JOURNAL OF COAL SCIENCE & ENGINEERING (CHINA)
ISSN 1006-9097
pp 294–298
Vol.14 No.2
June 2008
Research on mechanism of groundwater pollution from mine water in abandoned mines∗ WANG Lai-gui(王来贵)1, LI Xi-lin(李喜林)2, LIU Ling(刘
玲)2, HAN Liang(韩
( 1. College of Mechanics and Engineering, Liaoning Technical University, Fuxin Architectural Engineering, Liaoning Technical University, Fuxin Fuxin
亮)3
123000, China; 2. College of Civil and
123000, China; 3. Hydrographical Bureau of Fuxin,
123000, China )
Abstract In order to understand the mechanism and regularity of the groundwater contamination from mine water of abandoned mines, experiments were conducted on an abandoned coal mine in Fuxin, a representative city with lots of mine water in northeast China. The groundwater pollution from different contaminants of coal-mining voids (total hardness, SO 24 − , Cl and total Fe) and pollution factors transportation situation in the coal rock were simulated by soil column experiment under the conditions of mine water leaching and main water leaching (similar to rainwater leaching), and the water-rock interaction mechanism was discussed during mine water infiltration through saturated coal rock by application of principle of mass conservation, based on physical properties of coal rock, as well as monitored chemical composition. The results show that, compared with the clear water leaching process, trends of change in pollutant concentrations presented different characteristics in the mine water leaching process. Groundwater is contaminated by the water rock interactions such as migration & accumulation, adsorption & transformation, dissolution & desorption and ion exchange during the mine water permeation. The experiments also suggest that at first dissolution rate of some kinds of dissoluble salts is high, but it decreases with leaching time, even to zero during both the mine water leaching and main water leaching. Keywords groundwater pollution, mine water, abandoned mines, soil column experiment, water-rock interaction, pollution mechanism
Introduction There are approximate 1 000 mines that have been closed or will be closed in 426 mining cities in China currently [1]. Because of the cessation of drainage system that commonly follows mine closure, underground spaces, such as abandoned mining sites, roadways and so on, which are formed after mining, will be filled with water gradually and become underground lakes eventually. Substances with acidity or alkalinity, some with toxicity, heavy metal-bearing
minerals, organics, microbes and so on in accumulated mine water will contaminate groundwater in the mining district, and even cause environmental accidents. However, there are few reports concerned about contamination problems induced by abandonment in China. Compared with the water drained out of mine, accumulated mine water in abandoned mines is more likely to result in deep groundwater contamination, which is more secretly and more serious in the long run [2]. A great deal of research work about the effect of mine drainage on surface water and groundwater
∗ Supported by the National Natural Science Foundation of China(50434020, 50374042), Science & Technology Found of Liaoning Province (20022155); Specialized Research Fund for the Doctoral Program of Higher Education (20040147003) Tel: 86-418-3351087, E-mail:
[email protected]
WANG Laigui, et al. Research on mechanism of groundwater
has been conducted by scholars at home and abroad presently [3 7]. However, researches on the mechanism of groundwater contamination from abandoned mine are relatively less, meanwhile, groundwater pollution mechanism and contamination pattern after mine closure are different from that in mining phases [8]. So the primary objective of this study is to investigate water-rock interaction mechanism in the process of mine drainage leaching and to discuss contamination regularity of various contaminants and the transportation mechanism of pollution factors in saturated coal rock. The results will provide scientific basis for both prevention and harness of groundwater contamination in abandoned mining areas.
1
Materials and methods
Experimental device of soil column is shown in Fig.1 (90 cm is the effective length of the soil column, 13.2 cm is the diameter and 136.78 cm2 is the crosssectional area).
295
were air dried, smashed, put into the column and tamped in sequence for further experiments in laboratory. Soil column was leached for 48 h with distilled water to form saturated surrounding, followed by mine water leaching. In the process of experiment, measurements of substances in water samples comply with Chinese standard methods edited by State Environmental Protection Administration of China.
2
Experimental process
Mine water used for leaching experiments was taken from an abandoned pit in Wulong mining district. In order to make contamination transportation law more conspicuous in the laboratory and to compare interaction of different contaminants from mine water with rock influenced by mine water, the mine water was prepared by adding solution containing each ion Na, Ca, Mg and Fe ( in the form of pure NaCl, CaCl2, MgSO4 and FeSO4 respectively) evenly to the prepared mine water samples. Then the concentrations of these concerned ions in samples were higher. Some main constituents in two different experimental samples are shown in Table 2. Table 2
Fig.1
Experimental device of soil column
Rock samples used for experimental analysis were collected from Wulong Mine in Fuxin, where mining activities had been conducted for many years. According to the sequence of terrane deposition during historic periods, the very typical rock samples were selected for the leaching experiments, whose physical properties and chemical compositions (Table 1) of the samples were obtained respectively. Rock samples Table 1 Physical properties and main chemical composition of rock samples Properties of rock
Dry density (g/cm3)
Wet density for 2 h (g/cm3)
Saturated density for 12 h (g/cm3)
Porosity (%)
Coal rock
1.57
1.86
1.91
34.2
w(SiO2)
w(Al2O3)
w(Fe2O3)
w(CaO)
w(MgO)
61.13%
17.71%
10.32%
5.02%
4.38%
Properties of rock Coal rock
Concentrations of some constituents in water samples mg/L
Items
Total hardness
ρ (SO42 )
ρ (Cl )
Total Fe
pH
Mine water
980.0
581.2
332.4
12.505
7.60
Clear water
101.5
65.7
36.5
0.150
7.25
2.1
-
-
Mine water leaching experiment In order to keep hydrofacies and soils in a state of equalization, distilled water was injected by a constant current pump to leach soil in soil column for enough time. When the water in the soil column flowed smoothly, the formulated mine water solution in advance was injected in the soil column. After that, leachate samples were collected every 2 hours and solution concentrations were obtained by chemical experiments and analysis methods. And mine water leaching experiment was finished when solute concentrations were equilibrated with those in mine water solution prepared. 2.2 Clear water leaching experiment Clear water leaching experiment was conducted a day later after mine water leaching experiment. And this leaching experiment lasted for 3 days, meanwhile the leachate samples were collected every 4 hours.
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3
Journal of Coal Science & Engineering (China)
Results and discussion The charts of each component concentration var-
Fig.2
iation with time were drawn in the Fig.2 in mine water infiltration process according to experimental data obtained from two leaching experiments.
The exudate ionic concentration change chart under the condition of mine water and clear water leaching
Trends of change in pollutant concentrations (Fig.2(a)) were described under the condition of mine water leaching. Concentrations of total hardness, SO42 and Cl in exudate were lower at first, with ionic concentration values increasing, ionic contents out of exudate reached a maximum value, decreasing after that, even to equalization values at last. When the equalization values corresponded to monitored ionic concentration values in mine water, entrapment ability of soil column to pollutants was lost. There were obvious differences between ion Fe concentration change and rest monitored constituent concentration change. Ion Fe couldn’t be detected until mine water leaching experiment had been conducted for 8 h, showing apparent time lag. The release treads of ion Fe were significantly increased with leaching time (sharply increasing from 12 h to 24 h, slowing down after 24 h and reaching peak at 58 h) , however, decreased after 60 h, when the ion Fe concentration value almost corresponded to that in mine water. This meant that the sorption capacity of rock to ion Fe was great. Monitored various ion concentrations out of exudate (Fig.2(b)) were variable in clear water leaching experiment, following mine water leaching experiment. It could be concluded that concentration curves of total hardness, SO42 , Cl and total Fe were similar dur-
ing clear water leaching, whose trends were almost decreasing. Generally, many water-rock interactions such as migration accumulation, mechanical infiltration, transformation and self-purification, dissolution and sedimentation, ion exchange and absorption and so on would occur in infiltration process. Various actions were analysed as following. 3.1 Absorption and dissolution of soluble salts According to the law of mass conservation, for any kind of ion, ion qualitative addition of percolate in contrast to leachate containing monitored ions was equal to ion inlet from soluble salts of soil column due to dissolution, as is shown in the following equation: I
∑ (V C ) − VC i =1
i
i
0
= Δm,
(1)
where, Vi is the volume of samples collected each time, mL; Ci is the concentration of researched constituents obtained by consecutive sampling and monitoring, mg/L; I is the overall times of sampling; V is the overall volume of percolate, mL; C0 is the concentration of researched constituents in leachate, mg/L; Δm is the mass of some soluble salt entered exudates, mg. Generations of dissolution and desorption were bound to Δm >0 and generations of absorption and translation were bound to Δm <0, while Δm =0 de-
WANG Laigui, et al. Research on mechanism of groundwater
noted equilibrium state of absorption and dissolution for soluble salts in the soil column experiments discussed. The relationships between volume of leachate and total mass of contamination compositions from various soluble salts in soil column are presented by Equation (1) during each experiment. The ion qualities into exudate in mine water leaching experiment are calculated in Table 3. Table 3
The ion qualities into exudate in mine water leaching experiment mg
Volume of percolate(mL)
Total hardness
ρ (SO42 )
ρ (Cl )
ρ (Fe)
382
-136.985
-59.401
-47.215 2
-0.812 83
-
-
812
-173.062
-10.553
-74.692 2
-1.813 23
1 267
-235.306
42.773
-60.678 2
-3.063 73
1 767
-249.156
117.223
-29.528 2
-4.364 25
2 287
-238.756
237.187
22.679 8
-5.674 54
2 861
-199.954
359.104 6
62.630 2
-6.940 82
3 473
-41.262 2
549.069 4
107.612 2
-8.063 11
4 273
89.937 8
730.669 4
152.012 2
-9.338 96
5 014
253.847
851.007 8
192.767 2
-10.329 7
5 514
369.197
910.857 8
233.217 2
-11.565 7
6 053
609.375 4
947.887 1
269.869 2
-12.721 7
6 734
861.481 6
999.166 4
309.980 1
-13.315 6
7 434
1 009.602
1 121.386
348.410 1
-13.781 1
7 935
1 096.352
1 147.736
361.260 1
-14.279 5
8 535
1 235.002
1 227.609
394.615 6
-14.674 4
9 222
1 364.021
1 240.731
422.507 8
-15.133 1
9 794
1 484.370
1 271.447
459.973 8
10 318
1 580.314
1 302.521
10 838
1 670.638
11 688
297
obviously different from that of total hardness, SO42 and Cl during the whole mine leaching experiment, exhibiting absorption and translation all the time. The solution rates of soluble salts are given by: D = ΔmΔV0 . (2) Where the slope of each point on curves corresponds to the solution rate of soluble salts at the same time, showing that at the beginning of dissolution phases dissolution rates of soluble salts are high, but decreasing with leaching time, even to zero during both the mine water leaching and clear water leaching. The total ion qualities leaching out from soil column in clear water leaching experiment are shown in Table 4. It can be concluded that total hardness, SO42 , Cl and total Fe all generate dissolution during clear water leaching. Contaminants originally accumulate in rock ingressed into water flow as a result of clear water leaching, so groundwater is contaminated. Still we can find out that dissolution rates of monitored soluble salts are higher at the beginning, decreasing with time, even to zero still at the end of clear water leaching experiment. -
Table 4
Total hardness
ρ (SO42 ) -
ρ (Cl )
570
504.393
281.694
180.747
7.053 75
1 250
1 047.917
600.07
364.143
15.363 35
2 100
1 723.837
986.99
590.328
25.250 55
-15.570 2
3 100
2 430.437
1 422.99
832.628
34.748 55
484.497
-15.897 3
4 000
2 951.537
1 799.64
1 026.218
43.111 35
1 368.925
499.681
-16.329 1
4 740
3 300.151
2 067.076
1 160.232
49.003 97
1 814.118
1 414.23
513.706
-16.690 2
6 010
3 730.554
2 421.279
1 330.412
58.344 82
12 788
2 067.998
1 516.75
525.146
-17.014 1
7 160
4 010.349
2 673.129
1 452.657
66.344 22
13 504
2 027.332
1 562.216
517.556 4
-17.305 6
8 190
4 198.633
2 848.641
1 576.154
72.255 39
14 126
1 924.536
1 587.78
514.881 8
-17.673 8
9 770
4 430.893
3 058.939
1 701.132
79.937 35
16 174
1 892.210
1 651.473
555.841 8
-18.024 7
10 670
4 541.593
3 154.879
1 766.202
83.400 55
17 374
1 886.290
1 694.633
580.681 8
-18.235 4
12 070
4 663.533
3 276.399
1 858.042
87.729 35
18 329
1 892.018
1 712.423
593.192 3
-18.348 1
13 820
4 794.083
3 397.674
1 954.817
90.382 35
19 230
1 904.290
1 713.594
609.770 7
-18.387 8
15 920
4 947.383
3 530.184
2 037.347
92.530 65
17 120
5 007.983
3 590.304
2 087.627
93.340 65
18 680
5 099.555
3 636.636
2 146.751
94.159 65
21 580
5 179.015
3 699.856
2 239.261
95.328 35
26 080
5 265.865
3 829.906
2 370.211
96.268 85
It can be concluded from Table 3 that total hardness, SO42 , Cl are all adsorbed by the rock at first, and both dissolution and desorption occur following immediate adsorption equilibrium. The case of Fe is
Volume of Percolate(mL)
The ion qualities into exudate in clear water leaching experiment mg -
ρ (Fe)
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Journal of Coal Science & Engineering (China)
Ion exchange interaction At the preliminary stage of mine water leaching experiment, total hardness content out of exudate (Table 3) gradually increases and preferential water-rock interaction is dissolution of soluble salts. With the process of experiment, total hardness content out of exudate reaches a maximum value, following decrease of total hardness in leachate instead of keeping going up with hardness from soil column, which probably attributed to ion exchange interaction (Ca2+ and Mg2+, main elements of hardness, are withheld by rock and adsorbed by rock particles), as is shown in the following reactions: 2NaX+Ca2+=CaX2+2Na+,
References
3.2
[1]
[2]
2NaX+Mg2+=MgX2+2Na+.
4
Conclusions
(1) Not only the mine water itself in abandoned mine but also the pollutants of the mine water leached by the rainwater after transferring and accumulating in rock will contaminate groundwater to a great extent. (2) During mine water leaching experiment, concentrations of total hardness, SO42 and Cl in exudate are ranging from the lower to the higher at the beginning, reaching a maximum value, followed by decreasing, even to equalization values at last, when the equalization values correspond to monitored ionic concentration values in mine water. However, ion Fe detection shows apparent time lag, and the release treads of ion Fe are significantly increased within leaching time. Concentration curves of total hardness, SO42 , Cl and total Fe are similar during clear water leaching, whose trends are almost decreased. (3) The water-rock interaction mechanism in the process of mine water infiltration through saturated coal rock is summarized as follows: at the beginning of mine water infiltration total hardness, SO42 , Cl are all adsorbed by the rock, and dissolution, desorption and ion exchange occur following immediate adsorption equilibrium, and both adsorption and transformation continue all the time during mine water leaching, in contrast the ions concerned all show dissolution in the process of clear water leaching. (4) Dynamics characteristics to dissolution of soluble salinities monitored are clearly similar for both mine water leaching and clear water leaching, that is dissolution rates are relatively higher at first, decreasing with leaching time, even to zero eventually.
[3]
[4]
[5]
[6]
[7]
[8]
王来贵, 刘向峰, 吕明海, 等. 资源枯竭城市衍生灾害 中的环境岩石力学问题[J]. 岩石力学与工程学报, 2005, 24(15): 2 715-2 717. Wang Laigui, Liu Xiangfeng, Lü Minghai, et al. Problems of environmental rock mechanics in derived calamities of resources exhaustion cities[J]. Chinese Journal of Rock Mechanics and Engineering, 2005, 24(15): 2 715-2 717. 杨社锋, 方维萱, 胡瑞忠. 中国煤矿废弃物环境效应研 究 进 展 [J]. 矿 物 岩 石 地 球 化 学 学 报 , 2004, 23(3): 264-269. Yang Shefeng, Fang Weixuan, Hu Ruizhong. Advances in studying environmental impact and pollution control of coalmine waste in China[J]. Bulletin of Mineralogy Petrology and Geochemistry, 2004, 23(3): 264-269. Navarro A, Biester H, Mendoza J L, et al. Mercury speciation and mobilization in contaminated soils of the Valle del Azogue Hg mine (SE, Spain)[J]. Environ. Geol., 2006, 49: 1 089-1 101. Robert S H, Sherry L S, Theodore J W. Acid mine drainage flowing from abandoned gas Wells[J]. Mine Water and the Environment, 2005, 24: 104-106. Roel Cruz, Ignacio Gonzalez, Marcos Monroy. Electrochemical characterization of pyrrhotite reactivity under simulated weathering conditions[J]. Applied Geochemistry, 2005, 20(1): 109-121. Sanchez Espana J, Lopez Pamo E, Santofimia Pastor E, et al. The impact of acid mine drainage on the water quality of the Odiel River (Huelva, Spain): Evolution of precipitate mineralogy and aqueous geochemistry along the Concepcio Ntintillo Segment[J]. Water, Air, and Soil Pollution, 2006, 173: 121-149. 吴耀国, 沈照理, 钟佐燊, 等. 矿井水污染的地表水灌 溉入渗过程中的水岩作用[J]. 煤田地质与勘探, 2004, 32(3): 38-40. Wu Yaoguo, Shen Zhaoli, Zhong Zuoshen, et al. Water rock interaction during irrigation with polluted water by coal mine drainage[J]. Coal Geology & Exploration, 2004, 32(3): 38-40. 薛 强, 王惠芸, 刘建军. 采煤矿区地下水脆弱性评价 [J]. 辽宁工程技术大学学报, 2005, 24(1): 8-11. Xue Qiang, Wang Huiyun, Liu Jianjun. Study on groundwater vulnerability assessment in coal mining area[J]. Journal of Liaoning Technical University, 2005, 24(1): 811.