Hydrogeological Study of the Assasuni Region in Coastal Southwest Bangladesh – A combined fieldwork and modelling approach to understand changing flow directions

Abstract

The Bengal basin in Bangladesh is an area with a large variety in groundwater salinity. To improve freshwater security in the dry season the DeltaMAR project is set up to investigate the implementation of MAR-systems in the region. To improve the sustainability prediction of these MAR-systems it is important to know how different factors in a region are connected and what their implications are regarding the groundwater flow. This thesis combines results of a fieldwork and a modelling study to define the main connections between surface elevation, clay thickness, salinity, infiltration capacity and groundwater flow. This is done by defining distributions of the above-mentioned characteristics within a representative case study area of approximately 40 km2 (the Assasuni region). The study first confirmed that high elevated areas imply thin clay layers with relative fresh aquifer water due to recent rainwater infiltration. Low elevated areas can contain thin clay layers with saline aquifer water or thick clay layers with brackish aquifer water. Second, regarding the groundwater flow, elevation (including rivers) and pumping are found to be the main drivers. Surprisingly, it is found that groundwater flow reverses direction from high to low elevation during the wet season towards flow from low to high elevation during the dry season because of extensive pumping. Third, the fieldwork conclusions show the net yearly groundwater flow to currently be from the fresh elevated areas towards saline low region. This indicates that the fresh aquifers are not salinizing and the pumping is sustainable. Model results however show the instability of this flow and emphasize the possibility of aquifer salinization when pumping quantities increase or fresh recharge quantities decrease. This emphasizes the importance of quantifying the consequences of irrigation pumping with respect to fresh recharge when predicting the sustainability of a MAR-system.