UNDERSTANDING THE INFLUENCE OF HYDROSTRATIGRAPHY ON GROUNDWATER ARSENIC DISTRIBUTION IN THE GANGETIC DELTA AQUIFERS OF BANGLADESH AND INDIA

The Gangetic delta aquifers of the Bengal basin, transgressing across the Indo-Bangladesh border is known to be one of the most productive aquifer systems of the world. However, the groundwater of the aquifers is severely enriched with arsenic (As). There have been numerous studies to understand the controls of As distribution in the area, however, only providing a partial picture. More so, the regional hydrogeological controls, which dictate the groundwater flow paths and chemistry, and thus the arsenic fate and transport, remain less understood. This study is aimed to delineate the hydrostratigraphic architecture of the Gangetic Delta Aquifers in Bangladesh and India, and to understand its influence on the groundwater As distribution. The study area is bounded by the main Ganges river channel to the north and east, its distributary, the Bhagirathi-Hoogly River to the west and the Bay of Bengal to the south. A hydrostratigraphic model is being developed using about more than 2000 lithologs from a part of the study area. Towards the north, the model shows a very thick continuous shallow aquifer system, underlain by a basal aquitard in most places. Towards the south, the sediments show intricate interlayering of aquitard layers within the aquifer, dividing it into several vertically discontinuous aquifer units. The model shows presence of a deeper aquifer at places in the southern parts of the basin, separated from the shallow aquifer by thick clay aquitard. In the northern parts, deeper groundwater flows through the thick continuous aquifer result in arsenic enrichment in the deeper aquifers, while the aquitard interlayers towards the south prohibit the vertical inflow of shallow arsenic-enriched groundwater to deeper low-arsenic waters, thus providing a natural barrier against deep groundwater contamination. The poorly connected aquifer units towards the south leads to the development of distinct hydro-biogeochemical environments resulting in varied arsenic concentrations within these units. The geometry of the different aquifer units controls the groundwater flow paths and the hydro-biogeochemical environment therein, influencing the arsenic mobilization/sequestration within these units.