1 INTRODUCTION Ganges Delta Plains of Bangladesh and West Ben- gal, India constitute one of the largest deltas in the world. The major rivers viz., the Ganges, the Brah- maputra-Jamuna, the Meghna and their tributaries and distributaries have been the primary transporting media of the sediments in the depositional domain. This large delta can be divided into three broad flood-plain regions i.e., the Ganges flood-plain, the Jamuna flood-plain and the Meghna flood-plain. These flood-plain regions are also bounded by three well defined Quaternary basinal depressions Ganges- Mahananda depression, Jamuna depression, and Meghna depression (Figure 1). The Quaternary aqui- fers in the Ganges Delta Plains of Bangladesh are largely contaminated by arsenic. Arsenic contamina- tion has so far been observed in the aquifers within the Quaternary deposits situated in all the three flood-plain regions bounded by the basinal depres- sions. The sub-surface geology of the region bounded by the Ganges floodplain suggests that all the geologi- cal formations have a regional gradient towards south and southeast, indicating that the sedimenta- tion progressed in these directions throughout (Fig- ures 2 and 3). The Quaternary deposits of the flood- plain regions are typically characterised by finning upward sequence of channel migration. This has re- sulted in intercalation of channel-fill deposits with over-bank deposits. The sandy silt facies within overbank association are interpreted as levee depos- its formed by deposition of suspended sandy sedi- ments near the channel banks during over-bank floods (Imam et al. 1998). In addition to intercalated finning upward sequence of channel migration, the Quaternary deposits below finning upward sequence are composed mostly of clay, peaty clay, silty clay, clayey peat, peat and silty sand at variable depths. The occurrence of unusually thick clay which is ba- sically a carbonate mud has been the focal point of study. The physico-chemical studies of the clay have been made to understand the fate of arsenic transport from the overlying contaminated aquifers to under- lying aquifers. 2 GENESIS OF CLAY A group of detrital sedimentary rocks are commonly composed of clays, shales, mudstones, siltstones and marls. Two grades of particle size are recognised, the The role of clay in preventing arsenic dispersion in the Ganges delta plains of Bangladesh A. A. Khan Department of Geology, University of Dhaka, Bangladesh S. M. M. Alam Department of Geology, University of Dhaka, Bangladesh S. H. Akhter Department of Geology, University of Dhaka, Bangladesh ABSTRACT: A clay (carbonate mud) layer of about 110 m thick occurs immediate below the arsenic con- taminated aquifer zones and the geological evidences suggest for the clay to occur almost throughout the delta plain at variable depths. The clay is illitic in nature having three layers clay crystals like smectite. The domi- nant elemental constituents are Si, O, Al, and Ti with subordinate As, Fe, P, Ca, Mg, Na, Mn, S and C. The framework grains are highly coated with iron oxyhydroxide and to lesser extent with manganese oxyhydroxide together with arsenic. Sodium concentration shows a steady increase with depth in the clay layer. Carbon di- oxide ranges between 3.9 to 7.6 percent. The formation of soda ash (Na 2 CO 3 ) probably has forced the clay to be dispersed. The clay is very sticky, hard in drying and also calcareous in nature. The framework grain coat- ings have significantly deteriorated the pore geometry and subsequently reduced the vertical seepage potenti- ality. An extremely high resistivity signature for this clay supports the deterioration of pore-geometry. The ex- panding nature of clay resulted in diffusion of arsenic from pores in the overlying aquifers failing to compete with phosphate at bonding sites for adsorption. The clay has been interpreted as a "dispersed clay" and should act as a potential barrier to arsenic transport vertically. The frequent inter-fingering between the channel-fills and overbank-deposits in the arsenic contaminated regions also indicate that the clay is a potential barrier to lateral dispersal of arsenic. A.A. Khan, S.M.M. Alam & S.H. Akhter 1