Yueming Qi¹,Yuxiang Jiang²,Lanhe Yang¹,Yongguo Yang¹

（1.China University of Mining and Technology, Xuzhou,Jiangsu,221008,China；

2. Feicheng Mining Group Co. L td, Taian,Shangdong,271608 , China)

 

Abstract:Feicheng colliery is noted for its severely-threatened groundwater in Shandong province, China. After analyzing the data of the groundwater levels and water quality collected from 1963 to 2006, the authors find that the evolution of groundwater environment in the mine shows ever-increasing remarkable change difference in space and time under human influence. Furthermore several important hydrogeologic meanings in the evolution are revealed, including groundwater renovate rate, hydrogeochemical actions, hydraulic contact among aquifers, etc. Finally five primary factors are pointed out which contribute to the evolution, they are atmospheric precipitation, geologic structure, manually discharging water, mine drainage and sewage leakage in gangue stacks. This work will provide scientific foundation for following exploitation of deep coal seam, utilization of mine water and evaluation of  groundwater quality and quantity.

Keywords: groundwater environment, evolution,hydrogeology, renovate rate, Feicheng colliery

1 Intruduction

Feicheng colliery locates in Feicheng city, Shandong Province, in the north of Feicheng basin(Fig.1). Surrounding are low mountains that with the former Sinian Period granite-gneiss, Cambria system and Ordovician limestone. The colliery extends for 22 kilometers from west to east , 2 to 7 kilometers from north to south, and its area is 98 square kilometers. The geomorphology is a alluvial plain, the overall trend is high in northeast and low in southwest, From east to west, there are nine well fields in the colliery, they respectively are Yangzhuang, Caozhuang, Dafeng (closed in 2005), Taoyang, Xinlong, Baizhuang  Pingyin,Chazhuangand Guojiazhuang. Mean annual precipitation is 652.8mm, coal output is 6.810⁹kg/a, and mine drainage gets 4.2610¹⁰ kg/a, and it is noted for its severely-threatened groundwater.

2 Hydrogeological Conditions

2.1 Natural recharge, runoff and drain conditions before digging

Atmospheric precipitation is prime recharge source of groundwater. In the natural conditions before 1960, the groundwater flowed from the northeast to west or southwest, the southwest in the basin was the drain port of the surface water and groundwater.

2.2 Stratums

The strikes of the stratums are east-northeast and the dip angles are 6˚〜18˚, toward north. From the top down (or from new to old), the stratums are Quaternary System, Permian System , Carboniferous System, Ordovician System, Cambria System and the former Sinian Period.

3 Evolution of groundwater environment

Water environment, viz. the spatio-temporal conditions around water bodies. Usually people  point it to water quality, but hydrodynamical conditions also have important influence on the development of water quality. Thus, exactly to say, the water environment should refer to physical, chemical, biological conditions and the transformation during the formation, movement, change of nature water.

So the groundwater environment not only includes groundwater chemical environment, but also groundwater dynamical conditions. The evolution of groundwater environment is slow in natural conditions, only when big changes of external conditions occur, which may result in groundwater system changing greatly in a short term. Therefore, the authors will make a systematic exposition for spatio-temporal characteristics of groundwater environment from the hydrodynamic conditions (mainly water levels) and the hydrochemical conditions (water quality).

3.1 Trends of the water level fluctuations

Water levels are important indicator of groundwater dynamic conditions, especially for different aquifers, the differences of water levels show the way and degree of their hydraulic contact.

Without human influence, the evolution of groundwater dynamic environment is very slow. Among other factors, such as precipitation, not only affects the amount of groundwater safety exploitation, but also directly determines the supply amount of groundwater. Thus, the evolution of groundwater dynamic environment is resulted from many factors, Such as hydrogeological conditions, the intensity of human activities, weather and hydrologic factors ^[1], etc. Before 1960, the groundwater flow field was kept in the natural state in the colliery, the movement of groundwater was mainly  horizontal, and its vertical movement was unobvious, hydraulic contact between the aquifers was weak, for there were some clay seams and thick sandrock among them, which obstruct their contact, so the water levels in aquifers were different ,water pressure and types of water quality were also different from each other.

With coal-digging, the water levels of some aquifers descended and came into being descended water cones^[2]. Up to now, scores of water cones have been engendered in different dimensions in the area, and the directions of groundwater movement have been changed greatly, groundwater could not flow freely out from basin, but was extracted to the ground by human actions (such as mine drainage and water supply,etc.), thus the natural open groundwater system was evolved into independent relatively water cycle system in present.

Among main aquifers, due to the lack of recharge resources, a majority of hydrogeologic blocks in 4th limestone have not abundant groundwater except few blocks whose geological structures are complex, so the water level has been descended rapidly and 4th limestone was almost drained emptily in 1983. Whereas, trends of water levels in 5th limestone and Ordovician limestone have been descended gently until 2003, subsequently, again ascending and its level got +52.70m in January,2006(Fig.2).

In space scale(Magnitude is meter), groundwater quality shows obvious differences with the variation of space positions .

In vertical direction, take Baizhuang well field as an example, the following table(table 1) shows the water quality in aquifers and surface water.

Fig. 1 The flow field of groundwater in Feicheng  Colliery

 Table 1  Thecontents of main compositions in2004 inBaizhuang well field   unit: mg/L

Items	K+	Na+	Ca2+	Mg2+	NH4+	Cl－	SO42-	HCO3-	F－	NO2－	NO3	Hardness	TDS
Surface water	1.7	85.0	122.1	28.1	0.0	34.0	273.6	326.8	0.5	0.0	19.3	420.5	739.2
Quaternary System	0.3	22.3	164.7	23.9	0.0	65.0	133.3	325.4	0.2	0.0	94.9	509.9	680.9
Minewater	3.1	137.3	47.6	18.7	0.0	36.9	172.8	388.7	0.7	<0.004	14.1	195.8	647.8
4th limestone	1.4	185.0	13.7	5.4	0.6	24.0	35.8	422.5	1.6	<0.004	<0.4	56.5	501.0
5th limestone	1.6	25.3	77.9	24.5	0.0	27.0	60.6	304.2	0.4	0.2	6.5	295.3	386.7
Ordovician	0.8	15.2	82.9	24.3	0.0	30.0	49.5	284.4	0.4	0.0	16.6	307	378.2

 

Table 2  Thetypes of water quality in 4th limestone aquifer

Well fields	Yangzhuang	Caozhuang	Dafeng	Taoyang	Baizhuang	Guojiazhuang	Chazhuang
Types of water quality	HCO3-Na	SO4-Na	HCO3Cl-Ca	HCO3-Na	HCO3Cl-Na	HCO3-Ca	HCO3-Na

 

Table 3  The change of key ions in three main aquifers

Aquifers	Date	K++Na+	Ca2+	Mg2+	Cl－	SO42－	HCO3－
4th  limestone	1963.9	284.74	3.21	1.09	48.56	4.94	577.2
	1993.9	244.79	5.69	0.72	34.74	5.35	533.79
	2004.6	186.4	13.69	5.43	23.97	25.78	422.5
5th  limestone	1964.4	74.06	42.1	18.18	32.62	22.22	331.95
	1989.4	14.72	74.45	17.53	16.08	33.65	283.22
	1993.8	19.32	74.38	22.11	17.03	45.89	299.72
	2004.6	26.95	77.89	24.47	27.03	60.54	304.24
ordovician limestone	1963.9	29.14	64.61	25.86	21.81	72.84	273.85
	1993.8	8.51	76.61	19.83	15.85	31.28	287.86
	2004.6	15.99	82.94	24.25	29.99	49.53	284.41

 

Table 4   The way and characteristics of hydraulic contact

Items Types of contact	Way of contact	Hydrochemistry	Water level	Change route	Extents of contact
Horizontal direct contact	horizontal flow	horizontal change commonly	common change from high to low	long	common
Horizontal indirect contact	horizontal slow flow by water-hindering fault	horizontal change obviously	slow change from high to low	short	slow
Vertical indirect contact	leakage	vertical change obviously	level regime is different	short	weak
Vertical direct contact	conduct-water faults	vertical change unobviously	level regime is similar	long	intense

 

            From the table, we can find that the hydrochemical characteristics are different between the aquifers.

Since commonly there is a stable clay terrane under Quaternary System pore water aquifer that make it almost no hydraulic contact with Carboniferous-Permian coal system aquifers and Cambria- OrdovicianSystem karst aquifers, so Quaternary System pore water aquifer has its obvious characteristics that the contents of K⁺, F⁺ is low, while NO₃^-, Ca²⁺ is high, that agrees with the result of Zheng Ma’s research^[3].

Ordovician karst fissure water is the main type of groundwater in Feicheng basin, due to long-term massive drainage (such as mine drainage and water supply exploitation), make it in a good oxidative environment, in which nitrogen element is turned into saltpetre state (NO₃^-) or subsaltpetre state (NO₂^-), the ammonia ion (NH₄⁺) content is low or even little.

Opposite to the Ordovician limestone karst water, in the early days, 4th limestone karst water was in the deoxidization conditions, although its eventual recharge was also coming from Ordovician karst water, but the contents of NO₃^- and NO₂^- are very low, so is its hardness. While the contents of ammonia ions (NH₄⁺), potassium-natrium ions (K⁺, Na⁺,) and fluorine ion (F^-) are all high.

The water quality of 5th limestone karst water is similar to that of Ordovician limestone and the differences between them is less.

According to the analyzing of mine water quality, the contents of some ions such as SO₄^2-,F^- and hardness are all high, their main recharge sources come from 4th limestone and Ordovician limestone aquifers, so mine water is a mixed water. When it is mainly recharged by 4th limestone karst water, the contents of hardness and nitrogen are lower; when mainly recharged by Ordovician limestone karst water, they are higher. At the water warehouse of -150 and -230 m level in Baizhuang well field, where the compositions of mine water are similar with that in Ordovician limestone karst water ,it shows that there is close contact between them and should be paid great attention to.

Besides the vertical differences of water quality in aquifers, some differences are also shown in plane. The same aquifer, when it meet different geological structures or same structure while in different positions, viz. the geological conditions are different, then characteristics in hydrogeology and hydrochemistry are different, such as the change of the types of water quality in 4th limestone aquifer(Table 2).

In time scale(Magnitude is year),with time going on, there are obvious differences in water quality. The reasons are the geological environment is constantly changing although slowly, and human actions have exacerbated it. Take Baizhuang as an example(Table 3), though groundwater quality has good inheritances, the contents of main chemical compositions in the three different aquifers have been regular change trends in forty years.

4 Analyzing of influence factors

Changes of groundwater levels are mainly influenced  by the recharge and outflow conditions in the aquifer. If recharge amount is more than outflow’s, the water level will rise. contrarily, it will fall. While changes of water quality are complex, because the form and evolution of the chemical compositions in groundwater are a long-term geological development process and the water quality compositions are distributed in different depths of aquifers and different positions of tectonics, and affected by all kinds of  factors, so the groundwater in aquifers has its own hydrochemical characteristics^[4].

Nature quality of groundwater is mainly controlled by four key factors, they are geography, hydrogeological conditions, geological structures and physical chemistry functions ^[5],etc.

In addition, groundwater quality in colliery is changed along with space-time. In space, it has horizontal and vertical zone; in time, quality changes of nature groundwater are very slow, but once human actions destroy the geological environment, then they would change rapidly.

Generally, the factors that cause the change of water level are usually also the ones of water quality, which include natural factors and human factors.

4.1 Nature factors

Geological structures and atmospheric precipitation are two key nature factors. Among geological structures, fault structures play a major role in this area, faults can work as transmiting-water channels, or collecting-water corridor, or hindering-water barriers ^[6], etc. If the capability of transmiting-water is well for some faults, they can communicate traversed aquifers and keep them a unite hydraulic contact, such as F3 fault(Fig.1 and Fig.3).In reverse, if capability of transmiting water for the others is very bad, then, that will separate every aquifer and obstruct their hydraulic contact, such as  F23 fault（Fig.1）.

    Precipitation is a primary way to recharge groundwater. its differences with surface water and groundwater are not only showing in TDS but also in key ions and the contents of organic matters. Commonly it have richer SO₄² than the others, and its ion contents, [SO₄^2-]>[HCO₃^-]>[Cl^-]. Its infiltration not only changes the contents of main compositions and TDS of groundwater, but also changes the oxidation-deoxidization conditions for having much dissolved oxygen，thus the quality of groundwater changes. Furthermore, precipitation heightens the groundwater level, for instance, the water level in Ordovician aquifer increase rapidly during 2004 to 2006, a main reason is the rich precipitation recharge in this period.

4.2 Human factors

(1)Mine water discharging and groundwater exploitation. The water sources of mine drainage are Permian System sandstone fissures water and Carboniferous System lamina limestone karst fissures water, while groundwater explotation is mainly taken out from Ordovician limestone. According to some survey, the mean annual drainage is 4.2610⁷m³/a in this area. and groundwater exploitation amount reached 2.0810⁸ m³/a in 1996, whereas 2.7610⁸ m³/a in 1998^[7]. The original water level in Ordovician limestone aquifer is +63m around, has been descended for years, and reached the lowest water level -17.65m in July ,2003.

Both the human actions above make groundwater lever descend, hydraulic gradient increase, erode effect strengthen, and the water flow rate accelerate. Furthermore, because of descending of groundwater lever, the scopes of unsaturated zone expand correspondingly and differences of water level in different aquifers increase, which change hydrodynamic conditions and previous oxidation-deoxidization conditions.

(2) Mine drainage irrigating. As we know, water quality of mine drainage is worst among various samples. Due to the lack of surface water, mine drainage is used directly to irrigate, then irrigation water infiltrates in the underground and takes an adverse impact on the soil environment, thus the whole groundwater environment is destroyed to a certain extent.

(3)Sewage leakage in gangue stacks, which mainly affects shallow groundwater. There are twelve gangue stacks like hills, and the occupied area is 3.96 10⁵m² ,and the annual amount of produced gangue is 1.1010⁹ kg/a. the open gangue contains sulphur, while it is exposed to rain drip and air oxidation, it will create acidic water. If acidic water and other harmful substances flow into surface water system or underground aquifers, they will affect previous water quality. Moreover, if coal preparation waste water leaks that will also bring disadvantageous effects ^[8].

As a whole, natural factors and human factors can be united to work. For example, under mining conditions, geological structures such as hindering- water faults can be aroused to transit water under human disturbances.

5 Hydrogeologic Meanings of the Evolution

5.1 Calculating approximately lateral recharge amount and renovate rate of groundwater

In this area, if the infiltration coefficient adopts 0.3,then we can calculate the precipitation recharge amount, that is 0.1910⁸m³/a. Annual mine water drainage is 4.2610⁷m³/a, present groundwater exploitation assures 2.8010⁸m³/a, then groundwater is reduced 3.0410⁸m³/a, the reduced amount comes from lateral recharge, and it is composed of all kinds of karst water coming from south, west, northwest, east and northeast hill area(Fig.1). Since the groundwater levels have been increasing in the past three years, so its lateral recharge amount is more than 3.0410⁸m³/a. If taking the thickness of 5th limestone and Ordovician limestone as 830m and the rate of containing water as 0.1,then we can calculate that their water deposit amount is 81.3410⁸m³, after ignoring confined elastic deposit, groundwater renovate rate is（0.19+3.04）81.34=4%. Above these show that depth of water circulation is augmenting and the evolution of groundwater environment is obviously accelerating.

5.2 Making an opinion to hydrogeochemical actions

Many hydrochemical actions will happen when water flows in the subsurface, such as mix action. Taking mine water as an example, for its recharge comes from 4th limestone and Ordovician limestone, its quality type is HCO₃—CaMg or HCO₃SO₄ —CaMg or HCO₃—CaNa. But infiltration action happens in the whole route and it will have alternate ions absorption at the existence of clay terrane, etc. It is these actions that make the differences of water quality prominent when groundwater meet various rocks and minerals^[9] . So we can use hydrochemical data to analyze all kinds of hydrochemical actions and make certain the resources of TDS by chemistry and isotope equilibrium methods.

5.3 Judging the way and extents of hydraulic contact among aquifers

The differences of the water quality and water level in space also demonstrate the extents of the hydraulic contacts among aquifers. If there are similar characteristics in water quality and the trend of groundwater level regime that shows the hydraulic contacts among aquifers are intense. On the contrary, they are inadequate. Here four types of hydraulic contacts are summarized from water quality and water level, their ways and characteristics of hydraulic contacts show as Table 3.

6 Conclusions

From the analyses above, we conclude that:

(1)In the Feicheng collery, the evolution of groundwater environment shows ever-increasing remarkable change difference in space and time under human influence. In space, water quality has horizontal and vertical zone, the groundwater system is evolved to man-made independent relatively circle system from natural open large system. In time, the evolution of groundwater environment has been obviously accelerating since 1960.

(2)Several important hydrogeologic meanings were revealed from the evolution, they include that: Calculating approximately lateral recharge amount and renovate rate of groundwater; Making an opinion tohydrogeochemical actions; And judging the way and degree of hydraulic contact among aquifers,etc.

(3)There are five primary factors which contribute to the evolution: geological structures ,atmospheric precipitation, mine water discharging and groundwater exploitation, mine drainage irrigating, and sewage leakage of gangue stacks,etc.

(4)The work helps to validate the influences on hydrogeological conditions due to coal mine digging, provide scientific guidance for digging deep coal bed and also make certain some directions and goals for the utilization of mine water.

Acknowledgement

This work was supported by National natural Science Fundations of China (certification NO:40472146,59906014)

 

References

[1]      Junxiang Xu,Pin Xu. Groundwater pollution and control of Zibo mine. Coal Science and Technology, 2003 (10)

[2]      Zonghu Zhang. Evolution of groundwater environment in North China Plain. Beijing : Geology Press, 2000,57-60

[3]      Zhen Ma,Yonghou Duan. The sustainable utilization of groundwater resources in North Shandong plains. Hydrogeology and Engineering Geology, 2005, (2) : 1-9

[4]      Weizhou Pang,Weiguo Liu. Hydrogeology in coal mine. Beijing : Coal Industry Press, 1986,46-48

[5]      Zhaoli Shen etc.. Hydrogeochemical Basis (second edition). Beijing : Geology Press, 1993,98-99

[6]      Dachun Wang,Renquan Zhang,Yihong Shi etc.. Hydrogeology basis, Beijing:Geology Press, 2002,124-125

[7]      Jinming Li. Report about general programming of mine resources in Tai’an, 2002,10-50

[8]      Wen Wang etc..Development of coal resource and environment protection. Mining safety and environment protection, 2003 (5)

[9]      A.L. Mayo, E.C. Petersen, C. Kravits. Chemical evolution of coal mine drainage in a non－acid producing environment, Wasatch Plateau,Utah,USA.Journal of Hydrology. 2000, (236):1–16