Hydrological Sciences-Journal-dés Sciences Hydrologiques, 42(1) February 1997
67
Exploitation of alluvial aquifers having an
overlying zone of low permeability:
examples from Bangladesh
MOHAMMAD MIRJAHAN MIAH
Department of Water Resources Engineering, Bangladesh University of Technology,
Dhaka 1000, Bangladesh
K. R. RUSHTON
School of Civil Engineering, University of Birmingham, Birmingham B15 2TT, UK
Abstract Alluvial aquifers can often supply large quantities of water but if they are
overlain by a low permeability zone, the recharge may be restricted with the result
that the long term safe yield of the aquifer is greatly reduced. This paper describes a
study of an alluvial aquifer in Bangladesh where there is a low permeability layer
overlying the main aquifer. Pumping tests carried out in this aquifer were analysed
using a numerical model which represents both the main aquifer and the overlying
low permeability zone. Using the aquifer parameters deduced from the pumping test
analysis, the numerical model was then used to represent five years of pumping.
This long term simulation indicated that there would be a serious decline in the
pumped levels and that a water table would develop in the main aquifer and would
fall at a rate of almost five metres per year.
Exploitation d'aquifères alluviaux recouverts d'une couche peu
perméable: exemples du Bengladesh
Résumé Les nappes alluviales peuvent souvent fournir de grandes quantités d'eau
mais, si elles sont recouvertes d'une couche de terrains moins perméables,
l'infiltration peut être limitée et la production de la nappe garantie à long terme peut
s'en trouver considérablement réduite. Le présent article décrit l'étude d'une nappe
alluviale du Bangladesh où une couche moins perméable recouvre l'aquifère
principal. Les essais de débit effectués sur cette nappe ont été analysés à l'aide d'un
modèle numérique représentant à la fois la nappe principale et la couche moins
perméable. En utilisant les paramètres obtenus par les essais de débit, le modèle est
ensuite utilisé pour simuler cinq années de pompage. Cette simulation à long terme
indique que le niveau de l'eau dans le puits subirait un important rabattement tandis
que le niveau de la nappe s'abaisserait d'environ cinq mètres par an.
INTRODUCTION
Alluvial aquifers are used extensively in supplying water for irrigated agriculture; a
major concern is that the exploitation of the aquifer often leads to serious reductions
in the water table elevation and pumped levels. However, there are situations where
the main alluvial aquifer, which consists primarily of sand with layers of clay and
silt, is covered by an upper silty-clay layer. This is a common situation in
Bangladesh in the alluvial sediments of the three major rivers, viz. the Ganges, the
Bramaputra and the Meghna. Although significant potential recharge occurs during
the monsoon season, the upper silty clay layer regulates the flow into the main
aquifer. This paper explores the important flow mechanisms when pumping occurs
from an alluvial aquifer overlain by a low conductivity layer.
Open for discussion until 1 June 1997
68
M. M. Miah & K. R. Rushton
Studies of field conditions in areas with overlying clay layers have raised the
possibility that water can pass through the clay to recharge the main aquifer system.
Ahmad (1974) described early groundwater exploration studies in Bangladesh and
showed that in many areas there is in excess of 30 m of clay between the ground
surface and the main aquifers. He concluded that this lowers the water yielding
potential of the aquifer systems and suggested that these areas are unsuitable for high
capacity tubewells. In the Calcutta region of the Bengal Basin in India, much of the
aquifer consists of coarse to medium grained sand overlain by a thick clay zone
(Banerji, 1983). There is uncertainty about the possibility of recharge through the
clay layers; even in the early 1980s, there were indications that the groundwater
resources were not being fully recharged. In the Madras aquifer (Krishnasamy &
Sakthivadivel, 1986) pumping from the deeper aquifer zones has exceeded the rate at
which water can pass through the overlying clay layers with the result that new water
table conditions have formed beneath the clay layers resulting in a perched water
table above the clay layers. As well as limiting the recharge, the presence of the clay
layers can lead to subsidence. Ramnarong (1983) described the uppermost aquifer in
Bangkok which is overlain by a blue to grey to yellowish marine clay about 25 m
thick. The initial concept was that this clay was impermeable but water balance
studies indicated that vertical recharge could occur. The consequent settlement in
Bangkok confirmed that some water has drained out of the overlying clay.
The particular aquifer selected for this study is the Madhupur aquifer in Kapasia,
Bangladesh. Groundwater development in this area has been limited because the
hydrogeological conditions in the area are believed to be poor. However, a recent
study (Mott MacDonald International, 1990) revealed that the thickness of the
screenable formation in the top 120 m varies from 40 to 90 m. An examination of the
available borehole logs indicated that the aquifer is a complex mixture of sand, silt
and clay. A clay layer of thickness ranging from 5 to 30 m overlies the aquifer. The
average annual rainfall in the study area is 2370 mm. About 80% of this rainfall
occurs during the monsoon months of July-October.
The first stage of the analysis was to identify the important flow processes. Next,
a numerical model was developed to represent pumping from this aquifer. The twozone radial flow model of Rathod & Rushton (1991) was used but with developments
to represent flow mechanisms in the overlying low conductivity layer. Aquifer
parameters were chosen so that the model represented pumping tests and the
subsequent recovery. The same model was then used to examine the long term
response of the aquifer system and determine whether a deterioration of the aquifer
conditions might be likely to occur.
FORMULATION OF PROBLEM
First stage: description of the system
The first stage in the formulation of the problem was to develop a diagram which
represents the important flow mechanisms. This diagram did not need to represent all
the detailed complexities of the specific problem but it had to represent the major
Exploitation of alluvial aquifers
69
flow mechanisms. The formulation was considered in two parts: the flow through the
main aquifer and the response of the overlying low permeability layer. Figure 1
shows the aquifer system divided into two; the main aquifer is represented in
Fig. 1(b) and the overlying zone in Fig. 1(a).
RECH
(a)
overlying
zone
T
FWT
* INF* t • M M
(b)
upper
permeable zone
low permeability
zone
lower
permeable zone
Fig. 1 Idealization of the two parts of the aquifer system (a) overlying zone,
(b) main aquifer
RECH
(a)
wis.
-4—f-
-W-4—M
RECH
*
FWT
• GW
BASE
-4
f-
^H
XvGW
INF = VPERM
FWT =
WT-GW
WT - BASE
INF-RECH x D E L T
SYO
- BASE
WT - BASE
INF - RECH x D E L T
SYO
INF = VPERM
FWT =
m
RECH
if RECH < VPERM
INF = RECH
FWT= 0.0
if RECH > VPERM
rWT
INF = VPERM
INF
L.i
i-v-Gw
FWT=
INF-RECH
SYO
y n F
|T
Fig. 2 Method of calculating infiltration (INF) from the overlying zone (OZ) to the
main aquifer (MA).
70
M. M. Miah & K. R, Rushton
Considering first the main aquifer, detailed studies have shown that an alluvial
aquifer can be represented as a layered system (Kavalanekar et ah, 1992).
Figure 1(b) shows the major flows through an alluvial aquifer towards a pumped
borehole. This situation can be idealised as two permeable zones with an intermediate low permeability zone. The borehole draws water from both of the permeable zones. Inflow (INF) to the upper surface of the aquifer system can occur due
to recharge or flow from an overlying low permeability zone.
Next, inflows to the low permeability overlying zone (Fig. 1(a)) occur due to
recharge (RECH) from the soil zone. Another aspect of the water balance is the fall
of the water table (FWT) in the overlying zone. The quantity of water moving from
the overlying zone into the main aquifer system also depends on the vertical hydraulic gradient in the overlying zone. Figure 2 illustrates three possible conditions:
(a) the water table (WT) and the groundwater head (piezometric head) of the main
aquifer (GW) both lie within the overlying zone, Fig. 2(a); this means that the
main aquifer is under confined conditions;
(b) the groundwater head in the main aquifer (GW) is below the overlying zone, and
consequently unconfined conditions apply whilst perched water table conditions
apply in the overlying zone; and
(c) most of the water has drained out of the overlying aquifer with the result that the
water table (WT) is close to, or at the base of, the overlying zone.
Second stage: quantify vertical flow through overlying zone
The second stage of the formulation was to develop a method of analysing the
problem as described above. Numerical methods are suitable for this type of
problem. The two-zone radial flow model described by Rathod & Rushton (1991) can
represent the flow in the main aquifer as shown in Fig. 1(b); full derivations of the
finite difference equations were given in that paper. However it was necessary to
adapt the overlying layer simulation to represent the conditions in the low
permeability overlying zone.
The relevant equations for the overlying zone are shown in Fig. 2. Two quantities need to be calculated:
(i) the inflow, INF, from the overlying zone to the main aquifer; and
(ii) the fall in the water table in the overlying zone, FWT.
Four further parameters are required: VPERM, the vertical permeability of the
overlying zone; BASE, the elevation of the base of the overlying zone; SYO, the
specific yield of the overlying zone; and DELT, the time step of the calculation.
An examination of the equations in Fig. 2(a) shows that the value of the inflow to
the main aquifer, INF, is determined from a Darcy calculation in the vertical
direction. The vertical head gradient in the overlying zone equals (WT - GW) and
this acts over a vertical distance (WT - BASE). The fall in the water table in the
overlying zone, FWT, depends on the difference between the water draining to the
main aquifer, INF, and the recharge to the overlying zone, RECH; the fall is also
proportional to the size of the time step.
Exploitation of alluvial aquifers
71
For situation (b) the vertical hydraulic gradient is unity because atmospheric
conditions apply at the base of the overlying zone. In situation (c), the assumption
was made that any recharge moves immediately through the overlying zone to be
available to enter the main aquifer. In practice there would be a delay (Kruseman &
de Ridder, 1990); nevertheless, the overall water balance is valid.
This model can be used both to analyse pumping tests lasting for a number of
days and to consider the long term aquifer response.
Pumped well
KAP/13
Sand
x
47.7 m
D2J\ 54.8 m
D , A 54.8 m
Sand
X
87.9 m
o
10
20
30
40
100
Radial distance (m)
Fig. 3 Details of tube-well and observation piezometers used for pumping test at
KAP/13.
ANALYSIS OF PUMPING TESTS
Three pumping tests were carried out in the Madhupur aquifer. One test was selected
to illustrate the manner in which aquifer parameters can be estimated from the analysis of field results. The layout of the test site is shown in Fig. 3 and in Fig. 4 (a)-(c)
results are presented of field drawdowns in the pumping tube-well, KAP/13, and in
four observation piezometers. The tube-well penetrated 89.3 m below ground level
(82.7 m below the rest water level); the upper 12.8 m was a clay layer with the
remainder primarily of sand but with some clay zones; the slotted casing extended
from 29.6 to 87.8 m below ground level. Before pumping, the rest water level (RWL)
in the main tube-well was 6.6 m below the reference level. Pumping at a rate of
5141 m3 d"1 continued for three days and recovery was monitored for a further two
days. Drawdowns during the pumping and recovery phases were monitored in:
P the pumping tube-well, Fig. 4(a);
SI an observation piezometer, 10 m from the tube-well, 11.8 m below RWL,
Fig. 4(b);
72
M. M. Miah & K. R. Rushton
Time (day)
1&4
(a)
0.00
1E-3
I I I I 11
0.01
I
I I I I I I 11
0.1
I
I I I I I 111
1
M i l
10
I
I I I I I I il
4.00 —
1
12.00 .
(b)
1
(0
!
3.00 —I
Fig. 4 Pumping test at KAP/13: comparison between field and modelled results
(a) in pumped well; (b) in observation piezometers at 10 m from tube-well; arid
(c) in observation piezometers at 100 m from the tube-well.
Exploitation of alluvial aquifers
73
Dl an observation piezometer, 10 m from the tube-well, 54.8 m below RWL,
Fig. 4(b);
S2 an observation piezometer, 100 m from the tube-well, 11.8 m below RWL,
Fig. 4(c); and
D2 an observation piezometer, 100 m from the tube-well, 54.8 m below RWL,
Fig. 4(c).
The field results are represented in Figs.4(a)-4(c) by the discrete symbols while
the lines represent model results which will be described later.
Examination of the field results
A careful examination of the field responses at the five locations provides important
information about the aquifer response to pumping:
- in the pumped tube-well there was a sudden fall in the water level of about 5 m
which increases to 8 m during the three days of the test; the initial recovery to
4 m was rapid; the recovery was almost complete after two days;
- at the observation piezometers 10 m from the pumped tube-well, the responses of
the shallow (SI) and deep (Dl) piezometers were similar although during the
pumping phase the drawdown in the deep piezometer was slightly larger
suggesting that there are some low permeability zones in the main aquifer; the
initial drawdowns were smaller than in the pumped tube-well although the
general response during pumping and recovery was similar; and
- for the observation piezometers at 100 m there was little difference between the
shallow (S2) and the deep (D2) piezometers; however the response to pumping
was slower than in piezometers SI and Dl yet after about 0.01 day (about
15 min) the drawdown approached 0.3 m. After three days the drawdown was
increasing at a slower rate yet the drawdowns had not reached equilibrium. The
recovery curve is similar to the pumping curve; after two days the residual
drawdown was about 0.1 m.
Estimation of aquifer parameters using the two-zone model
These field drawdowns can be analysed using the two-zone numerical model. In that
model the mesh spacing increases logarithmically from the pumped tube-well; time
steps also increase logarithmically from 10"7 day during each phase of pumping or
recovery. The aim was to obtain the best match for the five time-drawdown curves
both for the pumping and the recovery phases using a single model. Particular
emphasis was placed on the observation piezometers at 100 m. Figure 4 shows the
match between the field readings and the modelled results shown by the continuous
lines. The parameters deduced from this analysis are as follows:
overlying layer:
vertical hydraulic conductivity =0.007 m d"1
specific yield = 0.03
M. M. Miah & K. R. Rushton
74
upper zone:
middle zone:
lower zone:
horizontal hydraulic conductivity = 11.5 m d"1
confined storage coefficient = 0.001
well loss factor = 4.0
vertical hydraulic conductivity = 0.01 m d '
equivalent thickness = 1.0 m
horizontal hydraulic conductivity = 11.5 m d"1
confined storage coefficient = 0.001
well loss factor = 3.0
The well loss factor represents the restriction on the movement of water through
gravel packs and the well screen into the tube-well; the hydraulic conductivity
adjacent to the tube-well equals the standard value divided by this factor.
An important issue is the tolerances of these parameters. Particular attention
needs to be paid to the vertical permeability and specific yield of the overlying layer.
Table 1 lists the drawdowns in piezometer S2, which was 100 m from the pumped
tube-well, at the end of the pumping phase (3 days) and recovery phase (5 days).
Different values of the vertical hydraulic conductivity and specific yield were
considered. Model results for the end of the pumping phase and after two days
recovery are included in Table 1.
Table 1 Sensitivity analysis of the overlying zone parameters; drawdowns in piezometer D2
Vertical hydraulic
conductivity (md1)
field
Specific
yield
Drawdown (m)
at 3 days
2.06
Drawdown (m)
at 5 days
0.08
0.001
0.004
0.007
0.010
0.03
0.03
0.03
0.03
2.39
2.16
2.00
1.89
0.34
0.18
0.12
0.09
0.007
0.15
1.99
0.09
The results in Table 1 demonstrate that the response in the distant observation
piezometer was sensitive to the vertical hydraulic conductivity; if the vertical
hydraulic conductivity was set too low the drawdowns did not level off at the end of
the pumping phase; if they were set too high the levelling off was too rapid and the
recovery was completed too soon. A vertical hydraulic conductivity of 0.007 m d"1
provided an adequate match between model and field results. Values of 0.004 m d"1
and 0.01 m d"1 were not acceptable.
The observation well responses were not very sensitive to the specific yield of
the overlying layer as indicated by the final line of Table 1. The lower value of 0.03
was used for the predictive studies to represent the slow drainage that occurs from
clay soils due to their low permeability.
Although reliable values of certain aquifer parameters were deduced by matching
Exploitation of alluvial aquifers
75
the model to the pumping and recovery field results, the analysis would have been
enhanced if piezometers had also been provided in the overlying lower permeability
layer.
REPRESENTATION OF LONG TERM RESPONSE
The next stage in the study was to consider the response when this aquifer system is
used to provide water for irrigation. The following assumptions were made which
allowed the formulation of a problem to represent a typical long term response.
For each year included in the simulation (the year started at the beginning of
December) the recharge and abstraction patterns were idealised as follows:
December-April:
May:
June-September:
October-November:
abstraction
no abstraction, no recharge
recharge
no abstraction, no recharge.
Further details are as follows (each year was assumed to consist of 12 months of
30 days):
Abstraction rate: 4900 m3 d1 for 12 h, recovery for 12 h; continuing for 0-150 days.
Spacing of the wells: 700 m by 700 m (equivalent outer radius 395 m), meaning the
intensity of irrigation was equivalent to 5 mm d"1 (0.5 x 4900 m3 d'7490 000 m2)
which is suitable for growing rice (total 750 mm in 150 days). In practice there
would be a variation in water demand during the land preparation and growing
season.
Recharge: the four recharge months were divided into ten day periods; total recharge
(mm) for each ten day period was as follows: 0, 30, 0, 15, 0, 30, 30, 0, 30, 30, 15,
0; hence the total annual recharge was 180 mm. In estimating this recharge, note was
taken of the high runoff which occurs during the intense monsoon rainfall. The effect
of higher recharge intensities is considered later.
Initial conditions: taken to be the same as for the pumping test with the initial
saturated depth of the overlying clay layer of 6.2 m.
Aquifer parameters: in general the parameters were the same as those deduced from
the pumping test; during the analysis of the pumping test it was not possible to
deduce a value for the specific yield of the main aquifer; from information in similar
areas it is taken to be 0.15 (Kavalanekar etal., 1992; Walton, 1987)
This standard example was called Example A. In the simulation, pumping and
recovery phases occurred each day and the time step was reduced to 10'7 at the start
of each phase. Figure 5 indicates the response in the pumped tube-well during the
first 150 days. The change in the pumping level between successive pumping and
recovery phases was about 7.0 m; when the non-pumping level was about 6 m the
maximum and minimum levels showed a change in slope due to the changing
M. M. Miah & K. R. Rushton
76
conditions in the overlying layer from condition (a) to condition (b) of Fig. 2. The
pumped drawdown after 150 days is 17.21 m and the non-pumping level is 10.25 m.
Figure 6 shows the response for a complete year. At the end of the pumping
season the pumped drawdown was 17.2 m; during the recharge period of June to the
end of September, drawdowns recovered to 8.6 m and remained at this value during
October and November when there was neither recharge nor abstraction.
Results for five years of pumping are summarized in Table 2. The maximum
pumped drawdown and final recovery level at the end of the first year are recorded
in the column headed Example A beneath the heading drawdowns. In addition results
are quoted at the end of the pumping phase and the end of the year for years 2, 3 and
Time (day)
0.00
J
0.00 •
160.00
80.00
40.00
I
L.
5.00 —
g
"p
10.00 •
&
20.00
Fig. 5 Predicted response in tube-well over 150 days with pumping for 12 h and
recovery for 12 h.
Time (day)
0.00
100.00
200.00
300.00
20.00 —J
Fig. 6 Predicted response in tube-well for one year of operation.
400.00
Exploitation of alluvial aquifers
77
5. During Year 5 the pumped drawdown reached 34.3 m with recovery to 23.8 m at
the end of the recharge period. This rapid fall in groundwater level averaged more
than 4.7 m per year which could mean that the aquifer resources would be exhausted
within a decade. The fall in the pumped levels to 34.3 m during Year 5 would be
likely to result in a deterioration in the tube-well performance.
Three further situations were considered with the results presented in Table 2.
For Example B: the spacing between the tube-wells was increased to 900 m which
meant that the equivalent radius of the area supplying a single tube-well was 508 m.
The abstraction rate remained at 4900 m3 d"1 which meant that the average intensity
of irrigation was reduced from 5.0 to 3.0 mm d"1 to compensate for the increased
area from which the tube-well withdrew water. Even with this reduced abstraction
rate the maximum pumped drawdown was 23.0 m compared to 34.3 m with the
standard spacing and the average groundwater head fell a total of 13.7 m in five
years, equivalent to 2.7 m each year.
In Example C all parameters were the same as for Example A apart from the
annual recharge which was increased from 180 to 300 mm year"1. As indicated in
Table 2 the fall in groundwater head during the five year period would be 19.9 m
which is equivalent to 4.0 m per year. Only if the quantity of water moving through
the overlying layer was equivalent to 750 mm in a year would no mining of the
groundwater occur. Although the annual rainfall is in excess of 2000 mm in a year,
the high intensity of much of the monsoon rainfall leads to very high runoff with the
result that the quantity available to move through the overlying zone would never be
as high as 750 mm in a year.
For Example D, the effect of pumping continuously was investigated. Instead of
pumping for 12 h with a recovery for 12 h, the pump operated continuously at half
the original pumping rate. Due to the continuous pumping, the pumped drawdowns
were less, 13.7 m compared to 17.2 m for the intermittent pumping during Year 1
Table 2 response to long term pumping for four different scenarios; the first five lines give the
specified parameters and the remaining results are drawdowns in the pumped well in metres.
Example no.
Details of problem:
Well spacing (m)
Irrigation intensity (mm"1)
Abstraction rate (m3 d"1)
Duration per day (h)
Annual recharge (mm)
Drawdowns (m) at:
End of 1st year pumping,
End of first year
End of 2nd year pumping,
End of third year
End of 3rd year pumping,
End of third year
End of 5fh year pumping,
End of fifth year
A
B
C
D
700
5.0
4900
12
180
900
3.0
4900
12
180
700
5.0
4900
12
300
700
5.0
2450
24
180
17.2
8.6
21.4
12.4
25.6
16.2
34.3
23.8
14.9
6.4
16.9
8.3
18.9
10.1
23.0
13.7
17.2
7.8
20.5
10.8
23.9
13.8
30.7
19.9
13.7
8.6
17.7
12.4
27.7
16.2
29.8
23.8
78
M. M. Miah & K. R. Rushton
and 29.8 m compared to 34.3 m during the fifth year of pumping. However, whether
the pumping was intermittent or continuous, the average drawdowns after five years
remained at 23.8 m.
CONCLUSIONS
This study has shown that, when an alluvial aquifer is overlain by a less permeable
layer, it is essential to consider the combined effect of the main aquifer and the
overlying layer, both when analysing pumping tests and when predicting the long
term behaviour of the aquifer system. A two-zone radial flow model was modified to
represent the important mechanisms of a water table forming in the main aquifer
leaving the overlying zone as a perched aquifer which still transmitted water.
The two-zone model was used to analyse pumping tests in the Madhupur aquifer
in Bangladesh. The model simulated successfully the pumping and recovery phases
of the pumping tests and reproduced the main features of the pumped and observation
piezometer responses. Values of many of the aquifer parameters were estimated but
the analysis would have been enhanced if piezometers had been provided in the
overlying layer.
The original workers who carried out the pumping tests suggested that the
aquifer would provide a high, sustainable yield. However, when the model, which
includes the effect of the overlying layer, was used to examine the long term
responses over a period of five years, questions were raised about the sustainability
of the yield. Each year a typical pattern of pumping and recharge is considered. The
responses were simulated for different values of irrigation intensity and annual
recharge. For each of the four cases considered, the groundwater head within the
main aquifer and the water level in the pumped well showed a rapid decline. The fall
in pumping level would be likely to lead to a deterioration in the efficiency of the
tube-wells. Consequently, as abstraction increases in this type of aquifer, it is
essential that monitoring is carried out in the overlying zone as well as in the main
aquifer to determine how rapidly the resource is being used.
This method of analysis is being applied to other areas. However, even before a
detailed study is carried out it is possible to conclude that mining of water from
aquifers used for extensive irrigation is likely to occur unless there is unusually high
recharge. For example, if the intention is to use the pumped water to irrigate a crop
in the dry season which requires more than 700 mm of water, the annual recharge is
unlikely to be this high and therefore mining of the groundwater is almost certain to
occur.
REFERENCES
Ahmad, N. (1974) Groundwater Resources of Pakistan. Ripon Printing Press, Lahore, Pakistan.
Banerji, A. K. (1983) Importance of evolving a management plan for groundwater development in the Calcutta Region
of the Bengal Basin. In: Groundwater in Water Resources Planning, 45-54. IAHS Publ. no. 142,
Kavalanekar, N. B., Sharma, S. C. & Rushton, K. R. (1992) Over-exploitation of an alluvial aquifer in Gujarat, India.
Exploitation of alluvial aquifers
79
Hydrol. Sci. J. 37, 329-346.
Krishnasamy, K. V. & Sakthivadivel, R. (1986) Regional modelling of non-linear flows in a multi-aquifer system.
UNESCO Regional Workshop on Groundwater Modelling, Roorkee, 85-105.
Kruseman, G. P. & de Ridder, N. A. (1990) Analysis and Evaluation of Pumping Test Data, Publication 47,
International Institute for Land Reclamation and Improvement, Wageningen, The Netherlands.
Mott MacDonald International Limited (1990) Report on exploration drilling in Kapasia Upazila, Working Paper 53,
IDA Deep Tube-well II Project, Bangladesh Agricultural Development Corporation, Ministry of Agriculture,
Government of Bangladesh.
Ramnarong, V. (1983) Environmental impacts of heavy groundwater development in Bangkok, Thailand. In:
International Conference on Groundwater and Man, Vol 2, 345-350. Australian Government Publishing Service,
Canberra, Australia.
Rathod, K. S. & Rushton, K. R. (1991) Interpretation of pumping from two-zone layered aquifers using a numerical
model. Ground Water 29, 499-509.
Walton, W. C. (1987) Groundwater Pumping Tests, Design and Analysis. Lewis Publishers, Michigan, USA.
Received 2 January 1996; accepted 11 June 1996