ORIGINAL ARTICLE Geochemical study of arsenic and other trace elements in groundwater and sediments of the Old Brahmaputra River Plain, Bangladesh Faruque Ahmed M. Hawa Bibi Hiroaki Ishiga Takehiko Fukushima Teruyuki Maruoka Received: 29 January 2009 / Accepted: 29 July 2009 / Published online: 13 August 2009 Ó Springer-Verlag 2009 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 lg/l, with an average value of 63 lg/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 yel- lowish 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 determi- nations suggest that clay horizons at *10 m depth were deposited at around 2,000 and 5,000 years BP ( 14 C ages) during the transgressive phase of sea-level change. Ele- vated 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. Keywords Arsenic Á Groundwater Á Sediment Á Peat Á Brahmaputra River Plain Á Bangladesh Introduction High concentrations of naturally occurring arsenic have been reported in groundwater from many regions, includ- ing Bangladesh, India, Nepal, Thailand, China, Taiwan, Vietnam, Chile, Hungary, and parts of the USA (Acharyya et al. 1999; Smedley and Kinniburgh 2002; Garcia-Sanchez and Alvarez-Ayuso 2003). Arsenic contamination of groundwater is now a serious hazard among the many environmental issues in Bangladesh. There has been increasing concern over the safety of groundwater from shallow aquifers in the Ganges, Brahmaputra, and Meghna (GBM) Delta, where As levels exceed the World Health Organization (WHO 2004) guideline of 10 and the 50 lg/l limit adopted in Bangladesh. Sediments act as both sources and sinks of As and other toxic trace elements (Guern et al. 2003), and the fate of trace metals is dependent on the biogeochemical transfor- mations that occur in the sediments (Peltier et al. 2003). The degradation of organic matter, present at high con- centrations in most wetland sediments, drives the formation of sediment redox gradients (Gaillard 1994). Arsenic is widely distributed as a trace constituent in rocks, soils, natural waters, and organisms, and it can be mobilized by F. Ahmed (&) Á T. Fukushima Á T. Maruoka Graduate School of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba 305-8572, Japan e-mail: frahmed2007@yahoo.com M. H. Bibi Department of Environmental Science and Technology, Saitama University, 255 Shimo-okubo, Saitama 338-8570, Japan H. Ishiga Department of Geoscience, Shimane University, Matsue 690-8504, Japan 123 Environ Earth Sci (2010) 60:1303–1316 DOI 10.1007/s12665-009-0270-7