DEVELOPMENT OF CONCEPTUAL HYDROGEOLOGICAL MODEL FOR EVALUATION OF THE AQUIFERS WITH LOW ARSENIC GROUNDWATER IN MATLAB UPAZILA, SOUTH-EASTERN BANGLADESH

The three-dimensional finite-difference groundwater model MODFLOW was used together with an analysis of the geological conditions in the process of developing a conceptual hydrogeological model of the southeastern part of the Bengal Basin. The conceptual model will be used in further studies to help target aquifers with low concentrations of arsenic in groundwater of Matlab Upazila, which could be a sustainable source of safe drinking water for disad-vantaged communities of the region. Only saturated flow was considered, and both a transient model and a steady state model were set up. The transient model was calibrated using the groundwater level data, and the steady state model was calibrated to match carbon 14 dating of the groundwater using the MODPATH code. The modelled area ranged from the Bay of Bengal in the south to the low lying beels and haors of Sylhet region in the north, and from the river Meghna in west to the Tertiary Tripura Hills in the east. During the initial phase of the study, it was found necessary to establish a groundwater model that covered a much larger area than was initially planned, as the groundwater with low concentrations of arsenic, below 30 m depth, is several thousand years old and thus probably have infiltrated far away from Matlab Upazila. The simulations together with the analysis of the geological conditions facilitate the establishment of a reliable conceptual model for the complex hydrogeological conditions. Some important conclusions can be drawn from the study: i) the vertical hydraulic conductivity of the aquifer is significantly less than the horizontal conductivity, ii) two flow systems can be identified; a deep regional flow system with recharge areas at the Tripura Hills or beyond, and a local, shallow flow system driven by local topography, iii) during the monsoon period the local flow system is more active, and iv) the impact of flooding during the monsoon season, must be included in future simulations to describe the subsurface groundwater system accurately.