Engineering Geology 124 (2012) 77–89

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Engineering Geology

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Sustainable development and utilization of groundwater resources considering landsubsidence in Suzhou, China

Xiaoqing Shi a,⁎, Rui Fang a, Jichun Wu a, Hongxia Xu a, YuanYuan Sun a, Jun Yu b

a School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, Chinab Geological Survey of Jiangsu Province, Nanjing, 210018, China

⁎ Corresponding author. Fax: +86 25 83686016.E-mail address: shixq@nju.edu.cn (X. Shi).

0013-7952/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.enggeo.2011.10.005

a b s t r a c t

a r t i c l e i n f o

Article history:Received 14 October 2010Received in revised form 10 October 2011Accepted 11 October 2011Available online 25 October 2011

Keywords:Groundwater exploitation banRegional land subsidenceDeformationNumerical simulation

Suzhou is located at the lower reaches of the Yangtze River in southeastern Jiangsu, China. It is part of theSu-Xi-Chang area including Suzhou, Wuxi and Changzhou. As one of the most developed areas in China,this region has suffered from severe land subsidence caused by extensive groundwater exploitation since1980s. The land subsidence was controlled by prohibition of groundwater exploration in the past severalyears. However, the surface water pollution prompted a new task of how to sustainably utilize the ground-water resource, especially to satisfy the emergency demands of water supply. In this paper, we took Suzhouas a representative case to discuss how to develop groundwater resources while controlling the landsubsidence. The relationship between the deformation and the groundwater level was analyzed, with focuson the deformation features after the period of groundwater exploitation ban. The results confirmed theconclusion by Shi et al. (2007, 2008a): even in the period of rising groundwater level, same units may man-ifest different deformation characteristics, such as elasticity, elasto-plasticity, and visco-elasto-plasticity, atdifferent locations of the cone of depression. A land subsidence model that couples a 3-D groundwatermodel and a 1-D deformation model was developed to simulate the groundwater level and deformation. Ahigh-resolution local grid (child model) for Suzhou was built based on the regional land subsidence modelof Su-Xi-Chang area by Wu et al. (2009). The model was used for a number of predictive scenarios up tothe year of 2012 to examine how to develop sustainable use of groundwater resources under the conditionsof land subsidence control. Our results indicated that about 3.08×107 m3/a groundwater could be providedas emergency and standby water source while meeting the land subsidence control target of 10 mm/a.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Excessive groundwater withdrawal has caused land subsidence innumerous regions throughout the world (Gambolati and Freeze,1973; Gambolati et al., 1974; Helm, 1975, 1976; Neuman et al.,1982; Bravo et al., 1991; Gambolati et al., 1991; Shearer, 1998;Larson et al., 2001; Teatini et al., 2006; Liu and Helm, 2008;Calderhead et al., 2010). Suzhou, located at the lower reaches of theYangtze River in southeastern Jiangsu, China, is a part of Su-Xi-Changarea including Suzhou, Wuxi and Changzhou (Figure 1). This area suf-fered from severe ground settlement caused by extensive groundwaterexploitation in the past several decades. Numerous efforts were devot-ed since 1995 to control the land subsidence and ground fissures(Table 1). By the end of 2005, an overall ban of deep groundwaterpumping in Su-Xi-Chang area was realized and the land subsidencerate slowed down with the gradual rise of groundwater level.

On the other hand, surface water (including lakes and rivers) hasbeen increasingly polluted due to industrial, domestic wastewater

rights reserved.

and agricultural runoff. In May 2007, Taihu Lake, the most importantfreshwater lake in Su-Xi-Chang area, was choked by blue-green algae,causing panic as over 200,000 people found their tap water wasundrinkable. This was China's most serious case of drinking waterpollution to date. Some measures were developed in the city to dealwith the water supply crisis. One of the main measures was to inves-tigate the reasonable development of groundwater resources undercondition of land subsidence control and to use groundwater as anemergency water supply source because of its fine quality.

The challenge is to balance groundwater usage and land subsidencecontrol for sustainable development. In principle, groundwater isrenewable and recoverable. Alley et al. (1999) defined groundwatersustainability as the development and use of ground water in a mannerthat can bemaintained for an indefinite timewithout causing unaccept-able environmental, economic, or social consequences. Obviously, weexpect that groundwater resources could be sustainably used, notonly to meet the requirement of urban development but also on theother hand, to control the land subsidence.

To address this challenge, the first and the most important issuewas to analyze the spatio-temporal characteristics of the groundwa-ter level and the land subsidence in the recent period of groundwater

Fig. 1. Geographical location of Suzhou City in China.

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exploitation ban. For the period before the ban, Shi et al. (2007,2008a) analyzed in detail the relationship between the deformationand the groundwater level in the Su-Xi-Chang area and the neighbor-ing Shanghai, which belongs to the Yangtze Delta Plain. They pointedout that the characteristics of aquifer system compaction are complexbecause of the difference in the types, compositions, and structures ofthe hydrostratigraphic units, and in the histories of groundwater levelchanges that the hydrostratigraphic units had experienced. Differenthydrostratigraphic units have different kinds of deformation and thesame unit may also exhibits different deformation characteristics,such as elasticity, elasto-plasticity, and visco-elasto-plasticity, at differ-ent locations of the depression cone or in different periods (Ye et al.,2005; Zhang et al., 2007; Shi et al., 2008a). However, the variation char-acteristics of the groundwater level and land subsidence in the recentperiod of groundwater exploitation ban were not thoroughly analyzed.After 2003, the newly built bedrock bench marks, extensometers andauto-monitor stations are used to monitor land subsidence in Suzhou.These latest monitoring data provide a sound support for the analysisof the land subsidence features after the pumping ban.

This paper discusses how to exploit the groundwater resourcestaking into account the effects on land subsidence control. The entire

Table 1The development and control of land subsidence in Su-Xi-Chang area.

Year Measures/actions Result

1990 With the rapid development of the economy and the severe pollution ofsurface water, the usage amount of groundwater is gradually increased.

The grarea.

1997 Began to restrict the groundwater exploitation The pi2000 Implemented a law for prohibiting the deep groundwater exploitation

progressivelyThe strecove

2003 A regional monitoring network has been set up, including 8 extensometergroups and about 100 observation wells.

2005 Achieved an overall banning of deep groundwater pumping The grin 10 m

Suzhou city was treated as a single area to avoid themodel errors causedby the artificial/administrative boundary. First, the spatio-temporalcharacteristics of groundwater level and land subsidencewas investigat-ed based on the long-time records of piezometric levels and landsubsidence both before and after the pumping ban. Then, a coupledtwo-step land subsidence model, which combines a 3D groundwaterflow and 1-D deformation model, was used to simulate dynamicevolutions of groundwater level and the related land sinking or rebound.Finally, the calibrated model was used to evaluate the groundwaterresources under different water demand scenarios considering land sub-sidence. The results were significant for the sustainable developmentand utilization of groundwater resources considering land subsidencein the cities along the Yangtze River.

2. Regional geology and hydrogeology

The study area stretches from Wuxi on the west to Shanghai onthe east (Figure 1). It is bounded by the Yangtze River on the north,and by the Taihu Lake on the south. The total area is 8488.42 km2, in-cluding downtown, Zhangjiagang, Changshu, Taicang, Kunshan andWujiang (6 counties in total).

s

oundwater was pumped all over the area and the subsidence spread to the whole

ezometric level rises gradually. But the area of land subsidence is still extended.rict restrictions of groundwater exploitation causes groundwater level rapidred. However, a regional depression cone of land subsidence has been formed.

oundwater level rose rapidly and the annual land subsidence rate was controlledm/a.

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This region lies in the lower Yangtze River alluvial plain. The aver-age elevation is less than 6 m above sea level. The Quaternary strataare widely distributed and the thickness of the unconsolidated depositsincreases from west to east and from south to north. Based on strati-graphic data and soil properties, the multilayer aquifer system consistsof four aquifer units, including an unconfined aquifer, three confinedaquifers, and four aquitard units. Fig. 2 presents the hydrostratigraphyalong a SW-NE cross-section through study area with the location ofthe cross section shown in Fig. 1.

3. Groundwater exploitation and land subsidence

With the rapid economic development and the severe pollution ofsurface water in Suzhou city, the groundwater usage increased gradu-ally. Overpumping from the aquifers resulted in a drop of the ground-water level, which caused an increase in effective stress and theconsolidation of sedimentary deposits. Land subsidence in this regionwas firstly discovered in the 1960s. Before the 1980s, most of thegroundwater pumping was distributed in downtown Suzhou, causingland subsidence there. With the urbanization and industrialization inthe surrounding counties after the 1990s, the groundwater pumpingincreased every year and distributed all over the area, as a result, thesubsidence spread to the whole area. The area affected by subsidenceis basically identical with that of the groundwater depression cone(Shi et al., 2008a).

Although the administration began to restrict groundwater exploi-tation after 1995, the groundwater pumping rate was still quite largeand varied in different counties. In 2000, the annual yield was greaterthan 40×107 m6 in the Zhangjiagang and Taicang counties, 30 to40×106 m3 in Changshu county, and 10 to 20×106 m3 in downtownSuzhou, Wujiang and Kunshan. The groundwater withdrawals for thefirst confined aquifer occurred mainly in Zhangjiagang. The exploita-tion of the second confined aquifer, which is the main aquifer, was inthe whole study area except Zhangjiagang. The exploitation of the

Fig. 2. Hydrostratigraphy along the cross-section line aModified from Wu et al., 2009.

third confined aquifer took place only in Taicang, Changshu andWujiang. Fig. 3 shows the groundwater pumping yield in entire Suzhoucity from 1995 to 2008. In 2000, the volume extracted from the secondaquifer accounted for 56.7% of the total, while 36.3% and 7% were fromthe first and third confined aquifer. The yield was rapidly reduced after2000 (Figure 3). Considering the change of the groundwater yield be-fore and after 2000, we could assume the year 2000 as the beginningof the groundwater pumping ban.

There are 8 extensometer groups and 92 observation wells in thestudy area (Figure 4). The data provided a large historical records ofdeformation and pore water pressure evolution that are importantfor the investigation of the special features of soil deformationwhen the groundwater level changes due to pumping/prohibition.

3.1. Groundwater level and land subsidence before the prohibition

Fig. 5 presents the typical groundwater level evolutions in themain pumping aquifer (second confined aquifer) and land subsidencein downtown Suzhou. The land subsidence correlates closely with thegroundwater level. From 1981 to 1995, groundwater was increasinglyover pumped. The groundwater level decreased annually and theannual subsidence increased. The maximum subsidence rate was90 mm/a in 1984–1987. After 1988, the annual groundwater yielddecreased gradually and consequently so did the subsidence rate. Asthe land subsidence became serious, the local government enactedsome policies to restrict the groundwater exploitation to controlland subsidence. The implementation of the policy had positiveeffects. As a result, the groundwater level started to rise after 1995(Figure 5). From 1995 to 2000, the groundwater level rose 5.4 m,and the annual subsidence was controlled in 30 mm.

Moreover, there are two special features of land subsidence in thestudy area (Zhang et al., 2007; Shi et al., 2008a). Firstly, land subsi-dence results from consolidation of both sandy and soft clay stratadue to long term pumping from the confined aquifers. The deformation

–a′ in Suzhou. The profile line is indicated in Fig. 1.

Fig. 3. Evolution of groundwater pumping from the confined aquifers in Suzhou City from 1995 to 2008.

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of the deep confined aquifers is the most important factor to the landsubsidence in recent years (Shi et al., 2007). Secondly, the same hydro-geological unit may also present different deformation characteristics,such as elasticity, elasto-plasticity, and visco-elasto-plasticity, at differ-ent locations of the depression cone or in different periods (Xue et al.,2005; Shi et al., 2007; Shi et al., 2008a). Fig. 6 shows the map of thegroundwater depression in the second aquifer and land subsidence in

Fig. 4. Distribution of observation wells in the thre

the Su-Xi-Chang Area in 2000. A cumulative subsidence larger than200 mm, 500 mm and 1000 mm affected a 5000 km2, 2000 km2 and350 km2 zone in the Su-Xi-Chang Area, respectively. Informationfrom extensometers and bedrock benchmarks is useful to analyze thetypical features of land subsidence in the study area. A more in depthanalysis about the subsidence features before the exploitation ban isreferred to Shi et al. (2008a).

e confined aquifers and extensometer groups.

Fig. 5. History of the groundwater pumping rates, groundwater level in the main pumping aquifer (2nd confined aquifer) and cumulative land subsidence in downtown SuzhouCity.

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3.2. Groundwater level and land subsidence after the prohibition

The groundwater level rose rapidly and the annual land subsi-dence rate was controlled in 10 mm after the prohibition of ground-water pumping in 2000 (Figure 5). Observations provided by thebedrock benchmarks and extensometers indicate that the study areacan be divided into three subareas according to the variations ofland subsidence after the prohibition.

Subarea I (settlement rebound): along the Yangtze River in thenorthern part of the study area, the groundwater level rose fast dueto abundant supply from Yangtze River. At present, the depth of thegroundwater level from the surface is less than 20 m. A significantgroundwater level rise was observed in the past 3 years. Fig. 7ashows the cumulative land subsidence recorded from extensometersJ4 to J8 after 2003. Due to the exploitation ban in the end of the 1990s,the subsidence rates kept decreasing at extensometers J4 and J6,which are both located in Weitang, extensometer J7, located inQiandeng, and extensometer J8. The annual subsidence rate duringthe first year after the ban was 2–3 mm/a, and became stable till2005. Fig. 6b indicates that extensometers J4, J6, J7 and J8 are locatedat the edge of the land subsidence depression cone, while J5 is in thecenter of a local depression cone in Zhangjiagang. In the 1980s, thegroundwater yield in Zhangjiagang was the largest in the study

Fig. 6. Map of the groundwater depression cones in the 2nd aquifer (a) an

area. In 1996, the groundwater yield in Zhangjiagang was morethan 8×107 m3/a and the annual land subsidence rate was morethan 40 mm/a. After 2000, land subsidence was still observed in theextensometer J5. The annual rate in J5 was about 22 mm in 2003.However, in 2004, a quick rise of groundwater level (3.7 m) occurred.After a certain period of rebound, the ground surface became stable.

A recovery of land subsidence occurred in extensometers J4 to J8from 2005 to 2008. For example, the rebound recorded at extensom-eter J7 in Qiandeng reached 8 mm during the three years. Groundwa-ter level in J6 (located in Weitang, close to the depression cone centerof both groundwater level and the land subsidence, Figure 6b) exhib-ited a rebound period, delayed with respect to that in the northernpart along the Yangtze River. The land subsidence slightly fluctuated(Figure 7a).

Subarea II (settlement controlled): the center of the study area,downtown Suzhou. At present, land subsidence is basically undercontrol. This zone was the center of the depression cone and landsubsidence bowl for a long time. After the implementation of the ex-ploitation ban, the groundwater level in this subarea kept rising. Forexample, the average groundwater level in the main pumping aquiferrose rapidly for about 15 m from 2000 to 2005 (Figure 5). However,the rebound shows different behavior from that of the subarea I.Using the extensometer F3, located in the downtown as an example,

d cumulated land subsidence (b) in the Su-Xi-Chang Area as of 2000.

Fig. 7. Time histories of cumulative land subsidence for each extensometer groups after 2003.

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the observed data indicate the land kept subsiding slightly thoughwith a decreasing rate and a rebound of 0.5 mm occurred in 2006(Figure 7b). The data demonstrate a cumulative soil compactionduring quick groundwater level rise.

Subarea III (settlement developed): the southern part of the studyarea. Because of slow recharge to the aquifers, the land subsidencerate was still large in this subarea. As illustrated in Fig. 7c, theannual land subsidence rate before 2003 reached >15 mm at theextensometer J3, located in the Songling Town. After the implementa-tion of the exploitation ban, the subsidence rate was much lower,maintaining at the rate of 4–6 mm/a. The land subsidence at exten-someter F4, located in the Shengze Town, was always in the fastdevelopment stage, with an average annual rate of more than40 mm. The ratio of compressed deformation in layers at differentdepths located at 31–94 m, 94–108 m, and deeper than 108 m belowthe surface is about 60%, 20% and 10%, respectively, which indicatesthe first confined aquifer and the second aquitards contributed morethan 80% of total subsidence and the compress deformation of the dee-per third confined aquifer cannot be ignored. The severe land subsi-dence showed a close correlation with the continuous loweringgroundwater level in Shengze in Wujiang County. In 2007, the ground-water level was 40 m deep from the surface. Although the present levelwas higher than that in 2000, it was still the lowest in the WujiangCounty.

From the analysis on the observed groundwater level and landsubsidence before and after the exploitation ban, the following con-clusions could be obtained.

(1) The groundwater level in the middle and southern subareasdropped greatly before the exploitation ban and was lowerthan the historical lowest level. The deformations of both thesandy aquifer and the clay aquitard are elasto-plastic, evenvisco-elasto-plastic. So even in the condition of the rapid riseof the groundwater level, the land in these subareas kept sub-siding, showing delayed effect.

(2) Conversely, the land subsidence observations indicate that arebound occurred in the subarea I due to the following tworeasons. First, the deposits in this subarea are mainly sandswith little clay. The deformation of the sandy layers would bemainly elastic when the groundwater level was relativelyhigh. Second, the extent of the groundwater level drop wasrelatively small. The level was higher than the historical lowestlevel. So the rebound occurred when the groundwater levelrose after the implementation of the exploitation ban.

(3) In summary, subarea I (the northern of study area along theYangtze River) has better recharge conditions. The groundwaterhas a certain exploitation potential and can be used reasonably.However, the groundwater exploitation should be strictly

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prohibited in themiddle (subarea II) and southern (subarea III) inorder to control the land subsidence development. These twoprinciples were used later to evaluate the sustainable groundwa-ter resources.

4. Mathematical model

Because of the complex deformation characteristics, which couldbe elastic, elasto-plastic, visco-elastic, and visco-elasto-plastic, themodified Merchant deformation model (Ye et al., 2005) was usedfor this regional land subsidence simulation.

The vertical deformation computed by the modified Merchantmodel is (Shi et al., 2008b; Wu et al., 2009):

ΔL ¼ ∫L

0

γα1ΔH þ 1α2η

γ Δt α1 þ α2ð ÞΔH1þ 1

α2ηΔt

dz ð1Þ

where ΔH=H−H0, with H the head at time step t and H0 the initialhead of every time step. α1 and α2 are the compression coefficients,η is the viscous coefficient, γ is the water specific weight. The valuesof α1 and α2 are determined by Eq. (2).

α1 ¼ αke1 if H > Hpαkv1 if H ≤ Hp

and α2 ¼ αke2 if H > Hpαkv2 if H ≤Hp

��ð2Þ

where αke1 and αkv1 are the coefficients of elastic compressibility andplastic compressibility of spring ‘a’, respectively; αke2and αkv2 are thecoefficients of elastic compressibility and plastic compressibility ofspring ‘b’, respectively, Hp is the lowest historical groundwater level,known as the pre-consolidation head.

The governing equation for groundwater flow is (Shi et al., 2008b;Wu et al., 2009):

∂∂xi

Ki;j∂H∂xj

!¼ γϕβ

∂H∂t þ γα1

∂H∂t

− 1α2η

γ α1 þ α2ð Þ H0−Hð Þ þ e−e01þ e0

� �i; j ¼ 1;2;3ð Þ

ð3Þ

where Ki, j is hydraulic conductivity, ϕ is porosity, β is the water com-pressibility, and e and e0 are the actual and initial void ratio,respectively.

The modified Merchant model was used to reproduce the elastic,elasto-plastic, visco-elastic, and visco-elasto-plastic models (Ye etal., 2005). For instance, supposing that the deformation of a zone isboth elastic and plastic, 1/η should be set to zero and the parametersof αke1 and αkv1 were chosen to reflect the elastic and plastic deforma-tion of the soil strata.

Considering soil consolidation over long times, both hydraulicconductivity and specific storage are expected to vary with porosity(Helm, 1976; Neuman et al., 1982; Rudolph and Frind, 1991; Adrianet al., 1999). A coupled two-step model was used (Shi et al., 2008a,b; Wu et al., 2009), which simulates hydraulic heads and deformationin two steps and couples them by relating hydraulic parameter valueswith hydraulic heads and deformation. The model parameters arefunctions of effective stress and void ratio, and were updated duringthe simulation.

The relation between hydraulic conductivity K and void ratio e de-veloped by Lambe and Whitman (1979) was adopted. For clays, theexpression is:

K eð Þ ¼ K010m e−e0ð Þ ð4Þ

and sands:

K eð Þ ¼ K0ee0

� �3 1þ e01þ e

� �ð5Þ

where m is a dimensionless parameter related to soil properties andranging from 2 to 3 in this case. K0 is the initial hydraulic conductivitywhen e=e0. The specific storage Ss=γ(α+ϕβ) is a function of bulkcompressibility α, porosity ϕ=e/(1+e), the water compressibilityβ and water specific weight γ. These two latter parameters were as-sumed to be constant and α is derived from the compression indexCc and swelling index Cs (Huang, 1983):

αkv ¼ 0:434Cc

1þ e0ð Þσ ′if σ ′≥ σp

αke ¼ 0:434Cs

1þ e0ð Þσ 0 if σ′b σp

ð6Þ

where σ ′ is the effective stress and σp is the pre-consolidation stress.

5. Numerical modeling

Solving a large regional land subsidence model is a challengingnumerical problem. Groundwater flow problems in heterogeneousporous media can be simulated more accurately using the traditionalfinite element method (FEM) with a fine mesh. This usually requiressignificant computational effort, especially when the study area islarge and consists of many aquifers and aquitards. In this work wechose the multiscale finite element method (MsFEM) (Hou and Wu,1997; Hou et al., 1999; Ye et al., 2004), which was applied successful-ly for Su-Xi-Chang area (Wu et al., 2009) and Yangtze Delta (Shi et al.,2008a).

5.1. Numerical representation of the aquifer system

The aquifer system in the study area is composed of the Quaternaryloose sediments. According to the geological architecture, the systemwas divided into the unconfined aquifer, first, second and thirdconfined aquifers. Due to the absence of aquitards in some areas,hydraulic connections exist to a certain extent between the neighboringaquifers. The records of piezometric levels in different aquifers showthat the variation of ground-water levels in the first confined aquiferis similar to that in the second confined aquifer (main pumpingaquifer), which indicates hydraulic connection between the twoaquifers. As a result, the groundwater flow was conceptualized as athree dimensional unsteady flow in the anisotropic and heterogeneousmedia. Groundwater pumping is the main discharge in the study area.The exploitation wells are distributed in the cities and counties on thebasis of the real destination. The top of the vertical one-dimensionaldeformation model is the land surface which is a free surface, and thebottom is represented by the bedrock which is considered as a fixedsurface.

5.1.1. Boundary conditions and model discretizationPrecipitation is the main recharge source for the groundwater. The

bottom of the aquifer system is the Pre-Quaternary impermeable bed-rock. Hence it is regarded as the no flow boundary. The northern part ofthe study area along the Yangtze River, where many groundwater ob-servational wells locates, was treated as a Dirichlet boundary. Thesouthern boundary around Taihu Lake and eastern boundary aroundShanghai City were treated as a Neumann boundary. The westernboundary is the administrative boundary to Wuxi City, which togetherwith Suzhou and Changzhou belongs to the Su-Xi-Chang Area. In orderto reduce the impact of uncertainty in the western boundary conditionon the simulation results, the telescopic mesh refinement (TMR)

84 X. Shi et al. / Engineering Geology 124 (2012) 77–89

method was used, i.e., the calibrated and validated 3D groundwaterflow model by Wu et al. (2009) was used as a parent model and therefined model of the study area as a child model. The submodel usesthe parent model solution as initial and boundary conditions.This method was most commonly accomplished using someform of interpolation of either heads, fluxes, or both, from the coarsegrid onto the boundaries of the child grid (for example, Ward et al.,1987; Leake and Claar, 1999; Davison and Lerner, 2000; Hunt et al.,2001).

The aquifer systemwas divided into 4 layers in the vertical directionwith an average thickness of about 50 m, since the aquifer was discre-tized into a series of wedge-shaped elements. There are totally 21,230elements and 29,903 nodes in the parent model for Su-Xi-Chang (Wuet al., 2009). The refined submodel totally has 88,870 nodes and155,079 elements.

5.1.2. Initial conditionsThe simulation spans the period from March 1996 to December

2007. The interval was divided into 48 time steps of 3 months. Thefirst 36 time steps from March 1996 to December 2004 were usedto calibrate the model and the other 12 time steps from March 2005to December 2007 were for the model verification. The observedgroundwater levels on December 31, 1995 were used as the initialcondition. The initial heads of all aquifers were obtained directly byKriging interpolation based on the observed data. Because there areonly a few observation wells in the aquitards which are basicallylocated in the center of the depression cone of land subsidence, it isnontrivial to directly obtain the initial heads for all the nodes in thestudy area by the interpolation method used for the aquifers. For thenodes in the neighbor of observations wells, the initial heads in aqui-tards were obtained by Kriging interpolation. The initial head forother nodes were obtained by calculation between the correspondingupper and lower nodes in the aquifers by linear interpolation, whichwere also modified during the model calibration.

5.1.3. Parameter zonationMany aquifer parameters have to be specified in the model.

Hydraulic conductivity, storage coefficient, and compressibility ofthe soil are considered as variables. The parameters to be determinedare: initial void ratio e0, the pre-consolidation stress σp, compressionindex Cc, swelling index Cs, initial hydraulic conductivity K0 , and theparameters m and η. Hydraulic conductivity is the most sensitiveparameter for the groundwater model. Pre-consolidation stress σp

and η are the decisive factors to control the deformation features.The former controls the deformation characteristics (which could beelastic, elasto-plastic, visco-elastic, and visco-elasto-plastic) in differ-ent locations and over different periods, and the latter controls theviscous deformations. Once pre-consolidation stress σp and η weredetermined for specific zone, initial void ratio e0, compression indexCc and swelling index Cs in turn were the sensitive parameters forthe deformation model. The parameter m is insensitive in the rangebetween 2 and 3 for this case.

Each layer was divided into several zones with different parametervalues based on the topography, geology, and soil and rock types. Thereare a total of 122 zones with different soil properties. Not only thezonation, but also the parameters of each zone are calibrated in thenumerical model. An initial estimation and a possible range for eachparameter were obtained from various geologic and hydrogeologic re-ports, pumping test data, stress–strain analysis from field data (Ye andXue, 2003) and literature (Riley 1984; Bravo et al., 1991). Because ofdata unavailability, the preconsolidation heads were estimated basedon the location and thickness of aquifers, the structure of the aquifersand aquitards, and the initial groundwater levels in each aquifer.Then the preconsolidation heads were adjusted during the modelcalibration, which brought some uncertainty to the model.

5.2. Model calibration and verification

Measured groundwater levels and deformation values from 92observation wells (24, 61, and 7 for confined aquifer 1, 2, and 3, re-spectively), 2 extensometer groups, and 6 bedrock benchmarks at36 times (from December 1995 to December 2004) were used tocalibrate and verify the coupled model by trial-and-error…Withmodel calibration and verification, about 81% of the head targets areshortened by an average absolute error less than 0.5 m or a relativeerror (defined as the ratio of the difference between the simulatedand the observed head to the simulated head) less than 10%. Thesimulated land subsidence values match the extensometers datawith an average absolute error less than 1.5 mm. Overall, thecorrelation coefficient of simulated and observed heads and deforma-tions at all monitoring locations and times is 0.89 and 0.81,respectively. The results suggest that the model is able to simulateheads and deformation at a satisfactory level. It should also benoted that the relative error for some observation wells is largerthan 10% or sometimes even larger than 20%. For some extensome-ters, the differences between the simulated and observed subsidencevalues are greater than 2 mm. The poor match may be caused bythe error/uncertainty from the model structure and relatedparameters.

To compare the temporal trends between the calculated and theobserved heads, four observation wells from different aquifers wererandomly selected from different locations in the study area. Calculat-ed and observed hydrographs at the selected observation wells havesimilar trends in general, and the calculated heads agree well withthe observed data (Figure 8). Note that the groundwater levels inthe first, second, and third confined aquifers obviously increase dueto the gradual decrease of the groundwater yield. From Fig. 8d, itcan be seen that the rising rate of the groundwater level in the thirdconfined aquifer is slower than that of the first and second confinedaquifers. Meanwhile, the groundwater level in the third confinedaquifer is also the lowest among the aquifers, which is very importantto understand why a continuous compaction of the soil layers is ob-served from extensometers J3 and F4. In the south of the study areain Wujiang county, the formation, of a piezometric depression conesin the third aquifer and a land subsidence bowl was likely due tothe limited recharge and the continuous pumping.

Although not shown here, the contour maps of simulated waterlevels were constructed and compared with the observed groundwatercontour maps for each confined aquifer. We believe that the overallfeatures of the spatial water-level distribution in the three confinedaquifers, such as the maximum drawdown and its location, were wellreproduced by the numerical model. Taking the main pumping aquiferas an example, the location and the shape of the depression conematchwell with the observed records during the model calibration andverification periods. The predicted groundwater level in Su-Xi-Changarea in 2000 is shown in Fig. 6a. Compared with the distribution ofthe depression cone before the exploitation ban, the −50 m and−60 m water-level contours do not exist anymore at the end of themodel calibration period. Only a −40 m water-level contour can beseen in some locations at downtown Suzhou, Taicang, and Kunshan.At the end of the model validation period, the −40 m water-levelcontour disappeared. The average groundwater level is between−20 m and −10 m in the first confined aquifer, and between−30 m and −20 m in the third confined aquifer. The poor rechargecondition results in the slow recovery in the third aquifer.

Similarly, 2 extensometer groups and 6 bedrock bench marks wereused for the deformation model calibration and verification. In order todistinguish the elastic, elasto-plastic, and visco-elasto-plastic parts,Fig. 9 represents the history of deformation rate at each time step.Comparison between observed and simulated deformation indicatesthat the land subsidence rate decreased in most of the study areawhen the groundwater exploitation was reduced in the 1990s (Figure

Fig. 8. Comparison between measured (crosses) and simulated piezometric head (solid lines) for some observation wells.

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9 a–c). Especially after the implementation of the exploitation ban in2000, rebound occurred in some locations (Figure 9a–b), showing anelastic-type behavior. All these extensometers are located inthe north of Suzhou city, along the Yangtze River. The simulated subsi-dence matches well the observations. Simulation results indicate thatthe aquifer condition along the Yangtze River can receive sufficientrecharge.

The records from extensometer F3, which is located in the centerof the depression cone of land subsidence, show a deformation ratealways greater than zero. This means that the soil layers keep com-pacting in spite of the quick groundwater level rise, showing delayedbehavior (Figure 9c).

Fig. 9. Comparison between measured (crosses) and computed (solid lines) deformation ratevalues indicate subsidence. To facilitate the distinction between rebound and settlement, a

Extensometer J3, located in theWujiang County, shows that the landsubsidence rate was small before 2002 and started increasingafterwards even though the groundwater level rose annually. This sug-gests that the deformation feature changes from elastic to elastic–plasticor to visco-elasto-plastic.

The contour map of the simulated cumulative land subsidence wasconstructed and compared with the observed ground settlement map(Figure 10). The simulated maximum ground settlement and itslocation are close to the observed values. The comparison indicatesthat the center of the land subsidence depression cone locates in down-town Suzhou city and new local cones appear in the Zhangjiagangcounty, east of Kunshan county and downtown Taicang county.

for selected extensometer groups. Note that negative values indicate rebound, positivedotted line is used for of the isoline 0 mm.

Fig. 10. Map of the cumulative land subsidence (mm) over the entire simulation period (a) as measured and (b) computed by the subsidence model. Positive values indicatesubsidence.

86 X. Shi et al. / Engineering Geology 124 (2012) 77–89

The simulated cumulative land subsidence from December 2004 to De-cember 2007 is provided in Fig. 11 to show the zoning feature of landsubsidence in the recent period after groundwater exploitation ban.In subarea I (the northern of the study area along the Yangtze River),the cumulative land subsidence is mostly negative, which indicatedthat land subsidence reversed over the last 3 years, especially innorthern Zhangjiagang. In the central subarea II, the cumulative landsubsidence is basically around zero, which indicated that landsubsidence is under control. In the southern part (subarea III), landsubsidence rate is still largely positive because of a delayed mechanismand the limited recharge to the aquifers.

5.3. Model prediction

The calibrated flow and deformation model provides a quantita-tive tool for examining how potential groundwater extraction couldaffect land subsidence in the study area. It is not efficient to usesimulation–optimization groundwater management modeling toevaluate the groundwater resources under the land subsidence con-trol condition because the forward run of the calibrated model takesmore than one week. The long execution time is due to thecomplexity of the nonlinear land subsidence and the convergence ofthe two steps coupling. Therefore, we at first predicted the ground-water level and land subsidence under the present groundwaterexploitation condition. Then the groundwater yield distribution foremergency purposes was determined by considering the restrictionson the groundwater levels in each aquifer and on land subsidencespecified by the government. The potential utilization of groundwater

resources was preliminarily obtained by a number of forward runsunder different scenarios with different groundwater extraction till2012.

The following principles are followed in the management strate-gies development:

(1) Control the groundwater level and land subsidence. Forsustainable groundwater uses, land subsidence cannot be exac-erbated, i.e. the existing land subsidence rate should be at leastmaintained.

(2) Temporal exploitation control. The yields in different scenariosshould satisfy the premises that the groundwater levels haveto rise gradually. With this principle, groundwater can beconsidered as an emergency water source.

(3) Spatial exploitation control. The planned location of thegroundwater pumping wells should avoid the followingareas: the present cones of land subsidence and groundwaterlevel depression and the zones with high density of pumpingwells. The pumping wells for emergency should be in theareas with good hydrogeological properties and rechargeconditions.

The final results of the validation period (2003–2007) were usedas initial condition for the future scenarios. The simulations cover a5-year using a 1-month time step.

Scenario 1. Prediction under the present groundwater exploitationcondition

In this scenario, it was assumed that groundwater withdrawals, thewell distribution, and the recharge rate remain the same as in 2007.

Fig. 11. Contour map of the simulated cumulative land subsidence from December 2004 to December 2007 in Suzhou City. Positive values indicate subsidence.

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Since the overall groundwater exploitation bar after 2005, only a fewextraction wells were retained for special industrial water. The ground-water yield is 4.4×106 m3 in 2007 as shown in Fig. 3. The model pre-dicts that the groundwater levels at the center of depression coneswill rise from −20m,−30 m,−35 m to−12 m,−25 m, and −30 min the first, second, and third aquifers, respectively, by the end of2012. The predicted cumulative land subsidence for the period2008–2012 (Figure 12a) is also different in the three zones. In subareaI, the groundwater level in each aquifer is expected to recover to−5 m and subsidence rebound because of the recharge condition.This indicates that subarea I could be selected as the location for pump-ing wells for emergency usage. In subarea II, i.e. the downtown Suzhou,the land keeps subsiding due to visco-plastic and visco-elasto-plasticdeformation. From 2008 to 2012, the average land subsidence in down-town is predicted to be about 5 mm and with maximum of 15 mm inthe center. The groundwater level rise in the subarea III (such asWujiang County) was predicted to be negligible. A new local land sub-sidence cone was expected to form near extensometer F4. To controlland subsidence, the groundwater yields in downtown Suzhou andWujiang county should be reduced asmuch as possible. Ourmodel pre-diction suggests that the present groundwater exploitation condition isconducive to land subsidence control and groundwater level rise.

Scenario 2. Prediction under interventions to provide the groundwaterin emergency conditions

Newly planned wells for emergency conditions were added in thefirst, second and third aquifers in the model. The unconfined aquiferwas not considered because its water can be easily polluted. Thewells were uniformly distributed in subarea I. No wells were plannedto be in the middle and the south of the study area.

In order to obtain maximum potential utilization of groundwaterresources for emergency, a number of forward runs were performedto predict the possible ground water level and land subsidenceevolution under different scenarios with different groundwaterpumping rates. Table 2 listed the final result of allowablegroundwater withdrawal for emergency in each county. The plannedpumping rates from the first, second and the third aquifer was1.71×107 m3/a, 8.7×106 m3/a and 5.0×106 m3/a, respectively. Thegroundwater level depression cone was predicted to become stableafter 2012 (not shown here). The lowest piezometric level of thethree aquifers were expected to reach −16 m, −30 m and −40 m,respectively. The cumulative land subsidence shown in Fig. 12b con-firms an almost constant sinking rate with values less than 10 mmover the 2008–2012 period in most of the northern part,and about 30 mm in downtown Suzhou. The cumulative land

Fig. 12. Contour map of predicted accumulative land subsidence from 2008 to 2012 for Scenario 1 (a) and Scenario 2 (b).

88 X. Shi et al. / Engineering Geology 124 (2012) 77–89

subsidence inWujiang County is a little greater than that computed inscenario 1.

6. Conclusions

This paper discusses how to assess sustainable groundwater ex-ploitation in consideration of land subsidence control at Suzhou,China. The main conclusions are:

(1) The comparative analysis of the land subsidence characteristicsbefore and after the groundwater exploration ban confirmedthe conclusions by Shi et al. (2007, 2008a, b) that even in theperiod of the groundwater level rising, an identical unit atdifferent locations of the depression cone may present differentdeformation characteristics, such as elasticity, elasto-plasticity,and visco-elasto-plasticity.

(2) The land subsidence after the groundwater exploration ban hasdifferent features in the three subareas. In subarea I in the

Table 2Allowable groundwater withdrawal for emergency in each county (unit: m3/d).

First confinedaquifer

Second confinedaquifer

Third confinedaquifer

Total

Zhangjiagang 22,800 7400 2100 32,300Changshu 13,600 9200 3200 26,000Taicang 7900 4740 5610 18,250Kunshan 2600 2510 2860 7970Wujiang 0 0 0 0Downtown 0 0 0 0Total 46,900 23,850 13770 84,520

northern part along the Yangtze River, the groundwater levelrises because of the sufficient recharge from the river and thedeformation is basically elastic. In the middle of the studyarea (subarea II), although the groundwater level rises rapidlyover the last years, the land keeps subsiding, which indicated adelayed deformation feature. In the southern area (subarea III),the deformation rate is still large due to slow recharge to thedeep aquifers and the deformation feature changes from elas-tics to elasto-plasticity, even to visco-elasto-plasticity.

(3) The rise of the ground water level and the elastic soil deforma-tion in subarea I suggest that the groundwater has a certain po-tential for further exploitation. From the point of view of arational usage of the groundwater resources and land subsi-dence control, a planned target of not deteriorating the landsubsidence in the study area was set. The calibrated coupledgroundwater flow and land subsidence model was adopted tosimulate the possible aquifer yields for emergency sources. Theresults show that a groundwater volume of 3.08×107 m3/acan be withdrawn in the case of water stress such as unexpecteddrought or pollution emergency in Suzhou city. The yield fromthe first, second, and third aquifers amounts to 55.5%, 28.2%and 16.3%, respectively. Under this circumstances, land subsi-dence rate was expected to be basically controlled for most ofthe study area within the target (10 mm/a).

Acknowledgment

The authors thank the anonymous reviewers whose comments andsuggestions very much improved the paper. This work is financiallysupported by the National Nature Science Foundation of China grants

89X. Shi et al. / Engineering Geology 124 (2012) 77–89

(no. 40702037 and 41172206) and the National Science Foundation forDistinguished Youth Scholar no. 40725010.

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