Seasonal changes in subsurface characteristics in the Lower Bengal Basin: Potential impacts on groundwater

The risk of groundwater depletion is most significant if anthropogenic withdrawals are high in regions where the subsurface characteristics do not favor surface water infiltration and natural recharge. However, such regions have not been identified systematically in the Indo-Gangetic basin, one of the most fertile alluvial aquifers in the world. This identification may be enabled by a study of how seasonal changes in subsurface characteristics affect groundwater levels. We conducted a resistivity survey in Nanoor block of the Lower Ganges Basin, where groundwater levels have declined steeply in recent decades, to determine how subsurface characteristics controlling infiltration and groundwater quality may change seasonally. Vertical Electrical Soundings (VES) using the Schlumberger electrode array were conducted over an 8 km² area with an electrode spacing of 180-200 meters during the pre-monsoon (April 2018) and post-monsoon (October 2018) seasons. Seasonal variations in the Dar- Zarrouk parameters: longitudinal conductance, transverse resistance, and the coefficient of anisotropy were evaluated. While the average longitudinal conductance remained unchanged across seasons, it increased in certain locations in post-monsoon, indicating potential risks of contamination from the surface. The transverse resistance significantly increased in the post-monsoon, suggesting reduced groundwater potential. Additionally, in some areas, the coefficient of anisotropy indicated increased compaction of overburden layers in the post-monsoon, suggesting decreased natural recharge potential. Finally, the thickness of the unsaturated zone increased significantly from pre-monsoon to post-monsoon, which may be attributed to groundwater withdrawal for irrigation during the Kharif cropping season. These results indicate that this region, heavily reliant on groundwater for irrigation, is characterized by subsurface properties that allow limited natural recharge potential. This study may provide a framework for managing groundwater resources in developing countries where anthropogenic withdrawals are likely to have a more significant impact on groundwater levels than reduced natural recharge due to changing rainfall characteristics.