Extremely high As concentrations in drinking water of the Ganges Delta (West Bengal and Bangladesh) has emerged as an issue of great concern in the past decade because of its serious impact on the health of millions of people. The distribution pattern of As concentrations in the Ganges Delta region is patchy and there are numerous As "hotspots". The present study is perhaps the first attempt in West Bengal to characterize such a hotspot by geophysical and geochemical methods, and to model the transport of the enrichment plume using a ID reactive transport model (PHREEQC). The study site is located along the Hooghly River, 60 km north of Kolkota City, near the city of Chakdaha. Total As concentrations in the groundwater range from 0. 5 to more than 6 pmol L-1; the WHO recommended maximum drinking water concentration is 0. 13 mu mol L-1 (i.e. 10 mu g L-1). Results show groundwater is in chemical equilibrium with siderite and calcite, a mineral phase previously shown to be an efficient trap for As(111). Groundwater redox potential is controlled by the Fe(OH)(3(am))/Fe2+ couple. The As(III) versus As(V) distribution (42% As(III) and 58% As(V), on average) is not at equilibrium with measured Eh values. No evidence of sulfide solid phases, such as As rich pyrite or arsenopyrite, was found. Although amorphous Fe dissolution is confirmed to play an important role in the release of As, selective dissolution extractions indicate that adsorption of As on carbonates and micas may also be an important component of As cycling in the sediment. Modelling results demonstrate the role of PO43-, HCO3- and Fe(II) in mobilizing the As plume, thereby increasing 4 3 the threat to the 75,000 inhabitants of Chakdaha. (C) 2007 Elsevier Ltd. All rights reserved.