Journal of Earth Science, Vol. 25, No. 3, p. 598–604, June 2014
Printed in China
DOI: 10.1007/s12583-014-0440-8
ISSN 1674-487X
Mechanics of Aquitard Drainage by Aquifer-System
Compaction and Its Implications for Water-Management in
the North China Plain
Chen Su, Zongyu Chen*, Jiang Chen, Yuhong Fei, Jingsheng Chen, Baoqian Duan
Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences, Shijiazhuang 050061, China
ABSTRACT: The deformation of aquitard is the main contribution to land subsidence in the North
China Plain, and the water released from aquitard compaction may be a large portion of the exploited
groundwater. In this study, the consolidation test was employed to understand the mechanics on the
drainage and deformation of aquitard. The results suggested the strain of aquitard mainly resulted
from the difference of hydraulic head between aquifers. And it was decreased with depth of aquitard at
the same hydrodynamic pressure. In contrast with the interbed within aquifers, the aquitard was deformable when it was compressed. The weakly bound water was significantly released when the void
ratio was about 0.44–0.45, and the EC of water released from the aquitard was decreased with the
compacting process. The data from the consolidation test suggested that the pumping of groundwater
from aquifer III might be less contribution to the land subsidence with respect to other aquifers in the
future.
KEY WORDS: Quaternary aquifer, clay, consolidation test, double layer, land subsidence.
1
INTRODUCTION
The groundwater has been intensively exploited for a long
time in the North China Plain (NCP), which has lead to seriously environmental problems, such as developing a regional
groundwater depression cone and land subsidence. The sustainability of groundwater resources has been drawn great attention by hydrogeologists (Pang et al., 2013; Foster et al.,
2004; Chen Z Y et al., 2001). Up to now, the water released
from aquitard compaction accompanying the decline of regional groundwater level has accounted for a large component of
groundwater exploitation (Li et al., 2012; Wang et al., 2007;
Zhang, 2003). In the NCP, 25%–44% of exploited water is
estimated from aquitard compaction by the study of Zhang et al.
(2009) and Shi et al. (2006). However, the amount of water
released from aquitard compaction in the future is not well
understood. Thus, understanding the mechanics of aquitard
drainage by aquifer-system compaction is urgent for water
management in the NCP. The methods common used are numerical simulation and water budget, but they have great uncertainty because of complexity of hydrogeology (Shu et al.,
2012; Zhou et al., 2012). The study of Cao et al. (2005) showed
that the consolidation test is the better way to simulate the
aquitard drainage. It could not only analyzed the deformation
of aquitard, but also record the quantity of aquitard drainage in
the process of compression.
*Corresponding author: [email protected]
© China University of Geosciences and Springer-Verlag Berlin
Heidelberg 2014
Manuscript received August 1, 2013.
Manuscript accepted February 18, 2014.
Some studies on the groundwater flow through aquitard
have been done by previous works, such as the test on the samples from the Bearpaw Formation and Cretaceous clay in Canada (Hendry and Wassenaar, 1999; Shaw and Hendry, 1998),
but the drainage of aquitard did not related to the deformation
of aquitard directly. Analyzing confining layer storage is
viewed as troublesome because of the additional computational
burden and because the hydraulic properties of confining layers
are poorly known (Konikow and Neuzil, 2007). The purpose of
this paper is to detect the mechanics of aquitard drainage by
aquifer-system compaction using the consolidation test of undisturbed core samples, and discuss the impacts on the exploited aquifer in the NCP.
2
STUDY AREA
The NCP, with an area of 139 000 km2, overlies a thick
Cenozoic sedimentary basin (Shao et al., 2009). The area has a
continental, semi-arid climate with a mean annual temperature
of 12–13 ℃, and the mean annual precipitation ranges from
500 to 600 mm (Chen et al., 2003).
The bedrock is composed of Archaeozoic gneiss and
Proterozic carbonate. The Cenozoic formation consists of thick
Tertiary and Quaternary deposits. The genetic type of Quaternary deposits is complex and their thickness varies considerably from 150 to 600 m. The sediments are dominated by fluvial
deposits in the piedmont plain, alluvial and lacustrine deposits
in the central plain and alluvial deposits with interbedded marine deposits in the littoral plain (Chen et al., 2003; Chen,
1999).
Based on the lithologic properties, geological age, the distribution of aquifers and aquitards, and hydrodynamic conditions, within the Quaternary there are 4 aquifers. The depth of
Su, C., Chen, Z. Y., Chen, J., et al., 2014. Mechanics of Aquitard Drainage by Aquifer-System Compaction and Its Implications for
Water-Management in the North China Plain. Journal of Earth Science, 25(3): 598–604, doi:10.1007/s12583-014-0440-8
Mechanics of Aquitard Drainage by Aquifer-System Compaction and Its Implications for Water-Management
4 RESULT AND DISCUSSION
4.1 Deformation of Aquitard
The pre-deformation of aquitard will be recovered limitedly after sampling from the borehole because of unloading of
the strata pressure, in which process the skeleton of aquitard
undergoes expansion (Freshley et al., 2002). Thus, it must be
corrected in the test study of deformation for using net stress
and net deformation. It’s well known that the deformation of
sample under stress equals to the product of its strain and
length, therefore, the stress-strain curve of aquitard is employed
to correct the pre-deformation (Fig. 5) (Chen et al., 1994).
W
Baoding
E 200
Ziya River Cangzhou Huanghua
0
I
I
II
II
3
Table 1
Samples
1#
2#
3#
4#
5#
IV
II 2
3
-400
IV
N1+2
0
1 I
-200
III
III
4
30 km
-600
5
Figure 1. The hydrogeological section of NCP. 1. Limestone;
2. boundary of aquifer; 3. direction of groundwater; 4.
boundary of brackish water; 5. sand layer (revised from
Chen, 1999).
The piezometric surface of
Confined aquifer (m)
MATERIALS AND METHODS
The core samples of aquitard were collected from borehole drilled at the Hengshui Testing Sites of Groundwater Sciences and Engineering. The site is located in the central flood
plain in NCP. The Quaternary formation consists of silt, clay
and sand. The depth of to the piezometric surface of confined
aquifer was measured 103.47 m in 2007.
Samples were taken from borehole by double tube technology. The borehole was drilled to 500 m depth. The lithology
and sampling location are shown in the Fig. 3. Five samples
were used for this experimental study. The samples 1#, 2#, 3#,
and 4# were sampled at depth of 40–50, 168–187, 215–227,
and 297–313 m, respectively. The samples were wrapped in
plastic after taken from borehole, and placed in a sealed plastic
box and sent to the laboratory for testing. Consequently, the
samples were in an initial condition of physical properties, and
the loss of water content was negligible.
The samples were prepared as a 14 cm long cylinder with
diameter of 50 mm. The density of samples was measured before the testing. The approximate moisture content of samples
was measured by Oven-Drying method before and after the
process of consolidation. The main physical property of samples is shown in Table 1. The sketch of equipment for testing is
shown in Fig. 4. The loading on testing sample was increased
gradually until no water released from the sample. The initial
load to the samples was 1 kN for slowly deformation, and then
gradually increased pressure from 2 to 180 kN. The deformation of samples, the volume and electrical conductivity (EC)
of released water were measured at different time. The raw data
obtained from the testing was shown in the Table 2.
Elevation (m a.s.l.)
bottom boundary of aquifer I is 40–60 m, and that of aquifer
II–IV are 120–170, 170–350 and 300–600 m, respectively
(Chen, 2001). The aquifer I is unconfined aquifer, and the others are confined aquifers. The corresponding geological formations are Holocene formation (Q1), Upper Pleistocene formation (Q2), Middle Pleistocene formation (Q3) and Lower
Pleistocene formation (Q4) (Fig. 1).
The unconfined aquifer has large area of brackish water
from central to coastal plain, which has limited use for exploitation. The deep aquifer has been intensively exploited since
mid 1970s, resulting in a large groundwater depression cone.
The maximum depth to the piezometric surface of confined
aquifer has reached 110 m and the areas with the depth to the
piezometric surface <40 m have reached 43.1% of the whole
region, while the area for the water level lower than sea level
has reached 8.7×104 km2, which account for 52.6% of the total
area. The decline of groundwater lever in water depression is
shown in Fig. 2. This decline of groundwater level has resulted
in land subsidence and water released from the aquitard compaction, which becomes a major contribution to the groundwater exploitation for deep confined aquifer (Wang and Li, 2004).
599
1973
20
1979
1985
Year
1991
1997
2003
40
60
80
100
120
Figure 2. Decline of groundwater level in water depression.
Physical property of samples in the testing
Before the consolidation
After the consolidation
Wet density Water content Wet density Water content Strain of samples
(g/cm3)
(%)
(g/cm3)
(%)
(%)
2.04
0.21
2.23
0.12
0.167
2.04
0.2
2.22
0.12
0.139
2.04
0.2
2.2
0.13
0.086
2.05
0.19
2.2
0.13
0.092
2.04
0.19
2.2
0.12
0.131
600
Chen Su, Zongyu Chen, Jiang Chen, Yuhong Fei, Jingsheng Chen and Baoqian Duan
Aquifer Depth Thick(m) ness (m)
group
Unconfined
I
aquifer
system
Stratigraphic column
40
1# Aquitard
10
II
168
Sand
19
2# Aquitard
12
3# Interbed
16
4# Interbed
18
5# Aquitard
215
Confined
aquifer
system
297
III
UBC=the final deformation in the reload process, and H=the
length of sample.
The σEC was the net vertical stress, which corresponded to
the net deformation DEC of sample. The strata pressure of samples could be calculated by its depth and the density.
The samples from aquitard and interbed within aquifers
were compressed in the consolidation testing with the same
loading. The strain of samples after deformation and stress
correction was shown in Fig. 6.
(1) The strain of different aquitards decreased with its
depth.
(2) The strain of interbed within aquifer was smaller than
that of aquitard.
As the theory of one-dimensional consolidation (Gibson
and Schiffman, 1981; Terzaghi, 1925), the pore void of soil was
378
Loading
⑤
Figure 3. The borehole profile.
Pressure
(KN)
5
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
Data of samples from the testing
③
Δh (mm)
②
1#
2#
3#
4#
5#
11.13
13.92
18.01
19.71
21
24.18
24.7
25.90
27.10
27.8
28.3
6.7
8.02
9.05
12.03
13.87
14.87
15.92
17.54
18.18
20.25
20.54
21.4
21.98
22.35
23
23.83
24.27
24.59
25.01
5.6
7.1
9.63
10.51
11.82
12.12
12.71
13.09
14.23
14.62
14.89
15.43
15.75
16.01
16.35
16.59
6.21
10.19
10.48
7.53
10.27
12.28
13.01
13.88
15.38
16.25
16.93
17.36
18.03
19.05
19.44
19.94
20.73
21.82
22.23
22.63
23.04
23.09
16.86
17.22
17.36
11.75
13.03
13.89
14.59
14.98
15.65
16.14
16.47
17.02
17.4
17.79
18.15
18.3
18.53
As is shown in Fig. 5, the strain of the sample was UA for
the loading of the strata pressure, and was recovered to UB after
taken from borehole. Then, the strain was UE when the loading
equaled to the strata pressure again, which was conducted in
the equipment shown in Fig. 4. The difference between UA and
UE was found very small and therefore the UBE could be regarded as the recovery. The net deformation could be written as
follow.
DEC=(UBC–UBE)×H
where DEC=net deformation of sample, UBE=the recovery of the
sample from borehole after unloading of the strata pressure,
①
④
⑦
⑧
⑥
1 .
Figure 4. The diagrams of consolidation equipment. ○
2 . plungers; ○
3 . piston; ○
4 . sample
Cylinder barrel; ○
5 . ruler; ○
6 . base plate; ○
7 . PTFE tubing; ○
8 .
chamber; ○
tube.
s A(=s E)
O
sC
①
UB
UA
UE
① Strata pressure
② Unload
③ Reload
B ③
A
②
Stress
E
UC
C
Strain
Figure 5. Stress-strain curve under aside-limit condition
(revised from Chen et al., 1994).
Strain
Table 2
0.20 0 . 167
0.15
0 . 139
0 . 086
0.10
0.05
0.00
0 . 131
0 . 092
Aquitard Aquitard Interbed Interbed Aquitard
1#
2#
3#
Sample
4#
5#
Figure 6. The strain of samples under the loading of 180 kN.
Mechanics of Aquitard Drainage by Aquifer-System Compaction and Its Implications for Water-Management
decreased with the increase of the pressure from the strata and
the formation time of strata. The aquitards between aquifers
and interbeds within aquifers had been compressed since the
formation time of pressure, and the clay in difference depth had
been in the different degree of consolidation before the
groundwater being exploited in 1960s. When the groundwater
level was lowered, as the principle of effective stress (Terzaghi,
1925), the pore-fluid pressure was transferred to the skeleton of
the aquifer system, and the aquitard and interbed were compressed. The strain of samples in the testing mainly resulted
from the difference of hydraulic head in aquifers.
The relatively low pumping rate of groundwater in aquifer
I and aquifer II resulted in the small difference of hydraulic
head between aquifer I and aquifer II. The strata pressure to
Sample 1# was lowest, and the formation time of strata was
shortest comparing with other samples. Hence, the strain of
Sample 1# was the greatest in the testing.
The groundwater in confined aquifers had been exploited
intensively to meet the water supply for the industrial and agricultural production since 1990s in the Hebei Plain, which had
caused the decline of the groundwater level in aquifer III significantly (Guo et al., 1995). The long-term difference of hydraulic head between aquifer II and III had resulted in large
compression of aquitard between aquifer II and III before testing. Hence, the strain of Sample 2# was much smaller than that
of Sample 1# in the testing.
The difference of strata stress between Sample 5# and
Sample 2# was large, which would result in different strain of
samples under the identical load. However, the change of
groundwater level in aquifer IV was small for the limited
amount of exploitation of groundwater, which decreased the
hydraulic pressure to compress the aquitard between aquifer III
and IV. At the same time, the aquitard between aquifer I and II
was compressed by the hydraulic press, which resulted in that
the strain of Sample 5# was nearly identical to that of Sample
2# in the testing.
The interbed within aquifers was thin or laterally discontinuous compared to the aquitard. The studies by Ding et al.
(2012) and Riley and Stewartson (1969) had demonstrated the
time delay caused by slow dissipation of transient overpressures and the permeability characteristic of clay. Thus, the vertical drainage of aquitard into adjacent exploited aquifers might
be slower than that of interbed in the identical thickness of
samples, and lag far behind the changes of water levels in adjacent aquifers. It was not susceptible to the transient fluctuate
of water level to compress for aquitard compared to the
interbed in the past, so the strain of Sample 3# and Sample 4#
were small in this study.
At present, the strain of interbed within aquifer V might be
large for the small fluctuate of water level, therefore, the
pumping of groundwater from aquifer V might be much contribution to the land subsidence. The aquitard between aquifer
II and aquifer III had been compressed for hydraulic pressure
largely, and the deforming capacity was relatively small. The
deformation of Sample 1# was much larger than that of Sample
2# at the small loading (Table 2). Hence, the pumping of
groundwater from aquifer III might be less contribution to the
land subsidence with respect to other aquifers. The loading of
samples in the testing was considerably larger than the real
pressure to aquitard by the decline of groundwater in the NCP,
therefore, the strain of aquitard in the nature could not reach the
deformation as samples in the testing. However, it was a trial
test to understand the characteristic of aquitard and to understand the tendency of deformation with the decline of groundwater level. When the thickness of aquitard and interbed were
checked clearly in the NCP, the potential land subsidence inferred from the deformation of the total samples would be understand as well.
4.2
Mechanics of Aquitard
Because the pore structure depends on the skeleton of
aquitard, the result in a permanent volume reduction of pore as
the pore fluid is “squeezed” out of the aquitards into the aquifers. In confined aquifer systems subject to large-scale overdraft, the volume of water derived from irreversible aquitard
compaction is essentially equal to the volume of subsidence
(Freshley et al., 2002). However, the drainage of aquitard was
less than the volume of the deformation of samples in the testing, this probably due to some gases filled in the pore of samples. When the strain of samples was nearly about 0.1, the
volume of about 2 mm deformation of sample was obviously
larger than the corresponding drainage of aquitard (Fig. 7). In
the testing, the EC of water was measured until no water released from the sample at certain load. R is the ratio between
the volume of the drainage from sample and the corresponding
volumetric change of sample.
4.2.1 Diffuse double layer theory
A double layer (DL) is a structure that appears on the surface of an object when it is exposed to a fluid. The DL refers to
2.0
13.81
15.85
18.1
20.16
22.31
24.95 26.09
1 810
R
1 690
1.0
1 570
0.0
0.034
0.049
0.066
R-strain curve of samples
0.085
0.102
Strain
0.120
0.141
1 430
0.151
Electrical conductivity
(ps/m)
The cumulative deformation of sample (mm)
11.98
601
EC -strain curve of water released from aquitard
Figure 7. The diagram of the results from the compaction test of Sample 2#.
602
Chen Su, Zongyu Chen, Jiang Chen, Yuhong Fei, Jingsheng Chen and Baoqian Duan
two parallel layers of charge surrounding the object (Fig. 8).
According to the result of electrical double layer of aquitard
(Mitchell and Kenichi, 2005), the surface negative charge
which comprises ions adsorbed directly onto the object due to a
host of chemical interactions is the first layer. The second layer
is composed of ions attracted to the surface charge via the
Coulomb force, electrically screening the first layer. The second layer is referred to as bound water membrane which consists of strong bound water and weakly bound water, respectively (Li et al., 1982). The gaps of particles decrease with the
increasing of effective pressure, and the weakly bound water
membrane is pressed and flows from high to low pressure,
which increase the thickness of membrane. When the microstructures of compacted clay are destroyed by the increase of
effective stress, the pore-fluid pressure is decreased and the
weakly water turns into the free water (Chen G et al., 2001;
Dixon et al., 1992).
Strong bound water
Weakly bound water
The double layer
Free
water
in the porous material (Revil, 1999; Revil and Glove, 1998).
As was showed in Fig. 7, the weakly bound water might
be released from the aquitard when the strain of sample was
nearly about 0.1 in the present study, and the EC of water released from the aquitard was decreased with the compaction
process.
The result of deformation of soil was the decrease in porosity. The release of bound water was directly affected by the
hydrophilic and the void ratio of aquitard, and when the void
ratio was less than critical value, the weakly bound water was
drained (Zhang et al., 2012). The void ratio was the main factor
for drainage since the lithology of sample was no changes.
According to the formula (Helm, 1975), the void ratio can
be calculated in the follow.
ei =e0–(1+e0)×Σ(Δhi /h0)
where e0=void ratio before the testing, ei=void ratio during the
testing, Δhi=the deformation of sample during the testing, and
h0=the length of sample before the testing.
From the project geology handbook 4nd (Chang et al.,
2007), the void ratio of samples before the testing could be
acquired by the water content, wet density and the type of soil.
In this study, the physical property parameters of samples was
measured before the testing and shown in Table 1. The empirical value of silt clay e0 is 0.5–0.6. When the strain of sample
was 0.01, the void ratio was 0.44–0.45, and the weakly bound
water might be significantly released.
4.2.2 Drainage of aquitard
The coulomb force between bound water and particle is as
high as 100–1 000 MPa within 1.0×10-8 m. The low compressibility of water results in large coefficient of compression itself.
When the water is at 0 ℃ by 40 MPa pressure, its volume is
reduced by 1.8%, and its density is 1.02 g/cm3 (Fine and
Millero, 1973). It follows from the Coulomb’s law that force
falls with the square of distance, therefore, the more bound
water close to the particles, the greater the density of water is.
The density of weakly bound water is about 1.00–1.25 g/cm3
(Zhang, 1980), therefore, when the weakly bound water is
turned into free water, the volume of water released from the
soil is greater than that of compaction of soil.
The initial drainage from aquitard was the free water with
high concentration of anions and cations. During the drainage
of weakly water, the microstructure and the surface properties
of the solid grains-water interface influenced directly the electrical conductivity. As the ionic strength decreased, the dominant paths for transport of the ion corresponding to the
counterion of the electrical double layer shifted from the pore
space to the solid grains-water interface. The anions and cations
did not move independently, and the membrane potential created by the charge polarization altered the velocity of the anions and influenced the mutual diffusivity coefficient of the salt
2
Figure 8. The diagrams of the electrical double layer (Shao
et al., 2011; Li et al., 1982).
IMPLICATIONS FOR WATER-MANAGEMENT
It’s critical to understand the relationship between the deformation of aquitard and its drainage of water. In this study,
the drainage and the deformation of the samples are recorded
and calibrated by the above-mentioned method. The drainage is
measured when the deformation of each sample is 1–2 mm.
Hence, the water released from aquitard by deformation per m2
was calculated by using the correlation between the average
drainage and deformation of each sample (Fig. 9).
It was reported that the area of land subsidence above 200
mm is 6.4×104 km2 by 2008 (Wu et al., 2010) in the decades of
over-pumping in the confined aquifer, and the groundwater is
overdrawn seriously when the development of land subsidence
is greater than 10 mm/a (Shi et al., 2010). If the rate of aquitard
deformation is about 10 mm/a with an area of 6.4×104 km2, the
water released from the aquitards compaction will be estimated
about 4.28×108 m3/a by the equation in Fig. 9. And if the exploitation of groundwater from the confined aquifer was identical to the past few years-about 30×108 m3/a (Shao et al., 2009)
Drainage of water (L/ m )
5
15
12
9
6
3
0
y =0.8191 x-1.5027
R =0.992
0
5
15
10
Deformation (mm)
20
Figure 9. The relationship between compaction and drainage of aquitard in the confined aquifer system.
Mechanics of Aquitard Drainage by Aquifer-System Compaction and Its Implications for Water-Management
in the future, then the water released from the aquitard compaction would be the 14.27% of exploitation and would be much
less than before. The contribution of the water released from
the aquitard compaction to the water supply would be decreasing in the future.
At present, the technique of intensometer which has many
sensors in a hole has been used inland subsidence monitoring in
Beijing, Tianjin and Cangzhou. However, the monitoring system in other cities in the North China is rare. In the next work,
the monitoring of groundwater level and the deformation of the
aquitard should be as priorities, especially in the serious area of
land subsidence. The groundwater is still referred to as the
main water source in some area. It is a critical issue for the
water resource management to control the land subsidence. The
strategy of regional exploitation of groundwater resource could
be made on the basis of the result of monitoring on the deformation of aquifer-system. However, the exploitation of
groundwater in the confined aquifers should be forbidden in the
long run, and the water source in the rivers and reservoirs
should be in full use.
6
CONCLUSION
(1) The strain of samples was mainly resulted from the
difference of hydraulic head between aquifers. The strain of
different aquitards is decreased with depth at the same hydrodynamic pressure, except for aquitard between aquifer IV and
V, which reflects the capacity of the aquitards in different depth
to deform in the future. The land subsidence is seriously environmental problems and should be controlled immediately and
therefore the groundwater in confined aquifers should be forbidden for control the land subsidence.
(2) When the void ratio is about 0.44–0.45, the weakly
bound water transformed into free water was released significantly from aquitard by the aquifer-system compaction, and the
EC of water released from the aquitard is decreased with the
compaction process. The correlation between the ultimate
drainage and deformation of aquitard in the testing was agreement with linear equation. In the next work, the monitor to the
water level and the compression of aquitard should be implemented widely for preventing regional decline of groundwater
level and the development of land subsidence in the next work.
(3) Groundwater is referred to as the main water resource
in arid and semi-arid area in north China. Hence, it is necessary
to test more samples from different boreholes for the consolidation test, such as Inner Mongolia and Gansu. It is a useful way
to detect the relationship between the drainage and deformation
of aquitard, which will be the key for the management of
groundwater. Besides, in the areas where the groundwater levels do not decline obviously, it is an early implication for the
composition of the exploitation of groundwater by the study of
the relationship between the drainage and deformation of
aquitard.
ACKNOWLEDGMENTS
This study was financially supported by the National
Basic Research Program of China (No. 2010CB428803) and
the National Natural Science Foundation of China (No.
41272252).
603
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