Groundwater modeling to understand the impact of pumping in the deep Late Pleistocene aquifers of the western Bengal Basin on arsenic migration

Abstract

Numerical modeling of groundwater flow and particle tracking have been applied to assess whether deep pumping in a part of western Bengal Basin will transport arsenic (As) from the shallow paleo-channel (SPC) Holocene aquifer to the deep aquifer of Late Pleistocene age. Current pumping has shifted the groundwater recharge zones at variable depths closer to the pumping wells compared to no-pumping condition. Large irrigation and domestic withdrawals over more than 50 years from the deep aquifer (depth > 70 m bgl) have drawn down some As-polluted groundwater from SPC aquifer into the deep paleo-channel (DPC) Holocene aquifer but not into the deep aquifer. Therefore, As in Late Pleistocene groundwaters beneath DPC has originated locally and has not been transported from the overlying As-polluted SPC aquifer. To obtain As-free drinking water for a long-time, wells fitted with hand-pump can be constructed in Late Pleistocene paleo-interfluvial (PI) aquifer made of brown sands underlying a capping of the last glacial maximum paleosol and even below it in the gray sands. In the paleo-channel aquifer, wells fitted with hand-pump may be constructed at depths > 200 m to provide safe drinking water. Caution is needed regarding development of high-yielding irrigation or drinking water wells in the deep aquifer.