Sultana, Sarmin; (2022) Hydrological loading influences on the Bengal Aquifer System of southern Bangladesh. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Hydrological loading influences on groundwater in the Bengal Aquifer System (BAS) have previously been recognized at individual sites in Bangladesh. This thesis interprets new and existing groundwater data to extend knowledge of BAS hydro-mechanical behaviour, to establish the significance of these coupled processes which under-pin groundwater variations in this environment. First, the thesis shows that BAS sensitivity to hydrological loading is extensive across southern Bangladesh, demonstrating a hydro-mechanical interpretation of groundwater hydrographs at 22 sites over 50,000 km2, where the data are unsupported by a solely hydraulic perspective. Secondly, the thesis applies geo-lysimetry at multiple sites to assess monsoonal changes in terrestrial water storage (ΔTWS) and its spatial variability across southern Bangladesh. This is the first application of geo-lysimetry across a large proportion of the sensing area of the GRACE satellites, allowing comparison of ΔTWS from the two independent approaches. One source of discrepancy is the anomalous effect due to surface water loads being focused along rivers rather than evenly distributed across the ground surface. Thirdly, the thesis further explores the influence of hydrological loading associated with a large river. Groundwater variations over two hydrological years along a linear transect of vertically-stacked piezometers adjacent to the Meghna river at Matlab, southern Bangladesh, are illustrated and analysed in the light of an existing 2D fully-coupled theoretical model of the hydro-mechanical aquifer. The effects on the BAS are proven to be extensive to at least 10 km from the river bank. This is the first field-based description of the hydro-mechanical responses of a thick aquifer system to loading from a large river. The thesis conclusions have significance for groundwater level monitoring and groundwater management in southern Bangladesh and similar environments, highlighting the potential shortcomings of groundwater data interpretation based on assumption of solely hydraulic aquifer behaviour. Results suggests that almost 50% of the national monitoring boreholes of Bangladesh are susceptible to being perturbed by mechanical loading.

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