, 2010a). Iron oxide minerals are common in sediments of the Gangetic plain (Acharyya, 2005 and Mukherjee, 2012) and the Bengal Deltaic plain (McArthur this website et al., 2001). These minerals are strong sorbents for As (Kocar et al., 2009). Arsenic may be desorbed from the surface of the dissolving Fe oxide, or released from within the mineral structure itself

(Harvey et al., 2002 and McArthur et al., 2004). Only a minority of groundwater samples at our study site were saturated with respect to Fe(III) (oxyhdr)oxide phases like ferrihydrite, hematite, lepidocrocite, goethite, maghemite, and Mg-Ferrite. However, this suggests that precipitation of Fe(III) phases from groundwater is thermodynamically favorable at these locations. McArthur et al. (2001) observed a positive correlation between As(III) and Fe2+ in West

Bengal and suggested they are coupled via reductive dissolution of As-bearing Fe(III) minerals. The study of Bhattacharya et al. (2003) in the aquifer of the Nawalparasi district also observed a positive correlation between As and Fe (r2 = 0.59). However, in this study As concentrations displayed poor correlation with most major cations (Mn, Ca and Na) including Fe (Fig. 5), which is consistent with the studies of Khadka et al. (2004) in the Nawalparasi district. There was also weak correlation between aqueous As(III) and HCO3−, which may be a consequence of local baseline alkalinity being generated mainly RG7422 by carbonate mineral weathering and nitrate reduction (Nath et al., 2008). Weak correlation between aqueous As and Fe in Gangetic plain aquifers has also been observed by others (Dowling et al., 2002 and van Geen et al., 2006a) and may indicate decoupling between mobilization of As and Fe2+. The behavior of Fe(III) oxides under reducing conditions is complex and although

Fe(III) oxides are important host phases for As, during either reductive dissolution or Fe(II)-catalyzed mineral transformation, the degree of As mobilization depends on the affinity of the original and transformed minerals for the arsenic species (e.g. Dixit and Hering, 2003). A variety of studies have shown that Fe(II)-catalyzed transformation of poorly crystalline Fe(III) oxides into more thermodynamically stable crystalline phases can retard As mobilization (e.g. Fendorf et al., GABA Receptor 2010b and Pedersen et al., 2006). In addition, the release of As during reductive dissolution of ferrihydrite can be substantially delayed compared to Fe2+, as As(V) continues to adsorb to residual ferrihydrite until surface sites are saturated, only then releasing As to the aqueous phase (Pedersen et al., 2006). This can have the effect of causing an apparent decoupling between Fe2+ and As mobilization. Decoupling between Fe2+ and As may also result from sorption of Fe2+ to other surfaces (i.e. clays) or precipitation of Fe(II) minerals, such as siderite. Groundwater in Nawalparasi is near saturated with respect to siderite in most samples (Fig. 7).