ONSHORE ENTRAPMENT OF SEA WATER IN COASTAL AQUIFERS BY RAPID COASTLINE PROGRADATION

Within the Bengal and Mississippi Deltas, onshore saline groundwater is found to depths of about 150-200 m extending up to 100 km onshore. This onshore seawater appears to be unrelated to anthropogenic mechanisms of emplacement. Both of these deltas have experienced relatively high Holocene sediment progadation rates resulting in the migration of the coastline oceanward by over 100 km during the past 10,000 years. In the Bengal Delta, at least 90 m of sediments have been deposited onshore during the Holocene. We developed a coupled sediment transport-coastal hydrogeologic model using the control volume finite element method. The code includes a novel mesh generation algorithm in order to take into account shoreline progradation. We found that the entrapment of seawater onshore can be described by the product of two dimensionless number groups. One is the ratio of the shoreline and groundwater velocities. The other is the Péclet number, the ratio of advection to diffusion. We conducted a sensitivity study in which we varied sediment hydraulic conductivity and dispersivity. Some simulations include a semi-confining unit. Assuming a shoreline progradation velocity of up to 15 m/yr, the current rate of clinoform progradation in the Bengal Basin, we found that an aquifer hydraulic conductivity of less than 10 m/day and a longitudinal dispersivity of less than 50m can sequester onshore seawater. The presence of a semi-permeable confining unit had the largest impact on onshore seawater entrapment. In 2022, we plan to conduct a towed control-source electromagnetic survey along inland rivers and onshore magnetotelluric soundings along the river banks in two transects of the Bengal Delta to better constrain the distribution of onshore freshwater and seawater.