Geochemical study of arsenic and other trace elements in groundwater and sediments of the Old Brahmaputra River Plain,Bangladesh |
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| Authors: | Faruque Ahmed M Hawa Bibi Hiroaki Ishiga Takehiko Fukushima Teruyuki Maruoka |
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| Institution: | (1) Graduate School of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba 305-8572, Japan;(2) Department of Environmental Science and Technology, Saitama University, 255 Shimo-okubo, Saitama 338-8570, Japan;(3) Department of Geoscience, Shimane University, Matsue 690-8504, Japan |
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| Abstract: | The geochemical study of groundwaters and core sediments from the Old Brahmaputra plain of Bangladesh was conducted to investigate
the distribution of arsenic and related trace elements. Groundwaters from tube wells are characterized by pH of 6.4–7.4, dissolved
oxygen (DO) of 0.8–1.8 mg/l, Ca contents of 5–50 mg/l, and Fe contents of 0.2–12.9 mg/l. Arsenic concentrations ranged from
8 to 251 μg/l, with an average value of 63 μg/l. A strong positive correlation exists between As and Fe (r
2 = 0.802; p = 0.001) concentrations in groundwater. The stratigraphic sequences in the cores consist of yellowish silty clays at top,
passing downward into grayish to yellowish clays and sands. The uppermost 3 m and lower parts (from 13 to 31 m) of the core
sediments are oxidized (average oxidation reduction potential (ORP) +170 and +220 mV, respectively), and the ORP values gradually
become negative from 3 to 13 m depths (−35 to −180 mV), indicating that anoxic conditions prevail in the shallow aquifers
of the Brahmaputra plain. Age determinations suggest that clay horizons at ~10 m depth were deposited at around 2,000 and
5,000 years BP (14C ages) during the transgressive phase of sea-level change. Elevated concentrations of As, Pb, Zn, Cu, Ni, Cr, and V are present
in the silts and clays, probably due to adsorption onto clay particles. Significant concentrations of As occur in black peat
and peaty sediments at depths between 9 and 13 m. A strong positive correlation between As and Fe was found in the sediments,
indicating As may be adsorbed onto Fe oxides in aquifer sediments. |
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