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Journal of Cleaner Production 202 (2018) 33e44
Contents lists available at ScienceDirect
Journal of Cleaner Production
journal homepage: www.elsevier.com/locate/jclepro
Goaf water storage and utilization in arid regions of northwest China:
A case study of Shennan coal mine district
Qiqing Wang a, Wenping Li a, *, Tao Li b, c, **, Xiaoqin Li a, Shiliang Liu a
a
School of Resources and Geosciences, China University of Mining and Technology, Xuzhou, 221116, China
School of Mining & Civil Engineering, Liupanshui Normal University, Liupanshui, 553004, China
c
Key Laboratory of Mine Geological Hazards Mechanism and Control, Xi’an, 710054, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 February 2018
Received in revised form
15 July 2018
Accepted 12 August 2018
Available online 13 August 2018
In arid and semiarid regions of northwest China, there are abundant shallow coal reserves throughout
the region, yet during the mining process, the shallow water resources available are typically depleted
due to the formation of fractures in the shallow subsurface. Water-preserved mining is the only option
for coal mining in arid regions of northwest China. Based on the geological and hydrogeological conditions of the study area, this paper investigates the basic conditions and mechanisms of goaf water
storage. A time series model for goaf water storage is proposed, along with a method for estimating the
quantity of goaf water storage. Finally, hydrochemical characteristics of mine water and water quality
characteristics after purification in the study area were analyzed. As indicated by the analysis results,
after simple purification, the goaf water can meet industrial, ecological and agricultural water consumption standards. In the end, combined with current monitoring results of goaf water pressure and
the status of goaf water utilization in the study area, this paper describes the quantity and the effect of
utilizing goaf water storage. Goaf water storage can effectively alleviate the scarcity of local production
and domestic water, and provides the most effective approach of large-scale water-preserved mining of
shallow coal seams in the arid regions of northwest China.
© 2018 Published by Elsevier Ltd.
Keywords:
Arid regions
Goaf water storage
Goaf water utilization
Water resources
1. Introduction
With the exhaustion of coal resources in the coal mine districts
of eastern China, the focus of coal resource development in China
has quickly shifted to the environmentally fragile arid and semiarid
regions of northwest China. Shaanxi, Inner Mongolia, Ningxia,
Gansu and Xinjiang are major coal-producing provinces to be
actively built by the state in the future with a total proven reserve of
about 776.9 billion tons of coal (as of 2012), roughly accounting for
54% of the total reserves of the country. In 2016, this region’s coal
yield reached 1.62 billion tons, accounting for 48% of the total coal
yield of the country. However, these five provinces of northwest
China are arid to semiarid regions (Fig. 1), where there is a severe
shortage of water resources, and account for only 1.6% of the total
* Corresponding author. School of Resources and Geosciences, China University of
Mining and Technology, Xuzhou, 221116, China
** Corresponding author. School of Mining & Civil Engineering, Liupanshui
Normal University, Liupanshui, 553004, China.
E-mail addresses: qiqw89@qq.com (Q. Wang), wp1igroup@163.com (W. Li),
qazwdx521@163.com (T. Li).
https://doi.org/10.1016/j.jclepro.2018.08.123
0959-6526/© 2018 Published by Elsevier Ltd.
water resources of the country. The fragile eco-geological environment has severely restricted and impacted the regional economic and social development in the region (Feng et al., 2000; Ji
et al., 2006; Chang et al., 2014). Where regional geographic,
geomorphic and stratigraphic conditions are suitable, water in the
shallow layer is an extremely precious resource (Qiao et al., 2017).
Such valuable water sources include the sand-layer phreatic water
within the upper Pleistocene Salawusu Formation (Q3s) distributed
extensively in the coal mine districts of northern Shaanxi Province
and the bottomland of the Maowusu desert, the surface reservoirs
(lakes), and the runoff water formed by atmospheric precipitation
in the loess hilly areas (Fig. 2). In recent years, large-scale, highintensity and extensive mining of coal resources in northwest China
has caused wide-range water seepage from the shallow layer into
the subsurface coalmines, further escalating the shortage of water
resources and seriously impacted production and life of local residents. Protection and proper utilization of the water resources in
connection with large-scale coal mining poses vital scientific and
technological problems in the arid regions of northwest China.
Targeting the above problems, there have been several multidisciplinary studies on “water-preserved mining” from many
34
Q. Wang et al. / Journal of Cleaner Production 202 (2018) 33e44
F1
List of symbols
a2
Q4eol
Q3s
Q2l
N2b
J2y
Qt
Q1
Q2
Q3
q1
q2
F
a1
Eolian sand of Holocene System
Upper Pleistocene Salawusu Formation
Middle Pleistocene Lishi Formation
Pliocene Baode Formation
Middle Jurassic Yan’an Formation
The total water inflow of goaf water storage, m3/d
The water inflow of atmospheric precipitation, m3/d
The water inflow of surface water, m3/d
The water inflow of groundwater, m3/d
The water inflow of precipitation within the scope of
goaf, m3/d
The water inflow of precipitation within the
peripheral catchment area of the goaf, m3/d
The area of the goaf, m2
The precipitation infiltration coefficient of the goaf
h
q3
q4
Q0
S0
S
m2
t
tm
Hm
m
The peripheral catchment area of the goaf, m2
The precipitation infiltration coefficient in the
peripheral catchment area of goaf
The precipitation intensity of the study area, m/d
The flow of surface water into the goaf, m3/d
The flow of surface water out of the goaf, m3/d
The water inflow of the goaf before enclosure, m3/d
The drawdown of the phreatic water level before goaf
closure, m
The drawdown of the phreatic water level in the goaf
water storage, m
The specific yield of the goaf caving zone
The time of goaf water storage, d
The time of full water storage needed by the goaf
caving zone, d
The development height of caving zone, m
The average specific yield of the goaf fissure zone
Fig. 1. Map of the distribution of arid and semiarid regions along with the Jurassic coal basin, China.
aspects. Some have explored the impact of coal mining on
groundwater (Kim et al., 1997; Karaman et al., 2001). Gu and Zhang
(2012) specified the influence principle of modern coal mining on
the groundwater environment and the trend of self-remediation in
the northwest China. Yu et al. (2017) and Wang et al. (2017) stated
the development characteristic of water-flowing fractured zones
under the influence of coal mining. Wang et al. (2012) provided a
mechanical model, predicting the height of water-flowing fractured
zone for a shallow seam covered with thin bedrock and thick
windblown sands. Many scholars have developed several waterpreservation mining techniques aimed at protecting impermeable
key strata, such as backfill mining (Liu et al., 2006), limited-height
mining (Miao et al., 2009), room-and-pillar water-preservation
mining (Shi and Hou, 2006) and water-preserved mining division
(Huang, 2010). In addition, the hydrochemical characteristics of
mine water (such as water quality characteristics and evolution
characteristics) have been extensively studied (Huisamen and
Wolkersdorfer, 2016; Bozau et al., 2017), along with the recycling
of mine inflow (Zhang, 2010; Gu, 2015). However, the above studies
mainly focused on the migration pathways through which coal
mining impacts water resources, but have yet systematically
studied the water bodies themselves. The quality of mine water and
the recycling of mine inflow have been investigated to some extent
(Li et al., 2013; Gao et al., 2016; Mativenga and Marnewick, 2018),
but there is a lack of studies on water resources confined in goafs.
Taking the Shennan coal mine district of northern Shaanxi
Q. Wang et al. / Journal of Cleaner Production 202 (2018) 33e44
(a) Sand-layer phreatic water
35
(b) Surface runoff
(c) Reservoir
Fig. 2. Shallow water resources in northwestern arid regions, China.
Province as an example, this paper investigates the formation
conditions and mechanisms of goaf water storage, establishing a
time series model for goaf water storage on this basis, and proposes
a time series method for predicting goaf water storage capacity. It
also analyzed the water quality characteristics within the goaf
water storage and the utilization of mine water in the study area.
The research findings provide a new approach for solving the
shortage of water resources within arid coal mining regions, and
are of great significance for the sustainable development of the
environment, economy and society in arid and semi-arid mining
areas of northwest China.
2. Study area
The Shennan coal mine district encompasses 363.4 km2 and
includes a total coal reserve of about 5.5 billion tons with an annual
yield well over 50 million tons (Yang et al., 2018). It is located in the
contiguous area between the Loess Plateau and the Maowusu
Desert in northern Shaanxi Province, and the geomorphological
features within this area include desert bottomlands, valleys and
loess hill gullies (Fig. 3). This area has an annual precipitation of
about 400 mm and an annual evaporation of over 1,700 mm (China
Meteorological Data Service Center, 2018). The region is currently
undergoing the initial stage for large-scale coal mining, but there
has already been an increasing shortage of available water resources (surface water and shallow groundwater). It is necessary to
plan the utilization of water resources. The strata from the earth
surface to coal seam in the study area are in ascending order
(Fig. 4):
(1) Sand-layer phreatic aquifer has a general thickness of
10e20 m, and consists of eolian sand of Holocene System
(Q4eol) and the upper Pleistocene Salawusu Formation (Q3s).
The eolian sand is mainly distributed in the desert bottomland, and constitutes a unified sand-layer phreatic aquifer
with the underlying Salawusu Formation, with a general
level depth of 5 m below the surface. The Salawusu Formation is a fluvio-lacustrine deposit dominated by silty-fine
sand and rich in groundwater. As the primary aquifer in the
coal mine district, it is exposed along the river valleys, and is
the target stratum of water-preserved mining.
(2) The loess of the middle Pleistocene Lishi Formation (Q2l) is
generally 20 m in thickness, and directly distributed on earth
surface in loess mountain ridge areas. It is a sandy-clay with a
coefficient of permeability of 0.04e0.13 m/d and is the direct
aquifuge of the sand-layer phreatic water (Chen et al., 2017).
It constitutes a key aquifuge for water-preserved mining
together with the underlying Baode laterite.
(3) The lateriteof the Pliocene Baode Formation (N2b) is generally 30 m in thickness, and, as a silty-clay with a coefficient of
permeability of 0.0016e0.017 m/d, is a relatively strong
natural impermeable layer.
(4) The weathered bedrock of the middle Jurassic Yan’an Formation (J2y) is generally 25 m in thickness, and consists of
mudstone, sandy mudstone and sandstone. It has well
developed weathering fissures accompanied by the occurrence of fissure water.
(5) The unweathered bedrock of the middle Jurassic Yan’an
Formation (J2y) is generally 29e250 m in thickness, and is the
coal-bearing strata. The first mining coal seam is the No.2-2
with a mining thickness of 6 m and a burial depth of
36
Q. Wang et al. / Journal of Cleaner Production 202 (2018) 33e44
Fig. 3. Study area and examples of typical landform.
Fig. 4. Typical profile sketch of the study area.
90e200 m. Coal mining exerts a direct influence on the
overlying key impermeable clay stratum, for which the water
within the sand layer experiences varying degrees of leakage.
Some coal seams have undergone spontaneous combustion,
and formed various forms of burnt rocks, and are accompanied by the occurrence of water resources to some extent.
3. Conditions and mechanism for goaf water storage
3.1. Conditions for goaf water storage
Goaf water storage should meet the following basic conditions:
(1) The coal-bearing strata are structurally intact and have low
permeability. The goaf surrounding rock mass of the coal
seam consist of an impermeable or extremely weakly
permeable stratum. For instance, the Jurassic stratum in the
Shennan coal mine district of northern Shaanxi Province
(similar regional conditions to the Jurassic coal seam of
northwest China) essentially has no faulting or jointing, and
is an overall thick-bedded formation. With a coefficient of
permeability of ranging from K ¼ 0.0000244e0.09 m/d, it is
an extremely weakly to weakly permeable stratum.
(2) The coal seam should be thick, or contain multiple seams and
subjected to large-scale coal mining, therefore containing
large goaf volume and a large water storage capacity. In the
Q. Wang et al. / Journal of Cleaner Production 202 (2018) 33e44
coal mines of northwest China, large-sized coal pillars
(generally greater than 50 m in size) remain in place between
panels, and panel goafs are stable water storage units with
high horizontal impermeability. Adopting panels as basic
water storage spaces, multiple panel goafs can be combined
for water storage, thus creating large water supply sources
for distributed goaf water storage. For instance, one panel of
goaf water storage space in the Shennan coal mine district
can store millions and millions of cubic meters of water.
(3) Recharge path: After coal mining, the overburden waterflowing fractured zone becomes a recharge path for goaf
water storage, and water converges in the goafs only in
places where the mining depth is relatively shallow, bedrock
and soil-stratum aquifuges are thin (a thin aquifuge resulting
in cross-flow leakage) and the path converges or almost
converges with water sources.
(4) Sufficient recharge water sources: Available goaf water
storage can occur only in regions with sufficient water
sources where water can continuously seep into goafs
through recharge paths consisting of mining-induced fractures. These water resources mainly include surface reservoirs, gully runoff, the Salawusu sand-layer phreatic water,
atmospheric precipitation, etc.
(5) Large-sized goaf water supply sources can be formed only in
regions with suitable hydrogeological conditions (such as
large water storage space, sufficient recharge water sources,
high impermeability of the surrounding rocks, stable goafs
and so forth). The regions of northwest China have an annual
evaporation of 1500e4,000 mm$a1, but their annual precipitation is only 200e400 mm$a1 (China Meteorological
Data Service Center, 2018). Through the mining-induced
fractures, water from the shallow layer can be transferred
underground for storage, thus avoiding the intense evaporation that occurs to surface water and increasing the total
amount of water resources in the coal mine district.
3.2. Mechanism of goaf water storage
The primary condition for the water storage in a goaf is that
water resources can be effectively and safely stored. In fact, a goaf as
the lowest point in the region has the lowest water potential energy
and water catchment capacity. Before coal mining, three types of
water (i.e., surface water, groundwater and atmospheric precipitation) are in constant and orderly circulation, and the hydraulic
connections among various types of water are in a relatively
balanced state. The sand-layer phreatic water can recharge water
within the burnt rocks and surface water; bedrock weathering
fissure water can recharge the surface water; in case of a flood
during the flood season, surface water can recharge water within
the burnt rocks; atmospheric precipitation serves as an ultimate
recharge source for all types of surface water and groundwater.
However, a fractured zone induced by coal mining may link surface
water and groundwater, change the hydraulic connections among
various types of water, promote various types of water to directly or
indirectly discharge to the lowest goafs through the water-flowing
fractured zone, and thus create goaf water storage. The mechanism
of the goaf water storage process under shallow coal mining in the
arid regions of northwest China is depicted in Fig. 5.
The amount of precipitation infiltration increased due to mining
of shallow coal seams in an arid region. Surface water resources in
an area prone to water leakage have also entered the goafs.
Although the evaporation amount of an underground water body
increases due to ground fissures, relative evaporation of surface
water is weak. Therefore, the total amount of evaporative water
37
resources decreases after coal mining; the increase of atmospheric
water transformation in a coal mining area is conducive to the
positive balance of the regional water system. In the study area,
although the recharge of groundwater increased after coal mining,
it was found that the groundwater near the goaf is draining into the
mine, and the surface water in the mining area is essentially
depleted. Without considering the mine water as available water
resources, the amount of water resources at the surface of the
mining area will continue to decrease year after year. Conversely,
water resources in the mining area itself after coal mining are
increasing year after year due to the drainage into the mine.
4. Evaluation for goaf water storage
The capacity and time of water storage are key to water utilization in goaf. The water quality is another concern for the utilization of water resources available in the goaf. In this study, the
dynamic evolution process on goaf water storage was analyzed. The
capacity and time of water storage in goaf were evaluated, and the
goaf water quality characteristics were tested. The flowchart of
evaluation for water storage in goaf is shown in Fig. 6.
4.1. Dynamic evolution analysis on goaf water storage
The total water inflow Qt of goaf water storage is calculated as
follows:
Qt ¼ Q1 þ Q2 þ Q3
(1)
where Qt is the total water inflow of goaf water storage, m3/d; Q1 is
the water inflow of atmospheric precipitation, m3/d; Q2 is the water
inflow of surface water, m3/d; and Q3 is the water inflow of
groundwater, m3/d.
After goaf formation, there are obvious differences between the
stopped area and the peripheral catchment area of the goaf (the
area affected by the cone of depression) in terms of the precipitation infiltration coefficient. However, the infiltration capacity of
atmospheric precipitation does not show any significant change
with the change of goaf water storage in either case. Thus, the water
inflow of atmospheric precipitation can be computed by the
following formula:
Q1 ¼ q1 þ q2
(2)
where Q1 is the water inflow of atmospheric precipitation, m3/d; q1
is the water inflow of precipitation within the scope of the goaf, m3/
d; q2 is the water inflow of precipitation within the peripheral
catchment area of the goaf, m3/d; q1 and q2 are computed by the
following formulae:
q1 ¼ F a1 h
(3)
q2 ¼ F1 a2 h
(4)
where F is the area of the goaf, m2; a1 is the precipitation infiltration coefficient of the goaf; F1 is the peripheral catchment area of
the goaf, m2; a2 is the precipitation infiltration coefficient in the
peripheral catchment area of the goaf; h is the precipitation intensity of the study area, m/d.
The amount of water entering the goaf from the surface water
does not show any obvious change with the change of goaf water
level, so the water inflow of surface water can be computed by the
following formula:
38
Q. Wang et al. / Journal of Cleaner Production 202 (2018) 33e44
Fig. 5. Water cycle in goaf, illustrating water pathways.
Q2 ¼ q3 q4
(5)
where q3 is the flow of the surface water into the goaf, m3/d; q4 is
the flow of surface water out of the goaf, m3/d.
Among the aquifers overlying the coal seams in the study area,
the aquifer directly recharging the goaf is mainly the sand-layer
phreatic aquifer, whose recharge effect on the goaf is directly
related to head difference. Thus, the amount of recharge of the
underground aquifer for the goaf is related to goaf water storage
level. According to Wei (1991), the groundwater inflow is directly
proportional to the 1/2 power of the water level decline ratio, and
can be computed by Formula (6):
sffiffiffiffiffi
S
Q3 ¼ Q0
S0
(6)
fracture zone, providing a spacious underground water storage
unit. There is an obvious difference in the specific yield between the
caving zone and fissure zone. Therefore, the water storage process
of a goaf includes two stages: the water storage process of caving
zone and the fissure zone (Fig. 8).
(1) Water storage process computation for the caving zone
The time dt of goaf water storage presents the drawdown
change dS of phreatic water level, so:
sffiffiffiffiffi !
S
Q1 þ Q2 þ Q0
dt ¼ F m2 dS
S0
(7)
where m2 is the specific yield of the goaf caving zone; assuming
0 ffi
, Formula (8) can be obtained through
K1 ¼ Q1 þ Q2 , K2 ¼ pQffiffiffi
S0
where Q0 is the water inflow of the goaf before enclosure, m3/d; S0
is the drawdown of the phreatic water level before goaf closure, m;
S is the drawdown of the phreatic water level in the goaf water
storage, m; the definition of the above variables S and S0 are shown
in Fig. 7.
Coal mining causes overburden failure to form a caving zone and
integration:
"
pffiffiffiffiffi#
pffiffiffiffiffi pffiffiffi 2F m2
K1 þ K2 S0
pffiffiffi
S0 S K1 ln
t¼
K2
K22
K1 þ K2 S
(8)
Q. Wang et al. / Journal of Cleaner Production 202 (2018) 33e44
39
Evaluation of goaf water storage
Dynamic evolution on goaf
water storage
Total water inflow of
goaf water storage (Qt)
Atmospheric precipitation
(Q1=q1+q2)
Precipitation
within the
scope of the
goaf (q1)
Analysis of goaf water
quality
The time of goaf
water storage (t)
Surface water
(Q2=q3-q4)
Precipitation within
the peripheral
catchment area of
the goaf (q2)
Surface
water
into the
goaf (q3)
Water storage
capacity and time for
the caving zone
Groundwater
(Q3= Q0 S S0 )
Surface
water out
of the
goaf (q4)
Proportional to the
1/2 power of the
water level decline
ratio (Q0 S S0 )
Cations, anions, permanent
hardness, pH, ect.
Water storage
capacity and time for
the fissure zone
Water storage capacity (Qs) and
time (tz) for the goaf
Design plan of goaf water
utilization
Water treatment and utilizationin in goaf
Fig. 6. Flowchart of evaluation for water storage in goaf.
Fig. 7. Sketch map for water storage variables in goaf.
wherein, Formula (8) is a transcendental equation, according to
which the approximate solution of S can be computed when time t
is given.
(2) Water storage process computation for the fissure zone
Through substituting S¼S0-Hm into Formula (8), the time tm it
takes for full water storage in the caving zone can be solved:
tm
"
pffiffiffiffiffi #
pffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2F m2
K1 þ K2 S0
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
S0 S0 Hm K1 ln
¼
K2
K22
K1 þ K2 S0 Hm
(9)
where tm is the time needed for full water storage within the goaf
caving zone, d; Hm is the development height of caving zone, m.
When t>tm, the caving zone is fully filled with water, and the
fissure zone begins to store water. During the time dt taken for the
40
Q. Wang et al. / Journal of Cleaner Production 202 (2018) 33e44
Fig. 8. Diagram of the water storage process stages in a goaf.
goaf caving zone to completely fill with water, the phreatic water
level changes by dS, so the following formula can be obtained
through integration:
"
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi#
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffi 2F m
K1 þ K2 S0 Hm
pffiffiffi
S0 Hm S K1 ln
t ¼ 2 K2
þ tm
K2
K1 þ K2 S
(10)
where m is the average specific yield of the goaf fissure zone.
When S ¼ 0, the time tz taken for the goaf to completely fill with
water is:
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi!
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2F m
K1 þ K2 S0 Hm
tz ¼ 2 K2 S0 Hm K1 ln
þ tm
K1
K2
(11)
By jointly solving Formulae (8) and (10), the relationship between the time t of goaf water storage and the drawdown S of the
phreatic water level can be obtained, and with S we further
determine the current goaf water storage Qs:
QS ¼ ðS0 SÞF m2 ; S0 Hm S S0
(12)
QS ¼ Hm F m2 þ ðS0 Hm SÞF m; 0 S S0 Hm
(13)
4.2. Analysis of goaf water quality
In the study area, there is plenty of water in the goaf that is also
shallow, which is a rare precious water resource in the in an arid
and water depleted area. In order to analyze the water quality
characteristics of the goaf in the study area, seven sets of water
samples were collected from the Shennan coal mine district and its
neighbouring mine water, residential well water and loose sandbed spring water. All were tested for constituents, including cat
2
ions (Ca2þþMg2þ, Fe2þþFe3þ, NHþ
4 ), anions (NO3 , NO2 , SO4 ,
HCO3 ), permanent hardness and pH. The water in the goaf can be
treated using a sedimentation tank to control the flow speed,
remove most of the suspended solids and chemical oxygen demand
(COD) in the water, reduce the chromaticity and adjust the pH
value.
consists of five sections with individual goafs within each section
interconnected: six goafs on the west side of the north wing
(N1201~N1211), four goafs on the east side of the north wing
(N1200-1~N1204), two goafs on the west side of the south wing
(S1205 and S1207), three goafs on the east side of the south wing
(S1217~S1221) and goaf S1210 on the south wing (Fig. 9). Fire dams
at relatively low goaf points under the coal mine determined by
contour mapping of coal mine district floor and other reference
data were mounted with water pressure monitors, i.e., MB1, MB2,
MB3, MB4 and MB5; their specific positions are shown in Fig. 9.
Currently, 95% of the water consumption in the study area
comes from goaf water and mine drainage. The production water of
the mine consists of three segments. First, that collected from the
fire dam of the air return way in goaf S1210 on the south wing,
connected with water supply pipeline F219, and supplied after
treatment via hole G2 to the south wing for production and fire
protection. The second portion is collected from the goafs on the
west side of the north wing, connected with water supply pipeline
F159, and supplied after treatment to the north wing for ecological
restoration. The third portion is collected from the goafs on the
south wing and north wing along pipeline G3 of the main haulage
roadway pipeline. It first arrives at the ground sewage treatment
station after going through the south wing sump, north wing sump
and central sump. It is then used as supplementary water for the
coal preparation plant after treatment. The domestic water of the
mine mainly comes from a water well sourced under the mine. The
water on the south wing comes from the water well through
auxiliary transportation along goaf S1210 at a depth of
4600e5,900 m, and, according to water quality tests, it meets
drinking water standards of the state. The water well flows under
gravity to the clean water sump along auxiliary transport way
S1210 at a depth of 2,700 m. After being pressurized, it is connected
with pipeline F159, and supplied via G5 to the surface for use as
domestic water. The roadway water well for working face N1206 on
the north wing is connected with water supply pipeline F219 via
the main haulage roadway, and supplied via G4 to the north wing
for use as industrial water by the Shaanxi Tiangong Construction
Co., Ltd.
5. Results and discussion
5.1. Estimation of goaf water storage capacity
4.3. Design plan of goaf water utilization
The Ningtiaota Coal Mine located to the west of the Shennan
coal mine district has an existing goaf area of about 6.97 km2 and
Working face N1201 of the Shennan coal mine district, which
intersects surface runoff of the Xinmin Gully, was taken as an
example. Through combining field surveys with the results of
Q. Wang et al. / Journal of Cleaner Production 202 (2018) 33e44
41
rection
Drainage di
n
Transportatio
N1209 goaf
roadway
roadway
Ventilating
0.5
0
N1211 goaf
1.0 km
N
er
Ground wat
g(N1)
intake drillin
Sump
face
N1206 coal
N1207 goaf
N1205 goaf
N1204 goaf
North area
N1203 goaf
MP2
N1202 goaf
MP1
N1201 goaf
N1200 goaf
eway
Main haulag
way
ad
ro
ting
Main ventila
ting roadway
Main ventila
af
N1200-1 go
d
Substation an
sump of north
area
t
Incline shaf
Mine water
ants
treatment pl
MP4
S1210 coalface
S1221 goaf
Settling pond
The annular
mp of south
su
t
af
Incline sh
area
S1219 goaf
South area
S1217 goaf
MP3
S1207 goaf
S1205 goaf
S1203 coalface
The central
d
substation an
sump
d
Substation an
h
sump of sout
area
Ground water
intake drilling(S1)
MP5
Fig. 9. Design plan of goaf water utilization in the Shennan coal mine district.
previous studies, parameters Q0, Q1, Q2, F, S0, m2, m and Hm were
respectively determined as 15 m3/h, 7.4 m3/h, 22.6 m3/h,
808,300 m2, 65 m, 0.15, 0.11 and 17 m. According to the above calculations, the caving zone of working face N1201 requires
206.1 104 m3 to fully fill the goaf over the course of 5.3 years. The
water-flowing fractured zone of the goaf can hold 632.9 104 m3 of
water, of which would take 14.1 years to reach full capacity. It
should be noted that the time taken for the goaf to completely fill
with water is also affected by many uncertainty factors, such
climate change or peripheral mining activities that were not taken
into account in the calculations.
5.2. The characteristics of goaf water quality
The water quality analysis results of the Shennan coal mine
district and its peripheral mine water, residential well water and
loose sand-bed spring water are in Table 1. Compared with natural
water, mine water does not shown any substantial change in any
index, except the obvious increase of permanent hardness and
SO2
4 . On the whole, the mine water was weakly alkaline, which can
be explained by the ultra-low sulfur coal deposited in the Shennan
coal mine district of northern Shaanxi Province.
The quality of goaf water is extremely complicated, and subjected to many factors, such as water filling sources, coal quality,
surrounding rock characteristics, residence time, hydrodynamic
environment, etc. The influence of coal quality on the initial water
quality within the goaf water storage is very significant, however,
with the long-term circulation of water resources, several harmful
elements have shown a significant remobilization. Thus the water
greatly benefits from purification. Before goaf water utilization, the
water should undergo simple purification treatment to meet relevant water consumption standards. Based on the water quality test
on adjacent coalmines (Miao et al., 2009), the water quality characteristics of goaf water storage after purification are provided in
42
Q. Wang et al. / Journal of Cleaner Production 202 (2018) 33e44
Table 1
Water quality analysis results for representative samples from Shennan and its neighbouring coal mine districts.
Sample sites
Cation (mg/L)
Springs from sand aquifer
Well water
Haolaigou opencast coal mine
Sunjiacha coal mine
Xinyao coal mine
Ningtiaota coal mine
Hongliulin coal mine
Anion (mg/L)
Ca2þþMg2þ
Fe2þþFe3þ
NHþ
4
NO
3
NO
2
SO24
HCO
3
53.7
51.2
120.2
109.2
133.22
78.1
55.6
0
0.02
0.04
not detected
not detected
0.08
0.08
0.02
0.06
not detected
0.16
not detected
0.03
0.03
9.0
9.0
not detected
18.0
0.6
6.3
2.5
not detected
not detected
not detected
0.99
0.16
0.004
0.003
11.1
15.2
120.2
140.7
184.1
138.6
142.5
183.9
168.7
323.3
265.3
353.1
155.6
186.2
Table 2
Water quality characteristics of treated mine water.
No.
Item
Standard
Treated
1
2
3
4
5
6
7
8
9
10
11
pH
turbidity (mg/L)
suspended solids (mg/L)
ammonia (mg/L)
volatile phenols (mg/L)
hexavalent chromium (mg/L)
sulfide (mg/L)
total number of bacteria (a/mL)
total hardness (mg/L)
COD (mg/L)
BOD5 (mg/L)
6.0e9.0
e
100.0
15.0
0.5
0.5
110.0
e
450.0
100.0
30.0
6.9
3.2
25
2.6
not detected
0.03
0.15
63
148.0
31.0
11.0
Table 2. The quality of goaf water can fully meet industrial,
ecological and agricultural water consumption standards after the
simple purification treatment.
5.3. Analysis of goaf water utilization in the Shennan coal mine
district
Fig. 10 provides the results of goaf water pressure monitoring
within various parts of the mine, from which it can be seen that no
goaf water pressure experienced any significant change on the
whole. Some of the water pressures fluctuated over the monitoring
period mainly because of typical drainage and utilization of the
goaf water throughout the monitoring period. The monitoring results indicate that S1210 (south wing) had the highest goaf water
pressure with a maximum water pressure of 202.9 kPa, equivalent
to a goaf water head height of 20.29 m (still lower than the height of
the caving zone of about 21.0 m). This suggests that the goaf water
Permanent hardness (German degree)
pH
0
0
1.9
4.4
5.4
3.3
3.2
7.4
7.3
7.4
7.2
7.4
7.8
7.1
storage of the study area is still in stage I, i.e., filling of the caving
zone. Its water quantity can be computed by Formula (12), wherein,
m2 was set at 0.15. According to the computation results provided in
Table 3, the goaf water quantity of the study area was about
2,061,000 m3 at the time of calculation.
Flow meters were used under the mine to monitor the water
draw rates of G1, G2, G3, G4 and G5, and data were collected every
5 min. According to the monitoring results in Fig. 11, G1 had a
maximum instantaneous flow rate of 84 m3/h, and an average
instantaneous flow rate of 1.5 m3/h; G2 had a maximum instantaneous flow rate of 386 m3/h, and an average instantaneous flow
rate of 217 m3/h; G3 had a maximum instantaneous flow rate of
111 m3/h, and an average instantaneous flow rate of 5 m3/h. The
water supply pipelines had regular rest periods, and were not under continuous 24-h service for production water supply. Production water of the entire mine had an average instantaneous flow
rate of 223.5 m3/h. G4 had a maximum instantaneous flow rate of
12 m3/h, and an average instantaneous flow rate of 1.3 m3/h; G5
had a maximum instantaneous flow rate of 190 m3/h, and an
average instantaneous flow rate of 99 m3/h. According to monitoring data, the water supply pipelines had normal rest periods, and
were not under continuous 24-h service for domestic water supply.
The domestic water of the entire mine had an average instantaneous flow rate of 100.3 m3/h. Currently the water consumption
and goaf water storage variations within the study area are
essentially balanced, suggesting that the goaf water quantity of the
study area can fully meet current industrial and agricultural water
demands.
Because of the entry of surface water bodies and groundwater
into the goafs, surface water has a tendency to be completely
depleted, and groundwater level drops abruptly. According to field
300
MB1
MB3
MB5
250
Water pressure (kPa)
200
MB2
MB4
150
100
50
25
20
15
10
6/1
6/4
6/7
6/10
6/13
6/16
6/19
6/22
6/25
6/28 7/1
Date
7/4
7/7
7/10
7/13
7/16
Fig. 10. Water pressure monitoring results of the goafs in the Shennan coal mine district.
7/19
7/22
7/25
Q. Wang et al. / Journal of Cleaner Production 202 (2018) 33e44
43
Table 3
Current water storage capacity of the goafs in the study area.
Goaf
Area (1 104m2)
Height of water (m)
Water storage in goafs (1 104m3)
N1201~N1211
N1200-1~N1204
S1205, S1207
S1217~S1221
S1210
Sum
337.5
172.0
112.3
72.6
2.1
2.2
1.7
1.5
1.8
18.6
111.4
43.9
25.3
19.6
5.9
206.1
Flowing rate (m3/h)
400
G1
G3
G5
300
200
G2
G4
100
0
6/1
6/4
6/7
6/10 6/13 6/16 6/19 6/22 6/25 6/28 7/1
Date
7/4
7/7
7/10 7/13 7/16 7/19 7/22 7/25
Fig. 11. Flow monitoring results of water consumption pipelines in the Shennan coal mine district.
(a) Unirrigated area
(b) Irrigated area utilizing goaf water storage
Fig. 12. Ecological environment within the study area.
surveys over a 15-year period, after the water level drops and
without artificial irrigation, some tall arbors within the ground
surface above the goafs have also been depleted, resulting in the
escalation of desertification in the region, as shown in Fig. 12 (a).
However, in areas where goaf water storage is used for irrigation,
various types of vegetation (Table 4) are obviously in better condition than those in unirrigated areas, as shown in Fig. 12 (b). This
suggests that it is feasible to use the reclaimed and treated water
obtained from goaf water storage for ecological restoration and
agriculture.
Table 4
Main vegetation types in irrigated and unirrigated areas.
Areas
Category
Names of vegetation
Areas
Category
Names of vegetation
Irrigated areas
Arbor
Pinus sylvestris L. var. mongholica Litv.
Populus L.
Pinus tabulaeformis Carr.
Salix psammophila
Caragana korshinskii Kom
Artemisia desertorum Spreng. Syst. Veg.
Hippophae rhamnoides Linn.
Cynanchum hancockianum (Maxim.) Al. Iljinski.
Cynanchum thesioides (Freyn) K. Schum
Aristida adscensionis Linn
Cleistogenes squarrosa (Trin.) Keng.
Potentilla chinensis Ser.
Agriophyllum squarrosum (L.) Moq.
Irrigated areas
Herbs
Unirrigated areas
Arbor
Heteropappus altaicus (Willd.) Novopokr
Hemerocallis citrina Baroni
Medicago sativa L.
Poa annua L.
Pinus sylvestris L. var. mongholica Litv.
Populus L.
Salix psammophila
Caragana korshinskii Kom
Artemisia desertorum Spreng. Syst. Veg.
Cynanchum hancockianum (Maxim.) Al. Iljinski.
Cynanchum thesioides (Freyn) K. Schum
Agriophyllum squarrosum (L.) Moq.
Tribulus terrestris L.
Shrub
Herbs
Shrub
Herbs
44
Q. Wang et al. / Journal of Cleaner Production 202 (2018) 33e44
6. Conclusions
Northwest China consists of typical arid and semiarid regions,
where surface water evaporation is far greater than precipitation. In
large-scale coal mining, there are extensive goafs throughout the
region, and underground goaf water storage can be used to increase
the total amount of water resources in the coal mine district. This
provides a new approach of effectively achieving “water-preserved
mining” and solving the shortage of industrial and agricultural
water in these regions.
Through analyzing the geological and hydrogeological conditions of the study area, this paper maintains that large-sized goaf
water supply sources can be formed only in coal mine districts with
suitable hydrogeological conditions (such as large goaf volumes,
sufficient recharge water sources, high impermeability of goaf
surrounding rocks, stable goafs, and so forth).
Beyond proposing that panels can be used as basic units to
implement goaf water storage, this paper provides the formulas for
calculating goaf water storage capacity and predicting water storage durations. The present situation of water storage in the goaf
with in the Shennan mining district was determined to still be
within stage I, i.e., filling of the caving zone, and the goaf water
quantity was about 2,061,000 m3. It also analyzed the water quality
characteristics of goaf water in northern Shaanxi Province, and
concluded that water quality exerts an insignificant influence on
the utilization of goaf water resources. Through investigating the
status of goaf water utilization in the study area, goaf water can be
used for industrial purpose and environmental restoration after
simple treatment, which further verifies the feasibility of using goaf
water storage to solve the shortage of water resources in arid
regions.
Acknowledgments
The authors express their gratitude to everyone that provided
assistance for the present study. The study was jointly supported by
the National Key Basic Research Program of China (973 Program)
(Grant No. 2015CB251601), the State Key Program of National
Natural Science Foundation of China (Grant No. 41430643) and the
China Postdoctoral Science Foundation (Grant No. 2018M630632).
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