Mineralogical profiling of alluvial sediments from arsenic-affected Ganges–Brahmaputra floodplain in central Bangladesh A. Uddin a,⇑ , M. Shamsudduha a,b , J.A. Saunders a , M.-K. Lee a , K.M. Ahmed c , M.T. Chowdhury c a Department of Geology and Geography, Auburn University, Auburn, AL 36849, USA b Department of Geography, University College London, Gower Street, London WC1E 6BT, UK c Department of Geology, University of Dhaka, Dhaka 1000, Bangladesh article info Article history: Available online 7 January 2011 abstract Mineral assemblages (heavy and light fractions) and sedimentological characteristics of the Quaternary alluvial aquifers were examined in the central Bengal Basin where As concentrations in groundwater are highly variable in space but generally decrease downward. Chemical compositions of sediment sam- ples from two vertical core profiles (2–150 m below ground level, bgl) were analyzed along with ground- water in moderately As-enriched aquifers in central Bangladesh (Manikganj district), and the As mobilization process in the alluvial aquifer is described. Heavy minerals such as biotite, magnetite, amphibole, apatite and authigenic goethite are abundant at shallow (<100 m below ground level (mbgl)) depths but less abundant at greater depths. It is interpreted that principal As-bearing minerals were derived from multiple sources, primarily from ophiolitic belts in the Indus-Tsangpo suture in the north- eastern Himalayan and Indo-Burman Mountain ranges. Authigenic and amorphous Fe-(oxy)hydroxide minerals that are generally formed in river channels in the aerobic environment are the major secondary As-carriers in alluvial sediments. Reductive dissolution (mediated by Fe-reducing bacteria) of Fe- (oxy)hydroxide minerals under anoxic chemical conditions is the primary mechanism responsible for releasing As into groundwater. Authigenic siderite that precipitates under reducing environment at greater depths decreases Fe and possibly As concentrations in groundwater. Presence of Fe(III) minerals in aquifers shows that reduction of these minerals is incomplete and this can release more As if further Fe-reduction takes place with increased supplies of organic matter (reactive C). Absence of authigenic pyrite suggests that SO 4 reduction (mediated by SO 4 -reducing bacteria) in Manikganj groundwater is lim- ited in contrast to the southeastern Bengal Basin where precipitation of arsenian pyrite is thought to sequester As from groundwater. Published by Elsevier Ltd. 1. Introduction Arsenic enrichment in drinking water-supplies poses a serious concern for the public health in many countries around the world including the Ganges–Brahmaputra–Meghna (GBM) Delta in Ban- gladesh and West Bengal, India where tens of millions of people are currently exposed to dangerous levels of the As (Bhattacharya et al., 1997; Acharyya et al., 2000; Smith et al., 2000; BGS and DPHE, 2001; Stüben et al., 2003; van Geen et al., 2003; Ahmed et al., 2004; Shamsudduha et al., 2008). Although As-enrichment in groundwater is common in many other countries such as Argen- tina, Cambodia, China, Mexico, Nepal, Pakistan, Taiwan, USA and Vietnam (Smedley and Kinniburgh, 2002; World Bank, 2005; van Geen et al., 2008), the health-affect of As poisoning is felt more se- verely in Bangladesh due to its high population density (1000 person per km 2 , BBS, 2008). Studies of groundwater As over the last few decades (BGS and DPHE, 2001 and references therein; Ahmed et al., 2004) have brought researchers to a general consensus that the occurrence of elevated As (>50 lg/L; Bangladesh drinking water standard is 50 lg/L) in alluvial aquifers in the Bengal Basin is geogenic and reductive dissolution of Fe(III)-oxyhydroxide min- erals due to microbial metabolism releases As into groundwaters (Bhattacharya et al., 1997; Nickson et al., 1998, 2000; McArthur et al., 2001, 2004; Islam et al., 2004; Zheng et al., 2005; Saunders et al., 2005; Shamsudduha et al., 2008). Many others have sug- gested that elevated As in aquifers can occur by other processes such as chemical weathering and dissolution of phyllosilicate min- erals (Pal et al., 2002; Pal and Mukherjee, 2009), for example, dis- solution of biotite (Sengupta et al., 2004; Shamsudduha, 2007; Seddique et al., 2008), reduction of magnetite (Pal et al., 2002; Swartz et al., 2004; Shamsudduha, 2007), and dissolution of apatite (Shamsudduha, 2007), or redox disequilibrium between phases (Mukherjee et al., 2008). High-As concentrations in sediments at very shallow depths (<50 m below ground level, bgl) are found to be associated with sulfide minerals such as pyrite (Polizzotto 0883-2927/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.apgeochem.2011.01.006 ⇑ Corresponding author. Tel.: +1 334 844 4885; fax: +1 334 844 4486. E-mail address: uddinas@auburn.edu (A. Uddin). Applied Geochemistry 26 (2011) 470–483 Contents lists available at ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem