Author: Md. Mahadi Hasan
The coastal belt of Bangladesh consists of 19 coastal districts out of 64 districts encompassing 147 upazilas including the exclusive economic zone (EEZ) and constitutes 32 percent of the area and 28 percent of the population of Bangladesh. The coastal belt of Bangladesh forms the lowest landmass and is part of the world’s largest delta called the Bengal Delta or Ganges-Brahmaputra Delta. The coast of Bangladesh is about 710km long. The southwestern coastal belt belongs to the Tidal Delta and parts of Satkhira district belongs to the inactive Ganges delta, Parts of Khulna district belongs to both active and inactive Ganges delta and parts of Bagerhat district belongs to the active Ganges delta respectively.
Based on geomorphologic features, the coastal areas of Bangladesh have been divided into three distinct regions viz. the eastern region, the central region and the western region and the study area belongs to the western region which is characterized by dense mangrove forest and deeply scoured tidal channels. According to PDO-ICZMP (2003), the coastal zone of Bangladesh has been classified into exposed coast and interior coast (figure) based on cyclonic storm surges, salinity and tidal influence. 12 districts out of 19 coastal districts comprising 48 Upazilas are exposed to the Bay of Bengal or lower estuaries and are defined as exposed coast and the remaining 99 Upazilas of the coastal districts are defined as interior coast.
The geology of the southwestern coastal belt of Bangladesh is characterized by active tectonism, sea level fluctuation and tidal activity. Tectonically the study area belongs to the Foredeep of the Bengal Deep (geosynclinal) Basin which is characterized by only mild or no folding. So the sedimentary layers are generally horizontal. The coastal zone is composed of Quaternary sediments carried by the Ganges-Brahmaputra-Meghna river system forming an overlapping deltaic arc. About 1.67 billion tons of sediment is carried annually by this mighty river system and 970 billion cubic meter of water is discharged into the Bay of Bengal. This huge load of sediments finds its way into the Bengal deep sea fan. The southwestern coastal belt belongs to the tidal delta and partly of both active and inactive Ganges delta and is composed of floodplain sediments mainly clay, silty clay, sandy clay and sand. Depositional pattern is very complex and it is difficult to find a regular or continuous succession but it shows a heterogeneous mixture of sand, silt and clay.
The southwestern coastal belt is crisscrossed by a large number of rivers and their tributaries. The major rivers of Satkhira district are the Morichap River, Kholpotua River, Betna River, Raimangal River, Hariabhanga River, Ichamati River, Betrabati River and Kalindi-Jamuna River. The major rivers of Bagerhat district are the Panguchi, Daratana, Madhumati, Pasur, Haringhata, Mongla, Baleswar, Bangra and Goshairkhali whereas the major rivers of Khulna district are the Rupsa-Pasur, Bhairab, Shibsha, Dharla, Bhadra, Ball, and Kobadak. The Rupsa River is formed by the union of Bhairab River and Atrai Rivers. Near Chalna, it changes its name to Pasur River and flows into the Bay of Bengal. The Pasur River is tributary of the Ganges River. The Bhairab River originates from Tengamari border of Meherpur district and bisects the Khulna city into two parts and passes through the Jessore town.
The southwestern coastal belt is experienced with three distinct seasons viz. hot humid summer from March to June, cool rainy monsoon from June to October and cool dry winter from October to March. About 1500 to 2000mm rainfalls occur annually over this area of which about 70% rainfalls occurs during the monsoon period whereas about 20% and 10% rainfall occur during the summer and winter period respectively. Evaporation changes throughout the year of the study area and ranges from 1.34mm to 4.3mm. Evaporation starts increasing from February and attains its peak at May when maximum temperature occurs and minimum evaporation occurs on December or January. The temperature of this area varies throughout the year. During the months of hot humid summer, the temperature reaches high of around 35.4°c in May and during the winter the temperature becomes as low as of 13.8°c in January.
The water level fluctuates between 1 to 2.6m in Khulna and 1.5 to 10m in Satkhira respectively. The water level occurs mostly within 1.75m in Khulna and 5m in Satkhira. The ground water level of this area is influenced by tidal activity. The river stage starts rising from the beginning of March whereas the ground water level starts rising from the late of April.
The southwestern coastal belt of Bangladesh especially the Khulna-Satkhira coastal belt is endowed with multi-layer prolific aquifers composed of deltaic sediments-a complex mixture of sand silt and clay. Hydrogeologically the southwestern coastal belt of Bangladesh belongs to the Holocene Coastal Plains (Zone-V). The sandy sediments forming the aquifers are not continuous but rather sand lenses interbedded with silts and clays with significant overlapping. Seven hydrostratigraphic units have been delineated which are, from top to bottom, aquitard 1, aquifer 1, aquitard 2, aquifer 2, aquitard 3, aquifer 3 and the aquitard 4.
The groundwater quality of the southwestern coastal area is poor and sometimes beyond the potable limit. The shallow aquifer is either saline or brackish or has high arsenic levels but at some places (Fakirhat, Bagerhat, Rampal) a fresh water bearing zone occur at the top. The middle aquifer is also saline or brackish. The lower aquifer is confined and free from vertical percolation of saline water into this aquifer. At some places this deep aquifer provides with fresh water but in most areas the water is saline or brackish. The coastal belt of Bangladesh is characterized by high chloride concentration. The chloride concentration exceeds the WHO drinking water standard (250 mg/l) and is unsuitable for drinking and even for irrigation. Arsenic (As) contamination varies from 20% to more than or equal 80% in this area. Though saline or brackish groundwater occur almost everywhere, some isolated pockets of fresh water can be found in some places where active flushing of fresh water takes place during the monsoon period.