Groundwater modeling to understand the impact of pumping in the deep Late Pleistocene aquifers of the western Bengal Bas
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ORIGINAL PAPER
Groundwater modeling to understand the impact of pumping in the deep Late Pleistocene aquifers of the western Bengal Basin on arsenic migration Pradip Kumar Sikdar 1 & Utsab Ghosal 1 & Surajit Chakraborty 1 Received: 15 June 2018 / Accepted: 23 November 2018 # Saudi Society for Geosciences 2018
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 longtime, 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. Keywords Bengal Basin . Jamuna sub-basin . Groundwater modeling . Pumping impacts . Arsenic migration
Introduction The World Health Organization’s and Indian acceptable value for arsenic (As) in drinking water is 10 μg/L (WHO 2011; BIS 2012). In Bengal Basin, As concentration in groundwater > 10 μg/L is mainly found in the shallow Holocene aquifer made of gray sands. The thickness of the Holocene aquifer varies between 35 (known as shallow paleo-channel (SPC) aquifer) and 70-m (known as deep paleo-channel (DPC) aquifer (Hoque et al. 2014; McArthur et al. 2008, 2011; Ghosal et al. 2015). The gray sands of the SPC aquifer were deposited above a Late Pleistocene surface known as last glacial maximum paleosol (LGMP), whereas those of the DPC aquifer were deposited in the river channels present during the Pleistocene time. Below the LGMP surface, Late Pleistocene
* Pradip Kumar Sikdar [email protected] 1
Department of Environment Management, Indian Institute of Social Welfare and Business Management, Kolkata 700073, India
brown sands of about 30–35-m thick occur followed at depth by gray sands, which is > 300-m thick. The groundwater in these Late Pleistocene aquifers below the LGMP contains < 10 μg/L As (Hoque et al. 2014; McArthur et al. 2008; Ghosal et al. 2015). As a result, Government of West Bengal resorted to installation of wells for irrigation and community purposes 152-m deep to obtain As-free
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