Migration of arsenic in multi-aquifer system of southern Bengal Basin: analysis via numerical modeling

A heterogeneous anisotropic steady-state groundwater flow model for the multi-aquifer system of a part of southern Bengal Basin shows that human intervention has changed the natural groundwater flow system. At present, the shallow groundwater flow is restricted within the aquifer, with very short travel time of tens of years and vertical path length. The deep aquifer is fed by surface water or rainwater from distant locations with travel time of thousands of years and has no hydraulic connection with the arsenic-rich shallow aquifer. Numerical simulations indicate that the future pumping of deep groundwater is not likely to drive in arsenic from the shallow aquifer. Therefore, new wells may be installed in the deep aquifer. High pumping of shallow unpolluted aquifer consisting of brown sand will drive in groundwater containing organic matter from the post-Last Glacial Maximum aquifer-aquitard system. The organic matter drives reduction of manganese oxides at strip interfaces between palaeo-channel and palaeo-interfluve. After the completion of manganese reduction, FeOOH reduction may take place in the marginal palaeo-interfluvial aquifer and release sorbed arsenic. Arsenic then moves into the interior of palaeo-interfluvial aquifer polluting its fresh groundwater. Arsenic migration rates ranges between 0.21 and 6.3 and 1.3 × 10−2 and 0.4 m/year in horizontal and vertical directions, respectively. Therefore, palaeo-interfluvial aquifer will remain arsenic-free for hundreds to thousands of years to supply safe drinking water.