Government of the People’s Republic of Bangladesh
Ministry of Local Government, Rural Development and Co-operative
Local Government Division
International Workshop
on
Arsenic Issue in Bangladesh
14-16th January 2002.
Country Paper
on
Hydrogeology Section
Prepared By
M. Nazrul Islam, Director General
and
Md. Nehal Uddin, Deputy Director
Geological Survey of Bangladesh
1.Introduction
1.1. After independence, the Government of Bangladesh undertook, a massive
program to provide bacteriologically safe drinking water for the people of the country.
As a result millions of shallow tube wells (hand pumps) were installed to extract safe
drinking water. Exact numbers of the tube wells are not known but the number may
be from 8 to12 millions. Simultaneously, for self-sufficiency in food, the cultivation
of high yielding variety of rice spread all over the country and large number of deep
and shallow tube wells for irrigation were sunk.
1.2. The latest information shows that in 61 out of 64 districts, tube wells in various
percentages are producing water with arsenic higher than the national standard of 0.05
milligram per liter (Fig. 1). In many Upazilas (sub-districts) more than 90% of the
tube wells are producing water that are not safe for drinking or cooking purposes. The
problem of arsenic contamination of the ground water became known from 1993 but
unfortunately it did not get the proper attention till very recently. By this time more
than 30 million people are already facing the problem and probably 70-80 million
people are threatened with the problem. This shows the enormous magnitude of the
problem faced by the country. Some other countries like India (West Bengal part of
the Bengal delta), China, Vietnam and Laos are also facing similar problem (Fig. 2).
1.3. By this time we have enough evidence that the wells with higher than permissible
level of arsenic have a direct relationship with the location and geological age of the
aquifers. We feel that the time has come to make the proper map of the aquifers, find
their geological age and relate them with the geological evolution of the GangesBrahmaputra-Meghna Delta Complex.
1.4. The authors acknowledge the help and guidance given by Mr. S. K M.Abdullah,
Chairman, Groundwater Task Force, Ministry of Local Government, Rural
Development and Co-operatives, Local Government Division, Bangladesh.
1
2. Geology of Bangladesh
2.1. Physiography of Bangladesh
2.1.1. Bangladesh belong to South Asia and lies between 20034´ and 26038´ N, and
88001´ and 92041´ E. The area of the country is 147,570 square km with more than
700 km long coastlines.
2.1.2. About 80% of the land are flat, intersected by numerous rivers and their
distributaries. The land area has a general slope of 1-20 from north to south (Fig. 3).
Physiographically Bangladesh can be divided into 7 divisions (Fig. 4). Each of these
divisions has distinguished characteristic of its own.
a) Hilly regions (in the eastern and northern frontier)
i) Chittagong Hill Tracts, ii) Hill Ranges of northeastern Sylhet, iii) Hills
along the narrow northern strip of Sylhet and Mymensingh
b) Pleistocene uplands
i) Barind Tract in the north western part, ii) Madhupur Tract in the
central part, iii) Lalmai Hill
c) Tippera Surface
d) Tista Fan (in the northern part)
e) Flood plains
i) Ganges flood plain, ii) Atrai flood plain, iii) Brahmaputra-Jamuna
flood plain, iii) Old Brahmaputra flood plain, iv) Meghna flood plain
f) Deltaic plain of the Ganges-Brahmaputra-Meghna Delta Complex
i) Inactive delta, ii) Active delta, iii) Tidal delta
g) Sylhet Depression and Inland marshes (Scattered all over Bangladesh)
2.1.3. The hilly areas occupy about 18% of the country that encompasses Chittagong,
Cox’s Bazar, Three Hill Tract Districts, Sylhet and adjacent areas. The Pleistocene
uplands cover about 10% of the country. The floodplains of the Ganges, the Atria,
Brahmaputra-Jamuna, the Old Brahmaputra, the Meghna rivers cover about 40% of
the country including the Tista Fan, Tippera Surface and Sylhet Depression. The delta
complexes in the southern part cover about 32% of the country. There are numerous
2
depressions/ inland marshes scattered and distributed all over the country. Among
these, the Sylhet Depression is a tectonically subsiding basin that is subsiding at a
much faster rate than the Bengal Basin as a whole.
2.1.4. The Bengal delta is a combination of three deltas namely the Ganges delta, the
old Brahmaputra-Meghna delta and the Ganges-Jamuna (the present Brahmaputra)Meghna delta. In some recent literature the name, "Ganges-Brahmaputra-Meghna
Delta Complex" has been used. Also, in the summer monsoon season when about 3
million cusecs of water passes through the delta, it behaves as a fluvial delta, whereas
in the winter when the volume of water passing through the delta drops to 250,000 to
300,000 cusecs(with 2 billion tons of sediments) it behaves as a tide dominated delta.
These unusual features make this delta one the most complex deltas in the world.
Holocene or Holocene sediments from a few hundred to thousands of meters cover
the Flood plains and the Delta.
2.2. Tectonic Set-up of the Country
2.2.1. Tectonically Bangladesh is divided broadly into the following divisions (Fig. 5
& 6)
1. Indian Platform and Shelf
a) Dinajpur slope (Himalayan Fore Deep); b) Rangpur Saddle, c) Bogra Slope
2. Hinge Zone (Eocene slope break)
3. Bengal Fore Deep
a) Folded flank, b) Basinal area; i) Sylhet Trough, ii) Chandpur-Barisal High,
v)
Patuakhali Depression (Hatia Trough)
2.2.2. Geologic evolution of the Bengal Basin starting from Upper Paleozoic time is
directly related with the break up of eastern Gondwanaland and collision of the Indian
plate with the Asian plate, it can be divided into four major stages: I. Permocarboniferous pre-breakup stage, II. Early Cretaceous Rift stage, III. Late CretaceousEocene Plate or drift stage, and IV. Oligocene-Holocene Orogenic stage. The
sedimentary cover of the basin with a maximum thickness of 20 km includes three
major lithostratigraphic units separated by three major unconformities. The western
part of Bangladesh is the platform shelf, whereas the eastern part of the country is
3
represented by the folded belt. The central part representing the most subsided part of
the basin comprises two major depressions at the north (Sylhet Trough) and south
(Patuakhali Depression). The transition zone from the shelf to basin is represented by
the hinge zone-a Eocene shelf/slope break. (Fig. 7 & 8). Rapid subsidence of the
foredeep of the Bengal Basin was compensated by the influx of huge amounts of
detritus originating from the nearby sources of the basin. Shallow water conditions
and in deltaic environment persisted. In addition to the western and Northern foreland
shelves, which were source areas earlier, the rising chains of the Himalayas and the
Indo-Burman Ranges were increasingly subjected to erosion and supplied much of the
sediments since the Mid-Miocene in the basinal area (Shamsuddin and Abdullah,
1997).
2.3. Stratigraphy
2.3.1. Bangladesh contain thick sediment (up to 20 km in the southern part)
sequences of Permian to Holocene. The sediment thickness is shallowest in northern
Bangladesh (114 m). Major part of the sediment is deposited by the GangesBrahmaputra-Meghna river systems during Miocene to Holocene time. In Bangladesh
the oldest sediment is the Permian Gondwana rock that lies over the Pre-Cambrian
Basement complex. Over the Gondwana rock successively lies the Cretaceous Rajmal
Trap, Paleocene Tura Sandstone, Eocene Sylhet limestone and Kopili, Oligocene
Barail, Miocene Bhuban, Bokabil and Tipam Sandstone, Pliocene Dihing and Dupi
Tila Sandstone, Pleistocene Madhupur Clay and Holocene Alluvium sediments ( Fig.
6, Table 1). This generalized sequence is not common in all parts of the country and
in some places many formations are missing due to depositional, non-depositional,
and post depositional erosion. The stratigraphic sequence is also variable between the
Fore Deep area and the Shelf areas.
4
Table 1. Generalized stratigraphic sequence of Bangladesh.
Age
Holocene
Pleistocene
PioPleistocene
Formation /
Group
Alluvium
~~~~~~~~~~~
Madhupur
Clay
~~~~~~~~~~~
Dihing
Dupi Tila
Formation
Miocene
Oligocene
Eocene
Paleocene
Cretaceous
Permian
PreCambrian
~~~~~~~~~~~
Tipam
Sandstone
~~~~~~~~~~~
Bokabil
~~~~~~~~~~~
Bhuban
~~~~~~~~~~~
Barail
~~~~~~~~~~~
Kopili
~~~~~~~~~~~
Sylhet
Limestone
~~~~~~~~~~~
Tura
Sandstone
~~~~~~~~~~~
Rajmahal Trap
~~~~~~~~~~~
Gondwana
~~~~~~~~~~~
Basement
Lithologic description
Comment
Sand, silt and clay: Grey; layered and
inter layered with peat and
decomposed vegetal matters.
~~~~~~~~~unconformity~~~~~~~~~
Red clay with patches of sandstone:
Deeply oxidized, variegated colors,
Contains ferruginous nodules and
laterite
~~~~~~~~~unconformity~~~~~~~~~
Sandstone: Variegated colors with
pebbles and laterites.
Sandstone with clay beds: Grey,
oxidized to variegated colors, coarse,
pebbles and petrified woods present
~~~~~~~~~unconformity~~~~~~~~~
Top: Girujan Clay: Grey
Bottom: Tipam Sandstone: Grey,
Medium to coarse with lignite bands.
~~~~~~~~~unconformity~~~~~~~~~
Sandstone and shale
~~~~~~~~~unconformity~~~~~~~~~
Sandstone and shale
~~~~~~~~~unconformity~~~~~~~~~
Sandstone
~~~~~~~~~unconformity~~~~~~~~~
Shale with sandstone and
fossiliferous beds
~~~~~~~~~unconformity~~~~~~~~~
Lime stone with sandstone beds
Top and Middle
& Lower
aquifers
~~~~~~~~~unconformity~~~~~~~~~
Sandstone
~~~~~~~~~unconformity~~~~~~~~~
Volcanic trap with sandstone and
shale
~~~~~~~~~unconformity~~~~~~~~~
Sandstone with shale and coal beds
~~~~~~~~~unconformity~~~~~~~~~
Granite, Granodiorite, Gneiss and
Schist
5
Stable Paleosol
Deep aquifers
Very deep
aquifers
Gas producing
zone
Limestone
deposits
Oldest exposed
rock of BD
Coal deposits
Igneous &
metamorphic
rock
2.3.2. In Bangladesh, the Quaternary (including Plio-Pleistocene) sediments are the
most important for the purpose of groundwater extraction. Hence, these are described
in some detail.
2.4. Quaternary of the Bengal Basin
2.4.1 The Brahmaputra and Ganges systems are draining the northern and southern
slope of the rising Himalayas. The terrigenous sediment is reworked and redistributed
by the strong current from the Bay of Bengal. These processes might have been
operated throughout the Plio-Pleistocene Period toward building this large delta and
the largest submarine fan in the world. Discharge of great volume of sediments from
the Himalayas and subsidence of the Bengal Basin are the major cause of this thick
sedimentation (Biswas and Agrawal, 1992). Sea level changes during the Upper
Pleistocene and Holocene (Fig. 9 & 10) influenced the sedimentation in the Bengal
Basin.
2.5. Pleistocene Sediments
2.5.1. The Pleistocene has the time span of 1.8 million years to up to 10,000 years BP.
This time span is important in the history of the Bengal Basin as well as of the world.
The mighty Himalayas took its present shape during this time. Also there were many
ups and downs in the earth's climatic condition during this period (Fig.9). The old
terraces (Pleistocene terraces) of Barind and Madhupur Tract area were also uplifted
during this time.
2.5.2. The presence of Toba ash on top of the Pleistocene upland surfaces in West
Bengal India (Acharyya and Basu, 1993), Bangladesh (Abdullah and Hasan, 1991)
and Bay of Bengal (Kudras et.al, 1999) indicates that these Pleistocene upland
deposits are older than 70,000 years BP.
2.5.3. In Bangladesh the Pleistocene deposits in the Madhupur and Barind upland
surfaces are characterized by deeply oxidized paleosol. The Clay is characterized by
red clay at the top and mottled clay at the bottom. The red clay is red to brick red and
yellow, massive, contains ferruginous nodules and laterite. The clay contains minor
amount of sand. The mottled clay is bluish to earthy gray with mottling of red, brown,
6
yellow and orange colors, The massive and sticky clay contains ferruginous nodules.
The concentration of the nodules varies from place to place. The thickness of the
mottled clay is variable but generally it is 15 m. Depending on the degree of oxidation
and hydration the color of the clay alters from red to yellow, orange and brown. The
clay contains small amount of sand which does not appreciably reduce its high
plasticity. The clay becomes soft and muddy in contacts with water and dries quickly,
unlike other clays, becomes very hard. This clay is impermeable and caps the
underlying Dui Tila sandstone aquifers locally (F.H. Khan, 1991).
2.5.4. Plio-Pleistocene Dupi Tila sandstone underlies the Pleistocene Red Clay and
Holocene Alluvial deposits overlies the Pleistocene Red Clay. The bottom of the
Pleistocene Clay (Top of the Dui Tila sandstone) is very much undulated and marked
by a prominent unconformity.
2.6. Sea Level Changes and Late Pleistocene-Holocene Sedimentation
2.6.1. Most of Bangladesh had low elevation throughout it's geologic history that
made it very much sensitive to the sea-level changes which influenced geologic
processes of weathering, erosion and deposition of sediments. Various authors
(Fairbridge, 1961; Pirzzoli, 1991, Acharyya, 1999)) presented sea level changes
curves. The Late Pleistocene includes the last glacial-interglacial periods between 128
to 10 Ka. (Fig. 9). The last glacial maximal was marked between 21 to 18 ka when the
sea level was in the lowest stand. During the lowest stand of sea level the Pleistocene
sediments were exposed, weathered and was eroded and incised by rivers. The basal
sand and gravel bed found at different places of the Bengal Basin was formed during
this time in the incised channels of the proto Ganges-Brahmaputra rivers and flood
plains. The sea level continued to rise from 18 ka to 12 ka resulting in transgression
and onlapping sedimentation and filling up of the entranced valleys by fluvial and or
fluvio-deltaic sand with scattered gravel. From 12 to 10 ka there was a regression (sea
level fall) as a result the upper sediments were exposed to aerial oxidation (Umitsu,
1993 and Acharyya, 2000) in different parts of the country.
2.6.2. The Holocene has a time span of 0 to 10,000 years BP. From 10 to 6 ka, the sea
level started rising again, and at ~6ka reached higher than 2 m than the present level.
During this time there was an extensive development of marine and fresh water peat.
7
After the post glacial optimum, the sea level dropped and initiated a phase of subdued
marine regression and migration of shore line to the present configuration. Islam and
Tooley (1999) have recognized 5 transgressive phases in the Holocene sediments NW
of Khulna City, each of which followed by a regressive phase (Fig. 10).
2.6.3. Umitsu (1993) divided the Late Pleistocene-Holocene sediments in the
Brahmaputra-Jamuna floodplain into 5 members on the basis upward coarsening or
fining of grain size. These five members has been broadly divided into Lower, Middle
and Upper members by Acharrya et.al (2000) in West Bengal and southern part of
Bangladesh.(Fig. 11). These divisions are shown in Table 2.
Table 2. Comparative divisions of the Late Pleistocene-Holocene sediments in
Bangladesh and West Bengal.
West Bengal
(Acharyya et al,
2000)
Unit I : fine sand
clay, peaty material
Bangladesh (Umitsu, 1993)
DFID report
Bangladesh (2001)
Upper most Unit: (5/6-0 ka);
fine silt and clay, peaty
material
Upper Unit: (10/8-5/6 ka);
fine sand and silt at bottom
and upper, coarser at middle
Unit II (10-7k);
fine to medium
sand with clay
intercalations
Middle Unit: (12-10ka);
coarse sand, silt in the upper,
fine sand, silt and clay at the
bottom
Lower Unit:(12-18ka); Sand
with scattered gravel; upper
part oxidized
Unit III (23.37
ka*); Sand with
scattered gravel
Lower most Unit:( ~18 ka);
gravel bed, ~ 10 m thick
18-5ka: Rapid base level
rise; Upward finings; Coarse
sediments at the bottom and
fine upwards.
21-18ka; Lowest strand of
sea level; Coarse material
deposition
28-21 ka: Rapi fall in base
level; Rapid valley incision.
Coarse sediment deposition.
128-28 ka: steady decline of
sea level. Slow erosion
Older
Late-Pleistocene-Early Holocene
Gravel Bed at the bottom, upper
Part oxidized.
Middle Holocene
Wide spread peat layers at the
upper part.
Upper
Holocene
Suggested
divisions
* Hait & others, 1996
8
2.6.4. Following is a brief description of the suggested divisions of the late
Pleistocene-Holocene sediments in the floodplain and the deltaic regions of
Bangladesh.
2.6.5. Late Pleistocene –Early Holocene (18 – 12/10 ka) is recognized by the basal
gravel bed with an average thickness of 10 meter. This gravel bed has a gradient of
3/10,000 towards south. This bed was deposited in valleys during the lowest strand of
sea level during the Late Pleistocene-Early Holocene. During this time the sea level
decreased to ~ 135 m at approximately 18000 years BP. At that time the shoreline was
at the outer margin of the continental shelf and Pleistocene and Tertiary sediments
were subjected to aerial erosion (Acharrya, et.al. 2000). The basal gravel and sand
was not uniformly developed as it was developed in the valleys. The depth of gravel
bed varies from place to place. (Generally it is within 18m to 100m). The period
beginning with 10,000 years marked an important change in the sedimentation in the
Ganges Delta. Rapid sea level rise marked the initiation and growth of deltaic
sedimentation. The Upper part (lower member of Umitsu) is composed of coarse sand
with scattered gravels. The coarser sediments are more in the upper reaches. The top
of the upper part is weathered and oxidized up to a depth of 15 meters which implies
that the sea level was low at least 10 meter during this time. Due to the rise of sea
level from 18-12 ka, the channels were infilled with coarse material.
2.6.6. Middle Holocene (12/10-8/6 ka): A regression of the sea occurred between
12,000 and 10,000 years BP. After that there was a transgression (rise in sea level
occurred from 10,000 – 8/6000ka). Early-mid Holocene transgressions led to back
flooding and over-topping of the entrenched alluvial channels that were formed
during low-stand setting. High sediment load from the rapidly eroding Himalayas
under the influence of tectonism and intense precipitation competed with rapid sealevel rise to continue sluggish deltaic sedimentation over entrenched valley fills and
adjacent floodplains, marshes, lagoons and estuaries. Clay and silt that are rich in
organic matter and peat interbedded with lenticular sand bodies from numerous
transient distributary channels characterized the unit. The Middle Holocene sediments
can be correlated with the Upper and Middle Unit of Umitsu (1993).
9
2.6.7. The Upper Holocene started from the 8000/6000 ka and continues till the
present time. The Upper Holocene sediment is composed of silt and clay. The sea
level started to rise from 5000/6000 years BP and is continuing. During this time, the
broad areas of Bengal lowland become marshy and the peat and wetland spread over
widely.
2.6.8. The basal gravel bed mark the bottom surface of the Late Pleistocene-Early
Holocene and the upper surface is formed of the oxidized sediment layer. It may be
possible to map the boundary between the Middle and Upper Holocene by widespread
peat layers (6000 years BP) developed at that time.
2.6.9. In the Ganges-Brahmaputra-Meghna Delta complex, vertical and horizontal
variations of the lithofacies in the Holocene is very high. It is difficult to correlate
them from one place to another even at short distances. Inter fingering of the
sediments are common. We must keep in mind that the Bengal Basin is one of the
fastest subsiding regions of the world. The high rate of subsidence along with similar
high rate of sediment deposition makes mapping of these units quite difficult. But
time and necessity dictates that this problem is solved.
3. Aquifer Systems in Bangladesh
3.1. Divisions of Aquifers
3.1.1. The country has been divided into 15 hydrogeological zones on the basis of the
geologic factors, aquifer characteristics and development constrains (UNDP report,
1982). These 15 zones have different potentialities for the development of
groundwater (Fig.12). In Bangladesh the aquifer systems are conventionally divided
into three groups (MPO, 1986, UNDP, 1982; DFID, 2001, Aggarwal and others,
2001) depending on their vertical distribution (Depth parameters; Fig. 13 & 14). The
three groups are as follows:
10
UNDP, 1982
DFID, 2001
Aggarwal & Others, 2001
i)
Upper aquifer
i)
Upper Shallow
i)
(composite aquifers)
ii)
Lower Shallow
ii)
Main aquifer
iii)
Deep Aqifer
iii)
Deep aquifer
1st aquifer (Type 1 &
2)
ii)
2nd aquifer (Type 3)
iii)
3rd aquifer (Type 4)
3.1.2. The aquifer systems can also be divided from the geological point of view (Fig.
15), which seems to be more logical. The major divisions are:
i)
Late Pleistocene-Holocene aquifers
a) Upper Holocene aquifers
b) Middle Holocene aquifers
c) Lower Holocene aquifers
ii)
Plio-Pleistocene aquifers
3.1.3. It should be mentioned here that none of these divisions are typical throughout
the country rather they depend on the geological characteristics of that particular area.
3.2. Conventional Aquifers
3.2.1. There is a silty clay/ clay layer on the top of the upper aquifer. In general the
thickness of the clay layer varies from 5-20 m but it may be less than one meter to
several hundred meters (Barind area). In some area (northwest Bangladesh) this clay
layer is absent. This aquifer is composed of fine to very fine sand with thin bands of
discontinuous clay. Thickness of this zone is variable. It is thin in NW Bangladesh
and is as much thick as 60 m in the south (UNDP, 1982). In Mongla port area this
aquifer is within the range of 24-150 m (EPC, 1998). Beneath the upper aquifer, there
is medium to coarse sand with discontinuous thin clay layers. This sand is considered
as the main aquifer in Bangladesh. Most of the deep tube wells are withdrawing water
from this aquifer. The thickness of this aquifer varies from 25 m to 85 m. Depth of
this aquifer depends on the geological nature of the particular area. The upper
composite and the main aquifer are separated by thin multi-layered discontinuous
clay. These two aquifers are hydraulically connected and may be termed as leaky
aquifers (Fig. 16). In the coastal belt there is a saline water zone between the upper
11
and main aquifer. In this region the main aquifer is considered as the deeper aquifer.
Beneath the main aquifer there is a thick clay layer in the coastal zone. Ground water
of the upper and the main aquifers are seriously affected by arsenic contamination in
most part of the country.
3.2.2. Beneath the main aquifer, there is a deep water bearing horizon called the deep
aquifer. This aquifer is separated from the main aquifer by a thick clay layer which
ranges in thickness 60 – 150 m (UNTCD, 1979). This aquifer is mostly composed of
medium sand but coarse to very coarse sands are also found in different places. The
thickness and depth of the deep aquifer depends on the geological nature of the area.
Generally the depth of the aquifer is considered as 200m below the ground level. In
the red soil area this aquifer ranges in depth 150 to 460 m, in the coastal belt it ranges
from 210 – 300 m. Near the foot hills the depth is shallower (30 m). It is reported that
the deep aquifer does not exist south of Khulna city (UNDP, 1982). Water in the deep
aquifer in the coastal area is sometimes called fossil water (Rus, 1985) because of its
isotopic dating of 20,000+ years (Aggarwal and others, 2001). The static water level
is constant over a long period of withdrawl in Khulna area, which indicates that the
water is recharged laterally from a long distance towards the coast by low hydraulic
gradient. (Fig.17). At present water from this deep aquifer are withdrawn in the city
areas of Dhaka and Khulna..
3.3. Division of the Aquifers from Geological Point of View
3.3.1. The aquifer system in Bangladesh are geologically controlled. The system
depends on the sedimentary characteristics, depositional environments, tectonic
features and other related parameters.
3.3.2. Plio-Pleistocene aquifers: The Pleistocene upland areas cover about 10% of
the country and includes the Madhupur and Barind Tract. In this region, there is a
thick silty clay layer (150 to 460 m) of Pleistocene age. Plio-Pleistocene Dupi Tila
sandstone lies beneath this layer. This sandstone is considered as a good aquifer in
this region. The conventional aquifer division systems UNDP (1982; DFID, 2001))
classified this aquifer as Deep Aquifer. The major portion of water for Dhaka City is
12
withdrawn from this aquifer and has not been reported to be affected by arsenic
contamination (Dhaka City withdraws about 1.2 billion liters of water per day). The
sandstone is medium to coarse and pebbly. The bottom of the Dupi Tila Sandstone is
at about 1000m depth at Kamta. In this area the Holocene alluvial sediment is absent.
Beneath the Dupi Tila sandstone is successively underlain by the Girujan Clay,
Tipam Sandstone, Bokabil, Bhuban and other Formations at greater depths. The
sandstones of these formations also contain water and are considered as good aquifers
(Table 1). The groundwater of Chittagong City is withdrawn from the Dupi Tila and
Tipam Sandstone.
3.3.2.1. The northern most part of the country is covered by the Tista fan deposit
which is composed of
coarse detritus ( sand and pebbles) derived from
the
Himalayan areas. The clay cover on the top of the aquifer is absent in this area. The
thickness of this deposit is variable. It is a few meters in Dinajpur area where as it
increases up to 395m at Tetulia in the extreme North of Bangladesh (Islam, et al,
1990). Beneath the Tista Fan deposit lies the coarse gravelly sand of Pleistocene
deposit. The thickness of the Pleistocene deposit is 10,703m at Tetulia (BOGMC Test
Hole, Shell, 1988). This is also a good aquifer and are not reported to be affected by
the arsenic contamination (Fig. 18).
3.3.3. Late Pleistocene-Holocene Aquifers: The Ganges-Bramaputra-Meghna
floodplain covers about 40% of the country. Here the Holocene alluvial deposits lie
over the Pleistocene Clay. Thickness of the Holocene deposits is highly variable. In
this area the aquifers mainly exists in the Holocene sediments. The Holocene aquifers
can be divided in to 3 major Divisions: each division contains a number of sand layers
that are interconnected. Their horizontal extensions in all directions are not for long
distances.
3.3.4. The late Pleistocene-Early Holocene aquifers: The Late Pleistocene Early
Holocene aquifer is characterized by the gravel bed (Umitsu, 1993) and sand with
scattered gravels. The average thickness of the gravel bed is 10 meters (Fig. 11). This
gravel bed has a gradient of 3/10,000 towards south. The basal gravel and sand was
not uniformly developed as it was developed in the valleys. The depth of gravel bed
varies from place to place. (Generally it is within 18 m to 100m). This aquifer
13
contains huge amount of water and is not normally affected by arsenic. The upper part
of the Late Pleistocene-Early Holocene aquifers is composed of sand with scattered
gravels. The top part is weathered and oxidized.
3.3.5. Middle Holocene aquifers: Above the Late Pleistocene_Early Holocene
aquifer lies the fine sand which becomes coarser in the upper part This sandy
sequence varies greatly both in vertically and horizontally. The upper part also
contains silt and peaty organic matters. These mid Holocene aquifers has been
considered as the main aquifer (UNDP, 1982) or Lower shallow aquifer (DFID, 2001)
in the floodplain and deltaic areas of Bangladesh. Most of hand tube wells withdraw
water from this aquifer and the water is severely affected by arsenic contamination.
3.3.6. Upper Holocene aquifers: The upper Holocene aquifers are not present in all
the deltaic and floodplain areas. The lower unit is composed of silt and clay at the
bottom, and fine sand at the top. The upper part is composed of silt and clay, and are
commonly found to be interbedded or mixed with medium sand. In UNDP (1982)
classification this aquifer is mentioned as upper composite aquifer and in DFID report
it is considered as upper shallow aquifers. Water of this upper aquifers are also
affected by the arsenic poisoning. The aquifer systems of the Holocene sediments is
leaky type from which the contamination spread vertically from one place to another.
3.3.6.1. The delta area covers about 32% of country. In this area the thickness of the
recent sediment is higher and The Dupi Tila sandstone is at greater depth. The
southern part of the deltaic plain constitute the coastal plain where the thickness of
the alluvial deposit is highest compared to other parts of the country. In the coastal
part the Holocene aquifers are affected by saline water.
4. Distribution of Arsenic in Groundwater of Bangladesh
4.1. Arsenic Distribution and Geology
4.1.1 Distribution of arsenic contaminated water in the country is related to the
geology (Fig. 19 & 20). Water in the aquifers of the Pleistocene and older sediments
are arsenic safe. The areas are in the eastern and northeastern hills region. The arsenic
distribution map (DFID, 2001) shows that the greater concentration is within the
14
deltaic and floodplain areas of the country whereas the Late Pleistocene and older
sediment covered areas are not affected. If we consider the aquifer conditions, then
the Mid and Upper Holocene aquifers are severely affected while the lower Holocene,
Pleistocene and older aquifers are not affected. Many researchers have found that the
tube wells at greater depth are not affected because the deeper tube wells are
withdrawing water either from the Pleistocene or the older Dupi Tila and Tipam
sandstone aquifers.
4.2. Arsenic Concentration in the Sediments
4.2.1. Naturally each geologic formation contains arsenic. Geological Survey of
Bangladesh has analysed the sediment samples from each geological formations. The
result is given in Table 3.
Table 3. Results of the analyses of rocks and sediments of Bangladesh for arsenic.
Rocks/sediments
of the Formation
Marine sediments
from the Bay of
Bengal
Alluvium
No. of
Arsenic
Sample
content in
analyzed
ppm
58
1-24
10
Peat
1
Less than
1.0 – 3.5
5
Madhupur Clay
Dihing
Dupi Tila
Girujan Clay
Tipam sandstone
Surma Group
Barail Sandstone
Sylhet Limestone
Tura Sandstone
Rajmahal Trap
Gondwana
Coal
ash
Pre-Cambrian
Rock
2
1
3
2
2
1
1
1
2-4
2.4
1.2-1.75
1.5-2.10
1.00
0.5
8.0
8.0
6
3.0-5.0
Remarks
Analyzed in Germany
Oxidised zone at ~1.5 m contains ~530
ppm. Mud layer at 105 ft=19ppm at
Debidwar(USGS)
Peat: High at Samta (262 mg/kg).
Not analyzed
Not analyzed
Not analyzed
15
4.2.2. The organic rich sediments are reported to have rich arsenic (peat layer at
Samta 260 ppm). An oxdised layer within the redox potential zone in the
Brahmanbaria area is reported to have up to 530 ppm of arsenic. High amount of
arsenic is also reported in the sediments of Bay of Bengal (up to 24 ppm). The release
mechanism of the arsenic from the sediment is not yet clearly understood. There are a
number of hypothesis of which the oxidation, reduction and oxidation-reduction is on
the top of the list. Some other hypothesis like the microbial activity and effect of
fertilizer have also been put forward.
5. Conclusions

The distribution of Arsenic in the ground water is related to the geology of the
country rather than depth.

Division of the aquifer system from the geological point of view, like the Upper
Holocene aquifer, Middle Holocene aquifer, Upper Pleistocene-Early Holocene
aquifer, Plio-Pleistocene aquifer and older aquifers is more logical than the depth
or the tube well pumping system as is customarily adopted in Bangladesh.

Most of the arsenic contaminated tube wells are drawing water from the Middle
and Upper Holocene sediments.

Aquifers in the Late Pleistocene-Early Holocene and older sediments are not yet
contaminated by arsenic.

Sediment analyses of the aquifers are very scanty.

Release mechanism of arsenic in the ground water is not very well understood,
though a number of hypothesis have been put forward.
6. Recommendations
6.1 Screening of all the Tubewells and Aquifer Mapping Issues

All the tube wells, both hand pumps and irrigation wells should be screened for
the Arsenic content in the water and unsafe ones marked. Those wells found to
have higher arsenic content than 0.05 mg/l should be sealed and those with less
16
than 0.05 mg/l should be monitored regularly at least once in a year. A database of
all water analyses for arsenic is prepared and updated regularly and made
available to the research community as well as to the interested people in general.

Testing facilities of tube well water should be made available to people at a
reasonable cost at the Upzila level.

Reference laboratories should be set up at regional level to validate and control
the analyses done in the Upzila levels.

Countrywide mapping of the aquifers of the Plio-Pleistocene, Late-PleistoceneHolocene to the Upper Holocene, Middle Holocene and Upper Holocene should
be undertaken as soon as possible.

Depth contour map of the top of the Late Pleistocene Madhupur Formation should
be prepared as soon as possible to identify the areas where ground water can be
withdrawn from the arsenic safe Dupi Tila Sand.
6.2. Groundwater Management Issues

Though the exact figure is not available it is considered that 99% of the ground
water withdrawn is used for irrigation. It may be considered whether it can be
made compulsory that irrigation tube wells should withdraw ground water only
from the Dupi Tila Sand where ever possible. In that case the Late Pleistoceneearly Holocene aquifer may be kept exclusively for withdrawal of water for
drinking and cooking purposes.

Arsenic safe aquifers must be protected from future contamination at any cost.

Research should be undertaken before any decision is taken to withdraw large
amount of water from the presently arsenic safe Late Pleistocene-early Holocene
aquifer. Till definite data are available about the recharge of these aquifers they
should not be allowed for exploitation.
6.3. Research Issues

Systematic work on the analyses of the aquifer sediments should be undertaken.
Chemical logs of the sediments in the different geologic districts from the Dupi
Tila sandstone (Plio-Pleistocene) to the most recent sediments should be prepared
as soon as possible.

Investigating into the release mechanism of arsenic in the ground water should be
undertaken on a priority basis.
17

Research on effect of arsenic contaminated irrigation water on crops should be
undertaken in a systematic way..

Research should be undertaken on risk of arsenic in food chain in relation to
livestock/humans.

Research should be undertaken on impact of arsenic contaminated water on soil
quality.
6.4. Legal and Administrative Issues

Groundwater should be considered as a natural resource and a suitable "Ground
water Act" should be enacted to control all activities regarding ground water
development and use.

There is no single organization in the country that deal with all aspects of ground
water though all our neighboring counties have organizations such as, "Ground
Water Commission", "Ground Water Board" or "Ground Water Agency" etc. The
Government should consider creation of such an organization bringing all the
personnel working on groundwater under one umbrella organization.

Coordination of all the government agencies and the NGOs should be given much
more importance than at present.

Local Government Institutions should be given as much importance as possible to
arrange arsenic safe water to the people.
7. References
Abdullah, S.K.M. and Hasan, A.K.M.S, 1991. Quaternaary Volcanic ash bed(?) in
Bangladesh. Jour. Geology Soc. Bangladesh; 10, 41-42.
Acharyya, S.K. and Basu. P.K, 1993. Toba ash on the Indian sub-continent and its
implications for correlation of Late Pleistocene alluvium. Quarternary
Research, 40; pp10-19.
Acharyya, S.K. 1999. Ganges delta development during the Quaternary in the Bengal
Basin and it relation to Arsenic Toxicity in Ground water; Abs.Int. Seminar on
Quaternary Development and Coastal Hydrodynamics of the Ganges Delta in
Bangladesh. Pp17
18
Acharyya, S.K., Lahiri, S., Raymahashay, B.C. and Bhowmik, A. 2000. Arsenic
toxicity in groundwater in parts of Bengal basin in India and Bangladesh: the
role of Quaternary Stratigraphy and Holocene sea-level fluctuation.
Environmental Geology; Springer-Verlog, pp1127-1137.
Aggarwal, P.K. with Basu, A., Poreda, J.R., Khukarni,K.M., Froelich,K., Tarafdar,
S.A., Ali, M., Ahmed, N., Hossain,A., Rahman,M., Amed,S.R.; 2000. Isotope
hydrology of groundwater in Bangladesh: Implications for characterization
and Mitigation of arsenic in Groundwater. IAEA-TC Project:BGD/8/016
Alam. M.K., Hasan, A.K.M.S., Khan, M.R., and Whitney, J.W., 1990. Geological
map of Bangladesh. Geological Survey of Bangladesh, Dhaka.
ANON, 2001. Hydrology; Summary Report. DPHE-DANIDA, Water Supply and
Sanitation Components. DPHE,WDB, DNIDA and Aqua Consultants.
Biwas, S.K., 1992. Tectonic evolution of the Bengal Foreland Basin since the Early
Pliocene and its implication on the development of the Bengal Fan. In Recent
Geoscientific Studies in the Bay of Bengal and the Andaman Sea. Geological
Suvey of India, Special Publication No. 29.p5-18.
DFID, 2001.
Arsenic contamination of Groundwater in Bangladesh., British
Geological Survey and Department of Public Health Engineering, Govt. of
Bangladesh. Final report Vol-2.
Fairbridge, R.W., 1961. Eustaic changes in the sea level. Physics and chemistry of the
earth. 4, pp99-185.
Hait, A.K., Das, H.K., Ghosh, S., Roy, A.K., Shaha, A.K., Chanda, S. 1996. New
dates of
Pleistocene subcrop samples from south Bengal, India. Indian
Journal of Earth Science,Vol. 23, p79-82.
Kudrass, H.R., Hofmann, A., Doose, H., Emeis, K, and Erlenkeuser, 1999.
Modulation and amplification of climatic changes in the northern hemisphere
by Indian summer monsoon during the last 80,0000 years. BGR Final Report,
Bengal shelf, SONNE Cruise-SO126 .
Islam, S.M, and Tooley, M.J. 1999. Coastal and sea level changes during the
Holocene in Bangladesh. Elsvier Science, Quaternary International, 55;pp 6175.
Khan, S.I, Williams, Van.S., Das, S.K., and Hassan, K.Z., 1990. Geology of
Panchagarh District, Rajshahi Division, Bangladesh. Records, Geol. Surv.
Bangladesh, 6, part 2.
19
Khan, F.H. 1990. Geology of Bangladesh. Willey, New Delhi.
Mannan, M.A and Mir Hamidov, 1981. The nature of gravitational field and it
relation of geotectonics of Bangladesh. Petrobangla Internal Report, Dhaka.
MPO, 1987. Groundwater Resources of Bangladesh. Technical Report Nr-5. Master
Plan organization, Dhaka-Harza Engineering USA in association with Sir
MacDonald and Parters, UK. Meta Consultant, USA and EPC Ltd. Dhaka.
Morgan, J.P, and McIntire, W.G., 1959. Quaternary Geolgy of the Bengal Basin, East
Pakistan and India. Bullein of the Geological Society of America. 70, pp319342. 1980
Pirzzoli, P.A., 1991. World atlas of Holocene Sea Level Changes. Elsvier
Oceanography series, 58. Elsvier, London.
Rus, J.S. 1985. Geohydrological investigations in Khulna. DPHE Water Supply and
Sanitation
Project,
DPHE.
Netherlands-Bangladesh
Development
Cooperation Program.
Shamsuddin, A.K.M and Abdullah S.K.M, 1997. Geologic Evolution of the Bengal
Basin and its implication in Hydrocarbon Exploration in Bangladesh. Indian
Jounal Of Geology, Vol.69, No. 2, p 93-121.
Umitsu, M., 1993. Late Quaternary sedimentary Environment and Landform
Evolution in the Bengal Lowland. Geographical Review of Japan, 60, pp164177
Umitsu, M., 1993. Late Quaternary sedimentary Environment and Landform
Evolution in the Bengal Lowland. Sedimentary Geology, 83, pp-177-186
UNDP 1982. Groundwater Survey, The hydrogeological Condition of Bangladesh.
UNDP Technical Report DP/UN/BGD-74-009/1.
20
Contents
Page No.
1
1.Introduction
2. Geology of Bangladesh
2
2.1. Physiography of Bangladesh
2
2.2. Tectonic Set-up of the Country
3
2.3. Stratigraphy
4
2.4. Quaternary of the Bengal Basin
6
2.5 Pleistocene Sediments
6
2.6. Sea Level Changes and Late Pleistocene-Holocene Sedimentation
7
3. Aquifer Systems in Bangladesh
10
3.1 Division of the Aquifers
10
3.2. Conventional Aquifers
11
3.3. Division of the Aquifers from Geological Point of View
12
4. Distribution of Arsenic in Groundwater of Bangladesh
14
4.1 Arsenic distribution and Geology
14
4.2. Arsenic Concentration in the Sediments
15
5. Conclusions
16
6. Recommendations
16
7. References
18
List of Figures
Fig. 1. Map of point source arsenic concentrations observed in
groundwaters in the national hydrochemical survey
Fig. 2. Documented cases of arsenic problems in groundwater related
to natural contamination.
Fig. 3. Contour map of Bangladesh
Fig. 4. Generalized physiographic map of Bangladesh
Fig. 5. Tectonic map of Bangladesh.
Fig. 6. Correlation of stratigraphic sections from the Himalayan
foredeep across the Bengal Basin.
Fig. 7. Map showing the Bengal Basin with Tectonic elements.
Fig. 8. Schematic crustal cross section through the line X-X’, Y-Y’ of
fig. 7
Fig. 9. Sea level changes during the last interglacial transition
21
Fig. 10. A sea level curve from Panigati based on the age and altitude
of 4 radiocarbon dated samples
Fig. 11. Profiles of Holocene sediments a) Section across Ganges
Delta in West Bengal, B) Section across Jamuna flood plain
and Ganges delta in Bangladesh.
Fig. 12. Grounwater development zones.
Fig. 13. Hydrological cross section from north-south across
Bangladesh
Fig. 14. Revised hydrological model based on isotopic and
hydrochemical data
Fig. 15. Schematic diagram of the aquifers on the geological point of
view.
Fig. 16. Geological cross section across the coastal area (E-W)
Fig. 17. Possible relationship of fresh and saline groundwater in SW
Bangladesh.
Fig. 18. Lithologic column at salbanhat Test Hole, Panchagarh
District.
Fig. 19. Map of smoothed groundwater arsenic concentration from
national hydrochemical survey.
Fig. 20. Geological map of Bangladesh
List of Tables
Table 1. Generalized stratigraphic sequence of Bangladesh.
Table 2. Comparative divisions of the Late Pleistocene-Holocene
sediments in Bangladesh and West Bengal.
Table 3. Results of the analyses of rocks and sediments of
Bangladesh for arsenic
22
5
8
15