47
4 Hydrogeology
4.1
INTRODUCTION
The deposits of thick unconsolidated Pleistocene and
Holocene alluvial sediments of the Ganges, Brahmaputra
and Meghna (GBM) delta system form one of the most
productive aquifer systems in the world. Most of this system is fully recharged each year by the annual monsoon
rains and floods. Deeper aquifers are exploited within the
coastal regions below shallow zones of saline water intrusion. Jones (1985), using data derived from oil and gas
exploration, suggested that fresh water may also be available from older Tertiary strata down to depths of 1800 m.
The BADC initiated development of groundwater in the
1960s to enable dry season irrigation of cereals. Study of
the hydrogeology and groundwater resources of Bangladesh was begun in the 1970s by BWDB under the guidance of the UNDP. UNICEF, recognising that large
quantities of groundwater existed at shallow depth, advocated the installation of large numbers of hand-drilled
boreholes equipped with suction pumps. Some 6–11 million hand-pumped tubewells are estimated to have been
installed to date, enabling the dramatic increase in the percentage of the population with access to ‘safe’ drinking
water during 1988-98 (Table 4.1). The maximum depth to
which tubewells can be drilled using traditional methods
without a power rig is shown in Figure 4.1.
Detailed investigations of the geology and hydrogeology of the Quaternary alluvial aquifers were initiated during the 1980s. Many of the available data were collected
during major irrigation projects undertaken in the northwestern, north-central and north-eastern parts of Bangladesh. During these projects, emphasis was placed upon
understanding the physical properties of the aquifers and
the design of deep tubewells. The nature of the data collected reflected the need to avoid screen blockage, the
reduction of borehole specific capacity and formation collapse due to sand pumping – problems that had caused the
failure of large numbers of similar boreholes in Pakistan.
Table 4.1. Percent of the population of Bangladesh with access to
safe drinking water
Year
Urban
Rural
Total
Reference
1983
1983-86
1985-87
1985-88
1988-91
1990-1998
29
29
24
24
82
99
43
43
49
49
81
95
42
41
46
46
84
95
UNICEF (1987)
UNICEF (1988)
UNICEF (1990)
UNICEF (1991)
UNICEF (1994)
UNICEF (2000)
Depth in metres
80 m (20% of population)
150 m (35% of population)
300 m (40% of population)
Stoney areas (5% population)
0
50
100 km
Figure 4.1. Maximum depth of drilling possible without a powered
rig (NWMP, 2000).
At present, the extensive abstraction of groundwater for
irrigation and domestic water supply is being questioned
because of its extensive contamination with arsenic.
The GBM is a large, low-lying fluvial and tidal delta
area whose surface dips southward away from a series of
fan deltas located along the Himalayan Main Boundary
Fault zone (Figures 3.1). The main channel of the Brahmaputra falls 20 m in 250 km between its confluence with
the Tista River at Kurigram and its confluence with the
Ganges at Aricha (Thorne et al., 1993). Patterns of rainfall,
runoff and recharge are outlined below, along with the
aquifer properties. These provide some indication of the
development potential of the shallow and deep aquifer systems. The variation of groundwater level is discussed with
reference to seasonality. Conceptual models of parts of
these aquifer systems are also presented.
4.2
AQUIFER DISTRIBUTION
The principal geomorphological units and depositional
environments of Bangladesh have been summarised in
Chapter 3. The landforms of the country can be divided
into three main types:
48
Arsenic contamination of groundwater in Bangladesh
Table 4.2. Main aquifer divisions within the fluvial and deltaic areas of Bangladesh
This study
UNDP, 1982
Fluvial area
Delta area
Grey highstand braided floodplain aquifer
Upper shallow aquifer Composite aquifer
(U Dhamrai Fm)
Grey coarse grained transgressive tract/lowstand
Lower shallow aquifer Main aquifer
aquifer in incised channels (L Dhamrai Fm)
Red-brown Dupi Tila of the Chandina area, and
Deep aquifers
Deep aquifer
Barind and Madhupur Tracts.
• the northern hills and fan deltas;
• the Early to Middle Pleistocene floodplains and terraces;
• the Late Pleistocene to Holocene fluvial floodplains
and delta areas.
The main aquifers are:
• Late Pleistocene to Holocene coarse sands, gravels and
cobbles of the Tista and Brahmaputra mega-fans and
basal fan delta gravels along the incised Brahmaputra
channel (Figures 3.5 and 3.27) (MMP, 1977; UNDP,
1982 and MMP 1983);
• Late Pleistocene to Holocene braided-river coarse
sands and gravels deposited along the incised palaeoGanges, lower Brahmaputra and Meghna main channels (Figures 3.5 and 3.26) (UNDP, 1982; MMP, 1983;
Davies et al., 1988; Davies and Exley, 1992; MMI, 1992
and Davies, 1994);
• Early to Middle Pleistocene stacked fluvial main channel medium to coarse sands at >150 m depth in the
Khulna, Noakhali, Jessore/Kushtia and western moribund Ganges Delta areas in the subsiding delta basin.
Younger Late Pleistocene to Holocene sands contain
saline groundwater at the coast (Figures 3.5, 3.17 and
3.26) (Haskoning/IWACO, 1981 and UNDP, 1982);
• Early to Middle Pleistocene red-brown medium to fine
sands underlie grey Holocene medium to fine sands in
the Old Brahmaputra and Chandina areas (Figures 3.1
and 3.18) (UNDP, 1982; MMP, 1983; MMI, 1992 and
Davies & Exley, 1992).
• Early to Middle Pleistocene coarse to fine fluvial sands
of the Dupi Tila Formation underlie the Madhupur and
Barind Tracts, capped by deposits of Madhupur Clay
Residuum (Welsh, 1966). The Madhupur sediments,
deposited during several pre-200 ka BP glacio-eustatic
cycles in former channels of the Brahmaputra, have
undergone several periods of flushing and weathering
resulting in the formation of red iron-oxide cements
and interbedded grey sticky clays (Figures 3.1 and 3.19)
(UNDP, 1982; MMP, 1983 and MMI, 1992). The aquifers that underlie much of the Barind Tract are also of
fluvial origin but are thinner with more clay (MMP,
1977).
The main features of the aquifer systems used in this study
and the earlier UNDP study are summarised in Table 4.2.
Grey highstand floodplain aquifer of dendritic distributary system
Grey transgressive tract/lowstand aquifer within
incised channels
Grey sub-150 m deep aquifers composed of
cyclic, vertically stacked aquifers in subsiding delta
In most of the groundwater studies undertaken in
Bangladesh, the aquifer system has not been divided stratigraphically. Conceptual models of hydrogeological conditions, based on simple lithological rather than stratigraphic
units, have been used to assess the engineering and hydraulic properties of aquifers and deep tubewell designs to
depths of about 150 m. The aquifers have been divided
into two groups according to colour and degree of weathering, factors that relate to relative age and aquifer properties. These groups are (Clark and Lea, 1992):
•
the grey sediments mainly deposited during the last
20 ka;
•
the red-brown sediments mainly older than 100 ka with
iron oxide cements and grey smectitic clays.
The three layer aquifer model (after UNDP, 1982 and
Barker and Herbert, 1989)
The most commonly used conceptual model which has
been applied to understand the effects of recharge and
abstraction in these aquifer systems has been the three
layer model of UNDP (1982) (Table 4.3). This was subsequently adopted for the National Water Plan assessments.
Barker et al. (1989) developed a three-layer model to
analyse detailed test-pumping data obtained from 16 sites
in the Dhamrai, Manikganj and Saturia area of the Brahmaputra-Jamuna valley. They concluded that:
•
the lower coarser-grained part of the shallow aquifer
was in general about 4 times more permeable than the
upper shallow aquifer;
•
transmissivities obtained by applying the Jacob method
of test-pumping data analysis tended to overestimate
the transmissivity due to the effect of leakage from the
upper layer to the lower layer;
Table 4.3. The three-layer aquifer model (after UNDP, 1982 and
Barker and Herbert, 1989)
Layer Description
1
2
3
Upper clay and silt
Upper Shallow or
Composite aquifer
Lower Shallow or
Main aquifer
Geology
Thickness
(m)
Upper clay and silt
5–15
Silty to fine sand
1–60
Medium to coarse grained
sand and gravel
5–75
Hydrogeology
5,000 mm in the north-east. Bangladesh experiences a
tropical monsoon climate with mean monthly minimum
temperatures from 10–12°C in January to 20–25°C in June
to August, and mean monthly maximum temperatures
from 25–28°C in January to 32–35°C in June to August.
Humidity and temperature increase during March to May
followed by a hot and very wet period from June to October.
Monthly evapotranspiration rises from 70 to 90 mm in
the coolest month of January to about 180 mm from
March to May and stabilises at between 115 and 145 mm
during the monsoon, before falling in November
(Table 4.5). Long-term monthly average rainfall data for
the four principal cities show strong seasonal patterns. Up
to 85% of the annual rainfall occurs during the May to
September monsoon. This coincides with the peak inflow
of the major rivers and annual flooding (Table 3.1). Less
than 5% of the mean annual rainfall occurs during the fivemonth dry season between November and March
(Table 4.5). During this period, when there is almost zero
effective rainfall, agriculture is not sustainable without irrigation. The need for water in these critical months has
been the driving force behind most of the groundwater
development programmes from which much of the knowledge of the regional hydrogeology has been gained.
The degree of flooding is very variable. Catastrophic
floods were recorded during 1987 and 1988 (Table 4.6)
and more recently in 1998. Flooding in Bangladesh has up
to three components depending on the location:
Table 4.4. The four-layer aquifer model of Bangladesh (after EPC/
MMP, 1991)
Layer Description
1
2
3
4
Thickness
(m)
Layer Geology
Upper alluvial sequence;
micaceous silts and fine
sands
Upper alluvial sequence
Upper Shallow Aquifer
medium to fine sands
Lower alluvial sequence
Lower Aquitard
clays and very fine sands
Lower alluvial sequence
Lower Shallow Aquifer medium to coarse sands
and gravels
Upper Aquitard
5–25
20–40
2–10
25–60
• the results obtained were consistent with hydraulic
conductivities of aquifer samples (taken during drilling)
based on a simple falling-head apparatus (Davies and
Herbert, 1990).
A more flexible, four-layer model was developed by EPC/
MMP (1991) so that vertical head differences could be
taken into account (Table 4.4). The subdivision of Bangladesh aquifers into three or four layers has proved adequate
for assessing the water balance for aquifers in much of the
country.
4.3
RAINFALL, RUNOFF AND RECHARGE
The headwaters of the major river systems that combine to
form the GBM system mainly drain parts of the Himalayan mountains and plains of India, Nepal and southern
Tibet. Only 7.5 per cent of their combined catchment area
of 1.5 million km2 lie in Bangladesh. The mean annual
rainfall in the headwaters ranges from 300 mm in Nepal to
11,615 mm at Cherrapunji on the Meghalaya Plateau.
Within Bangladesh, the mean annual rainfall rises from
1,250 mm in the western central region to more than
•
Tidal rise with the onset of the monsoon causes backup of the main rivers in the delta resulting in water level
rise during the first part of the monsoon. The tidal
height may reach a maximum of 4.5 m, high enough to
flood nearly 33% of the delta (Miah, 1988).
•
River flows increase and water levels rise during March
to May, initially affecting water levels in the floodplains
adjacent to the main channels. During April, the Brahmaputra starts to rise with snow melt from the Himalayas and the Meghna rises with pre-Monsoon rainfall
Table 4.5. Long term mean monthly rainfall and potential evapotranspiration for four cities in Bangladesh (Rashid, 1991)
Dhaka
Month
January
February
March
April
May
June
July
August
September
October
November
December
Annual total
Rainfall
1953–77
9
20
55
114
265
375
463
323
276
166
29
0
2095
Chittagong
ETo
Rainfall
1947–77
89
110
169
188
188
133
144
140
128
120
99
94
1602
7
15
53
119
242
589
759
547
279
60
61
10
2741
49
Rajshahi
ETo
Rainfall
1947–78
73
113
153
178
177
133
146
141
136
125
105
93
1573
13
10
29
81
266
520
439
319
279
160
9
1
2126
Khulna
ETo
Rainfall
1947–78
ETo
72
93
135
170
168
133
134
129
123
110
89
73
1429
8
19
36
93
184
350
393
286
280
161
25
15
1850
88
107
150
162
171
115
118
113
112
120
103
88
1447
50
Arsenic contamination of groundwater in Bangladesh
Table 4.6. Flooded areas 1954-1988
(from Miah, 1988 and Brammer, 1990a)
Usable recharge, mm a-1
< 300
Year
Flooded area
(km2)
% total land
area flooded
1954
1955
1956
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1980
1982
1983
1984
1985
1986
1987
1988
36920
50700
35620
28600
28860
37440
43160
31200
28600
33540
25740
37440
41600
42640
36475
20800
29900
52720
16590
28418
12548
10832
33077
3149
11112
28314
11427
4589
57491
82000
25.6
35.2
24.7
19.8
20.0
26
29.9
21.6
19.8
23.3
17.8
26
28.8
29.6
25.3
14.4
20.7
36.6
11.5
19.7
8.7
7.5
22.9
2.1
7.7
19.6
7.9
3.1
39.9
56.9
from the Shillong Hills. During May, the Ganges rises
with snow melt water. With the onset of the monsoon
rains proper in June-July, all three rivers rise rapidly, the
Brahmaputra peaking in July-August and the Ganges in
August-September. All river levels then fall rapidly during September to November following the end of the
monsoon. Floodwaters on the floodplains drain away
slowly during the first part of the dry season in
November-December (Brammer, 1990a).
• Rainfall starts with heavy pre-monsoon storms during
April–May, accumulating in depressions. The main
monsoon rainfall of June-August is increasingly ponded on the land by the rising water levels and is accompanied by infiltration from the adjacent rivers.
Thereafter, groundwater levels rise to above ground
surface (Brammer, 1990a). The rate of direct recharge
of rainwater is dependent upon soil type with the slowest recharge taking place through the Madhupur Clay
Residuum on the Pleistocene Tracts.
Estimates of potential and actual recharge can be made
using the specific yield of near-surface sediments and the
wet-dry seasonal difference in groundwater levels. It is
important to distinguish between actual and potential
300-600
600-900
> 900
no data
50
0
50
100 km
Figure 4.2. Actual recharge across Bangladesh (from DPHE/BGS/
MML, 1999).
recharge. The former is the quantity of water that infiltrates to the water table. This has normally been estimated
on an annual basis as a volume or as an equivalent depth of
water. The water table may rise to the ground surface during the monsoon so that all storage capacity is used up and
no more water is able to infiltrate. This ‘aquifer full’ condition means that further rain or flood water which would
have infiltrated instead becomes ‘rejected recharge’ or run
off and adds further to surface flooding. The potential
recharge is the quantity, based on climatic factors, which
could have infiltrated had vacant storage capacity existed,
and is thus the sum of actual and rejected recharge. Within
the Pleistocene Terrace areas, recharge occurs via incised
antecedent drainage channels that cut through the nearsurface clays into the underlying sandstones.
The most systematic description of recharge in Bangladesh is that provided by the MPO Potential Recharge
Model (DPHE/BGS/MML, 1999). The model is upazilabased and performs an annual water balance for the soil
zone using long-term meteorological and agricultural data.
A synthetic flood hydrograph is generated and applied
across the range of soil types. Eighteen crop types, twelve
of which are irrigated and four rainfed, are considered in
the model. The long-term average potential recharge is
estimated, and then a number of deductions are made to
take account of factors such as baseflow to rivers to derive
what is termed ‘actual recharge’. The distribution of actual
recharge across the country is shown in Figure 4.2. The
Hydrogeology
greatest scope for recharge is into the coarse-grained sediments that infill the incised channels along the Jamuna and
Meghna valleys, while the least is into the fine-grained sediments that underlie the western districts of Chuadanga
and Meherpur. In general, the rate of recharge through the
poorly permeable clays that cap the Pleistocene terraces is
less than through the unconsolidated micaceous silts on
the river floodplains because of lower hydraulic conductivities and a lesser tendency to flood.
4.4
51
Transmissivity, m2 d-1
< 500
500-1000
1000-2000
> 2000
no data
AQUIFER PROPERTIES
The locations of the main aquifers in Bangladesh is indicated by the distribution of average transmissivities
(UNDP, 1982) (Figure 4.3). Aquifer transmissivity is
related to sediment age, grain size and degree of weathering (Table 4.7).
Estimates of aquifer hydraulic conductivity and transmissivity have been obtained from:
• about 500 pumping tests conducted on deep tubewells
with observation piezometers (Pitman, 1981; UNDP,
1982);
• 7000 commissioning tests on BADC single production
deep tubewells (MMP, 1977; MMI, 1992);
• detailed test pumping and flow logging of 16 experimental boreholes with observation piezometers (Davies et al., 1988)
50
0
50
100 km
• tests on municipal tubewells (Welsh, 1966; Haskoning/
IWACO, 1981 and EPC/MMP, 1991).
Figure 4.3. Map of the variation in aquifer transmissivity across
Bangladesh.
These datasets can be seen as complementary. The aquifer
tests conducted with observation boreholes provide
detailed insights into the groundwater-flow regime in the
vicinity of wells and accurate aquifer parameter determinations. The simpler tests undertaken on a large number of
single production deep tubewells, when analysed using the
Logan approximation method, provide a broad geographical distribution of aquifer characteristics. These results
have been interpreted to give transmissivity values from
which estimates of hydraulic conductivity have been
derived after making assumptions about the degree of partial penetration and/or the relationship between screen
length and effective aquifer thickness. Table 4.8 summarises these aquifer test results.
Leaky to confined aquifer storage coefficients have
been determined from many pumping tests undertaken on
the Lower Shallow Aquifer and the Deep Aquifer, the latter in the Pleistocene Tract and Old Brahmaputra areas.
Average values fall within the narrow range of 1.3×10-5 to
6.7×10-3 (Table 4.8). The storage coefficient determines
the short-term water-level response close to pumping wells
but over periods of weeks and months, most aquifers in
Bangladesh exhibit either unconfined or semi-confined
responses. Also, long-term water-level decline is found
within the unconfined upper shallow aquifer over large
areas. As a consequence, interest has focused on the deter-
Table 4.7. The main aquifers in Bangladesh, their lithologies, relative ages and transmissivities (UNDP, 1982)
Aquifer
Brahmaputra - Tista Fan and Brahmaputra basal
gravels
Ganges, Lower Brahmaputra and Meghna main
channels
Deeper cyclic aquifers of main delta and coastal
areas
Old Brahmaputra and Chandina fluvial aquifers
and fine silts of the Sylhet basin
Madhupur and Barind Tract weathered fluvial
aquifers beneath surface clay residuum
Lithology
Grey coarse sand, gravel and cobbles
Grey coarse to medium sands and gravel
Grey medium to coarse sands
Age
Late Pleistocene and
Holocene
Late Pleistocene and
Holocene
Early to Mid Pleistocene
Red-brown medium to fine-grained weathered Early to Mid Pleistocene
sands
(Dupi Tila?)
Red-brown to grey medium to coarse sands and Early to Mid Pleistocene
interbedded clays
(Dupi Tila?)
Transmissivity
(m2 d–1)
3500–7000
3000–5000
1000–3000
300–3000
500–3000
52
Arsenic contamination of groundwater in Bangladesh
Table 4.8. Relationship between average aquifer test results and
geological formation
Aquifer Type/District or
Region
Table 4.9. Correlation of lithology with hydraulic conductivity and
specific yield (MMP/HTS, 1982; Davies and Herbert, 1990)
Transmissivity Storage
Ref
coefficient
(m2 d–1)
Deep Aquifer semi-confined by Upper Shallow Aquifer
(Chandina Formation)
Comilla District
1200
Noakhali District
617
Sylhet Floodplains
460
Lower Shallow Aquifer (Dhamrai Formation)
Dhaka (Dhamrai)
3480
Manikganj
4211
Tangail
2803
Upper Shallow Aquifer (Highstand Alluvium)
Bogra District
2380
Dinajpur District
2755
Nawabganj
3172
Pabna District
4316
Rangpur
4384
Jessore District
3660
Kushtia District
3780
Deep Aquifer (Old Deep Aquifer Alluvium)
Khulna District
3100
Deep Aquifer (Dupi Tila Formation)
Dhaka City
1333
Madhupur Tract
1161
Sylhet Hills
249
Barind Tract
1835
1.3×10–3
5.6×10–4
1
6
4
8.5×10–4
3.9×10–4
2.9×10–3
7
7
1
1.1×10–3
2.8×10–3
6.7×10–3
2.6×10–3
1.9×10–3
2.0×10–3
1
1
9
1
1
1
1
1.0×10–3
5
8.3×10–4
1.7×10–3
1.3×10–5
1.6×10–2
8
1,3,4
2
9
References: 1 UNDP (1982); 2 HTS/MMP (1967); 3 MMP/HTS
(1982); 4 MMI (1992); 5 Rus (1985); 6 MMI (1993); 7 Barker et al.
(1989); 8 EPC/MMP (1991); 9 Ahmed (1994).
mination of specific yield for the upper shallow aquifer.
Some results have been obtained by test pumping (Welsh,
1977; Pitman, 1981; Davies et al., 1988), and correlations
have also been made between specific yield and lithology
(Table 4.9).
Measurements of hydraulic conductivity were undertaken on a representative series of 150 grey sediment samples. These were obtained during the drilling of test
boreholes by reverse circulation using a simple falling-head
permeameter (Table 4.8; Davies and Herbert, 1990). The
results were correlated with aquifer parameters derived
from the analysis of detailed yield/drawdown and recovery
test-pumping data and found to be valid (Barker et al.,
1989). Such correlations, between lithological descriptions
and aquifer parameters, have since been used successfully
in water-resource planning and tubewell design. Table 4.9
provides an indication of the hydraulic conductivity values
obtained empirically from specific capacity (yield-drawdown) and lithological information accumulated in BADC
deep tubewell projects. In some parts of the country, the
permeabilities of grey sands are about twice those of
brown sands. It is now believed that most of the grey sands
belong to the Dhamrai Formation and most of the brown
sands to the Dupi Tila. The brown coloration of the latter
is indicates weathering by oxidation and leads to reduced
permeability. However, a simple correlation of permeability and colour cannot be applied to all areas.
Characteristic
hydraulic
conductivity
(m d–1)
Lithology
Clay
Silt
Very fine sand
Fine sand
Fine – medium sand
Medium – fine sand
Medium sand
Medium – coarse sand
Coarse – medium sand
Coarse sand
Gravel (clayey)
4.5
Characteristic
specific yield
(%)
Terraces
Floodplains
Terraces
Floodplains
—
—
8
13
17
21
25
34
38
46
25
—
0.4
—
12
26
43
57
61
63
95
40
0.5
4
—
8
—
—
20
—
—
25
30
3
5
—
16
—
—
20
—
—
25
30
GROUNDWATER ABSTRACTION AND TUBEWELLS
Groundwater abstraction is from a large number of handpump tubewells (HTWs) for domestic supply, shallow and
deep tubewells (STWs and DTWs) for irrigation and public water supply (PWS) boreholes for domestic supply in
cities and district towns. There are also an increasing
number of hand-pump deep tubewells (HDTWs). ‘Deep’
here refers to the depth of the screened interval – the
water table is invariably shallow which means that a simple
suction hand pump can still be used even for these deep
well.
Considerable uncertainty surrounds the exact number
of the various types of well present in Bangladesh but estimates are: HTW, 6–11 million; STW, 0.5 million, and
DTW, 55,000. Irrigation wells (STWs and DTWs) are typically shallow (<100 m) with multiple screens in an unconfined aquifer. The water level is commonly near the surface
and within the limit of suction pumps (7 m). The pump
intake is set above the screen level, but the screens are set
lower (typically 30 m bgl for STW and 100 m bgl for
DTW), depending on where the appropriate coarse lithology is encountered. Pumping of this type of well causes
vertical gradients to be developed as the well induces flow
from the water table to the well screen. This depletion of
the water table is replenished during the wet season as long
as total abstraction does not exceed the available resources.
Deep hand-pump tubewells are currently being
installed by DPHE and others in areas with arsenic-contaminated shallow groundwaters. Their major disadvantage
is the cost, usually at least ten times greater than for a typical HTW.
Shallow tubewells (STWs) of 15 L s–1 (0.5 cusecs)
capacity are constructed using 75 mm or 100 mm diameter
pipe and screen. Well losses due to uphole friction loss can
be large and have a marked effect upon the specific capac-
Hydrogeology
Table 4.10. Approximate wet season regional groundwater gradients
(BWDB, 1993)
Location
North
Central
Southern
Gradient (m km–1)
Gradient (m km–1)
Maximum
Minimum
2
0.5
0.1
0.5
0.1
0.01
ity of the suction pump used as the pumped water
approaches about 5 m below ground surface. Deep tubewells (DTWs) are constructed with 36 mm upper well casing and 15 mm diameter or 20 mm diameter lower well
casing and screen. These borehole are equipped with deepset shaft drive turbine pumps powered by surfacemounted diesel or electric motors to produce 58 L s–1 of
groundwater from the main shallow aquifer. The pumping
efficiencies of such boreholes can be high. Similar designs
of boreholes and pumps are used for supply of groundwater to larger towns where dry-season drawdowns can be
large, as in Dhaka and Joydebpur.
Well design and construction in Bangladesh can have a
marked effect on groundwater use over the dry season.
Typically, the shallow water table combined with deep set
screens in boreholes means that water has to be drawn
tens of metres up a borehole. This causes large frictional
losses and can lead to significant reduction of well use over
an irrigation season. For example, hand pumps built using
38 mm diameter casing can cause very high frictional
losses in the well and create 1–2 m more drawdown. The
large frictional losses can increase the pumping head
beyond the practical limit of suction pumps (c. 7 m) and
result in the well being unusable for the rest of the dry season. This has led to the use of the more expensive forcemode Tara pump in some areas, particularly north-western
Bangladesh.
4.6
GROUNDWATER LEVELS
The water table or piezometric surface within soft unconsolidated alluvial sediment aquifers of the large, low-lying
and gently sloping GBM floodplain and delta system is
invariably shallow allowing easy installation of cheap handdrilled tubewells. Regional hydraulic gradients are very low,
reflecting the low topographic gradients (Table 4.10). In
the southern coastal areas, the piezometric surface in the
deep aquifer is approximately 1.0–1.5 m above mean sea
level and so in low lying areas, the deep wells can be artesian.
In addition to the influence of the strongly seasonal
rainfall, runoff and recharge, other features of the Bangladesh aquifer systems are:
• The maximum depth to groundwater is very similar in
all years despite significant differences in demand for
irrigation water. Increased abstraction has lowered dryseason water levels and drawdowns with succeeding
years especially within the less transmissive ‘brown’
sediments.
53
•
The aquifers are effectively full from August to October, so that any excess potential recharge is rejected by
the groundwater system during the latter part of the
monsoon season.
•
Although peak groundwater levels vary between years,
they tend to be the same at the start of the irrigation
season in January. This suggests that groundwater levels are controlled by local base levels in the rivers and
that any additional recharge during the monsoon is lost
as increased baseflow in November and December.
•
In areas of high abstraction from deep aquifers with
brown sediments, groundwater levels often fail to
recover fully by the start of the following dry season
indicating possible over-abstraction, as in Dhaka and
Joydebpur.
Groundwater levels are recorded at a nationwide network
of piezometers maintained by BWDB. The system was
established with the assistance of the UNDP. Currently
1230 water levels are recorded weekly (every Monday at
6.00 a.m.). All of these are for shallow wells. In addition,
BWDB maintain 20 autographic water level recorders for
daily records. Some of the above data are available on a PC
database (Microsoft Access) but the data have not yet been
fully verified. DPHE also record wet- and dry-season
water levels with a one-per-union network of approximately 4400 wells.
As mentioned above, groundwater levels are affected
by tidal surges due to cyclones and monsoons (southern
half of the delta), groundwater abstraction and increased
river flow due to melt waters from the Himalayas and from
monsoon rainfall.
The decline in water levels due to abstraction for irrigation during the dry season through the use of shallow and
deep tubewells can be significant, especially in areas of
thick near-surface silt and very fine sand layers with low
specific yields. In low-lying areas of increased annual
abstraction for irrigation, as in the Jamuna and Ganges
delta floodplains, shallow tubewell use may be halted due
to decline of water levels below the suction level before the
end of the dry season. In such areas, crop irrigation has to
be completed using water from deep tubewells. Such a
regional decline in water level renders many hand-operated
suction pumps inoperative towards the end of the dry season.
In the Madhupur and Barind Tracts, where water levels
are relatively deep, only DTWs can be used to supply
groundwater for irrigation. Tara hand pumps are now used
for domestic supply in areas with deeper water levels. In
the Old Brahmaputra floodplain and Chandina areas, both
shallow and deep tubewells are used, but drawdowns in the
less permeable lower aquifer can be much greater than
those in the upper highstand aquifer.
Examples of hydrographs (Figure 4.4) from the main
aquifers are used to indicate the annual amplitude of seasonal water-level change, the effects of annual recharge
and the effects of increased abstraction for irrigation or
urban supply.
54
Arsenic contamination of groundwater in Bangladesh
1991, indicative of early rains or the early onset of the
monsoon during that year. In all three years, peak levels
were reached between mid July and early October. The
amplitude of annual fluctuations indicates that suction
mode hand pumps and shallow tubewells can be used in
this area.
10
0
-1 0
Groundwater level (m PWD)
-2 0
Motijheel, Dhaka city (DH-123)
1974
1978
1982
1986
1990
15
10
5
Adamdighi, Bogra (BO-10)
1974
1978
1982
1986
1990
1986
1990
5
0
Sarsa, Jessore, (JE-13)
1974
1978
1982
Water level (m bgl)
Year
-5
-15
Joydepur BADC A884/1
1985 1986 1987 1988 1989 1990 1991
Water level (m bgl)
Year
0
Shallow aquifer
-5
Deep aquifer
-10
-15
Dhaka draws its domestic water supplies from a series of
deep tubewells installed within the Dupi Tila aquifer. The
volume of groundwater abstracted has grown rapidly over
the last 20 years. The groundwater from the Dupi Tila
aquifer beneath Dhaka is free of arsenic contamination.
The monitoring point at Mothijheel was changed in 1983
from a shallow dug well in the Madhupur Clay to a piezometer screened at 30 m depth in the Dupi Tila Formation. Back projection of the piezometer trend suggests that
the Dupi Tila aquifer became unconfined in the early
1970s, while a perched water table was maintained in the
Madhupur Clay, presumably from sources of urban
recharge such as leaking water mains, storm drains and
sewers. A seasonal fluctuation of 2 m has been recorded
within the perched aquifer. Within the Dupi Tila aquifer, a
1 m seasonal fluctuation is recorded. Between 1979 and
1989, the water level has declined by 10 m at an average of
1 m a-1, indicative of over abstraction from the Dupi Tila
aquifer below Dhaka.
Adamdighi, Bogra District
-10
-20
Mothijheel, Dhaka City
Kishorganj
J F M A M J J A S O N D J F M A M J
1991
Year
1992
Figure 4.4. Examples of hydrographs from selected sites in the
main aquifers of Bangladesh.
Dhamrai, Dhaka District
At Dhamrai, an autographic water-level recorder was
installed in the lower shallow or main aquifer of the Brahmaputra floodplain. Seasonal water-level fluctuations of
about 5 m have been recorded. Dry-season base water levels are about –10.0 m below datum, whereas wet season
peaks increase from –5.5 m in 1989 to –4.0 m in 1991.
Base levels in 1989 and 1990 were reached at the end of
April whereas base level was reached during mid March in
The hydrograph from Adamdighi is representative of an
area of intensive irrigation using groundwater from highstand shallow aquifer deposits which are not affected by
arsenic contamination. Between 1980 and 1993, irrigation
increased from about 10% to practically 100% of the irrigable area as a result of the installation of large numbers of
private shallow tubewells. During 1972–1978 the annual
seasonal fluctuation of water level was 3.5–4.5 m. Between
1978–1989 the annual seasonal fluctuation increased to
4.5–6.5 m. Between 1989–1993, the base level of seasonal
fluctuation has remained at 6.5 m below aquifer full levels.
During years of heavy rainfall, recharge is achieved to give
the aquifer-full level at 12.5 m above datum. However, during years of low rainfall, water levels fail to recover to aquifer-full levels, notably during 1989 when water levels
recovered to only 8.5 m above datum, some 4 m below the
aquifer-full level. Dry-season base levels continued to
decline during the 1993–2000 period and so shallow tubewell operation in this area will now be difficult during the
latter part of the dry season. The zone of intermittent aeration reaches down to about 6.5 m bgl.
Sarsa, Jessore District
The hydrograph from Sarsa is representative of an area of
intensive irrigation within the moribund Ganges delta area
where groundwater is badly affected by arsenic contamination. About 77% of the irrigable area was irrigated using
groundwater of which 70% of this was obtained from shallow tubewells. The monitoring site was changed from a
dug well to a piezometer in 1988. The natural seasonal
Hydrogeology
fluctuation is about 3 m from an aquifer-full level of 5 m
above datum. The piezometer hydrograph is very peaky in
nature with dry season base levels reaching –0.5 m below
datum. Wet season peaks are normally reached at the end
of October, but frequently do not recover to the aquiferfull level during years of low rainfall. Thus, the zone of
intermittent aeration reaches down to 6 m below ground
level indicating that shallow tubewells and hand-pumped
boreholes may become inoperative during the latter part of
the dry season, especially where screens are deep-set and
friction losses during pumping become significant.
55
Depth to groundwater, m bgl
< 5.0
5.0-7.5
7.5-10.0
> 10.0
no data
Joydebpur, Gazipur District
At Joydebpur, an autographic water-level recorder has
been installed within the Dupi Tila aquifer of the southern
Madhupur Tract. The hydrograph recorded seasonal water
level fluctuations of the order of 8–10 m. Dry-season base
water levels are about –12 to –16 m below datum whereas
wet-season peaks reached about –6 m below datum during
1985 to 1988, with a decrease to –7 m below datum during
1990 and 1991. Base levels in 1985 to 1989 and 1991 were
reached in mid-April whereas base level was reached in
mid-March in 1990, indicative of early rains or the early
onset of the monsoon during that year. Peak levels were
reached by September during 1985 to 1989, but were only
reached by the end of October in 1990 and 1991. The
amplitude of annual fluctuations indicate that suction
mode hand pumps or shallow tubewells cannot be used in
this area.
Kishoreganj, Kishoreganj District
The hydrographs (Figure 4.4) show water levels from a
shallow piezometer in the highstand upper shallow aquifer
and a deep piezometer in the underlying Dupi Tila aquifer.
Within the upper shallow aquifer, water levels fluctuate
between a dry-season level at –5 m bgl and September–October wet season peak at –2 m bgl. The deep aquifer water level declines to –13 m bgl during the dry season.
These levels indicate that shallow tubewells and hand
pumps can be installed within the upper shallow aquifer,
whereas only deep tubewells can be used for abstraction
from the Dupi Tila aquifer. Arsenic contamination occurs
sporadically within the shallow aquifer. Therefore contaminated water may be drawn down into the underlying Dupi
Tila aquifer by pumping.
The hydrographs from Adamdighi, Sarsa and Dhamrai
reflect the concern that high-density usage of STWs and
DTWs could lower water levels in various parts of Bangladesh sufficiently to affect operation of HTWs during the
late dry season. UNICEF and DPHE are now installing
Tara pumps in such areas where late dry-season water levels lie at 6 m or more bgl (Figure 4.5). Maximum depths to
groundwater reflect the seasonal variation, except where
there is significant drawdown due to over abstraction in
urban parts of Dhaka and Chittagong. The influence of
geology and geomorphology is shown by the deeper water
levels found in the Madhupur and Barind Tracts.
In summary, water-level fluctuations at a particular site
reflect the aquifer, its proximity to major rivers, and
abstraction rates. Grey alluvial sediments, making up the
50
0
50
100 km
Figure 4.5. Map indicating the maximum depth to groundwater.
Sources: water level data for 1964–1993 from BWDB, BADC,
DPHE and DWASA; analysed by EPC/MMP (1994). Upazila
boundaries from WARPO/EGIS Databank.
upper and lower shallow aquifers, have increased drawdowns due to irrigation abstraction but are fully recharged
during the annual monsoon.
Grey delta sediments are finer-grained than alluvial
sediments and therefore have a lower hydraulic conductivity. Irrigation abstraction therefore causes a greater drawdown than for the alluvial aquifers. During years with a
relatively ‘dry’ monsoon, full recovery may not occur.
The deep aquifers with red-brown sediments have
much larger drawdowns in the dry season but normally
show a full recovery by the end of the monsoon. Only in
areas of very high abstraction (major cities and industrial
centres) is the annual recovery incomplete.
In general, groundwater gradients over the country are
low, typically between 1 m km–1 (1:1000) in the north of
the country to as low as 0.01 m km–1 (1:100,000) in the
south.
4.7
GROUNDWATER USAGE
Irrigation coverage has been increasing steadily with time
as shown in Figure 4.6. This map and the following statistics are derived from the National Minor Irrigation Census
1996/1997 undertaken by the National Minor Irrigation
Development Project, Ministry for Agriculture and Food,
Government of Bangladesh.
56
Arsenic contamination of groundwater in Bangladesh
4.0
3.5
Area (MHa)
3.0
2.5
2.0
1.5
1.0
0.5
0.0
1982 1984 1986 1988 1990 1992 1994 1996 1998
Year
Figure 4.6. Change since 1982 in total irrigated area in Bangladesh.
Table 4.11. Summary of change in use of irrigation technologies,
expressed as a percentage of the overall irrigation volume
Mode of irrigation
Groundwater
Shallow tubewell
Deep tubewell
Manual operated pump unit
Surface water
Low-lift pump
Traditional
Canal
1982–83
1996–97
24
15
1
56
13
1
22
28
10
15
5
10
The total area under irrigation coverage has risen from
1.52 million hectares (Mha) in 1982–1983 to 3.79 Mha by
1996–1997. The increase is largely attributable to the
installation of different types of irrigation wells, particularly shallow wells. In addition, the proportion of irrigation
drawn from groundwater has also changed significantly
(Table 4.11). In 1982–1983, groundwater represented 40%
of the total irrigation consumption. This had risen to 70%
by 1996–1997. Table 4.12 shows the number of units of
irrigation equipment that were potentially available during
Figure 4.7. Distribution of irrigation technologies used in Bangladesh about 1996.
the year 1996–1997.
Figure 4.7 shows the distribution of irrigation technologies in Bangladesh. It is clear from the map that groundwater irrigation is extensive in the north-west and western
parts of the south-west and north-central regions.
Groundwater irrigation is less extensive in eastern part of
the north-east, in the south-east and south-eastern part of
the south-west regions. There is almost no irrigation in the
hill districts.
Table 4.12. Summary of irrigation abstraction modes operating in Bangladesh during 1996-1997
Units operating
Units nonoperating
Total units
shallow tubewells
(STW)
600,276
13,284
613,559
Deep set STW
26,245
615
26,860
Very deep set STW
3,313
99
3,412
All shallow tubewells
629,834
13,998
643,831
Force mode tubewells
201
18
219
Deep tubewells
25,210
5,663
30,873
Low lift pumps
62,875
2,949
65,824
Aquifers
Technology
Highstand grey fine to medium sands within floodplains, with shallow
(<5 m) water table
Fine to medium sands within floodplains, fairly deep (<7 m) dry
season water table
Fine to medium sands within floodplains with deep (<10 m) dry
season water tables
Grey or red-brown transgressive tract medium to fine sands with a
deep water table
Transgressive to lowstand grey and red-brown coarse grained sediments
Very shallow aquifers and open bodies of water
Hydrogeology
4.8
GROUNDWATER FLOW AND AQUIFER FLUSHING
Regional groundwater flow
Groundwater generally flows through the fluvial sediments
of the northern part of the GBM system from north to
south, mainly through the coarse sands and gravels of the
lower shallow aquifer (Figure 3.5). South of the Hinge
Zone, within the delta area, stacked main channel deposits
from several cycles of glacio-eustatic deposition form a
series of fining-upward aquifer units separated by very fine
sand, silt and clay aquicludes (deep aquifer sediment cycles
2–4 in Figure 3.5). Within the coastal zone the shallow and
deep aquifers 1a, 1b and 2 have been intruded by saline
water. The deep aquifer cycle 3 contains freshwater that
probably flowed along stacked channel deposits from the
present Ganges and Padma Rivers. This indicates a possible recharge mechanism to deeper aquifers found in the
coastal zone where they form sources of arsenic-free
water.
Basin-wide flow
Most flow probably takes place through the infilled,
incised channels under the major rivers. It is therefore necessary to examine flow through a typical incised channel in
order to attempt to quantify the flow in different aquifers.
This will enable the extent of flushing to be estimated. By
selecting an estimate of the gradient at the present day and
one likely to have been in existence 10 ka BP, it is possible
to define two rates of flushing. A section is shown across
the Brahmaputra River just before its confluence with the
Ganges River (Figure 4.8). The infilled trench is split into a
near-surface aquitard and three aquifers:
• the aquitard (layer 1) is composed of micaceous overbank silts and micaceous fine sands 10–20 m thick.
These have a high porosity but low permeability;
Table 4.13. Estimates of flow and time for flushing for the aquifer
units of the Brahmaputra Channel between Faridpur and Dhamrai
under present-day gradients
Upper part
of lower
shallow
Upper
shallow
Aquifer
Lower
shallow
Approx. age (ka BP)
5 to 8
~10
15 to 18
0.08
0.08
0.08
Gradient (m km–1)
Width (km)
45
45
45
950
1325
2325
Transmissivity (m2 d–1)
3420
4770
8370
Flow (m3 d–1)
Thickness (m)
45
55
40
1.69×10–3 1.93×10–3 4.65×10–3
Seepage velocity (m d–1)
Porosity (-)
0.05
0.2
0.3
3.38×10–2 9.64×10–3 1.55×10–2
Darcy velocity (m d–1)
Volume of groundwater (m3) 2.531×1011 1.238×1011 1.350×1011
Time to replace one pore
20
71
44
volume (ka)
•
an upper shallow aquifer (layer 2, highstand) is composed
of micaceous fine to medium sands 25–35 m thick;
•
the upper part of the lower shallow aquifer (layer 3, lowstand) is composed of micaceous medium to fine sands
about 0–30 m thick. Layer 2 is separated from Layer 3
by an intermittent grey to red clay layer. The main part
of the lower shallow aquifer (layer 4, lowstand) is composed of coarse to medium sands, gravels and basal
cobbles about 50–65 m thick.
The present-day flows are calculated for each of these
three aquifer units and are given in Table 4.13. These estimates show that with the present hydraulic gradient, it will
take about 44 ka to flush the lower shallow aquifer (lowstand) once (one pore volume) given a flow rate of
Faridpur
Dhamrai
Faridpur
test Bh
Jamuna
main channel
Ghior
Bh
BGS exploration boreholes
7
3
6 16 14 5
GRAIN SIZE
0m
Depth below ground surface (metres)
Silt
River bed
load
Fine sand
Fine to medium sand
45m
Medium sand
Intermittent clay
at Holocene base
Coarse sand, gravel
and cobbles
Palaeosol
Layer 1
Dupi Tila
sandstone
Layer 2
Layer 3
140m
57
brown
orange
s
thered lluvial sand
a
e
w
a
to
ised in onsolidated
c
in
l
e
Chann en brown c
to gre
Layer 4
Pre-100ka BP lowstand
to transgressive tract of
weathered brown sands
0
Figure 4.8. Geological cross-section through the Jamuna Channel alluvial deposits showing the four-layer aquifer structure.
km
10
58
Arsenic contamination of groundwater in Bangladesh
Table 4.14. Estimates of flow and time for flushing for the aquifer
units of the Brahmaputra Channel between Faridpur and Dhamrai
under early Holocene gradient
Aquifer
Upper part
of presentday lower
shallow
Layer
Lower
shallow
Approx. age (ka BP)
~10
15 to 18
0.28
0.28
Gradient (m km–1)
Width (km)
45
45
1325
2325
Transmissivity (m2 d–1)
Flow (m3 d–1)
16695
29295
Thickness (m)
55
40
6.75×10–3 1.63×10–3
Seepage velocity (m d–1)
Porosity (-)
0.2
0.3
3.37×10–2 5.43×10–2
Darcy velocity (m d–1)
Volume of groundwater (m3) 1.238×1011 1.350×1011
Time to replace one pore
20
13
volume (ka)
Table 4.15. Estimates of flow rates and time for flushing for Upper
Ganges, Lower Ganges and Mahananda Channel sequences at
Chapai Nawabganj under present-day gradients
Aquifer
Upper
Ganges
Table 4.16. Estimates of flowrates and time for flushing for a cross
section through Faridpur (see Figure 4.9)
Lower
Ganges
Approx. age (ka BP)
2–5
5–15
0.08
0.08
Gradient (m km–1)
Width (km)
5
5
570
2500
Transmissivity (m2 d–1)
Flow (m3 d–1)
228
1000
Thickness (m)
40
80
1.14×10–3 2.5×10–3
Seepage velocity (m d–1)
Porosity (–)
0.05
0.1
2.28×10–2 2.50×10–2
Darcy velocity (m d–1)
Volume of groundwater (m3) 1.000×109 4.000×109
Time to replace one pore
12
11
volume (ka)
Mahananda
2–5
0.08
4
350
112
40
7.00×10–4
0.05
1.40×10–2
8.000×109
20
8400 m3 d–1. By comparison, it is calculated to take about
71 ka to flush the upper part of the lower shallow aquifer
once at a flow rate of 4770 m3 d–1. This difference is
largely due to differences in the porosities of the aquifers.
It is assumed that porosity increases with depth as the
sediments become progressively coarser. The calculated
time needed to replace the groundwater is 20–71 ka. The
ages of the lowstand sediments are between 15 ka and
18 ka BP whereas the highstand sediments are about 10 ka
old. This implies that the highstand deposits forming the
widely-exploited upper shallow aquifer will not have even
been completely flushed once since deposition and will
therefore tend to contain concentrations of arsenic greater
than in the lower, coarser parts of the system.
At the beginning of the Holocene (10 ka BP), hydraulic
gradients were likely to have been higher: increasing from
20 m in 250 km (1:12,500) at the present time to 70 m in
250 km (1:3600) at 10 ka BP. The evidence for this is a
combination of lower sea levels and higher river gradients
Column A Column B Column C Column D
a. Block transmissivities (m2 d–1)
1
1870
2a
1140
2b
780
3a
120
3b
1830
3c
940
3d
1220
125
920
780
120
1830
940
1220
1230
5020
b. Block throughflow rates (m3 d–1)
1
598.4
2a
364.8
2b
249.6
3a
38.4
3b
585.6
3c
300.8
3d
390.4
40
294.4
249.6
38.4
585.6
300.8
390.4
492
2008
c. Time to replace block volume (a)
1
12362
184932
2a
11265
11167
2b
10976
13172
3a
114155
85616
3b
2339
4678
3c
5465
5465
3d
8772
10527
430
1830
940
1400
172
585.6
300.8
560
12529
10233
27875
3509
5465
9173
150
240
40
1720
1830
940
1220
60
96
16
688
585.6
300.8
488
68493
14269
321062
6969
3509
5465
8772
demonstrated by the coarse nature of base load carried by
the rivers (Davies et al., 1988). This implies that the rate of
flow through the aquifers was greater and therefore the
early flushing was more rapid, a single pore volume being
displaced in a much shorter time (Table 4.14). Comparing
the results with those for the Lower Shallow aquifer under
present-day gradients (Table 4.14) shows a decrease of
flushing time from 44 ka to 13 ka. This demonstrates that
the groundwater in the lower aquifer could have been
flushed at least once since deposition.
A similar calculation can be carried out for flow along
the transmissive parts of the aquifer system for Chapai
Nawabganj. Table 4.15 demonstrates that stored groundwater from sediments underlying the Mahananda River
(Figure 3.8) is the slowest to be replaced. This takes about
20 ka as opposed to 10–12 ka for the groundwater in the
Ganges sediments. This corresponds with the higher
arsenic concentrations observed in the groundwater from
the Mahananda sediments. Flow in the Barind Tract is predominately from east to west. This has not been considered in the present calculations as any groundwater
flowing through the Barind Tract flows out via a series of
springs along the faulted junction between the Mahananda
sediments and the Barind Tract.
The calculation of through flow and the time to replace
volume of groundwater for Faridpur is complicated by the
distribution of sediments (Figure 3.12). To investigate the
distribution of flow rates, the section has been divided into
a series of blocks (Figure 4.9). The transmissivity, flow rate
and time to replace the volume of each block were deter-
Hydrogeology
59
;y y;y; y;y; y;y;
;y ;y;y;y y;y; y;y; y;
y; ;y;y ;y y;y; y;
;
y
;
y
;y ;y;y y;
;y y;y;y; ;y;y ;y;y
;y ;y;y;y y;y; ;y;y ;y
;y ;y;y;y ;y;y y;y; ;y
;y ;y;y ;y ;y;y y;
;
y
;
y
;
y
;
y
;
y
;
y
y; ;y;y;y y;y; ;y;y
y; y;y;y; ;y;y y;y; ;y
West
TTW12
BWDB16
Goalchamat BH
TTW8
TTW6
East
0
1A
1B
UPPER
SHALLOW
AQUIFER
HIGHSTAND LEVEL
40
Depth below ground surface (metres)
1D
1C
2aA
2aB
2aD
2aC
80
2bA
2bD
LOWER
SHALLOW
AQUIFER
2bB
EROSION SURFACE
2cD
2bC
120
3aC
3aA
160
200
3aB
3aD
3bB
DEEP
AQUIFERS,
STACKED
CHANNEL
DEPOSITS
3bC
3bA
3cA
3cB
3cC
3dA
3dB
3dC
3bD
Figure 4.9. Hydrogeological cross section through the shallow and deep
aquifers of the Faridpur area. shows
the blocks used for the calculation of
aquifer throughflow rates.
3cD
3dD
y;
y;
;
y
KEY
220
CYCLE OF SEDIMENTATION
Non-aquifer
Cycle 1a - Highstand (10-0ka BP)
Aquifer block
Cycle 1b - Transgressive Tract and
Lowstand (20-10ka BP)
0
km
5
Cycle 2 - (130-110ka BP?)
Cycle 3 - (>220ka BP)
mined (Tables 4.16a-c). These and other aquifer parameters are summarised for the main aquifer units in
Table 4.17. The deep aquifer consists of a series of stacked
channels infilled with coarse material. This is obviously
very transmissive and will exhibit a significantly shorter
time for flushing than the upper part of the aquifer. This is
confirmed in Table 4.16c which indicates that it takes
some 2.5 ka–5 ka to flush the deep aquifer once. This is in
contrast to the corresponding upper aquifer which takes
between 10 ka–15 ka to be flushed once. Since the sediments making up the deep aquifer were deposited over
140 ka ago, it is likely that this part of the system has been
flushed quite a few times.
These simple estimates of throughflow and times to
flush have demonstrated that the transmissive parts of the
aquifer system can be flushed in some 10 ka. In some
cases, where sediments are very transmissive, flushing is
estimated to take less than 5 ka. Since the highstand
sequence was deposited around 10 ka ago, transmissive
sediments that predate this will have already been flushed
to some extent.
The deltaic and alluvial sedimentary environments in
Bangladesh are such that major rivers have been alternatively incising and filling the same channels for at least 1
million years. This has led to a series of incised channels
surrounded by infills of finer sediments. Both of these
facies contain fining-upward sequences, but the incised
channels are coarser than the surrounding areas with the
sequence starting with medium to coarse sand. In contrast,
the areas between the channels contain thick sequences of
clay that will tend to inhibit recharge and locally reduce
groundwater flow rates.
Therefore the sediments in the incised channels are
highly transmissive with a transmissivity of approximately
3000 m2 d–1 and a porosity of 20%. They are likely to be
flushed in approximately 10 ka. The surrounding sediments have a medium/low transmissivity (300 m2 d–1) and
a high porosity (60%) and will take some 300 ka to flush
under a similar gradient. This is an important distinction as
the finer sediments will tend not to be flushed within a glacial cycle.
4.9
CONCEPTUAL MODEL OF SEASONAL FLOW PATTERNS
Seasonal groundwater movement due to climatic and
abstraction controls is most evident within the shallow
aquifer systems of Bangladesh. Therefore the mechanisms
of seasonal recharge to, and discharge from, the shallow
aquifer systems need to be understood. A segment of the
Faridpur-Dhamrai cross section was selected to investigate
Table 4.17. Summary of aquifer parameters for the upper shallow,
lower shallow and deep aquifers at Faridpur
Aquifer
Approx. age (ka BP)
Gradient (m km–1)
Width (km)
Transmissivity (m2 d–1)
Flow (m3 d–1)
Thickness (m)
Porosity (-)
Time to replace one pore
volume (ka)
Upper
Lower shallow
shallow
5 to 8
0.08
18
125–1870
1190
45–60
0.10–0.15
12–185
8–23
0.08
18
40–5020
3287
75–90
0.05–0.20
10–321
Deep
>140
0.08
18
120–1830
6311
90
0.10–0.15
2.3–114
60
Arsenic contamination of groundwater in Bangladesh
FLOOD PLAIN
MAIN RIVER
CHANNEL
MAIN RIVER
CHANNEL
PLEISTOCENE
TERRACE
FLOOD PLAIN
PLEISTOCENE
TERRACE
Surface runoff into down-cut
rivers the main zones of recharge
to underlying sandy aquifers
Red clay
residuum
Silts and
clays
Channel infill of
fine to medium
sands
Micaceous
fine to
medium
sands
Hand
pumped
BH
Shallow
tube
well
Depth of scour
Brown medium sand
WATER LEVELS
Deep
tube well
Early dry season
Late dry season after
pumping
Grey micaceous
medium to fine sand
Following tidal rise at start
of the monsoon season
Deep
tube well
Rise following increased
river flow during May-June
Rise during July-August
heavy rainfall
Figure 4.10. Conceptual model – basic hydrogeological units and
main irrigation pumping methods.
seasonal patterns of water inflow and outflow from the
main rivers and the adjacent floodplains. Seasonal aspects
of water flow are also considered as well as the influence
of rainfall on the floodplain and Pleistocene Tracts
(Figure 4.10).
The selected segment corresponds with a transect from
the main channel of the Brahmaputra across the adjacent
floodplain to the Madhupur Pleistocene Terrace. The
floodplain is underlain by grey, micaceous medium to fine
sands belonging to the upper shallow aquifer and is capped
by very micaceous low-permeability silts and clays. The
Pleistocene Terrace is underlain by deep aquifer sediments
capped by Red Clay Residuum that is incised by antecedent
drainage channels. Groundwater is abstracted from the
shallow aquifer in the floodplain using hand pumps and
shallow and deep irrigation tubewells. Only deep tubewells
are used in the Pleistocene Tract. The main river is
assumed to be in hydraulic continuity with the shallow
aquifer system (Figure 4.10).
During the December to March dry season, groundwater is abstracted in the floodplain and Pleistocene Tract
areas for the irrigation of crops. Shallow tubewells will be
used in the floodplain until water levels decline to below
suction level, maybe by March, from when the irrigation
will be completed using deep tubewells. Regional drawdowns of the order of only 2–3 m will result from the
abstraction of groundwater for irrigation within the floodplain area. In the Pleistocene Tract, water-level drawdowns
of the order of 10 m are more typical (Figure 4.11).
With the onset of the monsoon season toward the end
of April and the associated tidal rise in the Bay of Bengal,
water levels begin to rise. There is a general rise of water
levels within the aquifers beneath the floodplains but no
change within the Pleistocene Tracts.
During May and June, melt water from the Himalayas
will have reached Bangladesh, causing a further rise in river
levels. This results in a rise of water levels in the floodplain
due to lateral flow of water into the fine to medium sands
Figure 4.11. Conceptual model – water flow patterns and resultant
water levels during the dry season and the following wet season.
The effects of irrigation and flooding during the onset and course of
the monsoon season are shown.
below the surface silts causing a rise in water level adjacent
to the river. The effect of this moves into the floodplain
away from the river but causes no change within the Pleistocene Tracts.
The monsoon rains start in earnest during August.
Groundwater and river levels rise further in response to
these rains (Figure 4.11) and cause:
•
flooding of floodplains;
•
recharge through floodplain soils and near-surface silts,
filling the shallow aquifer and therefore rejecting further potential recharge;
•
flow along antecedent channels within the Pleistocene
terrace areas with direct recharge to sand layers below
the clay residuum;
•
seepage through the clay residuum surface.
Following the end of the monsoon in December, water
levels rapidly decline in the main channel, in the Madhupur
aquifer beneath the clay residuum and in the floodplains.
Surface runoff and water-level declines are slow in the
near-surface silts and in the very micaceous fine to
medium sands. There is delayed seepage from the nearsurface silt layers. Excess water flows from the Pleistocene
Tracts and the floodplain towards the river through the
shallow aquifer (Figure 4.12).
4.10
SUMMARY
4.10.1 Groundwater flow and aquifer flushing
The five main sedimentary aquifer units considered in this
study were deposited within distinct fluvial and delta plain
Hydrogeology
MAIN RIVER
CHANNEL
FLOOD PLAIN
PLEISTOCENE
TERRACE
WATER LEVELS
Post monsoon
Early dry season
Direction of groundwater
flow during main dry
season
Figure 4.12. Conceptual model – water flow patterns and water
level change following the end of the monsoon season and during
the early dry season.
environments as part of the GBM system. These are:
• Late Pleistocene to Holocene Tista mega-fanglomerate and
Brahmaputra channel basal gravel aquifers composed of
coarse sands, gravels and cobbles;
• Late Pleistocene to Holocene Ganges, Lower Brahmaputra
and Meghna main-channel shallow aquifers composed of
braided and meandering river sediments;
• Early to Middle Pleistocene coastal and moribund Ganges
delta deep aquifers composed of stacked, main channel
medium to coarse sands at >130 m;
• Early to Middle Pleistocene Old Brahmaputra and
Chandina deep aquifers composed of red-brown medium
to fine sands underlying Holocene grey medium to fine
sands;
• Early to Middle Pleistocene Madhupur and Barind Tract
aquifers composed of coarse to fine fluvial sands of the
Dupi Tila Formation, confined by near-surface clay
residuum deposits.
Various conceptual models have been devised mainly for
the study of the fluvial aquifers of the northern half of the
GBM system. There has been a tendency to differentiate
between the aquifers composed of younger grey sediments
and those consisting of older red-brown sediments that
occur within the GBM system. Three- and four-layer models have been developed and applied to understand the
effects of recharge, abstraction and throughflow within the
red-brown and grey aquifers.
Groundwater systems are strongly influenced by the
annual monsoon rainfall and its intensity. There are distinct wet and dry seasons with flooding common during
the wet season. Irrigation using groundwater is necessary
during the dry season. Annual flooding of floodplains
occurs as a combination of increased river flow due to
melt water from the Himalayas, tidal-level increase in the
61
Bay of Bengal and intense monsoon rainfall, which seems
to have become more intense in recent years. Usually the
whole aquifer system receives sufficient recharge to
become full by the end of each monsoon season.
Aquifer physical properties have been summarised
from a number of published accounts. Hydraulic conductivities determined for grey sediments are estimated to be
in the range 0.4–100 m d–1. Those for red-brown sediments are in the range 0.2–50 m d–1. These give a ratio of
hydraulic conductivities of 2:1 for grey:red-brown sediments. There has been little investigation of the deep aquifer and, therefore reliable aquifer parameters for this
aquifer are as yet largely unknown. Borehole logs indicate
the high degree of sediment heterogeneity across Bangladesh.
The thicknesses of near-surface silt and very fine sand
layers govern the availability of groundwater to HTW and
STW pumps. Therefore it is important to set the aquifer
system in the correct sedimentological context.
In general, groundwater gradients over the country are
low, typically between 1 m km–1 in the north of the country to as low as 0.01 m km–1 in the south. These low gradients may be a strong factor in determining the
groundwater chemistry and chemical heterogeneity due to
low rates of flushing of the aquifers. Groundwater head
and corresponding gradients are difficult to define adequately.
By the latter part of the dry season, groundwater levels
can become depressed by abstraction from STWs and private domestic tubewells. Use of these can be restricted as
water levels decline. This results in an increase of DTW
use, further depressing groundwater levels. The seasonal
cycle of groundwater heads is influenced by irrigation
abstraction which takes place only during the dry season.
The effect of seasonality differs across Bangladesh.
In general, the regional groundwater flow in the aquifers of Bangladesh is from north to south, with local variation near major rivers. However, the regional flows are not
well understood. Evidence suggests that there is some
interaction between groundwater flow within the fluvial
deposits in incised channels and the flow within the
stacked channels in the delta sequences. In the coastal
region, from Khulna to Lakshmipur, there is known to be
fresh groundwater at depth, below shallower saline aquifers. The water in this aquifer also has a very low arsenic
concentration and there is also anecdotal evidence that
artesian flow sometimes occurs from boreholes in the
deep coastal aquifer.
Groundwater flushing rates have been estimated for
the Brahmaputra valley, the River Mahananda–Chapai
Nawabganj area and Faridpur. These are summarised
below. The Brahmaputra valley or channel consists of
highstand, transgressive and lowstand sediments which,
under present-day conditions with a gradient of
0.1 m km–1, are flushed once in approximately 20 ka (highstand), 70 ka (transgressive tract) and 44 ka (lowstand).
Since the highstand deposits are less than 10 ka old, they
have not yet been completely flushed since deposition. At
the beginning of the highstand period 10 ka BP, flow gradients were of the order of 0.3 m km–1. Flushing of the
transgressive tract is estimated to take approximately 20 ka
while flushing of the lowstand is estimated to take approx-
62
Arsenic contamination of groundwater in Bangladesh
imately 12 ka. Therefore it is likely that the lowstand sediments of the Brahmaputra valley will have been flushed at
least once since deposition while the highstand deposits
may have only been flushed once.
In the Chapai Nawabganj section, the rate of groundwater flow through the upper and lower Ganges sediments
and the Mahananda river alluvium was estimated to take
12 ka for the Upper Ganges, 11 ka for the Lower Ganges
and 20 ka for the Mahananda. Therefore only the Lower
Ganges sediments would have been completely flushed
since deposition.
In Faridpur, the flushing times (one pore volume) for
the upper shallow aquifer, lower shallow aquifer and deep
aquifers were estimated to be 12–185 ka. Therefore none
of these deposits would have been flushed completely
since deposition. Within the lower shallow aquifer, estimated times required for a single flushing varied between
10 ka for the coarse-grained sediments of the incised channel up to 320 ka for the fine-grained sediments found on
either side of these sediments. Therefore only the coarse
channel sediments would have been flushed and completely since deposition. Within the deep aquifers, times
required for a single flush are estimated to vary between
2 ka and 115 ka. Since all of these sediments are greater
than 140 ka old, even the finest-grained sediments will
probably have been flushed at least once, while the coarse
basal sediments will have been flushed many times.
4.10.2 Implications for arsenic
Clearly the variation of the arsenic concentration in
groundwaters can be related in part to the the past history
of groundwater flushing of the aquifers. This in turn
depends on the age of the sediment, the hydraulic properties of the aquifer and past and present groundwater flow
regimes. Other factors related to the source of arsenic and
its mobilisation are also likely to be important but, all other
things being equal, the following hydrogeological factors
are likely to be contributory.
Low arsenic concentrations may be associated with:
• coarse sands – at the base of incised channels in fluvial
areas (possibly stacked channels in delta regions);
• relatively high hydraulic conductivity, medium porosity;
• high present-day groundwater gradients and/or historically high gradients due to the influence of the past glacial maximum;
• relatively rapid flushing, some 2–10 ka per pore volume;
•
sediments greater than 10 ka old.
High arsenic concentrations may be associated with:
•
areas with low recharge, either because of relatively low
rainfall, high evaporation or high runoff;
•
silts and fine sands within alluvial floodplains and delta
areas leading to low groundwater flow rates;
•
other horizons with a low hydraulic conductivity;
•
areas with low groundwater gradients even at the time
of the last glacial maximum;
•
areas where flushing takes 50–200 ka per pore volume
even during the last glacial maximum;
•
areas with low gradients at the present time leading to
flushing times exceeding 200 ka;
•
regions of especially low flow, perhaps inside river
meanders, in closed basins and in the dead zones of
aquifers.
The deep aquifer can be seen to be largely free of arsenic
and could be a possible source of irrigation and drinking
water. Over much of Bangladesh, the groundwater in the
shallow aquifer is known to be arsenic contaminated. Since
pumping will induce flow both laterally and vertically,
exploiting the lower shallow and the deep aquifers by
pumping can be viewed as a competition between the lateral or regional flows and vertical flows. The ratio of these
two inflows to the well has important implications for the
movements of contaminants. Typically, high arsenic concentrations are found in the younger and shallower aquifers. Creation of vertical gradients will therefore result in
the arsenic being transported down into the deeper parts
of the system and the concentration in the deeper wells
will tend to increase with time until the source of arsenic in
the shallow aquifer becomes depleted.
Lateral inflows to a deep aquifer will be derived from
areas with generally low arsenic concentrations but may
also include a contribution from shallower, and potentially
contaminated, horizons. Therefore, determining the ratio
of the lateral flows to the vertical flows will be important
in determining the impact of pumping deeper parts of the
system.
The likely pattern of flow to a well also has implications for the design of boreholes. It is desirable to have the
shortest feasible length of screen placed at the deepest
level to maximise the travel time between the upper aquifer
and the well. It is also important to avoid construction of
wells with multiple screens in different horizons especially
where the shallow groundwaters are known or expected to
be contaminated with arsenic and other elements.