In heterogeneous geological environments, understanding hydrogeology at the necessary scale is a difficult task. In fluvio-deltaic terrains depositional elements are often arranged in a linear fashion and are spatially discontinuous and varied in nature. These characteristics have implication for groundwater flow.
The upper few hundred meters of the floodplain sediments of Bangladesh contain unconsolidated Quaternary fluvio-deltaic sediments that hold an essential source of water, meeting almost 80% of the national water-demand. However, many issues concerning the hydrogeological framework remain poorly understood. Many 'deep' tubewells have been drilled beyond the shallow aquifer system in the past few years, yet the existence of a shallow and a deeper aquifer as a separate entity has been questioned. In a study of about 5000 km2 area in the eastern Bangladesh we have used 10 deep logs from the petroleum industry, 576 drillers' logs, and 12 bore-hole geophysical logs to delineate the hydrogeological basement, to characterise the upper 250 m of the sedimentary column - the exploited part of the aquifer system, and to investigate the local scale spatial variability.
Petroleum industry logs indicate some structure on the hydraulic basement with a surface at between 600 and 1500 m depth. The surface of the 'Upper Marine Shale' of Miocene age extending across the region as indicated in existing literature. For the uppermost 250 m drillers' logs are used to interpret 'lithofacies' and 'hydrofacies' in 3D. Two broad hydrofacies were coded numerically in order to identify internal variability within the aquifer materials and to investigate potential layering of the finer grained sediments. A single aquifer is observed across the region with the potential for hydraulic connection between a shallower and a deeper zone. Intermediate finer materials between these zones do not form a continuous layer of low permeability. Three dimensional curvilinear features containing coarser sediments identified within the aquifer may correspond to paleo-channels and be acting as preferential groundwater flow paths. Geophysical logs are interpreted in terms of sedimentary-facies. Thin clay layers are present locally but they are not laterally persistent. The paper will present a conceptual basis for incorporating this hydrogeological variability into groundwater flow models.