THE ROLE OF AQUIFER ARCHITECTURE IN CONTROLLING GROUNDWATER ARSENIC IN THE GANGES DELTA: IMPLICATIONS FOR IDENTIFYING SAFE ZONES (Invited Presentation)

The Ganges River delta aquifer system, spanning India and Bangladesh, is a crucial freshwater resource for millions but is plagued by severe arsenic (As) contamination. This study delves into the role of aquifer geometry in shaping groundwater chemistry and the distribution of As, with a focus on identifying deeper As-safe zones. Our regional-scale hydrostratigraphic model delineates three distinct aquifer subsystems: an unconfined aquifer (Type I), a semi-confined vertically segregated aquifer (Type II), and a nearly confined multilayered aquifer (Type III). The Type I aquifer, characterized by Ca-Mg-HCO₃-rich waters, reveals homogenous hydrogeochemical signatures, suggesting significant recharge infiltration under high vertical hydraulic gradients. Conversely, Type II and Type III aquifers exhibit greater dominance of Na-Cl hydrogeochemical facies at shallow and intermediate depths, transitioning to Na-HCO₃ facies in deeper layers. This variability reflects horizontally stratified groundwater flows dictated by the geometry of the intervening aquitards.

Arsenic distribution is profoundly influenced by hydrostratigraphy. In the Type I aquifer, As concentrations remain relatively uniform across depths. However, in the Type II and Type III aquifers, As levels decrease sharply with depth, with intervening aquitards acting as barriers that prevent the downward migration of As or organic matter-rich water, thereby safeguarding deeper aquifer zones. Our findings underscore the critical importance of hydrostratigraphic architecture in controlling groundwater chemistry and As distribution within the Ganges River delta. The delineation of these aquifer subsystems and their distinct geochemical behaviors provides a scientific foundation for identifying and delineating As-safe zones, enabling effective planning and successful installation of tubewells in these targeted safe zones.