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Md. Nehal Uddin, Geological Survey of Bangladesh (Bangladesh)
John W Whiteny, United states Geological Survey (United States)
M. Mustofa Alam, Dhak University (Bangladesh)
ASM Woobaid Ullah, Dhak University (Bangladesh)
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Abundant groundwater occurs in the sandy sequences of the Pleistocene-Holocene sediments of the Bengal basin, which was thought to be safe for drinking and domestic use. It has been observed that millions of tube wells within shallow depth are contaminated with higher than permissible limit of arsenic.
Bengal basin is one of the most extensive areas of geologically young alluvial deposition in the world but the progress in dating the sediments of this GBM deltaic basin remains limited. Total 49 samples from five boreholes of different settings were dated by the OSL method at Luminescence dating Laboratory, University of Illinois, Chicago, USA. Two samples from over bank recent deposits and two samples from near surface deposits were also dated. Depth wise sediment dating has been used for the calculation of sedimentation rate of shallow bore holes (50 m) at Brahmanbaria, Savar, and Meherpur, and for two the deep bore holes (450 m) at Srirampur and Rajoir. Samples from Pleistocene sediments up to ± 100,000 show fairly good results.
The thickness of Holocene (<10,000 years old) sediment across the delta ranges from 24 meters to 42-44 meters. The depth-wise sequential dating of the samples from different location of Bengal delta, Bangladesh shows that the rate of sedimentation is not same throughout the Holocene and Pleistocene. The range sedimentation rate varies between 0.3 mm/year and 17 mm/year.
An age unconformity of several tens of thousands of years often is found below the Holocene deposits and is sometimes marked by oxidation and rare paleosols indicating subaerial exposure of the delta surface. In Srirampur bore hole, mainly gray deltaic sediment at a depth of 104 meters is approximately 115,000 years old and at a depth of 158 meters, the lowest sample to yield a finite age, the sediment is 140,000 to 160,000 years old. At Rajoir the grey medium sand at 79 meter depth is 60,000 years and at 139 meter depth the micaceous sand shows infinite age. The long-term aggradation rate at Srirampur and Rajoir for the entire late Pleistocene is about 1 mm/yr. Gravel and coarse sand between 60 and 120 meters at both the sites were deposited due to the down-cutting of the incised rivers during the worldwide sea level lowering at the last glacial times.
The depth versus age plots of the sediment samples from the mid deltaic regions shows that the sedimentation is high within 4 to 36 meter depth. This high sedimentation is from the 3000 to 7200 years of the Holocene age when the sea level was high.
The aquifers within the high sedimentation zone show high concentration of arsenic whereas the low sedimentation zone within Pleistocene-Holocene shows low (within allowable limit) concentration of arsenic.
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