Journal of Earth Science, Vol. 25, No. 3, p. 550–562, June 2014 ISSN 1674-487X Printed in China DOI: 10.1007/s12583-014-0447-1
Cui, Y. L., Su, C., Shao, J. L., et al., 2014. Development and Application of a Regional Land Subsidence Model for the Plain of Tianjin. Journal of Earth Science, 25(3): 550–562, doi:10.1007/s12583-014-0447-1
Development and Application of a Regional Land Subsidence Model for the Plain of Tianjin
1. School of Water Resources and Environment, China University of Geosciences, Beijing 100083, China 2. Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences, Shijiazhuang 050061, China
3. Tianjin Institute of Geological Survey, Tianjin 300191, China
ABSTRACT: In this paper, a coupled numerical groundwater and land subsidence model was devel-oped for the Tianjin Plain. The model was employed to investigate the groundwater resources and their changes over the last decade, and to further predict the changing patterns of the groundwater level and associated land subsidence in future. First, according to the regional hydrogeology, the simulation area was defined with an area of 10.6×103 km2, which was divided into six aquifer units. A coupled ground-water and land subsidence numerical model was built by using Modflow2005 and the land subsidence simulation module SUB (subsidence and aquifer-system compaction), in which the groundwater flow was modeled as three-dimensional unsteady flow and the land subsidence simulation was based on one-dimensional consolidation theory. The model was then calibrated by using the groundwater level contour lines, hydrographs, and land subsidence hydrographs over the period of 1998–2008. In addition, groundwater balance analysis of the simulation period indicated that under multi-year groundwater withdrawal condition the cross-flow recharge, compression release, and lateral boundary inflow con-tributed 44.43%, 32.14%, and 21.88% to the deep aquifer recharge, respectively. Finally, the model was applied to predict the changing patterns of the groundwater levels and the associated variations in land subsidence under the control of groundwater exploitation after implementation of the south-to-north water diversion project. The simulation results demonstrated that the groundwater level may gradually increase year by year with an decrease in the groundwater withdrawal; and the land in dominated land subsidence regions including the urban area, Dagang, Hangu, Jinghai, Wuqing, and Jinnan, may re-bound at an average rate of 2–3 mm/a, and the land subsidence rate in the other regions may decrease. KEY WORDS: groundwater, land subsidence, simulation, modflow, SUB, the south-to-north water di-version.
1 INTRODUCTION
Land subsidence is a complicated environmental geologi-cal problem, which is a great concern for the human being. Land subsidence features with a slow decrease in the land ele-vation resulted from a number of reasons, and severe land sub-sidence could become a geological hazard (Dong et al., 2013; Xue and Xie, 2007). Land subsidence can be caused by both natural and human reasons, of which the dominant one should be the excess withdrawal of groundwater. Many cities and re-gions around the world have been heavily affected by land subsidence, e.g., Venice (Lewis and Schrefler, 2007), Mexico (Ortega-Guerrero et al., 1999), San Joaquin (Deverel and Leighton, 2010), Wairakei-Tauhara (O’Sullivan et al., 2010) and China’s Tianjin, Shanghai and Jiangsu (Xue et al., 2005).
increasingly developed in recent years; however, deterministic modeling is still the main approach to simulate land subsidence process. These deterministic models consist of groundwater flow models and subsidence models. By such a combination, the deterministic models can be categorized as two-step calcu-lation models, partial coupling models and full coupling models (Li and Zhou, 2006). Land subsidence models are often based on the one-dimensional Terzaghi theory (Terzaghi, 1925), Boit consolidation theory (Biot, 1941), fluid-solid coupling theory (Ran and Gu, 1998) or visco-elastic-plastic deformation meth-od (Wu et al., 2010). A typical regional aquifer model normally covers an area of tens of thousands of square kilometers, and the simulated aquifers are normally divided into 2–10 simula-tion sublayers or even more (Zhou and Li, 2009). Constrained by regional data availability of the study area and the complex-ity of the regional hydrogeology, simulation of the large-scale long-duration land subsidence of the groundwater system still remains a great challenge (Ji et al., 2009; Alley et al., 2002). Few coupled groundwater and land subsidence models have been built for specific regions though Xue et al. (2008) at-tempted to build coupling model for China’s Yangtze Delta using a modified Merchant model (Ye et al., 2011; Xue et al.,
Development and Application of a Regional Land Subsidence Model for the Plain of Tianjin 551
2008). Groundwater is the main water supply source for Tianjin.
However, Excessive withdrawal of groundwater in this region in the past years had resulted in land subsidence at different levels in the vast plain area south of the Baodi Fault. Up to 2002, the Tianjin City had recorded a land subsidence area of 4 080 km2 with a cumulative subsidence of more than 1 000 mm, which was 44% of the deep confined water area. The cumula-tive subsidence depths at the subsidence centers since 1959 are 2.882 m for Tianjin, 3.182 m for Tanggu urban area, and 2.98 m for Hangu urban area (Wang and Li, 2004). Land subsidence in Tianjin has drawn great attention of the government, de-partments, and researchers, who carried out extensive hydroge-ological and engineering geological work as well as groundwa-ter and land subsidence observation and withdrawal survey in this area (Wang et al., 2007; Yu et al., 2007). Wang and Li (2004) calculated the water balance in the plain region of Tian-jin over a period of 10 years from 1991 to 2000, and concluded that water released from cohesive soil contributed around 40% to the deep confined water withdrawal (Wang and Li, 2004). To simulate land subsidence process, Guo et al. (1998) established a quasi-three-dimensional flow and one-dimensional compres-sion model. Based on the IBS (Interbed-Storage Package) sub-sidence package developed by the United States Geological Survey, Xiaodong Cui developed an IDP (Interbed Drainage Package) to simulate the land subsidence in Tianjin by taking into account the lag of pressure variation for cohesive soil pore water relative to the head of the aquifer (Cui, 1998). Xu et al. (2010) simulated the groundwater variations using GMS (groundwater modeling system) and predicted the land subsid-ence in Tianjin’s Binhai New District (Xu et al., 2010). These efforts have improved the city’s research on land subsidence, but the spatial scales were fairly small (using administrative units as the modeling area and covering generally a few hun-dreds square kilometers), the simulation justification periods were relatively short, and the conceptualization of the simula-tion target, i.e., unconsolidated sedimentary layer, was rough. Research findings have confirmed that the head of an aquitard changed later than that of an aquifer, and consequently the deformation from an aquitard came later than that of the aq-uifer (Helm, 1976). Thus, to correctly simulate the defor-mation of an aquitard, the lag phenomenon needs to be con-sidered (Shearer, 1998). Investigation of the above-mentioned issues may significantly improve the accuracy of the land subsidence simulation.
This study aims to develop a coupled groundwater flow and land subsidence numerical model for the entire plain region of Tianjin by considering the lag water release of water-bearing media on the basis of the groundwater levels and land subsid-ence observations in the plain region of Tianjin. Modflow2005 program package (Harbaugh, 2005) developed by the United States Geological Survey and SUB program package (Hoff-mann et al., 2003) are employed. The numerical model simu-lates the water discharge in relation to time, the lag drainage and compression of interbeds (aquitards) with a view to inves-tigate the quantitative relation between groundwater withdraw-al and land subsidence, and determines the composition and variation of groundwater resources in confined aquifers. This
study is the first attempt to simulate both the groundwater flow and the land subsidence in the entire plain region of Tianjin. In addition, the calibrated numerical model will be used to fore-cast the trend of land subsidence under the control of ground-water exploitation after implementation of south-to-north water diversion project. Hence, it is important for the generation of the groundwater management strategies, and it provides scien-tific basis to control the land subsidence. 2 DESCRIPTION OF STUDY AREA
The study area covers the entire plain region of Tianjin (Fig. 1) and includes 13 counties (districts), facing Bohai on the southeast, the Yanshan Range on the north and the plain regions of Beijing and Hebei on the northwest, with an area of about 10.6×103 km2. The plain area of Tianjin lies in the downstream of the Haihe Basin, in a warm temperate subhumid continental monsoon climate zone where the mean annual precipitation is 582 mm. The study area is composed of three geomorphic units. North of the Baodi Fault is the piedmont alluvial plain, while south of the fault covers alluvial-lacustrine and marine plains.
The upper part of the alluvial lacustrine fan to the north of the Baodi Fault typically composes of coarse sand and pebbly medium-course or medium sand, comprising singular phreatic aquifers. Aquifers under the alluvial lacustrine fan are com-posed of multi-layered medium-fine sand, with a cumulative thickness of generally 30–50 m. The primary groundwater re-charge resources are precipitation and mountain-front lateral recharge.
The water-bearing in the central plain region south of the Baodi Fault mainly consists of river alluvium and interfluvial shallow lacustrine deposit overlain by marine deposit. These aquifers are mostly characterized with multi-layered silty-fine sand that thins out southwards or eastwards. Salt water is con-tained at shallow levels in the absolute majority of Aquifer I at the topmost part. Groundwater recharge and discharge mainly include atmospheric, infiltration, and evaporation discharge. The groundwater chemical types are mainly Cl·SO4-Na·Mg and Cl-Na with a mineralization rate of more than 10 g/L. More than one freshwater aquifer exists under the salt water body, consisting mostly of silty-fine sand. These aquifers vary signif-icantly in cumulative thickness, generally in the range of 20–60 m. Overall, the groundwater circulation conditions deteriorate in a descending order vertically.
In the coastal plain, the unconsolidated rock pore water- bearing rock series are composed typically of river alluvium and marine deposit. The shallow ground water is salt water. The aquifers are generally composed of silt of weak water abun-dance. The deep confined freshwater aquifers under the salt water body consist dominantly of silty-fine sand.
Vertically, the strata in the region can be divided in a de-scending order into Holocene-Tianjin Formation (Q4), Late Pleistocene-Tanggu Formation (Q3), Mid-Pleistocene-Tonglou Formation (Q2) and Early Pleistocene-Yangliuqing Formation (Q1). As aquifers are relatively developed in the upper member of the Neogene Minghuazhen Formation, they are also exploit-ed as deep confined water. Based on the hydrogeological char-acteristics and consideration of the current development and utilization degree of groundwater, the Quaternary system is
552 Yali Cui, Chen Su, Jingli Shao, Yabin Wang and Xiaoyuan Cao
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Bohai Bay
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Figure 1. General information of the study area. 1. Location of borehole; 2. observation bore of land subsidence; 3. observa-tion bore of groundwater level; 4. groundwater flow; 5. boundary of district; 6. fault; 7. salinity boundary; 8. hatch; 9. shal-low coarse sand and range gravel distribution, coarse sand and fine sand distribution; 10. constant heads; 11. general heads; 12. specific flow boundary (the north boundary of I–II layer simulation zone); 13. Baodi fracture (the north boundary of III–IV). traditionally divided into four aquifers according to the age of the formation, and the Neogene Minghuazhen Formation (N2) is divided into two aquifers, thus totally 6 aquifers (Fig. 2).
Groundwater pumping in the plain region of Tianjin start-ed at the beginning of the 20th century. Over the past two dec-ades, the groundwater withdrawal has amounted to 600×106– 1 150×106 m3 (Wang et al., 2007). In the last century, ground-water withdrawal in the downtown area was mainly occurred in aquifers II and III. In the recent years, much less groundwater has been pumped in the downtown area and withdrawal opera-tion has deepened into aquifers IV, V and VI in outskirts like Tanggu and Hangu. Extensive withdrawal of groundwater has accounted for the considerable declines of groundwater levels of deep aquifers in the central south of the city, and also given rise to different levels of subsidence in the vast plain region south of the Baodi Fault. This has resulted in four subsidence centers in the downtown area: Hebei Street, Outside Beizhan, Hedong Dawangzhuang, and Dazhigu-Chengtangzhuang, and a
number of land subsidence cones center in Tanggu, Hangu, Dagang and the lower reaches of Haihe. The subsiding regions have been connected to the land subsidence area in the neigh-boring Hebei Province and form part of the land subsidence area in the North China Plain (Zhang et al., 2009).
3 CONPLED GROUNDWATER FLOW LAND SUBSIDENCE MODEL 3.1 Conceptual Model
Different from the small-scale coupled groundwater flow and land subsidence model for Tianjin Plain that treated aquitards as a separate layer in the model (Cui, 1998; Guo et al., 1998), we conceptualized the study area into six aquifer units (Fig. 2), each of the aquifer units contains both sand and cohe-sive soil layers that were considered as a whole: Aquifer I is a phreatic aquifer. Aquifers II–VI are confined aquifers. Aquitards were distributed in each of these aquifers, and the quantification of cross-flow between the aquifers was obtained
Development and Application of a Regional Land Subsidence Model for the Plain of Tianjin 553
Figure 2. Hydrogeological cross section of the study area. 1. Quaternary Lower Pleistocene; 2. Quaternary Middle Pleisto-cene; 3. Quaternary Pleistocene; 4. Quaternary Holocene; 5. Neogene Pliocene; 6. Cenozoic ago; 7. auifer group number; 8. salt water roof and floor boundary (tooth body of salt water); 9. the water levels of unconfined groundwater; 10. Cenozoic overburden and bedrock boundary; 11. aquifer; 12. hydrous boundaries; 13. fault.
by adjusting the vertical infiltration parameters. The floor of Aquifer Unit VI was assumed to be -900 m. The simulation spatial domain covered the entire plain region of Tianjin (Fig. 1) with an area of 10.6×103 km2. The northern boundary of aqui-fers I and II was an interface between a mountainous area (the Yanshan Range) and the plain region, and a study showed that the groundwater flowed from the mountainous area to the plain region. Hence, it was processed into a known flow boundary (Wang, 2007). The phenomena of that the groundwater of the coastal zone was suffering from seawater intrusion showed the aquifer I was connected with Bohai (Zhang, 2012). Therefore, the eastern boundary of Aquifer I along the Bohai Bay was processed into a known level boundary (with a water level of zero m). Because of the water flow and exchange vary with the groundwater level, the lateral boundaries of all the other aqui-fers were processed into the general head boundaries (GHBs) (Harbaugh, 2005). Vertically, the free surface of phreatic aqui-fers was taken as the upper boundary through which water was exchanged vertically with sources outside the system. Beyond that, the aquifer was generalized as isotropic and heterogene-ous.
Under natural conditions, both phreatic water and the con-fined water from the aquifers flow generally in a piedmont plain-central plain-onshore sequence. The intensive ground water withdrawal over the past decades, however, has already changed the groundwater flow field and resulted in different levels of groundwater depression cones, which leads to the fact that the groundwater in majority region flows into its respective depression cone centers.
Hydrogeological parameters are important indicators for
the distribution of source sink terms and the hydrogeological structure of a region. These parameters include the precipitation infiltration parameter (α), permeability coefficient (K), and specific yield (μ) for phreatic aquifers, permeability coefficient (K) of confined water aquifers, and specific storage (Ss) of groundwater layers. Hydrogeological zonation was mainly based on lithology and water budget. The initial hydrogeologi-cal parameters for each zone were based on the hydrogeologi-cal reports (Wang, 2007). After examining the hydrogeological conditions, the hydrogeological parameter zones and initial values were given for all the parameter zones including 44 zones of K and Ss and 32 zones of μ in all 6 aquifer units. The precipitation infiltration parameter was divided into 20 zones and the value range from 0.13 to 0.26.
Because less water was withdrawn from phreatic aquifers and the level of these aquifers did not decline significantly, the compression water released from phreatic aquifers was not considered in our study. While the deep confined water in the study area was mainly recharged by cross-flow, due to artificial pumping, the water level varies largely and tended to decline over time, constituting the dominant contributor to land sub-sidence. Thus, aquifers II–VI was taken as the compression layers that made up to five layers. To facilitate the simulation, the initial subsidence was set to 0 m (Li et al., 2012).
According to the hydrogeology conditions of the study area, recharge factors for the groundwater system included atmospheric precipitation infiltration, agricultural irrigation infiltration, lateral boundary inflow, and land subsidence com-pression water release. Discharge factors included groundwater withdrawal (agricultural, household, industrial, forest, livestock
554 Yali Cui, Chen Su, Jingli Shao, Yabin Wang and Xiaoyuan Cao
and fishing), evaporation discharge, and groundwater lateral outflow. 3.2 Mathematical Model
Groundwater flow in the study area can be expressed as follows (Hoffmann et al., 2003)
⎪⎪
⎩
⎪⎪
⎨
⎧
++∂∂−⎟⎟
⎠
⎞⎜⎜⎝
⎛∂∂+⎟⎟
⎠
⎞⎜⎜⎝
⎛∂∂+⎟⎟
⎠
⎞⎜⎜⎝
⎛∂∂=
∂∂
∂∂=+⎟⎟
⎠
⎞⎜⎜⎝
⎛∂∂
∂∂+⎟⎟
⎠
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⎛∂∂
∂∂+⎟⎟
⎠
⎞⎜⎜⎝
⎛∂∂
∂∂
pp)(Kz
h
z
hK
y
hK
x
hK
t
hμ
t
hSε
z
hK
zy
hK
yx
hK
x
z
2
z
22
sz
(1)
where h-level elevation of the groundwater system (m); K-horizontal permeability coefficient of the water-bearing me-dia (m/d); Kz-vertical permeability coefficient of the water- bearing media (m/d); ε-source sink term of an aquifer (L/d) including water released or stored by the water-bearing media before elastic deformation; Ss-storage rate in the aquifer below the free surface (L/m); p-infiltration rate and evaporation rate of the phreatic aquifer (m/d); μ-specific yield for phreatic aqui-fers.
The initial and boundary conditions are described as fol-lows
⎪⎪⎪
⎩
⎪⎪⎪
⎨
⎧
=∂∂
=
=∂∂−
−
Γ
Γ
Γ
),,,(
),,,(
0)(
3
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1
tzyxqnhK
Htzyxh
zhK
hh
n
nr
σ
(2)
where Γ1- the general head boundaries; Γ2- a known level boundary; Γ3- a known flow boundary; n- the normal direction of the boundary surface; Kn- the normal permeability coeffi-cient of the boundary surface (m/d); hr- groundwater level of Γ1 (m); σ- resistance coefficient of Γ1 (d).
Soil deformation of aquifers caused by level drop is de-scribed by
Δb=SskbΔh (3)
where: Δb- deformation of the water-bearing media caused by level change in the confined aquifer (m); b- thickness of an aquifer unit (m); Δh- evel change in the water-bearing media (m); Ssk- skeleton storage rate of the water-bearing media (L/m).
⎩⎨⎧
′≤′′>′
=min
min
hhShhS
Sskv
skesk (4)
where Sske- elastic skeleton storage rate (1/m); Sskv- non-elastic skeleton storage rate (L/m); h'- calculated head for the present period (m); h'min- previous minimum head corresponding to the previous consolidation stress (m).
For cohesive soil interbeds of smaller thicknesses (≤1.5 m), hysteresis effect was not considered for land subsidence. For lag cohesive soil interbeds of larger thicknesses (≥1.5 m), their consolidation process was conceptualized into a double drain-age process that is expressed as
'2
2 's
V
Sh hz K t
∂ ∂=∂ ∂
(5)
where h- head of the cohesive soil system (m); z- vertical coor-dinate (m); S's- storage rate of the cohesive soil (L/m); K'V- vertical permeability of the cohesive soil (m/d); t- permeation time (d).
Within the same simulation layer, lag interbeds with the same vertical permeability coefficient and the same elastic or non-elastic storage rate were combined into a lag interbed sys-tem. Equivalent interbeds and aquifers had the water exchange through two interfaces (the upper boundary and the lower boundary).
totalequivj jQ n Q= (6)
where Qj- water exchanged through the two interfaces at node j (m3/d); nequiv- equivalent proportional factor; total
jQ - total water exchanged through the interbed system (m3/d).
By combining these equations, a coupled groundwater flow and land subsidence model can be derived, which can be further incorporated in water flow equations with the heads on the interbed boundary at different times. Released water from the interbeds was eventually stacked into the source sink terms in the flow equation for calculation.
When the head drop in an aquifer is known, 93% of the ultimate subsidence in an aquitard is achieved within the time factor τ0 (Riley, 1969). By comparing τ0 with the time step, we can decide whether lysteresis effect should be considered in the calculation. The lag diffusion of head and the water exchange in the lag release process may be processed using the vertical double drainage diffusion equations (Hoffmann et al., 2003; Carslaw and Jaeger, 1959).
Within the same aquifer, non-lag interbeds and lag interbeds can be combined into an interbed system, and the water exchange volume can be calculated by correlating the interbed system and the aquifer head, which can then be stacked into the flow equation as a source-sink term before the groundwater flow model can be coupled with the land subsid-ence model. 3.3 Numerical Model 3.3.1 Spatial-temporal discretization
The simulation area was the plain region of Tianjin, the groundwater flow and land subsidence of which were simulated using the groundwater simulation software GMS. The model grid consisted of 346 rows and 248 columns; each cell repre-sented 500 m×500 m for each layer. Vertically, the area was discretized into 6 layers as described earlier.
According to the available data, we selected the period from Jan. 1998 to Dec. 2008 as the identification and justifica-tion period for the model, of which each month was a stress period, (totally 132). Each stress period was divided into two time steps. 3.3.2 Data processing
The source sink terms of the study area included point, linear, and areal elements. The withdrawal and boundary inflow were processed in the form of pumping wells and injection wells. The withdrawal was distributed equally among the grid points for each stress period taking towns as the statistical units.
Development and Application of a Regional Land Subsidence Model for the Plain of Tianjin 555
The boundary inflow was distributed among the piedmont grids for each stress period. The precipitation infiltration and irriga-tion infiltration were processed using the Recharge module. The evaporation was simulated using the Evapotranspiration module by assuming that the local evaporation depth was 4–5 m. The general head boundary was simulated using the General-Head module, assuming that the boundary data varied with the stress period.
The water contours of the aquifers were obtained using the simultaneously measurements and long-term observations as of the end of December 1997 before the water level at each grid point was derived. The water level at the general head bounda-ry was processed using the gradual change method. This was processed as an arithmetic progression using the available wa-ter level record of the stress period so that the boundary head was gradually transitioned to its ultimate state attempting to minimize the effect of the boundary on the model.
Conceptualization of cohesive soil was the key point for our subsidence modeling. Cohesive soils smaller than or equal to 1.5 m were regarded as non-lag cohesive soils, while those larger than 1.5 m were taken as lag cohesive soils. According to the available record of 275 boreholes, the number of sand and cohesive soil layers in each aquifer and the thickness corre-sponding to each layer were measured. The non-lag cohesive soil layers and aquifer sand layers were conceptualized together into non-lag compression layers, while lag cohesive soil layers within the same aquifer were regarded as a lag compression layer. According to the storage rate (L/m) and vertical permea-bility coefficient obtained from the lab measurements, the me-chanical parameters were computed, including the elastic skel-eton storage coefficient and non-elastic skeleton storage coeffi-cient for non-lag compression layers, the elastic skeleton stor-age coefficient, non-elastic skeleton storage coefficient, equiv-alent thickness and proportional factor for lag compression layers. Then the values of the parameters for each model grid were derived using the Kriging interpolation algorithm. 3.4 Prediction Scenario
Based on the developed model, the potential groundwater
level and land subsidence in the simulation area in future were predicted. According to the water supply plan after the south-to-north diversion (Tianjin Municipal Water Conserva-tion Bureau, 2003), extensive withdrawal will be implemented on deep aquifers under the following principles: first, less in-dustrial and domestic water will be withdrawn (from central-ized sources and captive wells), especially from regions that have been excessively over-withdrawn and have cone centers; secondly, withdrawal will be adjusted or reduced in and below aquifer unit IV where the recharge conditions are poor, the withdrawal quantity is smaller but may result in great or quick decline of the groundwater level, thus more easily giving rise land subsidence; groundwater withdrawal from aquifer units II and III will be adjusted according to the recoverable resources.
The groundwater exploitation in the model for prediction was derived from the average pumping rate from 1998–2008, and depended on the results of the development of the cone of depression in each aquifer unit and the land subsidence from the model from 1998–2008. The pumping rate applied was the average groundwater exploitation from 1998–2008 multiplied by a factor less than 1. As a result of the widely distributed cones of depressions in the aquifer units V and VI, the factor was 0 except the Jinna with a small pumping rate to meet the water supply. Analogously, the exploitation in other aquifer units is showed in Table 1. The groundwater exploitation should meet the water supply plan from the Tianjin Municipal Water Conservation Bureau (2003). Based on the above analy-sis, the calculated groundwater withdrawal rate for Tianjin under the south-to-north diversion plan was 322×106 m3/a. The average groundwater withdrawal over the simulation period of 1998–2008 reduced by 513×106 m3/a. The groundwater levels of the aquifers in 2010 were used as the initial water levels for the prediction model. The prediction period was 11 years.
4 RESULTS AND DISCUSSION 4.1 Simulation Results of the Water Level and Land Subsidence
The model was identified using the estimation-correction method under the following principles: (a) the simulated
Table 1 Proposed withdrawal plan in 106 m3/a after the implementation of the south-to-north water diversion
Place Aquifer I Aquifer II Aquifer III Aquifer IV Aquifer V Aquifer VI Confinedaquifers
Note: The proposed withdrawal from confined aquifer units excluding Jixian is 60.16×106 m3/a.
556 Yali Cui, Chen Su, Jingli Shao, Yabin Wang and Xiaoyuan Cao
groundwater contour and land subsidence counter should be similar to the measured ones in terms of the cone and variation; (b) the simulated groundwater level and land subsidence dura-tion curves should match the measured ones; (c) in terms of balance, the simulated groundwater balance should generally agree with the actual balance; and (d) the identified hydrogeo-logical parameters should agree with the actual hydrogeological conditions. During the process of identifying the model, data from 316 long-term groundwater level observation bores and 6 groups of land subsidence monitoring bores as well as ground-water contours of each aquifer over a period of 7 years were used.
Figures 3 and 4 present the ground water contour lines fit-ting results of aquifers III and IV at of the end of the simulation period (the end of 2008). Figure 5 illustrates the groundwater
hydrograph fitting curves of 4 typical observation bores. Data from 6 groups of land subsidence monitoring bores were avail-able for the simulation area. Figure 6 shows the cumulative subsidence fitting curves of two typical land subsidence moni-toring bores. It can be observed that the simulation results match well with the measured groundwater flow field, the groundwater level, and land subsidence hydrographs. Table 2 lists the hydrogeological parameters after identification and justification of the model.
Errors from the model came mainly from the statistical error of the source sink terms, conceptualization error of the formation structure, diversity of the spatial distribution of hy-drogeological parameters, and the processing of boundary con-ditions.
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Figure 3. Measured (solid lines) and simulated (dashed lines) water levels in Aquifer III, 2008.
Development and Application of a Regional Land Subsidence Model for the Plain of Tianjin 557
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Figure 4. Measured (solid lines) and simulated (dashed lines) water levels in Aquifer IV, 2008.
The water levels of the 316 long-term observation bores over the 132 stress periods were calculated using residual error and variance analysis (totaling 41 712 data). As indicated in Table 3, the mean absolute residual error ≤1 and standard error ≤0.05 vary from aquifer to aquifer. Because the lack of ob-served data, larger errors mostly occur in the groundwater de-pression cones in the south and the deep aquifers. The absolute average residual errors in the northern regions like Jixian and Baodi are smaller.
In this study, the withdrawal was evenly distributed to the grids of each of the towns, which was somewhat different from the actual withdrawal. Besides, as the water level at the grid center was used to represent the water level variation over the 500 m×500 m region, when performing single-point dynamic curve fitting, especially when the hydraulic gradient was large,
there could be considerable errors. The representativeness of hydrogeological parameters in field test and the uncertainty of hydrogeological boundary values may both result in unsatis-factory simulation results.
Though it is noted that the differences between the ob-served and simulated water heads were considerable at several observation points or wells, the simulation results generally matched well with the measured groundwater flow field，and the hydrogeological parameters were in a reasonable range. It didn’t significantly affect the average changing trends in the district and in the entire plain region of Tianjin in the process of prediction. The variation of the predicted value of groundwater level and land subsidence were consistent in that of actual val-ue, and the whole of change of land subsidence and groundwa-ter flow could be predicted reasonably.
558 Yali Cui, Chen Su, Jingli Shao, Yabin Wang and Xiaoyuan Cao
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Figure 5. Observed VS simulated hydrographs at observa-tion wells. (a) H1 observation well of layer 2; (b) H2 obser-vation well of layer 3; (c) H3 observation well of layer 4; (d) H4 observation well of layer 5. 4.2 Composition of Groundwater Withdrawal
Table 4 shows the groundwater balance results of the
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Figure 6. Observed VS simulated land subsidence at obser-vation points. (a) S3 observed point; (b) S4 observed point. study area from January 1998 to December 2008. The analysis of the water balance in each year and each aquifer unit provides the following results.
(1) The main sources of recharge for the groundwater sys-tem in the study area include precipitation infiltration, agricul-tural irrigation infiltration, and lateral boundary inflow, and contributed 56.36%, 14.68%, and 11.63% of the total recharge rate, respectively. The cross-flow from the lower boundary only accounted for around 0.66%. Artificial withdrawal and phreatic water evaporation were the main discharge factors for ground-water, contributing 47.40% and 50.21% to the total discharge, respectively. In the 11 years’ simulation period, the average groundwater recharge was 1 626.15×106 m3/a, and the dis-charge was 1 761.89×106 m3/a. In general, groundwater had a negative balance with a recharge/discharge difference of -135.72×106 m3/a.
(2) The confined aquifers underlying the phreatic aquifers were the main groundwater withdrawal targets as well as layers prone to land subsidence. As the storage space of deep confined water was relatively concealed and water circulation went slowly, cross-flow recharge became the main source of
Table 2 Model parameters
Parameter Aquifer I Aquifer II Aquifer III Aquifer IVAquifer V Aquifer VI
Total recharge-to-discharge difference -121.51 -14.21 -135.72
Aquifer storage variable -121.51 -14.37
recharge for deep confined water, contributing 44.43% to the total recharge. When large amounts of confined water had been withdrawn, the head will decrease at a high rate and gave rise to compaction water release from cohesive soil. This contrib-uted 32.14% to the total recharge. Inflow from lateral bounda-ries contributed 21.88%. Deep confined water was mainly consumed by artificial withdrawal, which contributed 96.3% to the total withdrawal.
(3) Vertically, the withdrawal tended to reduce as the layer became deeper (Fig. 7). From the withdrawal composition of different layers, withdrawal from Aquifer II consists mainly of cross-flow recharge, which accounted for 75.93% of the total; the withdrawal from Aquifer VI mainly came from the com-pression release from water-bearing media, which accounted for 63.71% of the total. These results demonstrated that, alt-hough the withdrawal from deep aquifers (aquifers V and VI) did not contribute much to the confined water withdrawal, it resulted in high percentage of compression release from cohe-sive soil, therefore contributing more to land subsidence under the same withdrawal intensity.
(4) Overall, the groundwater withdrawal over the 1998-2008 period was declining, and percent compression release from water-bearing media decreased slightly over time, from 33.37% to 29.61% of the total withdrawal. 4.3 Groundwater Prediction under the Proposed South-to-North Diversion Plan
Based on the Terzaghi’s theory of one-dimensional con-solidation, the total stress is equal to the sum of pore water pressure and effective stress. When pumping rate of groundwa-ter increase, groundwater heads as well as pore water pressure will decrease. Because the total stress is a constant value, the effective stress will increase. This will eventually result in the compression of soil. On the contrary, when pumping rate of
groundwater decrease, groundwater heads as well as pore water pressure will increase. This will then result in the expansion of soil. The surface of aquifer also will rebound.
According to the groundwater withdrawal plan described above, after an 11-year operation of the model, the groundwater flow field, groundwater level, and land subsidence variations of the aquifers were predicted. Figure 8 shows the groundwater level variation and cumulative land subsidence in the aquifers for the four land subsidence centers of the simulation area: Dagang, central zone, Ninghe, and Tanggu.
After pressure withdrawal measurements are taken, the groundwater level will eventually recover to some extent. In
Figure 7. Composition of water withdrawal from confined aquifers. (a) Composition of main withdrawal from differ-ent aquifers; (b) withdrawal and its composition vary with time. 1. Net lateral infiltration; 2. net interlayer leakage; 3. rate of net water released from storage accompanying both elastic and inelastic compaction in the interbeds; 4. pump-ing rate.
560 Yali Cui, Chen Su, Jingli Shao, Yabin Wang and Xiaoyuan Cao
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Figure 8. Groundwater levels and cumulative land subsid-ence in 11 years. (a) Dagang H5; (b) city center H6; (c) Ninghe H7; (d) Tanggu H8. For the purpose of cumulative land subsidence herein, “+” stands for land subsidence and “-” for land rebound.
the downtown area, the cumulative land subsidence rebound over the 11 years will be close to 15 mm. In Dagang, the cu-mulative land subsidence rebound over the 11 years will be close to 47 mm, and the rebound will take place at an average rate of 2–3 mm/a. Land subsidence in Ninghe and Tanggu will also be alleviated to some extent (Fig. 8). In other regions, the groundwater levels in the aquifers will more or less go back up. As predicted, during the first 5 years, the maximum groundwa-ter level recovery rate in the aquifer units will be close to 4 m/a and most of the regions will recover at the rate of 1–2 m/a. During the last 6 years, the maximum water level recovery rate will be close to 2 m/a and most of the regions will be less than 1 m/a. It should be noted that, however, corresponding to Fig. 7, the adjusted withdrawal from Aquifer units V and VI is tre-
mendous (Table 1), but there is still land subsidence occurring in some places (Table 5). This result suggests that land subsid-ence attributed to many reasons, and withdrawal is only one of them. The formation lithology and cohesive soil distribution are also important contributors to land subsidence.
In fact, the south-to-north water diversion project in Tian-jin had not completed as expected, therefore, the withdrawal condition from 2010–2012 is not the same as the value provid-ed in Table 1. However, the development of the land subsidence is clearly investigated under the proposed south-to-north diver-sion plan, and it will be an valuable implication for the groundwater management.
Hence, the shallow aquifer should be served as the main exploited aquifers in future in order to control the development of land subsidence and the to meet large amounts of ground-water for water supply. The groundwater in aquifer units V and VI should not be suggested to be exploited, and that in aquifer units II and III could be exploited marginally. It would be an effective way to control the development of land subsidence.
5 CONCLUSIONS
In this study, the land subsidence was simulated using SUB program package. Considering lag drainage and compres-sion processes, the hydrogeological conditions were identified using the actual data from 1998–2008, which improved the accurate description of the groundwater system in Tianjin under coastal deposition that characterized great deposition thickness, widespread interbeds, and complicated structure.
Results from this study indicated that confined aquifers were the main withdrawal targets for Tianjin. Withdrawal from these aquifers mainly came from the cross-flow recharge of the overlying Aquifer I, compression release from soils, and boundary inflow, which contributed 45%, 33%, and 20%, to the withdrawal from confined aquifer, respectively. Groundwater withdrawal was the dominant discharge factor for deep groundwater and contributed 92% to the total deep water dis-charge.
It is an important and effective solution by adjusting the withdrawal quantity and withdrawal layout to reduce the occur-rence of the alleviate land subsidence. As the groundwater level recovers, the land in the key controlling regions, including the downtown area, Dagang, Hangu, Jinghai, Wuqing and Jinnan will rebound at the rate of 2–3 mm/a. The land subsidence rate in Ninghe, Dongli, Tanggu and Jinnan will also reduce slightly.
A multi-year coupled groundwater flow and land subsid-ence numerical model can be used to analyze the composition of groundwater withdrawal, describe the quantitative relation-ship between groundwater withdrawal vs groundwater level
Table 5 Predicted land subsidence by layer in mm
Total land subsidence Aquifer II Aquifer III Aquifer IV Aquifer V Aquifer VI Avg Max Avg Max Avg Max Avg Max Avg Max Avg Max-39.3 84.4 -5.6 19.6 -7.4 91 -9.8 42.4 -8.8 11.9 -7.9 11.3
Note: “+” stands for land subsidence and “-” for land rebound.
Development and Application of a Regional Land Subsidence Model for the Plain of Tianjin 561
of regional groundwater resources and control of land subsid-ence. Nevertheless, the land subsidence is caused by a variety of reasons; in addition, we were not able to obtain as much monitoring data as necessary. In addition, since the simulation period was not long enough, the precision of the numerical models using the existing software could be improved. We are expecting groundwater and land subsidence measurements of 2009–2013, and will take into account the correlation of verti-cal permeability coefficient to cohesive soil compression to improve the model and increase its precision. ACKNOWLEDGMENTS
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