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Quaternary Geology and Aquifer Systems in the Ganges-Brahmaputra-Meghna Delta Complex, Bangladesh

 

ABSTRACT

People of Bangladesh face a serious problem of groundwater contamination by arsenic.  Millions of tube wells within the depth of 10 to 50 meters are contaminated with higher than permissible limit of arsenic. The pattern of distribution of arsenic in the ground water is related to the geology of the country. The tube wells in the Pleistocene Uplands, the hilly areas and the Tista Fan areas are not affected by the arsenic contamination, whereas the tube wells in the flood plain and deltaic plains of Holocene sediments are severely affected. The exact mechanism of the arsenic release and its movement in the sediment and groundwater is yet to be resolved.

During the Pleistocene and Holocene time large volume of sediments were laid down in the Ganges-Brahmaputra-Meghna (GBM) Delta Complex by the mighty rivers that built up the delta and aquifer systems. The input of the sediments and their distribution was largely controlled by tectonic activities (rising Himalayas) and the climatic changes in the region. The unconsolidated sediments of the Late Pleistocene and Holocene unconformably lie over the Madhupur Clay Formation. The presence of Toba ash on top of the Madhupur Clay Formation of the Pleistocene Uplands indicates that these are older than 75,000 years.

The sea-level changes influenced geologic processes of weathering, erosion and deposition of sedimentation in the Bengal delta. Late Pleistocene-Holocene sediments of the GMB Delta Complex are important for groundwater withdrawal.  It is proposed to divide these sediments into 3 major units based on changes in the nature of sedimentation due to sea level fluctuations. These divisions are i) Late Pleistocene–Early Holocene unit, ii) Middle Holocene unit and iii) Upper Holocene unit.

During the lowest stand of sea level in Late Pleistocene (21-18 ka) sediments were exposed, weathered and was eroded and incised by the rivers. The basal sand and gravel bed and sand with scattered gravel found at different places of the GMB Delta Complex were formed during this time in the incised channels and flood plains of the proto Ganges-Brahmaputra rivers. Upper part of this sediment is oxidized which can be used to map this unit.

The Middle Holocene (12/10ka-6ka) sediments are characterized by lenticular sand bodies with clay, silt and peat layers that were developed in a transgressive phase of the sea level. Carbon14 dating show that much of the peat layers were most probably deposited during a high stand of sea level (about 2 m above the present level) at around 6ka. The oxidized zone at the bottom and large-scale development of peat at the top may be used to map this unit.

The Upper Holocene (6ka–0) sediments are recognized by the interlayered/interfingered silt, clay and fine sand which forms the upper most part of the sedimentary column of the delta and the recent flood plains of the GMB Delta Complex. Upper Holocene sediments are not found in the slightly uplifted parts of the delta such as the Chandina Formaion (Tippera Surface and equivalent).

Historically the Late Pleistocene-Holocene aquifers of the country have been conventionally divided into three divisions on the basis of depth. These three divisions can be summarized as; i) Upper Aquifer (normal range 10 to 100 m depth), ii) Main Aquifer ( normal range 100 to 200 m depth), and iii) Deep Aquifer. The Upper and Main aquifers are hydraulically connected. Major portion of drinking and irrigation water is withdrawn from the main aquifer. The Upper and Main aquifers are found to be severely affected by arsenic. Beneath the Main Aquifer lies the Deep Aquifer. Till now, water from this aquifer is not much affected by arsenic contamination.

The sedimentation rate and subsidence in the whole of the Bengal Basin were not uniform throughout the Quaternary. Sediments of very different nature or of different geological age can be found at similar depths. Considering the above fact, an attempt has been made to divide the aquifer systems in the GMB Delta Complex from a geological point of view, which seems to be comparatively more logical than the conventional divisions based only on depth. These divisions are i) Plio-Pleistocene aquifers, ii)Late Pleistocene – Early Holocene aquifers, iii) Middle Holocene aquifers and iv)Upper Holocene aquifers.  These divisions are matched with the sedimentary units of the Quaternary in the GMB Delta Complex.

The Plio-Pleistocne Aquifer: The Plio-Pleistocene aquifers of the Dupitila Formation lies beneath the Pleistocene Madhupur Clay Formation. This aquifer is composed of light grey to yellowish brown, medium to coarse sand with pebble beds. All of the water for Dhaka City is withdrawn from this aquifer but the water is till now arsenic safe.

The Late Pleistocene-Early Holocene Aquifer: The Late Pleistocene-Early Holocene aquifers are not continuous all over the country.  This to some extent corresponds to the Deep Aquifer. The sediments of this aquifer to some extent correspond to the Late Pleistocene-Early Holocene Unit of the sediment section. Water within this aquifer is found to be arsenic safe but heavy withdrawal from this aquifer needs further study.

Middle Holocene Aquifer: Above the Late Pleistocene-Early Holocene aquifer lies the fine sand which becomes coarser in the upper part This sandy sequence varies greatly both vertically and horizontally. The upper part also contains silt and peaty organic matters. Water from this aquifer has been dated  as about 3000 years old. Most of the ground water in Bangladesh is withdrawn from this aquifer and the water is severely affected by arsenic contamination.

Upper Holocene Aquifer: The Upper Holocene aquifers are developed all over the deltaic and flood plain areas. This aquifer somewhat corresponds to the Upper aquifer of the conventional system. Water from this aquifer has been dated as about 100 years old and also affected by the arsenic contamination. Each of the Holocene aquifer contains a number of sand layers/lenses that are stacked and interconnected which makes them of leaky type from which the contamination spread vertically from one place to another.

Detailed mapping of the aquifer systems from the geological point of view can help in the planning for systematic withdraw of arsenic free groundwater.  This new approach need coordinated and integrated works of detailed lithofacies analysis, age dating of the sediments and water along with shallow geophysical surveys and palynofacies analysis.

 

Source:

Quaternary Geology and Aquifer Systems in the Ganges-Brahmaputra-Meghna Delta Complex, Bangladesh.

By Md. Nehal Uddin and S.K.M Abdullah

 

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