) to establish a model for calculating groundwater mixing ratios in coal mining districts.

GEOLOGICAL BACKGROUND

Huaibei Coalfield, in the north of Anhui Province, is the largest coal mining area in the North China Plain, and contains significant coal resources (Figure 1). The area has a marine-continental climate, with an annual average temperature of 14.9 °C. The average annual precipitation and evaporation are about 867.0 and 832.4 mm, respectively, and more than 50% of the total precipitation falls from June to September (Chen et al. 2014). There is little surface water in the study area, so groundwater is the main source of water for industrial and domestic uses.

Figure 1

Location and geological setting of the study area in northern Anhui Province, China.

Figure 1

Location and geological setting of the study area in northern Anhui Province, China.

The Huaibei Coalfield has three mining areas: Suixiao, Suxian, and Linhuan. In the Suxian area, the coal-bearing formations are distributed mainly through the Carboniferous and Permian, with a total thickness exceeding 1,300 m (Zheng et al. 2008). The formations in the study area comprise strata from the Cambrian, Ordovician, Carboniferous, Permian, and lower Triassic periods. The groundwater system in the Suxian coal-mining area includes four principal aquifers – a Quaternary aquifer (QA), a sandstone aquifer (SA), an aquifer from the Taiyuan formation (TA), and an Ordovician limestone aquifer (OA) – all of which pose threats to the mine (Figure 2).

Figure 2

Hydrogeological profile of the multilayer aquifer system studied.

Figure 2

Hydrogeological profile of the multilayer aquifer system studied.

The four aquifers differ in lithology, thickness, and water-bearing capacity:

  • The QA consists of conglomerate, sand, and clay, is between 11.7 and 132.9 m below sea level (bsl), and has a permeability (K) of 0.00043 to 0.0456 m/d.

  • The SA, characterized by sandstone, siltstone layers, and a coal seam, is between 459.4 and 864.6 m bsl, and its K is 0.0008 to 12.5 m/d. It is underlain by an aquitard that does not develop cracks or fractures, and is between 67 and 200 m thick.

  • The TA, a limestone, is evenly distributed throughout the district. It is between 8.36 m above sea level (asl) and 174.56 m bsl, and K is between 0.021 and 3.11 m/d.

  • The OA consists of gray or dark-gray thick-bedded limestone. The main aquitard between underlying TA and the OA is mudstone between 30 and 120 m thick.

Because of the aquifer's lithology, SA groundwater is mainly static reserves, with poor liquidity. The groundwaters in the TA and OA move laterally, and the flow directions are controlled by the Sunan anticline and syncline. In the TA and OA, groundwater runoff is from both the east and west into the middle. The exploitation of deep coal resources has led to recharge leakage throughout the aquifer system, especially in the QA, TA, and OA.

SAMPLE COLLECTION AND ANALYSIS

Thirty two samples of river- and groundwater were collected from 6 coal mines in July 2015 – see Table 1 for information about the sampling and the sampling sites. Every fifth sample was collected in duplicate to check the accuracy of the analyses. River water samples were put into 2.5 L plastic barrels pre-rinsed 3 times with the water at the sampling site. Information about the site longitude and latitude, and water pH, conductivity, and total dissolved solids (TDS) content, was recorded in situ. Groundwater samples were collected from drainage holes in mine alleys and filtered through 0.45-μm membranes into sterilized polyethylene bottles pre-washed in line with the cleaning procedures for standard trace element analysis. All samples were analyzed for major ion concentrations, and hydrogen and oxygen isotopic contents. Major ions were determined in the analytical laboratory of the Department of Coal Geology, Anhui Province, China. The K+ + Na+, SO42− 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 precisions of δ18O and δD were ± 0.2‰ and 2‰, respectively.

Table 1

Sampling information – Suxian coal-mining district

SampleSourceMineWater level elevation (m)LocationLatitude (N)Longitude (E)
River Luling 9.8 Tuohe River 33.46025° 117.15302° 
River Taoyuan 10.0 Yunliang River 33.50755° 117.00642 
River Zhuxianzhuang 12.8 Xinbian River 33.62951° 117.1242° 
River Zhuxianzhuang 13.5 Tuohe River   
River Qianyingzi 8.6 Huihe River 33.48469° 116.94886° 
River Qinan 9.6 Huihe River 33.43632° 117.01678° 
QA Zhuxianzhuang −13.7 Hydrologic hole-84-22 33.63886° 117.09230° 
QA Zhuxianzhuang −126.4 Hydrologic hole-84-14   
QA Qianyingzi −14.0 Hydrologic hole-2 33.47780° 116.93623° 
10 QA Qianyingzi −11.8 Hydrologic hole-5 33.48311° 116.92879° 
11 QA Qidong −466.0 Drain hole in South   
12 QA Qinan −12.7 Observation bh 02-4 33.46178° 116.98812° 
13 QA Zouzhuang −21.0 Observation bh 10-2 33.40815° 116.86013° 
14 SA Qianyingzi −520.0 E3211 coal face   
15 SA Qianyingzi −608.0 W3213 roadway   
16 SA Qidong −600.0 Hydrologic hole-7124   
17 SA Qinan −486.1 101 track lane   
18 SA Zouzhuang −440.0 32 transport roadways   
19 TA Luling 24.0 Ventilating shaft 33.57288° 117.17794° 
20 TA Taoyuan −400.0 1,035 coal face 33.50732° 116.99301° 
21 TA Taoyuan −800.0 Drain hole II4 aea   
22 TA Zhuxianzhuang −130.0 Mine track riseII5   
23 TA Qianyingzi −5.1 Hydrologic hole-30B4 33.48235° 116.93677° 
24 TA Qidong −17.0 Hydrologic hole-ST4 3338862° 117.07508° 
25 TA Qinan −538.5 3,682 coal face   
26 TA Zouzhuang 0.6 Observation bh 11-1 33.43855° 116.85635° 
27 OA Luling 24.0 Luling middle school hole 33.51991° 117.18179° 
28 OA Taoyuan −9.9 Observation bh 2014-2   
29 OA Zhuxianzhuang −2.7 Hydrologic hole 84-24 33.63434° 117.13338° 
30 OA Qianyingzi −8.3 Hydrologic hole-T4 33.48391° 116.94859° 
31 OA Qidong −10.0 Observation bh 33.40287° 117.12053° 
32 OA Qinan −9.1 Observation bh 07-1 33.47383° 116.97445° 
SampleSourceMineWater level elevation (m)LocationLatitude (N)Longitude (E)
River Luling 9.8 Tuohe River 33.46025° 117.15302° 
River Taoyuan 10.0 Yunliang River 33.50755° 117.00642 
River Zhuxianzhuang 12.8 Xinbian River 33.62951° 117.1242° 
River Zhuxianzhuang 13.5 Tuohe River   
River Qianyingzi 8.6 Huihe River 33.48469° 116.94886° 
River Qinan 9.6 Huihe River 33.43632° 117.01678° 
QA Zhuxianzhuang −13.7 Hydrologic hole-84-22 33.63886° 117.09230° 
QA Zhuxianzhuang −126.4 Hydrologic hole-84-14   
QA Qianyingzi −14.0 Hydrologic hole-2 33.47780° 116.93623° 
10 QA Qianyingzi −11.8 Hydrologic hole-5 33.48311° 116.92879° 
11 QA Qidong −466.0 Drain hole in South   
12 QA Qinan −12.7 Observation bh 02-4 33.46178° 116.98812° 
13 QA Zouzhuang −21.0 Observation bh 10-2 33.40815° 116.86013° 
14 SA Qianyingzi −520.0 E3211 coal face   
15 SA Qianyingzi −608.0 W3213 roadway   
16 SA Qidong −600.0 Hydrologic hole-7124   
17 SA Qinan −486.1 101 track lane   
18 SA Zouzhuang −440.0 32 transport roadways   
19 TA Luling 24.0 Ventilating shaft 33.57288° 117.17794° 
20 TA Taoyuan −400.0 1,035 coal face 33.50732° 116.99301° 
21 TA Taoyuan −800.0 Drain hole II4 aea   
22 TA Zhuxianzhuang −130.0 Mine track riseII5   
23 TA Qianyingzi −5.1 Hydrologic hole-30B4 33.48235° 116.93677° 
24 TA Qidong −17.0 Hydrologic hole-ST4 3338862° 117.07508° 
25 TA Qinan −538.5 3,682 coal face   
26 TA Zouzhuang 0.6 Observation bh 11-1 33.43855° 116.85635° 
27 OA Luling 24.0 Luling middle school hole 33.51991° 117.18179° 
28 OA Taoyuan −9.9 Observation bh 2014-2   
29 OA Zhuxianzhuang −2.7 Hydrologic hole 84-24 33.63434° 117.13338° 
30 OA Qianyingzi −8.3 Hydrologic hole-T4 33.48391° 116.94859° 
31 OA Qidong −10.0 Observation bh 33.40287° 117.12053° 
32 OA Qinan −9.1 Observation bh 07-1 33.47383° 116.97445° 

RESULTS AND DISCUSSION

Hydrogeochemical characterization

Information about the major ion concentrations in the water samples is presented in Table 2 and Figure 3. The ionic balance was checked using AqQa software (RockWare Inc 2003). The pH of river water was between 8.1 and 8.9, indicating that the water is alkaline. The river waters' TDS was between 983 and 2,680 mg/l. Cation concentrations in the river water decreased in the order: K+ + Na+ > Ca2+ > Mg2+, and anion concentrations: SO42− > Cl > HCO3 > CO32−. The river water types were Cl-Na·K and SO4-Na·K, and the principal ions Na+ + K+, Cl, and SO42−.

Table 2

Major parameter values in the water samples

SampleK+ + Na+Ca2+Mg2+ClSO42−HCO3CO32−pHTDSWater typeδDδ18O
166.6 105. 60.6 336. 162.2 283.1 0.0 8.15 1,300. Na-Cl −41.0 −4.80 
470.4 72.1 46.6 490.8 432.4 211.2 37.5 8.91 2,680.0 Na-Cl −52.41 −6.69 
176.9 45.3 49.6 142.4 260.5 244.3 17.5 8.44 983.0 Na-SO4 −33.36 −3.01 
314.7 80.2 97.2 223.1 638.0 394.4 0.0 8.12 1,910.0 Na-SO4 −36.06 −3.73 
483.4 62.0 65.5 237.2 847.1 227.0 43.2 8.60 2,225.0 Na-SO4 −36.42 −4.01 
420.2 65.6 59.4 269.4 560.6 363.9 35.0 8.53 2,010.0 Na-SO4 −43.17 −5.34 
104.0 22.7 12.3 152.7 2.5 150.1 0.0 7.78 495.0 Na-Cl −57.77 −7.64 
143.1 4.0 4.9 158.7 4.1 0.0 72.0 10.70 539.0 Na-Cl −57.57 −7.42 
37.9 48.1 20.1 22.3 7.2 99.2 35.0 9.33 229.0 Ca-HCO3 −57.72 −7.98 
10 199.7 1.6 4.9 135.6 5.8 62.6 126.1 10.02 707.0 Na-CO3 
11 109.6 198.4 137.5 238.5 610.0 399.5 0.0 7.27 1,810.0 Mg-SO4 −63.42 −8.59 
12 148.2 4.0 9.8 118.7 4.5 151.3 45.6 9.25 491.0 Na-HCO3 −61.58 −8.36 
13 101.1 4.0 7.3 75.0 14.8 75.7 48.0 10.10 370.0 Na-CO3 −52.08 −7.02 
14 519.1 10.5 4.9 120.4 55.2 1,157.0 0.0 7.81 1,680.0 Na-HCO3 −63.24 −8.43 
15 865.4 10.5 4.9 144.8 96.7 1,981.9 0.0 7.80 2,870.0 Na-HCO3 −63.68 −8.38 
16 593.0 4.1 9.3 229.9 40.3 1,185.8 0.0 8.04 2,100.0 Na-HCO3 −63.57 −8.48 
17 449.3 12.1 3.9 155.2 182.3 749.3 0.0 8.20 1,550.0 Na-HCO3 −64.48 −8.28 
18 1,104.7 105.3 109.2 254.0 2,357.6 369.0 0.0 7.44 4,740.0 Na-SO4 −66.27 −8.94 
19 28.1 4.5 7.1 15.4 4.5 49.4 24.0 9.40 123.0 Na-HCO3 −71.83 −10.15 
20 174.4 210.5 76.1 205.9 681.0 267.2 0.0 7.86 1,900.0 Ca-SO4 −62.10 −8.31 
21 197.4 266.4 77.8 212.8 806.5 335.9 0.0 7.70 2,190.0 Ca-SO4 −62.16 −8.37 
22 413.7 112.0 68.9 155.2 540.0 451.6 187.7 7.80 1,530.0 Na-SO4 −63.15 −8.44 
23 95.4 5.3 9.1 34.3 2.5 221.4 20.0 8.62 333.0 Na-HCO3 −54.45 −7.46 
24 50.5 4.9 6.4 20.6 7.4 104.3 17.5 9.26 172.0 Na-HCO3 −54.45 −7.49 
25 194.6 204.8 109.0 248.8 676.7 399.5 0.0 7.82 1,950.0 Ca-SO4 −63.91 −8.33 
26 43.1 4.8 6.4 14.0 15.6 73.2 21.6 10.10 150.0 Na-CO3 −49.09 −6.64 
27 141.6 78.8 17.4 54.9 4.3 207.6 81.1 9.43 587.0 Na-HCO3 −42.76 −5.40 
28 190.4 117.4 62.4 181.9 562.5 132.3 7.5 8.80 1,310.0 Na-SO4 −60.23 −8.07 
29 77.1 4.1 3.0 56.6 2.5 100.8 23.5 9.13 279.0 Na-HCO3 −50.57 −6.73 
30 130.1 9.3 4.7 49.8 181.7 3.6 39.0 10.45 691.0 Na-SO4 −47.98 −6.94 
31 62.8 5.7 3.9 32.6 42.4 43.3 32.5 10.54 276.0 Na-HCO3 −47.69 −6.50 
32 98.4 16.2 13.3 97.8 136.7 20.4 7.5 8.90 446.0 Na-SO4 −55.80 −7.51 
SampleK+ + Na+Ca2+Mg2+ClSO42−HCO3CO32−pHTDSWater typeδDδ18O
166.6 105. 60.6 336. 162.2 283.1 0.0 8.15 1,300. Na-Cl −41.0 −4.80 
470.4 72.1 46.6 490.8 432.4 211.2 37.5 8.91 2,680.0 Na-Cl −52.41 −6.69 
176.9 45.3 49.6 142.4 260.5 244.3 17.5 8.44 983.0 Na-SO4 −33.36 −3.01 
314.7 80.2 97.2 223.1 638.0 394.4 0.0 8.12 1,910.0 Na-SO4 −36.06 −3.73 
483.4 62.0 65.5 237.2 847.1 227.0 43.2 8.60 2,225.0 Na-SO4 −36.42 −4.01 
420.2 65.6 59.4 269.4 560.6 363.9 35.0 8.53 2,010.0 Na-SO4 −43.17 −5.34 
104.0 22.7 12.3 152.7 2.5 150.1 0.0 7.78 495.0 Na-Cl −57.77 −7.64 
143.1 4.0 4.9 158.7 4.1 0.0 72.0 10.70 539.0 Na-Cl −57.57 −7.42 
37.9 48.1 20.1 22.3 7.2 99.2 35.0 9.33 229.0 Ca-HCO3 −57.72 −7.98 
10 199.7 1.6 4.9 135.6 5.8 62.6 126.1 10.02 707.0 Na-CO3 
11 109.6 198.4 137.5 238.5 610.0 399.5 0.0 7.27 1,810.0 Mg-SO4 −63.42 −8.59 
12 148.2 4.0 9.8 118.7 4.5 151.3 45.6 9.25 491.0 Na-HCO3 −61.58 −8.36 
13 101.1 4.0 7.3 75.0 14.8 75.7 48.0 10.10 370.0 Na-CO3 −52.08 −7.02 
14 519.1 10.5 4.9 120.4 55.2 1,157.0 0.0 7.81 1,680.0 Na-HCO3 −63.24 −8.43 
15 865.4 10.5 4.9 144.8 96.7 1,981.9 0.0 7.80 2,870.0 Na-HCO3 −63.68 −8.38 
16 593.0 4.1 9.3 229.9 40.3 1,185.8 0.0 8.04 2,100.0 Na-HCO3 −63.57 −8.48 
17 449.3 12.1 3.9 155.2 182.3 749.3 0.0 8.20 1,550.0 Na-HCO3 −64.48 −8.28 
18 1,104.7 105.3 109.2 254.0 2,357.6 369.0 0.0 7.44 4,740.0 Na-SO4 −66.27 −8.94 
19 28.1 4.5 7.1 15.4 4.5 49.4 24.0 9.40 123.0 Na-HCO3 −71.83 −10.15 
20 174.4 210.5 76.1 205.9 681.0 267.2 0.0 7.86 1,900.0 Ca-SO4 −62.10 −8.31 
21 197.4 266.4 77.8 212.8 806.5 335.9 0.0 7.70 2,190.0 Ca-SO4 −62.16 −8.37 
22 413.7 112.0 68.9 155.2 540.0 451.6 187.7 7.80 1,530.0 Na-SO4 −63.15 −8.44 
23 95.4 5.3 9.1 34.3 2.5 221.4 20.0 8.62 333.0 Na-HCO3 −54.45 −7.46 
24 50.5 4.9 6.4 20.6 7.4 104.3 17.5 9.26 172.0 Na-HCO3 −54.45 −7.49 
25 194.6 204.8 109.0 248.8 676.7 399.5 0.0 7.82 1,950.0 Ca-SO4 −63.91 −8.33 
26 43.1 4.8 6.4 14.0 15.6 73.2 21.6 10.10 150.0 Na-CO3 −49.09 −6.64 
27 141.6 78.8 17.4 54.9 4.3 207.6 81.1 9.43 587.0 Na-HCO3 −42.76 −5.40 
28 190.4 117.4 62.4 181.9 562.5 132.3 7.5 8.80 1,310.0 Na-SO4 −60.23 −8.07 
29 77.1 4.1 3.0 56.6 2.5 100.8 23.5 9.13 279.0 Na-HCO3 −50.57 −6.73 
30 130.1 9.3 4.7 49.8 181.7 3.6 39.0 10.45 691.0 Na-SO4 −47.98 −6.94 
31 62.8 5.7 3.9 32.6 42.4 43.3 32.5 10.54 276.0 Na-HCO3 −47.69 −6.50 
32 98.4 16.2 13.3 97.8 136.7 20.4 7.5 8.90 446.0 Na-SO4 −55.80 −7.51 
Figure 3

Piper diagrams of waters from Suxian coal mining district.

Figure 3

Piper diagrams of waters from Suxian coal mining district.

The ionic compositions of the groundwaters from the different aquifers were variable and complex, and the water chemistry inconsistent.

The groundwater pH and TDS in QA ranged from 7.3 to 10.7, and from 229 to 1,810 mg/l, respectively, and the similarity of the QA and river water hydrochemical characteristics suggest that the QA is mainly recharged by surface water. SA groundwaters were of the Na-HCO3 type, indicating long residence time in the aquifer and extensive water-rock interaction. TDS concentrations were highest in SA, ranging from 1,550 to 4,740 mg/l. TA groundwaters had high Ca concentrations, with TDS in the range 123 to 2,190 mg/l. The TDS concentration range was lowest in the OA groundwaters in OA (276 to 1,310 mg/l), suggesting quick runoff, good recharge and limited water-rock interaction. TDS concentrations were low in the QA and OA.

Isotopic characterization

Information about the δD and δ18O concentrations is given in Table 2 and Figure 4, but these data must be seen in a wider context to provide some insight into the isotopic characteristics of the ground and surface waters. This is achieved by reference to the global meteoric water line (GMWL), local meteoric water line (LMWL), and local surface water line (LSWL) for δD and δ18O. The GMWL, as defined by Craig (1961), is described by δD = 8*δ18O + 10.56; the LMWL was summarized by Zhang (1989) as δD = 7.9*δ18O + 8.2, and the LSWL was summarized as δD = 6.74*δ18O − 3.33 (Gui et al. 2005). These lines and the groundwaters' isotopic signatures are shown in Figure 4.

Figure 4

δD and δ18O of water samples from Suxian coal mining district.

Figure 4

δD and δ18O of water samples from Suxian coal mining district.

The δD and δ18O contents were highest in the river water, and ranged from −52.41 to −33.36‰, and from −6.69 to −3.01‰, respectively, and show clear heavy isotope enrichment. The samples were collected in July, when the climate was relatively dry, and the δD and δ18O concentrations were probably controlled by evaporation.

The contents of δD and δ18O in groundwater from QA ranged from −63.42 to −52.08‰ and from −8.59‰ to −7.02‰. The δ18O and δD contents in groundwater from OA varied from −8.07‰ to −5.40‰, and from −60.23‰ to −42.76‰, respectively. The plots for the QA and OA groundwaters are distributed along the LSWL, with similar line slopes to that of the GMWL, indicating that rainfall and surface water recharge may have occurred, and this view is supported by previous work (Chen et al. 2008). The δD and δ18O concentrations in the SA, however, were lower, ranging from −66.27 to −63.24‰, and −8.94 to −8.28‰, respectively, and plotted away from the LSWL. The δD and δ18O concentrations in LA groundwater ranged from −71.83 to −49.09‰, and −10.15 to −6.64‰, respectively.

Influences on the isotopic contents of water samples

Usually, the sources of solutes in water bodies are complex, and can be influenced by both natural conditions and human activities. If not influenced by other factors, the TDS concentrations in river water derived directly from precipitation are low (Gibbs 1970). The TDS concentrations in river water in this study were high, however, which could indicate strong evaporative effects, although other factors can also have the same effects. The river waters also have the highest δD and δ18O concentrations, which is a strong indication of evaporation.

Figure 5 shows that the river water concentrations of δD and δ18O generally fell with increasing TDS – that is, the δD, δ18O, and TDS were negatively correlated, and the δD and δ18O concentrations in river water are influenced by evaporation and other factors. The negative TDS – δD, δ18O, correlations suggest that water-rock interactions may also influence the δD and δ18O contents of water bodies (Chen et al. 2008).

Figure 5

Plots of TDS-δD, δ18O in waters from Suxian coal mining district.

Figure 5

Plots of TDS-δD, δ18O in waters from Suxian coal mining district.

The TDS concentrations in QA and OA groundwater were lower than those in the SA and TA, suggesting that the QA and OA are recharged by precipitation and the groundwaters are not subject to evaporation. This is also suggested by the δD and δ18O properties (Figure 4). The isotopic properties of QA and OA groundwater show evidence of water-rock exchange reactions, leading to slight isotopic enrichment of the groundwater. Further, δD, δ18O, and TDS in QA and OA groundwater were negatively correlated (Figure 5). The TA groundwater TDS concentrations were higher than those of QA and OA, indicating water–rock interaction occurrence in the TA – that is, carbonate dissolution has more influence on groundwater in TA than in QA and OA.

The δD-δ18O concentrations in waters from the river, QA, TA, and OA were all similar (Figure 5), with negative TDS-δD, -δ18O correlations. The general decrease in stable isotope concentrations as TDS increased, suggests that the isotope concentrations in all four sources are controlled mainly by water–rock interactions. The SA groundwater differed from the other waters, however. Its TDS concentrations were higher than in the other aquifers, and the δD and δ18O concentrations were the lowest in any groundwater sampled. Further, the δD and δ18O concentrations in SA groundwater were stable, and did not vary as the TDS concentration changed. These properties indicate that SA groundwater is dominated by static reserves, with either slow or zero flow.

Fisher discriminant analysis

The results presented above show that ground and surface waters have variable features inherited either from their immediate environment through water–rock interactions or from evaporation. The property variations can be used to identify the water types and origins. Previous studies have shown that discriminant analysis is an efficient way to identify water sources and types (Chen et al. 2013). The data from this study were analyzed using Fisher discriminant analysis (SPSS version 19) (IBM Corp. 2010), and the results are presented in Tables 3 and 4, and Figure 6. Two discriminant functions were obtained, and the related parameters are listed in Table 3. The Fisher discriminant functions can be expressed as:
formula
formula
Table 3

Fisher discriminant function parameter values for waters in Suxian coal mining district

ParameterF1F2
TDS 0.002 -.002 
δ18−4.76 −13.16 
Ca2+ −0.03 0.05 
Cl 0.01 −0.08 
HCO3 0.006 0.004 
Constant −15.80 −48.93 
Variance (%) 60.0 38.2 
Eigenvalue 8.79 5.59 
RA (central value) 5.29 −0.86 
QA (central value) −1.30 −1.01 
SA (central value) −0.01 4.92 
TA (central value) −2.42 −0.72 
OA (central value) −0.75 −1.27 
ParameterF1F2
TDS 0.002 -.002 
δ18−4.76 −13.16 
Ca2+ −0.03 0.05 
Cl 0.01 −0.08 
HCO3 0.006 0.004 
Constant −15.80 −48.93 
Variance (%) 60.0 38.2 
Eigenvalue 8.79 5.59 
RA (central value) 5.29 −0.86 
QA (central value) −1.30 −1.01 
SA (central value) −0.01 4.92 
TA (central value) −2.42 −0.72 
OA (central value) −0.75 −1.27 
Table 4

Discriminant results of water and actual source in the study area

SampleActual sourceDiscriminated sourceSampleActual sourceDiscriminated source
River River 17 SA SA 
River River 18 SA SA 
River River 19 TA TA 
River River 20 TA TA 
River River 21 TA TA 
River River 22 TA TA 
QA QA 23 TA QA 
QA QA 24 TA QA 
QA TA 25 TA TA 
10 QA OA 26 TA OA 
11 QA TA 27 OA OA 
12 QA QA 28 OA QA 
13 QA OA 29 OA OA 
14 SA SA 30 OA OA 
15 SA SA 31 OA OA 
16 SA SA 32 OA QA 
SampleActual sourceDiscriminated sourceSampleActual sourceDiscriminated source
River River 17 SA SA 
River River 18 SA SA 
River River 19 TA TA 
River River 20 TA TA 
River River 21 TA TA 
River River 22 TA TA 
QA QA 23 TA QA 
QA QA 24 TA QA 
QA TA 25 TA TA 
10 QA OA 26 TA OA 
11 QA TA 27 OA OA 
12 QA QA 28 OA QA 
13 QA OA 29 OA OA 
14 SA SA 30 OA OA 
15 SA SA 31 OA OA 
16 SA SA 32 OA QA 
Figure 6

Fisher discriminant diagram for waters in the study area.

Figure 6

Fisher discriminant diagram for waters in the study area.

Functions F1 and F2 are the first and second discriminant functions, and X1, X2, X3, X4, and X5 represent the concentrations of TDS, 18O, Ca2+, Cl, and HCO3, respectively. In total, 98.2% of the variance was explained by the first and second functions, with eigenvalues of 8.79 and 5.59, respectively – that is, most of the ground and surface water information can be interpreted from these two functions, including identifying confined groundwaters.

The discriminant analysis results – Table 4 – show that nine samples were inconsistent with their actual sources. For example, the samples from the river and SA were all recognized correctly, but there were 4, 3, and 2 discriminated erroneous samples from QA, TA, and OA, respectively. The similarity of the hydrochemical and isotopic characteristics of some groundwaters in QA, TA, and QA, suggests that that the three aquifers OA are hydraulically connected. The waters fell into three clear groups between the first and second functions on function score scatter diagrams (Figure 6), comprising the river water, and sandstone and limestone aquifer components. Figure 6 also shows the mixing processes between QA, TA, and OA waters suggested in earlier discussion of hydrochemical characteristics.

Calculating water mixing ratios

Previous studies have shown that mixing processes are common in multi-aquifer systems (Hamed et al. 2011). This is particularly true in coal-mining areas, where the mining activity may strengthen the hydraulic connectivity between aquifers, resulting in groundwater mixing (Carreo'n-Diazconti et al. 2003). Major ion geochemistry and environmental isotopes have also been used successfully to identify the origins and mixing ratios in multi-layer aquifer systems (Hamed et al. 2011). Multivariate statistical analysis, like principal component and discriminant analysis, have also been used to establish models for calculating groundwater mixing ratios in multi-aquifer systems (Chen et al. 2014).

As shown, the groundwater in the study area is recharged mainly by precipitation or river water, and is subject to the influence of water–rock interactions. The geological conditions and hydrochemical properties indicate that the carbonate and silicate contents in TA and SA are the two principal controls on the hydrochemical features, so the waters in the rivers, TA, and SA can be considered the three end-members of the multi-aquifer system in the Suxian area. The discriminant functions scores of the 32 water samples are plotted in Figure 7, where they are surrounded by a triangle whose vertices – A, B, and C – reflect the water recharge end-members in the area.

  • A represents river water, recharged by precipitation. It has higher δ18O and δD values because of evaporation.

  • B represents SA groundwater, which had the highest TDS, and lowest δ18O and δD concentrations because of the long aquifer residence time.

  • C represents groundwater influenced by carbonate-rock interactions. While the QA, OA, and TA groundwaters are all influenced by carbonate–rock interactions, their extent varies according to the flow rate in the aquifer – higher flow rates reduce the interaction opportunities.

Figure 7

Calculating water mixing ratio calculation model for Suxian coal mining district.

Figure 7

Calculating water mixing ratio calculation model for Suxian coal mining district.

A conceptual model was developed to calculate the groundwater mixing ratio, based on the three end-members and groundwater sampling points. The calculation method is based on Laaksoharju et al.'s mass balance model (1999). In this, if dot (O) – the sample investigated – is within ABC, the mixing degree is assessed mainly by calculating the end-member groundwater mixing ratios of A, B, and C (Figure 7). End-member A's mixing ratios can be represented by Equation (1):
formula
(1)

B and C's mixing ratios can be calculated similarly.

CONCLUSIONS

The major ion contents, and hydrogen and oxygen isotopic properties of 32 ground and river water samples collected in the Suxian coal-mining area, Anhui Province, China, were determined. Discussion of the hydrochemical characteristics, main influences, and mixing processes is based on traditional graphs and discriminant analysis. The conclusions are as follows:

  • (1)

    All waters sampled were alkaline, while TDS concentrations differed according to the source. The TDS concentrations were highest in SA and lowest in OA groundwaters, with its concentrations in river water, and TA and QA groundwaters in between and in that order. δD and δ18O concentrations were highest in river waters and lowest in SA groundwater. Distribution of the QA and OA groundwater isotope values along the LSWL, and the distribution line's slope being close to that of the GMWL, indicate that the QA and OA are recharged by rainfall and from surface water.

  • (2)

    The relatively higher TDS concentrations, and the δD and δ18O enrichment in river water reflect evaporation in the relatively dry climate. The QA, OA, and TA groundwaters were influenced mainly by water–rock interactions. The carbonate rocks in the TA may have offered the maximum water-rock interaction capacity. Such interactions were weak in the QA and OA, where groundwater flow rates were relatively high. SA groundwater has different characteristics, and the δD and δ18O concentrations determined were stable, indicating that SA groundwater is dominated by static reserves.

  • (3)

    Two Fisher discriminant functions were determined and explained 98.2% of the variance. The discriminant diagrams were divided into three groups, which can be considered the three water source end-members. The 32 points on the diagram were surrounded by a triangle, with vertices A, B and C, and a conceptual model was established to calculate groundwater mixing ratios between the end-members and the groundwater sampling points.

ACKNOWLEDGEMENTS

The study was supported by the ‘excellent top-notch talents project’ in colleges and universities of Anhui Province (gxbjZD2020091), the Natural Science Foundation of Anhui Province (1708085QE125) and the National Natural Science Foundation of China (41773100).

DATA AVAILABILITY STATEMENT

All relevant data are included in the paper or its Supplementary Information.

REFERENCES

Carreo'n-Diazconti
C.
Nelson
S. T.
Mayo
A. L.
Tingey
D. G.
Smith
M.
2003
A mixed groundwater system at Midway, UT: discriminating superimposed local and regional discharge
.
Journal of Hydrology
273
,
119
138
.
Chen
L. W.
Gui
H. R.
Yin
X. X.
Qian
J. Z.
2008
Composing characteristics of 18O and D and current field in deep groundwater
.
Journal of China University of Mining & Technology
37
(
6
),
854
859
.
Craig
H.
1961
Isotopic variation in meteoric water
.
Science
133
,
1702
1703
.
Gibbs
R. J.
1970
Mechanisms controlling word water chemistry
.
Science
170
,
1088
1090
.
Gui
H. R.
Chen
L. W.
Song
X. M.
2005
Drift features of oxygen and hydrogen stable isotopes in deep groundwater in mining area of northern Anhui
.
Journal of Harbin Institute of Technology
37
,
111
114
(in Chinese)
.
Gui
H. R.
Sun
L. H.
Chen
L. W.
2011
Rare earth element geochemistry of groundwater from a deep seated sandstone aquifer, northern Anhui Province, China
.
Mining Science and Technology
21
,
477
482
.
Hamed
Y.
Dassi
L.
Tarki
M.
Ahmadi
R.
Mehdi
K.
Dhia
H.
2011
Groundwater origins and mixing pattern in the multilayer aquifer system of the Gafsa-south mining district: a chemical and isotopic approach
.
Environmental Earth Science
63
,
1355
1368
.
IBM Corp
.
2010
IBM SPSS Statistics for Windows, Version 19.0
.
IBM Corp
,
Armonk, NY
.
Laaksoharju
M.
Skarman
C.
Skarman
E.
1999
Multivariate mixing and mass balance (M3) calculations, a new tool for decoding hydrogeochemical information
.
Applied Geochemistry
14
,
867
871
.
RockWare, Inc.
.
2003
Aq·QA 1.0
.
Golden, Colorado, USA
.
Software available from: http://www.aqqa.com.
Zhang
H. P.
1989
Study on the background value of stable isotopes in precipitations of China
.
Site Investigation Science and Technology
1
,
6
13
(in Chinese)
.
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