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Arsenic contamination in groundwater and hydrogeological background in Samta village, western Bangladesh(<Special issue>elucidating the mechanism of arsenic contamination of groundwater in Asia with special reference of research in Bangladesh)

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  • バングラデシュ西部シャムタ村における地下水ヒ素汚染と水文地質的背景(<特集>アジア地下水ヒ素汚染の機構解明-とくにバングラデシュ調査について-)
  • バングラデシュ西部シャムタ村における地下水ヒ素汚染と水文地質的背景
  • バングラデシュ セイブ シャムタムラ ニ オケル チカスイ ヒソ オセン ト スイモン チシツテキ ハイケイ

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Abstract

<p>The groundwater contamination by arsenic has become a serious problem for human health in Bangladesh and West Bengal State of India. Since 1996, the Asia Arsenic Network (AAN) and the Research Group for Applied Geology (RGAG) have started their activities to tackle the arsenic problem in Bangladesh. The AAN and RGAG selected Samta village in Jessore district, western Bangladesh as a pilot village for their activities. The RGAG has carried out hydrogeological investigations including core sample analysis and groundwater monitoring at constructed observation wells since 1998 to reveal the source and the mechanism of the arsenic contamination. The AAN and the research group of Miyazaki University measured concentration of arsenic and groundwater quality at the observation wells and selected private wells. It is known by the AAN that the concentration of As is higher than the Bangladesh drinking water standard (=0.05mg/l) in the southern part of the village, whereas the As concentration is lower than the standard in the northeastern part. Samta village is bounded on the east by the Betna river, that flows from north to south with bending toward west. The geomorphology of Samta village, which is situated on the Ganges delta and has elevation between about 5 to 7m, is subdivided into natural levee, gentle slope and back swamp. The continuous core drilling was performed up to a depth of 64m at the central part of the village to investigate subsurface hydrogeologic conditions. A total of 23 observation wells were drilled with collecting core samples in Samta village and Deuli village, which is located east of Samta village. Soil auger survey and interpretation of existing deep tube wells' data were also carried out. As a result, it is found that the sediments of Samta village within a depth of 200m consist of lower sandy layer (ls), lower muddy layer (lm), upper sandy layer (us), upper muddy layer (um), upper most sandy layer (ums), upper most muddy layer (umm), and embankment (b). The sediments are hydrogeologically classified into the unconfined aquifer (ums) having perched water, the first aquitard (um), the first confined aquifer (us), the second aquitard (lm), and the second confined aquifer (ls). The private tube wells extract groundwater from the first aquifer, which is contaminated by As. The deep tube wells constructed by the Department of Public Health Engineering (DPHE) use the groundwater from the second aquifer, which is not contaminated by As so far. Gradual rise in rainy season and sharp drop in dry season were characterized by the changes in piezometric head in the first aquifer. A typical recession curve was not found from the hydrograph, indicating that the changes are affected by vertical leakage caused by the pumpage from the deeper part of the first aquifer for irrigation use. The result of As content test shows that the As content is high in the first aquitard (um), ranging from 3.0 to 261.5mg/kg. Particularly, peaty samples in the layer have high As contents from 46.5 to 261.5mg/kg. On the other hand, the As content in (umm), (ums), and (us) was relatively low (0.7 to 23.0mg/kg). The distribution of As content in (um) shows that high As content is found in the central to southern Deuli and in the central to southern Samta. The As concentration in groundwater of the first aquifer was measured by the field kit using Hironaka method, AAS in Miyazaki University, and SDDC spectrophotometry in DPHE Rajshahi laboratory. The results show that the As concentration is high in the southern part of Samta and low in the northeastern part. The measurement at private wells shows that As concentration in October 1997 (rainy season) is higher than that in March 1997 and May 1998 (dry season). The As concentration measured at the observation wells having different well depths and screen depths at the same location shows that the groundwater in the upper part of (us) has</p><p>(View PDF for the rest of the abstract.)</p>

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