geosciences journal 2009
DESCRIPTION
Groundwater supply in the Nobi Plain, Central JapanTRANSCRIPT
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
}
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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
154 Adrian H. Gallardo, Atsunao Marui, Shinji Takeda, and Fumio Okuda
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.
Groundwater supply under land subsidence constrains in the Nobi Plain 155
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
156 Adrian H. Gallardo, Atsunao Marui, Shinji Takeda, and Fumio Okuda
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.
Groundwater supply under land subsidence constrains in the Nobi Plain 157
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
158 Adrian H. Gallardo, Atsunao Marui, Shinji Takeda, and Fumio Okuda
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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9 0.78 0.40 45
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18 1.20 0.70 50
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Manuscript received May 7, 2008
Manuscript accepted March 3, 2009
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