Geosciences Journal 2009

Geosciences Journal

Vol. 13, No. 2, p. 151 − 159, June 2009

DOI 10.1007/s12303-009-0014-4

ⓒ The Association of Korean Geoscience Societies and Springer 2009

Groundwater supply under land subsidence constrains in the Nobi Plain

ABSTRACT: Groundwater overdraft resulted in land subsidence

throughout the Nobi Plain, central Japan. To cope with the growing

water demand in the region, a numerical model was used to deter-

mine the maximum withdrawal capacity of two confined aquifers

without causing undesirable consequences. Results were validated

against field data and by analytical solutions. The analysis focused

in Aburashima, a site expected to experience a rapid development

in the forthcoming years. Calculations showed the water availabil-

ity in the upper aquifer is limited. Moreover, seasonal fluctuations

in heads reduce its extraction capacity up to 44%. In contrast, stor-

age is substantially higher in the deep aquifer. Larger quantities

and lower extraction costs make this layer a more reliable source

for water supply. Findings from this study will be used by authorities

to update the current legislation on groundwater abstraction. Nev-

ertheless, it is argued that to achieve a long-term sustainability, poli-

cies should not limit solely to control regulations but also to economical

strategies and the expansion of the infrastructure system.

Key words: groundwater extraction, safe yield, land subsidence, numer-

ical modeling, sustainability

1. INTRODUCTION

Land subsidence due to excessive extraction of ground-

water has affected the Nobi Plain in central Japan for sev-

eral decades. In the 1920’s, the piezometric levels of the

confined aquifers were above the ground surface in most of

the plain (Iida et al., 1977). However, from 1945 there was

a dramatic increase in abstraction rates due to the industrial

and agricultural activities recovering from the war. The

result was a rapid decline in groundwater levels and the

consequent land subsidence. This phenomena increased

exponentially in the following years, with areas sinking

over 20 cm in 1973 (Yamamoto, 1984). Adoption of strict

regulations on pumping managed to mitigate the problem,

and the area affected by subsidence rates over 1 cm was

effectively reduced from 283 km2 in 1975 to 9 km2 in 2004

(METI, 2006). Furthermore, the land surface at some

points, as Gôcho and Matsunaka in the east-southeast of

the plain, experienced a rebound between 20 and 25 mm

(METI, 2006).

Management authorities are responsible not only for pro-

tecting the water resources, but also for ensuring a sufficient

supply. Despite the success of the implemented policies,

the expansion of the region’s economy is forecasted to be

accompanied by new developments and higher water con-

sumption, which will exert a significant impact on the estab-

lished equilibrium. It is anticipated that conflicts between

water availability and consumer needs will be inevitable

unless the exploitation schemes are optimized in response

to the actual socio-economic circumstances. Understanding

of the groundwater movement will ensure its proper utili-

zation (Don et al., 2005). In addition, establishing the lim-

its of pumpage for a sustainable supply requires accurate

information about the safe yield of the groundwater sys-

tem. Safe yield refers to the rate at which groundwater can

be withdrawn from an aquifer without causing an undesir-

able effect (Dottridge and Jaber, 1999; Heath and Spruill,

2003). In this context, the prefecture of Gifu supported the

present investigation as a basis to update the policies on

water resources in accordance with the increasing demand.

The study focused on Aburashima, at the tripartite bound-

ary of Gifu, Mie, and Aichi prefectures, all of them concerned

with the environmental management of the basin. Moreover,

the site faces the possibility of new developments in the

forthcoming years, which makes it especially susceptible

to adverse consequences.

Then, the main objectives of the present work are to

develop and calibrate a numerical simulation of the Nobi

plain with special emphasis on Aburashima as a mean to

understand the groundwater flow in the region, and based

on these results, calculate the maximum amount of water

that can be extracted from two confined aquifers without

detrimental impacts on the environment. The groundwater

availability was analyzed in relation with the wells number

and distribution, and the accuracy of the calculations val-

idated against analytical solutions.

2. AREA OF STUDY

The Nobi plain occupies an area of about 1,800 km2 over

the prefectures of Gifu, Mie and Aichi, in central Japan.

Adrian H. Gallardo*Atsunao MaruiShinji TakedaFumio Okuda

}

}

AIST, Geological Survey of Japan, Higashi 1-1-1, Central 7, Tsukuba 305-8567, Japan

Hytec Co. Yodogawa-ku, Miyahara 2-11-9, Osaka 532-0003, Japan

*Corresponding author: [email protected] [email protected]

152 Adrian H. Gallardo, Atsunao Marui, Shinji Takeda, and Fumio Okuda

With a density of approximately 1,000 hab/km2, it is one of

the most populated districts of the country, just after the

Kanto plain, in the surroundings of Tokyo. The plain is

bounded by the Ise Bay to the south, the Yoro Mountains

to the west, and the Owari Hills to the north and east. The

Ibi, Nagara, Kiso, and Shonai are the dominant rivers in

the region (Fig. 1). The surface elevation ranges from 0 m

at the coast to nearly 25 m toward the mountains although

a large part of the terrain is below sea level as a result of

land subsidence.

The basin is filled with sediments from the Paleozoic to

the Holocene dipping westward (Fig. 2). They reach a thick-

ness of more than 360 m in the western part of the plain

(Adachi and Kuwahara, 1980). For the purposes of the

present study, the strata in consideration are those above −200

m, the base of the second aquifer at Aburashima. The shal-

lower unit corresponds to the Nanyo formation, formed by

alluvium sands shifting into clays to the bottom deposited dur-

ing the post glacial transgression in the Holocene (Saka-

moto et al., 1984). A small-scale unconformity separates it from

the Nobi Fm, a 20 m unit dominated by the interbedding

of clays and silts. Below, the First Gravel Bed, also knows

as G-1, constitutes the upper aquifer in the region. It thick-

ness averages 10 to 20 m, mainly composed of river gravels

deposited during the last glacial advance in the Pleistocene.

The Atsuta Fm underlies the aquifer. The unit is divided

into lower clays reaching a thickness of 40 m at Aburash-

ima, and upper sands of similar thickness deposited in the

inner bay and surroundings during the Last Interglacial and

early half of the Last Glacial times (Sakamoto et al., 1984).

The Second Gravel Bed or G-2 situates below, and constitutes

the deepest formation considered. These gravels represent

another confined aquifer that provides a large amount of the

water for industrial use in the region (Yoshida et al., 1991).

The G-2 has a thickness of approximately 25 m, but it is

found at variable depths due to the strata dipping. Drilling

at Aburashima intercepted the unit from −175 to −200 m.

3. METHODS

Except for very simple systems, analytical solutions of

groundwater flow are rarely possible therefore, various

numerical methods must be employed (Don et al., 2005).

A regional model of the Nobi Plain was developed and cal-

ibrated in steady state using the three-dimensional finite-

difference code MODFLOW (McDonald and Harbaugh,

1988), and then, a sub model was constructed by refining

the mesh size in the area of interest. The simulation was used

to evaluate the maximum amount of groundwater available

Fig. 1. Map of the Nobi Plain.

Fig. 2. Cross section showing the aqui-fers system in the vicinities of Tsush-ima (modified from Sakamoto et al., 1984).

Groundwater supply under land subsidence constrains in the Nobi Plain 153

for pumping, providing also a scientific tool to assess alter-

natives to optimize the extraction rates. The main risk of

subsidence derives from the overexploitation of the First

Gravel Bed and therefore, the safe yield estimated for this

aquifer was verified against analytical solutions.

Sediment parameters were derived from three explor-

atory wells drilled for this study at Aburashima, to a maxi-

mum depth of −200 m. In addition, the geologic framework

was determined from a regular grid of nearly 80 geological

sections distributed throughout the Plain (METI, 2006).

Other information required by the simulation as climatic data,

topography, river conditions, wells locations and charac-

teristics, heads, and abstraction schemes, were compiled

and integrated from a number of existing reports and data-

bases.

3.1. Model Formulation

The model covers and area of 46 by 55 km discretized

in a mesh of 314 rows and 198 columns. To get a better

representation of head contours, cells were especially

refined around Aburashima to a maximum of 20 m by side.

The domain was preferentially delimited by impermeable

boundaries. An exception is the Ise Bay, which was rep-

resented as a constant head of 0 m (Fig. 3). Based on records

from the Ministry of Land, Infrastructure and Transporta-

tion of Japan (2007), specified heads were imposed along

the major rivers of the region. The city of Nagoya was

excluded from the investigation as it constitutes an inde-

pendent entity for the purposes of resources management.

Seven layers with a general tilt westward defined the geol-

ogy of the system. The aquifers were divided into two lay-

ers while the rest corresponded to the different confining

units (Table 1).

Physical properties were specified for each layer follow-

ing the analyses of soil cores. Hydraulic conductivity of the

aquifers and the sandy member of the Atsuta Fm are in the

order of 2×10-3 cm/sec. In contrast, values within the con-

fining units ranged from 1.9×10-3 to 9.9×10-8 cm/sec, reflecting

the heterogeneity of the sedimentary sequence. Hydrogeo-

logical information below the Second Gravel Bed was more

limited. Based on the approach of Rayne et al. (2001), ver-

tical hydraulic conductivity was calculated using an esti-

mated anisotropy ratio (kh/kv) of 10.

Recharge in the area occurs mainly through rainfall infil-

tration. Initially, recharge to the aquifers was assumed as

the difference between precipitation and evapotranspira-

tion. A shallow water table and the predominance of allu-

vial sands near the surface suggest a rapid and effective

percolation of water into aquifers, with a negligible par-

ticipation of overland flow as a recharge mechanism. Since

other variables as land use, soil structure, topography, sea-

son, and irrigation may also exert some effect, the obtained

values were adjusted during calibration to better reflect real

conditions. Rainfall for the period 1986-2005 was derived

from records of the Japan Meteorological Agency at three

monitoring stations, while evapotranspiration was calcu-

lated by the Thornthwaite method (1948). The highest recharge

corresponded to the Ogaki station in the north, with an aver-

age of 1876 mm/yr. Values reduced to 1676 mm/yr at Aichi

Fig. 3. Model grid and boundary conditions.

Table 1. Stratigraphic units and main physical properties of sediments at Aburashima

Formation Lithology Depth (m) Hydraulic conductivity (cm/sec) Model Layer

Nany Fine sands - silt - clay 0 - 36 3.2 × 10-3 to 9.9 × 10-8 1-2

Nobi Clays - silt 36 - 55 4.2 × 10-4 to 7.8 × 10-8 1-2

First Gravel Bed Gravel - sands 55 - 79 2.1 × 10-3 to 1.1 × 10-4 3

Atsuta (upper member) Fine sands 79 - 133 1.9 × 10-3 to 6.9 × 10-7 4

Atsuta (lower member) Sands - clays 133 - 175 2.1 × 10-3 to 4.4 × 10-8 5

Second Gravel Bed Gravel - sands 175 - 200 2 × 10-3 to 3.3 × 10-3 6

Others various > 200 2.1 × 10-3 7

o


station in the center of the plain, and to 1596 mm/yr in

Kuwana, in proximities of the Ise Bay.

Aquifers are subjected to intensive exploitation. Abstrac-

tion rates were simulated by 372 wells extracting water

from the upper aquifer to a maximum of 2120 m3/day, and

by 287 wells pumping up to 3050 m3/day from the Second

Gravel Bed. Wells in Aichi were represented using a mesh

of 1 km by side designed by the prefecture, while at Gifu

prefecture wells were essentially distributed around indus-

trial centers.

After construction, the model was run under steady state

with the aim at establishing how the aquifers of the Nobi

Plain function. Since most of the data was available until

2004, the simulation period was specified from January to

December of that year. This approach facilitated also com-

parisons of model computations with records on ground-

water heads.

4. MODEL CALIBRATION AND LIMITATIONS

Calibration consisted in adjusting the values of recharge

and river stages within a reasonable range until calculated

heads compared favorably with values recorded at 69 wells

during 2004. In general, the close agreement between mea-

sured and computed heads suggests a satisfactory calibra-

tion. An exception takes place at Ichinomiya, where results

were less accurate (Fig. 4). In here, discrepancies may be

attributed to an insufficient grid refinement. In effect, grid

cells of approximately 500 m by side were unable to

appropriately mimic the high hydraulic gradient resulting

from pumping activities at the site. Even though residuals

could be minimized by increasing the grid resolution, this

was not feasible as a further mesh refinement translated

into numerical instability, limiting thus the capacity of the

model to reproduce flow conditions at a few specific sites.

The mean residual throughout the plain is 0.048 m, which

can be regarded as a very acceptable value considering that

observed heads range from -12 to 18 m. More accurate results

were attained at Aburashima, where differences between

observed and calculated heads reduced to 0.17 and 0.28 m

for the First and Second Gravel aquifers respectively. The

lowest residual throughout the model was established at the

Tsushima station, about 4 km northeast of the site of interest,

with a mean residual of 0.01 m.

Errors are considered to be acceptable if the ratio of the

root mean squared error to the total head loss is minimized

(Anderson and Woessner, 1992; Meriano and Eyles, 2003).

Considering a benchmark of 10% (Waterloo Hydrogeologic,

2003), the 6.7% estimated for the normalized root squared

(NRMS) is low enough to indicate a successful simulation.

In addition, other attributes as a nearly null mass-balance

error, a high correlation coefficient (0.92), and a histogram

displaying a quasi-normal distribution of the residuals are

all indications of the reliability of the model outputs.

Model limitations must be pointed out too. Data was mostly

available on annual intervals therefore, pumping in the model

represented the mean abstraction rates over one year. By

averaging the pumping out over the entire year, the higher

drawdown occurring during the summer months is lost

(Larson et al., 2005). This may result in an overestimation

of the safe yield of the aquifers, as the temporal drop in the

groundwater heads is not taken into account. This limita-

tion in the approach was overcome by constructing an inde-

pendent scenario to analyze the permissible withdrawal

rates when groundwater level is at its lowest. In addition to

temporal averaging, pumping rates in Gifu prefecture had

to be spatially averaged as well. Industry is the main ground-

water user throughout Gifu however, information provided

by the prefecture is arranged by city, and does not detail

the location and pumping volumes of specific wells. In

view of this, the modeling approach consisted in distrib-

uting the user’s abstraction into a number of wells uni-

formly scattered around industrial centers at each city in

the prefecture. As expected, this approach is valid to quan-

tify the bulk of groundwater extraction but could present

some errors when examined at a detailed scale, as wells

would deviate from their exact location. Finally, recharge

was assumed to be dependant on climatic conditions ignoring

the effects of land and vegetation cover. However, these

estimates were revised during the calibration, and yielded

the best outcomes when dividing the plain in 11 zones with

a recharge rate between 0 and 900 mm/yr.

A sensitivity analysis was performed to evaluate the

uncertainties associated with variations in the most rele-

vant input parameters. Thus, changes in recharge rates upFig. 4. Relation between calculated and observed heads over theNobi Plain.


to ±50% were not corresponded with important fluctua-

tions in heads at Aburashima and therefore, results are

nearly independent of this parameter. In contrast, the model is

especially sensitive to changes in river conditions, as cal-

culated heads for the shallow aquifer decreased linearly

with a reduction in river stages (Fig. 5). This highlights the

importance of obtaining accurate estimates of river param-

eters to correctly approximate piezometric heads.

5. RESULTS AND DISCUSSION

5.1. Groundwater Flow

The simulation shows that groundwater flow originates

at the foothills of the north-northeast, converges toward

central part of the Plain, and discharges mainly through the

Shounai River. The center of the Plain is characterized by

a high density of industrial wells that causes a low hydrau-

lic head and alters the direction of the natural flow (Uchida

et al., 2003).

Water levels usually drop to about -7 m, although the

drawdown increases drastically at sites subjected to intensive

exploitation, as Gifu and Ogaki city (Fig. 6). At Aburash-

ima, groundwater flows mainly from north-northwest to

south-southeast with velocities in the order of 10-6 to 10-7 cm/

sec in the horizontal, and 10-8 cm/sec in the vertical direc-

tion. Simulated heads are -1.02 m for the shallow aquifer,

and -1.62 m for the Second Gravel Bed. These values are

in good agreement with the -1.05 and -1.75 m registered

during mid to late 2006, for the upper and lower aquifer

respectively.

Water balance calculations indicate that the total volume

of groundwater flowing in and out is 1.2×106 m3/d for the

upper aquifer, and 2.7×105 m3/d for the deep one. Most of

the recharge to the shallow unit derives from rainfall infil-

tration (83%), the rest from upward flow from the Atsuta

Fm (16%) and leakage through the river bed (1%). Pump-

ing (48%) and fluxes to deeper formations (46%) are the

main routes of groundwater outflow, while 6% of the infil-

trated water returns to uppermost formations through con-

vective pathlines. Groundwater inflow to the deep aquifer

occurs mainly by vertical infiltration through the confining

Fig. 5. Sensitivity analysis of groundwater heads at Aburashima inrelation to a decrease in river stages.

Fig. 6. Contours map of the simulated groundwater within theshallow aquifer.

Table 2. Water balance for the First and Second Gravel Bed

Inflows Outflows

G-1 G-2 G-1 G-2

Downward flow 9.96 × 105 4.58 × 105 1.64 × 105 1.16 × 105

Upward flow 1.87 × 105 6.81 × 104 1.0.2 × 105 6.16 × 104

River leakage 1.38 × 104

Ise bay 2.28 × 103 2.28 × 103 9.47 × 103 1.44 × 104

Pumping 5.72 × 105 8.36 × 104

Total 1.2 × 106 1.2 × 106 2.75 × 105 2.76 × 105


units (59%). However, upward flows play a significant role

in the aquifer recharge as well (Table 2).

5.2. Groundwater Safe Yield

The flow model was ultimately utilized to determine the

abstraction capacity at Aburashima, without causing irre-

versible compaction and land subsidence of the site. The

solution was achieved by establishing a constrain condition

in which water heads must not fall beyond a certain level

otherwise, subsidence effects will arise.

Long-term monitoring data showed that subsidence has

occurred at G cho station when groundwater in the shal-

low aquifer dropped more than 0.8 m respect the average

piezometric level (METI, 2007). Given its geographical prox-

imity, similar aquifers geometry, and equivalent hydraulic

properties, data at Gôcho can be extrapolated to Aburash-

ima with confidence. Thus, it was considered that land sub-

sidence will also occur in Aburashima if the mean groundwater

level there declines more than 0.8 m. As previously explained

however, model predictions in the area of interest differ

0.17 m from field measurements. This uncertainty needed

to be taken into account therefore, the maximum draw-

down for subsidence to occur was set at 0.63 m respect the

mean groundwater level. These estimates reduce the amount

of water that can be withdrawn from the shallow aquifer

but ensure the induced drawdown remains within accept-

able limits. On the other hand, records for the period 1985-

1986 showed that subsidence at Tsushima and Saya ended

when groundwater heads in the Second Gravel Bed recov-

ered above a level of -10 m. This leads to the conclusion

that for simulation purposes, groundwater exploitation will

be sustainable if piezometric levels do not decline more

than 0.63 m in the shallow aquifer, and more than 10 m in

the G-2 formation (Fig. 7).

Model results indicate that the maximum withdrawal

sustainable for Aburashima is 27.7 m3/d for the upper aqui-

fer, and 776.4 m3/d for the lower unit. This scenario assumes

water will be withdrawn from a single well. A possible

design to maximize the groundwater production consists in

arranging several wells within a circle centered in the site

of interest. The more the number of wells the higher the

extraction volumes, as a larger area of the aquifers is cov-

ered (Fig. 8). Nevertheless, groundwater exploitation depends

not only on the supply capacity of the aquifers but also on

the cost of lifting the water to surface. Thus, an optimal

abstraction scheme involves maximum pumping with min-

imum costs. Considering the extraction volumes over 1 year

and the current cost for a production bore in the area, the

optimum withdrawal from the shallow aquifer is attained

by 5 wells distributed in a radius of 250 m (Gallardo et al.,

2008). Although less cost-effective, pumping from 9 wells

yields 65% more of water (247 m3/d) constituting a valid

o

Fig. 7. Schematic representation of the permissible drawdownused for the simulation.

Fig. 8. Maximum pumping rate from wells at Aburashima.

Fig. 9. Minimum price of extracted groundwater.


option to meet larger demands (Fig. 9). As anticipated, install-

ing more wells translate into even larger pumping volumes

however, the cost of water steadily increases and deviates

from the optimal scheme of groundwater exploitation.

It is more beneficial to extract water from the lower

aquifer, as there are both 6.7 times increase in production

along with a similar drop in prices respect the shallow unit

(Hytec Co. 2007). Pumping volumes are again maximized

with 5 wells however, the trade-off curve between extrac-

tion and price shows that as the pumpage increases, the

cost of water increases approximately linearly too. As a

result, the main constrain to meet the future demand could

be not the supply capacity of the aquifer system but the

high cost of the produced water.

Calculations above assumed that the components of the

hydrologic cycle do not change in time. In reality, water

levels in the aquifers decline between May and September

due to greater evapotranspiration and a higher demand for

irrigation in rice fields. Moreover, occasional droughts and

natural fluctuations may further lower the piezometric

heads causing a temporary overexploitation of the aquifers.

Records for the second half of 2006 showed that ground-

water levels may decline up to 24 cm respect the values

adopted for the simulation. With a permissible drawdown

of 10 m, these water fluctuations would produce a negli-

gible effect on the deep aquifer. In contrast, the limited

storage capacity of the G-1 means the upper unit is highly

sensitive even to small variations in piezometric levels: if

groundwater levels drop to a minimum, the supply from

the shallow aquifer reduces between 39 and 44%, approx-

imately 8.5 to 12 m3/d by well. This makes clear that agri-

cultural activities cannot be overlooked when establishing

protection policies. Climatic variability and frequent changes

in irrigation strategies complicate the estimation of sus-

tainable rates of water abstraction in the region. To cope

with these uncertainties, it is wise to adopt a precautionary

principle, which gives preference to risk-averse decisions

and restricts investments that might irreversibly impact the

ecosystem (Young, 1993, Gomboso 1997). In this line, the

safe yield of the aquifer was calculated as the quantity of

groundwater that can be extracted at the minimum head

observed, plus a safety factor of 20 %. Although somewhat

conservative, the calculated permissible withdrawal takes

into account periods with significant water declines, keep-

ing exploitation of aquifers in Aburashima within safe lim-

its throughout the year (Table 3).

5.3. Analysis Validation

Traditionally, land subsidence has been associated to the

overdraft of the shallow aquifer. Then, a validation test was

conducted to confirm whether the simulation provided an

appropriate solution of the safe yield in the upper unit. If

the permissible abstraction rates calculated by the model at

Aburashima are correct, the groundwater drawdown should

not exceed 0.8 m otherwise, subsidence will occur. In this

context, the decline in hydraulic heads was predicted by

the solution of Hantush and Jacob (1955) for leaky aqui-

fers. It was assumed there is no storage in the confining

clays, which resulted in a conservative solution. In effect,

if there is significant storage in the confining layer, then

part of the flow during the initial time period will come

from this storage, attenuating the drawdown (Fetter, 2000).

The solution is expressed as:

(1)

where h0-h is the drawdown; Q is the pumping rate, equiv-

alent to the aquifer safe yield; T is the aquifer transmissivity,

and W (u, r/B) is a function defined by

and (2)

with r as the radial distance from the pumping well, S cor-

responds to storativity, t is time, K' is the vertical hydraulic

conductivity of the leaky layer, and b' is the thickness of this

leaky layer.

Analytical results indicate that for one well pumping at

the safe yield value of 12 m3/d, the cone of depression reaches

equilibrium within one month. The predicted maximum

drawdown is 0.41 m at the well itself. As groundwater pro-

duction proceeds, the cone would expand approximately

150 m, with a piezometric decline in the order of 0.1 m.

There is little difference in the drawdown patterns when

placing 5 wells. However, the time required to reach equi-

librium extends to about 45 days. The asymmetrical dis-

tribution of the production wells would cause an irregular

cone of depression preferentially elongated in the north-

south direction (Fig. 10). The groundwater level is expected

to vary between 0.26 to 0.61 m in the wells adjacencies,

gradually recovering towards the peripheries of the draw-

down zone. Extracting water from 9 wells is still within the

permissible drawdown limit. In this scenario the cone of

depression expanded about 400 m by side, with a maximum

drop in water levels of 0.78 m. A higher number of wells may

exceed the subsidence threshold at the wells themselves,

but no effect is predicted a few meters away from them

(Table 4). In view of this, it is confirmed that maintaining

h0 h–Q

4πT----------W u r, B⁄( )=

ur2

S

4Tt--------=

r

B--- r

K′

T′b′----------=

Table 3. Optimal safe yield from the shallow aquifer

UnitNo. of

wells

Maximum abstraction

(m3/day)

Safe yield

(m3/day)

Upper Aquifer

1 15.4 12

5 91.7 73

9 151.8 121

Lower Aquifer

1 776 621

5 2059 1647

9 2479 1983


the abstraction rates within the safe yield values calculated

by the model ensures a sustainable exploitation of the aqui-

fer and eliminates the risk of land subsidence.

6. SUMMARY AND CONCLUSIONS

Groundwater plays a fundamental role to sustain the

industrial and agricultural production of the Nobi Plain.

Nevertheless, overexploitation of the resource has resulted

in land subsidence over several decades. A number of reg-

ulations helped to mitigate the problem but the constant

development of the region, especially at suburban areas,

poses serious concern about the ability to meet the future

demand of water. A numerical simulation supported by

analytical solutions was used to estimate the maximum

pumping capacity from aquifers at Aburashima, a site fac-

ing the possibility of economical developments in the short

term. Results indicated that the supply capacity of the

upper aquifer is constrained by a limited permissible draw-

down and by seasonal fluctuations of groundwater levels.

However, land subsidence would not be a major threat for

pumping rates in the range 27.7 to 150 m3/d. In contrast,

there is a higher storage in the deeper aquifer, as it allows

for a more significant drop in piezometric levels. Moreover,

this aquifer is relatively isolated from extreme climatic events

and urban development, providing a safeguard against droughts

and pollution. Higher quantity and quality, and a more effi-

cient relationship extraction-price make the lower aquifer a

more reliable source of water.

The present work will be used by local authorities to sci-

entifically update strategies on groundwater management.

Estimates of water availability will be useful to develop

appropriate policies that reconcile the needs of freshwater

with a safe exploitation of the aquifers. Rather than simply

restrict the abstraction rates, decision-makers must need to

consider a range of political and economical measures to

ensure the sustainable utilization of groundwater. In addi-

tion, changes in climatic conditions are expected to lead to

more frequent dry periods accompanied by declines in pie-

zometric levels, so water conflicts would tend to worse

unless alternative regulations are also implemented. As an

example, new directions may include but are not limited to,

subsidies for cultivation of crops requiring minimum irri-

gation, soft credits and technical assistance to industries

implementing low-water production processes, financial

incentives for users switching exploitation from the shal-

low to deeper aquifers and, increase in the cost of energy

for stakeholders demanding groundwater beyond a pre-

defined threshold. Simultaneously, it is imperative to expand

the infrastructure of the region to divert surface waters into

a distribution network which will permit to reduce the depen-

dence on groundwater throughout the region. Although costly

and time consuming, construction of reservoirs, aqueducts,

and pipelines would constitute the ultimate solution to the

subsidence issue.

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Table 4. Maximum drawdown predicted by the Hantush and Jacobsolution

No. of

wells

Maximum drawdown

at production wells (m)

Maximum drawdown

50 m from wells (m)

Time to steady

state (days)

1 0.41 0.1 30

5 0.61 0.25 45

9 0.78 0.40 45

13 0.95 0.50 50

18 1.20 0.70 50


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Manuscript received May 7, 2008

Manuscript accepted March 3, 2009

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Technology

Geosciences Journal 2009