DELINEATING THE HYDROSTRATIGRAPHIC ARCHITECTURE OF THE GANGES RIVER DELTA AQUIFER SYSTEM: IMPLICATIONS FOR GROUNDWATER ARSENIC CONTAMINATION

The Ganges River Delta, shared between India and Bangladesh is one of the largest and most densely populated deltas in the world. The delta aquifer system serves as a major source of drinking water for its inhabitants. However, the delta faces the challenge of widespread arsenic contamination in its groundwater, posing serious health risks to millions of people. This research project focuses on understanding the hydrostratigraphic architecture and its control on the depth distribution of dissolved arsenic across the delta aquifer system. Deploying a total of approximately 3000 borehole lithologs sourced from various government databases from both India and Bangladesh, we developed the lithological model of the Ganges River Delta Aquifer System. The model was constructed using a three-dimensional interpolation method with a horizontal bias. The model indicates that the northwestern part of the delta consists of a continuous sand layer with little or no confinement, while the southern and eastern regions exhibit multiple aquifers layering intercalated by confining units. This understanding of the subsurface architecture coupled with the existing data on arsenic depth distribution from previous studies provides valuable insights into the potential transport pathways of dissolved arsenic in the aquifer system. The observed decline in arsenic concentrations with depth towards the south and east of the delta, as documented in previous studies, can be attributed to the presence of confining units that limit the migration of shallow arsenic-contaminated water into deeper aquifers. In contrast, the continuous and largely unconfined aquifer to the north shows no significant decline in arsenic concentrations with depth. The findings from the model provide crucial information for the decision-makers and stakeholders to target the potentially safe aquifers within the delta as drinking water sources.