Geochemical composition and the level of Arsenic (As) contamination of groundwater in the Ganges Delta Plain, southwestern Bangladesh were elucidated. Hydrogeochemical data of tube well samples suggested that the groundwater is mostly Ca-Mg-HCO3 type with bicarbonate (HCO3-) as the dominant anion, though other type waters are also observed. In contrast, the elevated EC, Cl- and high content of Na+ relative to Ca2+, Mg2+ and K+ in six groundwater samples suggest their saline origin. Low concentrations of NO3- and SO42-, and high concentrations of dissolved organic carbon (DOC), HCO3- and PO43- indicate the reducing conditions of subsurface aquifer where sediments are deposited with abundant organic matter. The total As concentration in the analyzed samples is very high (0.0431-1.352 mg/L) along with high Fe (2.791-17.058 mg/L) and relatively low Mn (0.134-1.972 mg/L) at different depths. Distinct relationship of As with Fe and Mn, and strong correlation with DOC suggests that the biodegradation of organic matter and reductive dissolution of Fe-oxyhydroxide is considered to be the dominant processes to release As in aquifers. Moreover, negative correlation between As and SO42- demonstrates the As may not be directly mobilized from sulfide minerals like arsenopyrite. (C) 2008 Elsevier B.V. All rights reserved.