assessment of groundwater changing trends through the generalized large well method with...
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ORIGINAL ARTICLE
Assessment of groundwater changing trends throughthe generalized large well method with confined–unconfined flowmodel in open-pit mine area
Tianxin Li • Hongqing Song • Gang Huang •
Ying Bi • Xiyao Li
Received: 10 February 2013 / Accepted: 15 May 2014
� Springer-Verlag Berlin Heidelberg 2014
Abstract In this paper, a simple but accurate method
(generalized large well method) is presented to assess
groundwater level trends during mine exploitation. This
method includes a mathematical model of confined–
unconfined well flow and a corresponding analytical solu-
tion. Based on the method, a case study was analyzed with
data from the Yimin open-pit mine. As a result, the radius
of groundwater level rose, along with the increase of the
exploitation intensity. Moreover, a suitable value of
pumping flow could be beneficial to understanding poten-
tial groundwater contamination concerns. Additionally, it
has also been predicted that the groundwater level of the
Yimin open-pit mine will change within the next 3 years.
The Yimin open-pit mine case study demonstrates the
validity of the analytical method explained herein. The
presented methodology provides a theoretical foundation
for assessment of groundwater changing trends in other
open-pit mines with similar hydrogeological conditions.
Keywords Generalized large well method � Confined–
unconfined flow model � China � Groundwater protection �Open-pit mine
Introduction
In recent years, open-pit mines have been rapidly expanded
and developed to meet the demands of social and economic
development in China (Han et al. 2009). Mine exploitation,
including underground mining and surface mining, has a
detrimental influence on the natural environment. Mining
has been known to cause serious damage to the ground-
water system through the reduction of water resources, the
pollution of groundwater, and the decline of aquifer levels
(Dhakate et al. 2008). Therefore, understanding how to
assess the changes in groundwater trends during mine
exploitation is critical to addressing environmental pro-
tection concerns. The movement of groundwater in the
stratum is a complicated process, and it is often difficult to
retrieve accurate data of the geology and hydrogeology in
mining areas, especially underground (Cidu et al. 2009; Li
et al. 2011). These factors make it difficult to address the
problems between groundwater utilization and protection,
and open-pit mine development (Qiao et al. 2011). One of
the greatest concerns is that, once the aquifer water level
declines or water becomes polluted, it is more difficult to
reverse its condition (Wang et al. 2011a, b). In order to
achieve sustainable development and reach a secure status
regarding both quantity and quality of groundwater bodies,
some management methods for environmental protection
should be introduced, particularly in predicting ground-
water change and controlling contamination prior to mine
exploitation (Jimenez-Madrid et al. 2012).
Most previous studies have focused on the assessment of
contamination of groundwater via heavy or toxic metals,
nitrate, chromium, and the reactive transport of contamina-
tion from the mines to groundwater, etc. (Bicalho and Batiot-
GuilheC 2011; Li et al. 2012; Hajhamad and Almasri 2009).
Some studies have emphasized that the decrease of recharge
T. Li � H. Song (&) � G. Huang � Y. Bi
School of Civil and Environmental Engineering, University of
Science and Technology Beijing, Beijing 100083, China
e-mail: [email protected]
X. Li
Huaneng Yimin Coal and Electricity Corporation,
Hulunbeier 021000, Inner Mongolia Autonomous Region, China
123
Environ Earth Sci
DOI 10.1007/s12665-014-3357-8
has a direct impact on groundwater resources (Wu et al.
2009; Moustadraf et al. 2008; Puraji and Soni 2008). Others
have focused on the land subsidence caused by groundwater
table changes (Hu et al. 2009; Tia et al. 2011). Thus, in past
research, a set of models has been established to analyze the
results, which were useful for predicting the change of
groundwater after mines were exploited (Rolland et al. 2001;
Samato et al. 2004; Rapantova et al. 2007). Previously
established models were successfully developed for pro-
tecting the environment and saving water resources (Gao
2011). However, because the strata are complicated and
anisotropic, it is difficult to construct an accurate model to
describe strata properties. The previous models also dem-
onstrate limited abilities to predict the trends of groundwater
change. In order to improve understanding of the actual
groundwater conditions, it is necessary to build a more
accurate model.
As for open-pit mining, hydraulic head drawdown drops
in the process of pit dewatering, which often causes the
water table of the pumping well to drop under the roof of
the confined aquifer, thus forming groundwater confined–
unconfined well flow (Wu et al. 2004). Therefore, this
paper attempts to present a groundwater model to delin-
eate, analyze, and assess the groundwater changes during
the exploitation of an open-pit mine. As an analytical
method, this model has been validated in the present study
and can be applied to study and predict the trend of
groundwater before exploitation in an open-pit mine.
Model and methods
Description of the generalized large well methods
and assumption of model
Generalized large well method is a method that takes into
account all of the drainage wells in the open-pit mine as
one large well with equivalence in area. Therefore, the
water flowing into the large well is as much as that flowing
into all of the drainage wells (Moldovan et al. 2008).
According to the principle of groundwater dynamics, the
hydrogeologic boundary of the open-pit mine may be
represented by an unlimited supply boundary model (Wang
et al. 2011a, b). Based on this the water yield of the large
well can be approximately calculated based on Dupuit’s
basic equation for steady flow.
In the theory, with the process of dewatering, drainage,
and pumping in a confined aquifer, especially during the
dewatering of groundwater in an open-pit mine, the water
table of mine or pumping well often goes under the con-
fined aquifer for two main reasons: (1) high drawdown of
hydraulic head and (2) in some regions, pumping ground-
water in the fixed water table for a long time with low,
natural hydraulic head will transform groundwater flow
into unconfined water flow in the practical open-pit mine
(Chen and Lin 2011). The whole process is a transforma-
tion from pressure flow to non-pressure flow, with which
the confined–unconfined flow is formed. At the same time,
a phreatic aquifer, which is considered a free surface
change with the time, has the characteristics of unsteady
flow (Xi et al. 2010), as shown in Fig. 1. So the confined–
unconfined, unsteady groundwater flow is formed during
the process of the exploitation of open-pit mine.
According to the geological conditions of different
mines, aquifers in this model are divided into two layers:
the upper layer is phreatic aquifer and the lower layer is
confined aquifer, whose thickness is M, as shown in Fig. 1.
In order to establish the equation of confined–unconfined
well flow, the following conditions are assumed: (1) the
aquifer is taken as an elastomer, being isotropic, horizon-
tally distributed, and of the same thickness; (2) percolation
ought to satisfy Darcy’s law; (3) for a fully penetrating
well, water is assumed to flow into the well evenly; (4)
recharge and evaporation ought to be neglected; (5) the
dropping of the hydraulic head results in the discharge of
groundwater from the storage being instantaneously com-
pleted; (6) hydraulic head surface is horizontal before
pumping; (7) the boundary condition of the hydraulic head
is H0 and the equivalent radius of the well is r0; and (8) the
aquifer is extended unlimitedly as well as laterally (Guo
et al. 2008; Hussien 2013).
Governing equation of confined–unconfined flow model
Based on the analysis above, the ‘‘generalized large well
method’’ is applied to calculate the total pump discharge of
the mine. According to confined–unconfined unsteady flow
(Xi et al. 2010) in existence, a mathematical model
describing the whole process of confined–unconfined
unsteady groundwater flow in an open-pit mine is presented
as follows (Chen et al. 2006):
Homogeneous confined aquifer zone equation:
u ¼ KM H �M=2ð Þ ð1aÞ
Homogeneous unconfined aquifer zone equation:
u ¼ Kh2=2 ð1bÞ
Flow rate in confined–unconfined aquifer equation
(Chen et al. 2006):
Q ¼ 2prKMdH
dr¼ 2pr
dudr
Confined aquiferð Þ ð2aÞ
Q ¼ 2prKhdh
dr¼ 2pr
dudr
Unconfined aquiferð Þ ð2bÞ
In Eq. (2a), (2b), the rate of the cross session is Qr,
which can be presented as
Environ Earth Sci
123
Qr ¼ �2prdudr
ð3Þ
Through the deviation of Eq. (3), the following equation
is obtained:
dQr ¼ �2p ro2uor2þ ou
or
� �dr ð4Þ
As shown in Fig. 1, taking infinitesimal as the research
object, the following equation is produced:
�dQr � dt ¼ 2pr � dr � dH � l; ð5Þ
where l is specific yield, l = le in confined area, and
l = ld in unconfined area (Chen and Lin 2011). Substi-
tuting Eq. (4) into Eq. (5), an equation can be obtained as
follows:
1
lo2uor2þ 1
r
ouor
� �¼ oH
otð6Þ
Based on Eq. (1a) and Eq. (1b), further equations in
which T is the transmissibility coefficient of an aquifer are
obtained as follows:
ouot¼ du
dH
oH
ot¼ oH
ot
d
dHKMH � KM2
2
� �
¼ ToH
otConfined aquiferð Þ ð7Þ
ouot¼ du
dH
oH
ot¼ oH
ot
d
dH
1
2KH2
� �
¼ ToH
otðUnconfined aquifer) ð8Þ
Substituting Eqs. (7) and (8) into Eq. (6), the governing
equations of confined–unconfined areas can be combined
into one:
T
lo2uor2þ 1
r
ouor
� �¼ ou
otð9Þ
Here, the hydraulic conductivity (T) and specific yield
(l) are variables associated with the pumping flow of
confined–unconfined water. Average Tm and lm, which
correspond to different periods, are assumed to be constant.
Thus, Eq. (9) can be linearized, and the defined problem
can be described as (Guo et al. 2008):
Tm
lm
o2uor2þ 1
r
ouor
� �¼ ou
otr0� r\1; t [ 0ð Þ
u r; 0ð Þ ¼ u0 r0� r\1ð Þu 1; tð Þ ¼ u0 t [ 0ð Þ
limr!r0
2prouor¼ Q consð Þ
8>>>>>>><>>>>>>>:
; ð10Þ
in which u0 ¼ KMH0 � 12
KM2,r0 ¼ffiffiffiFp
q¼ 0:564
ffiffiffiffiFp
;
where
F area of mine (m2)
H confined aquifer hydraulic head (m)
H0 initial hydraulic head (m)
K permeability coefficient (m/s)
M confined aquifer thickness (m)
Q flow rate of the whole pumping wells (m3/s)
r radius of groundwater (m)
r0 reference radius (m)
Tm different period corresponding to average hydraulic
conductivity (m2/s)
t time (s)
lm different period corresponding to average specific
yield
u Jilin Gaussian potential function
Fig. 1 Sketch map of the
confined–unconfined
groundwater flow
Environ Earth Sci
123
Analytical solution
Based on the characteristics of groundwater flow in open-
pit mines, an analytical model of special pumping flow
transformation from confined water to unconfined water is
derived as follows:
Boltzmann function transformation (Zhang et al. 2010)
equation u ¼ r2
4at, where a ¼ Tm
lm
Through substitution, the differential equation can be
changed into an ordinary differential equation as shown in
Eq. (11):
ud2udu2þ 1þ uð Þ du
du¼ 0
u uð Þju!1¼ u0
limu!u0
uouou¼ Q
4pconsð Þ
ð11Þ
8>>>><>>>>:
Solving Eq. (11), an equation can be obtained as
follows:
u uð Þ ¼ c1 �Z1
u
e�x
xdxþ c2 ð12Þ
Based on boundary conditions, coefficients c1 and c2 can
be expressed as
c1 ¼ �Q
4pe�u0
c2 ¼ u0 ¼ KMH0 �1
2KM2;
where u0 ¼ r20
4at
The expression of potential function can be presented as
u uð Þ ¼ � Q4pe�u0
�R1
ue�x
xdxþ u0; note W uð Þ ¼
R1u
e�x
xdx,
Du r; tð Þ ¼ u0 � u r; tð Þ ¼ Q
4pe�u0W r2
�4
Tm
lm
t
� �ð13Þ
Substituting Eq. (1a) into Eq. (13), the equation of
unconfined area is
2MH0 �M2 � h2 ¼ Q
2pKe�u0W r2
�4
Tm
lm
t
� �ð14Þ
Substituting Eq. (1b) into Eq. (12), the equation of
confined area is
H ¼ H0 �Q
4pKMe�u0W r2
�4
Tm
lm
t
� �ð15Þ
Thus, the analytical solution models Eqs. (14) and (15)
have been obtained. The physical meaning of the param-
eters is the same as above.
Groundwater transforms from confined well flow into
unconfined–confined well flow. At the stage of confined
well flow, when hydraulic head is greater than or equal to
confined aquifer thickness (hw C M), if r = r0, t = tc, and
u0 ¼ r20
.4 Tm
lmtc, the following equation can be obtained:
H0 �M ¼ Q
4pKMe�r2
0
�4Tm
lmtc
W r20
�4
Tm
lm
t
� �ð16Þ
In Eq. (16), all parameters except tc are given, and thus
the time of transforming from confined well flow into
unconfined–confined well flow (tc) can be calculated.
At the stage of unconfined–confined well flow, when
hw \ M, Tm and lm vary over time. With the advance of
time, changes of hw can be investigated based on the
materials affecting hydrogeological conditions of the mine
(Chen and Lin 2011). Therefore, based on Eq. (14), the
following equation can be obtained:
2MH0 �M2 � h2w ¼
Q
2pKe�r2
0
�4Tm
lmtc
W r20
�4
Tm
lm
t
� �ð17Þ
According to Eq. (17), when the water table in the well
equals hw:Tm
lmt, can be calculated. By substituting this into
Eq. (14) and (15), the depression cone curve corresponding
to the water table in the well (hw) can be obtained.
Results and discussion
Study area
Located in the north of the Inner Mongolia Autonomous
Region, north of China, the studied area covers nearly
65 km2 (Fig. 2). The open-pit mine is a synclinal basin
surrounded by low mountains and hills. The landform of
the south area is the highest point at 906.50 m in altitude
while the altitude of the central area ranges from 644 to
781 m with a gentle slope. The Yimin River, 359 km in
length, flows into the east mine and covers a basin area of
*9,105 km2.
The area is in the mid-temperate zone of China. The
average annual precipitation was 354.73 mm during the
sampling period (2003–2008) and was distributed unevenly
throughout the year with the majority falling from June to
September, and the average annual evaporation was
1,166.0 mm.
Groundwater in the Yimin open-pit mine is developed
from two aquifer types: sand and gravel aquifers (also
called unconsolidated deposits), and bedrock aquifers.
According to borehole data, regional aquifers are divided
into three groups based on vertical direction: quaternary
sand and gravel aquifers, the tertiary sand and gravel
aquifers, and coal seams, respectively. The top aquifer
consists of quaternary sand and gravel aquifer, which flows
continuously. It is distributed widely above the coal seams.
Environ Earth Sci
123
The lithology of an aquifer is made up of sand, gravel,
pebbles, coarse sandstone, and medium sandstone, and
belongs to the category of porous aquifer. The tertiary sand
and gravel aquifer, which consists of well-sorted sand,
gravel, and coarse sandstone, also belongs to the porous
aquifer category. Its hydraulic conductivity and aquosity
are stronger, so it is closely connected with groundwater.
The coal seams aquifer includes coarse sand and gravel,
whose thickness ranges from 40 to 90 m. This aquifer has
the strongest hydraulic conductivity and aquosity. Atmo-
spheric precipitation and recharge from the other aquifers
are the main sources of groundwater.
Confined–unconfined flow model validation
Based on the long observed data of hydrology in the
studied area from the years 2008–2011, some parameter
values were obtained, as listed in Table 1. According to the
Fig. 2 Map of the study area
Table 1 The actual measuring
of parameters in the study areaSymbol H0 (m) K (m/s) M (m) r0 (m) hw (m)
2008 2009 2010 2011
Value 660 3.7209 9 10-4 80 1,009.3 72 69 65 61
Fig. 3 The comparison of
analytical value and real data in
2008–2011
Environ Earth Sci
123
materials concerning hydrogeological conditions of the
mine, pumping discharge obtained is the annual average,
which varies from year to year.
The data of average hydraulic head each year, which
was observed in the studied area, changed with the distance
from the center of the mine to the measured position. The
observed data and calculated values from analytical solu-
tions are shown in Fig. 3. The figure shows that model
predicted values derived here matched well with the
practical data.
The comparison concludes that the trend of groundwater
between calculated value and real value was very close
during the 4 years of study, which was almost the same
around the center of the open-pit mine. The groundwater
depression curve derived by the formula, which is deduced
from the model, is more precise. Therefore, the calculated
value has been validated correctly and may be used to
predict the trend of groundwater in future years. It also can
be applied in other open-pit mines with the same envi-
ronmental and hydrogeological conditions as the Yimin
study site.
Analysis of the trend of groundwater
with the exploitation quantity
We obtained the trend of the groundwater radius with
Eq. (15) in the studied area from 2008 to 2011 in Fig. 4.
The figure clearly shows that the radius of groundwater
gradually increased and the hydraulic head decreased in the
same water table each year. When the hydraulic head was
652 m, the value of groundwater radius was slightly more
than 2,500 m in 2008, but it reached the maximum value of
about 3,700 m in 2011. From 2008 to 2011, the value of
groundwater radius decreased by 1,200 m, which is due to
excessive exploitation.
We acquired exploitation quantity and groundwater
radius from 2008 to 2011, as shown in Table 2, which
showed that the difference of exploitation quantity between
2008 and 2009 reached a maximum of 4.42 9 106 (t), but
the difference between 2010 and 2011 reached the mini-
mum of only 3.3 9 105 (t). As a result, the radius of
depression cone curve increased much from 2008 to 2009.
And from Table 2, it can be seen that the radius of
groundwater increases along with an increase in the
quantity of exploitation.
Analysis the changing trend of groundwater at different
flow rate
We took the year of 2011 as an example to analyze the
radius of groundwater in the condition of different flow rate
values as shown in Fig. 5.
As shown in Fig. 5, the hydraulic head reached the
lowest point of 620 m when Q was at its highest value of
1.09 9 105 m3/day. The radius of groundwater also
reached the maximum value of *5,000 m in the same
water table at 653 m. While the hydraulic head was only
653 m the radius of groundwater was *1,000 m when
Q = 3.89 9 104 m3/day. The lower the flow rate, the
better the groundwater could be recovered. This illustrates
that the pump discharge is relevant to the trend of
groundwater change, which agrees with the results of
Table 2. By changing the values of flow rate it was dis-
covered that the trend of groundwater decreased while the
value of flow rate increased. However, in the real envi-
ronment of open-pit mine exploitation the groundwater
condition was often badly damaged and difficult to recover,
which caused negative effects such as groundwater deple-
tion, ground subsidence, and pollution of groundwater.
According to the analysis above, in the real situation, a
suitable value of pumping flow for rational exploitation and
better protection of groundwater can be achieved.
Groundwater trend prediction
The analytical model was used to predict the trend of
groundwater change over the next 3 years, as shown in
Fig. 6. Pumping discharge (Q) is calculated based on the
pumping discharge over several years.
It can be seen clearly that the changing trend of
groundwater from 2012 to 2014 followed the same trend
seen in 2011. With increased time the hydraulic head value
of the groundwater increased gradually, and the radius of
groundwater declined in the same water table. This shows
that the groundwater table is seriously damaged and that
proper control of several factors, including pumping, is
needed. As shown in Fig. 6 the analytical model managed
Fig. 4 The trend of groundwater in the study area in 2008–2011
Environ Earth Sci
123
to predict the changing trend of groundwater for the
3 years that followed the original study.
The groundwater system and its movement are com-
plicated; therefore, the best way to protect groundwater
during the exploitation of open-pit mines is to know how it
is changing. The analytical model presented herein can be
used to predict the trend of groundwater prior to exploi-
tation. Thus, the model provides a beneficial, basic method
to improve protection of groundwater resources and guid-
ance on how to conduct exploitation operations.
Conclusions
In the present study, based on the special water flow model
(the confined–unconfined, unsteady groundwater pumping
flow model), an analytical model has been derived to cal-
culate the radius of groundwater changes. The Yimin open-
pit mine study area provides long-term observational and
quantitative data. Through the analysis, it can be seen that
calculated values and real values were very close, and the
accuracy of the model was validated. In addition, the trend
of groundwater from 2008 to 2011 has been analyzed,
through which a conclusion that the radius of groundwater
increased along with the increase of the quantity of
exploitation can be drawn. An increase in flow rate values
was found to correspond to a decrease in the groundwater
table. In fact, in the real situation, a suitable value of
pumping flow can be obtained for rational exploitation and
groundwater protection, and it will not be beyond the
condition of recovering. This analytical model also offered
an effective method for predicting future groundwater
changes. Therefore, the use of this model is a suitable
method for studying and predicting the trend of ground-
water change before exploitation, and hopefully to instruct
people on useful methods of protecting groundwater
resources.
Acknowledgments The authors gratefully acknowledge the Public
Welfare Industry Special Research of China under Grant 200909063,
the Fundamental Research Funds for the Central Universities under
Grant FRFTP12003A and the Nature Science Foundation under Grant
41303059 for financial support. The authors also express gratitude to
researchers of Yimin open-pit mine for their cooperation.
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