Palaeosol Control of Arsenic Pollution: The Bengal Basin in West Bengal, India

Abstract

Groundwater in the Bengal Basin is badly polluted by arsenic (As) which adversely affects human health. To provide low-As groundwater for As mitigation, it was sought across 235 km 2 of central West Bengal, in the western part of the basin. By drilling 76 boreholes and chemical analysis of 535 water wells, groundwater with <10 μg/L As in shallow aquifers was found under one-third of a study area. The groundwater is in late Pleistocene palaeo-interfluvial aquifers of weathered brown sand that are capped by a palaeosol of red clay. The aquifers form two N-S trending lineaments that are bounded on the east by an As-polluted deep palaeo-channel aquifer and separated by a shallower palaeo-channel aquifer. The depth to the top of the palaeo-interfluvial aquifers is mostly between 35 and 38 m below ground level (mbgl). The palaeo-interfluvial aquifers are overlain by shallow palaeo-channel aquifers of gray sand in which groundwater is usually As-polluted. The palaeosol now protects the palaeo-interfluvial aquifers from downward migration of As-polluted groundwater in overlying shallow palaeo-channel aquifers.The depth to the palaeo-interfluvial aquifers of 35 to 38 mbgl makes the cost of their exploitation affordable to most of the rural poor of West Bengal, who can install a well cheaply to depths up to 60 mbgl. The protection against pollution afforded by the palaeosol means that the palaeo-interfluvial aquifers will provide a long-term source of low-As groundwater to mitigate As pollution of groundwater in the shallower, heavily used, palaeo-channel aquifers. This option for mitigation is cheap to employ and instantly available.

Integration of aquifer geology, groundwater flow and arsenic distribution in deltaic aquifers - A unifying concept

“…The palaeo‐interfluvial brown sediments have adsorptive capacity for As, and a clay unit of low permeability protects them from the invasion of As‐rich water from above (McArthur et al, ). The palaeosol model has been validated by field observations (Ghosal, Sikdar, & McArthur, ; Hoque, McArthur, & Sikdar, ; Hoque et al ., ; McArthur, Nath, Banerjee, Purohit, & Grassineau, ); however, it does not explain the heterogeneities of As concentration within the palaeo‐channel itself nor the effective absence of As at depth in the aquifer (approximately >150 m) irrespective of palaeo‐channel/palaeo‐interfluvial distinctions. These protective palaeosols are laterally discontinuous and the thickness of brown sands is limited (Hoque et al .…”

“…The most recent palaeosol appears to be locally punctured by palaeo‐channel sand deposits (Ghosal et al . , ; Hoque et al . , , ; McArthur, Nath, et al .…”

Variations in hydrostratigraphy and groundwater quality between major geomorphic units of the Western Ganges Delta plain, SW Bangladesh

“…The northern part shows single aquifer system and aquifer complexity increases towards the south where it ends up as a multi-layer aquifer system. Several studies in Bangladesh (Ravenscroft 2003;Ahmed et al 2004;Shamsudduha and Uddin 2007;Shamsudduha et al 2011;Ayers et al 2016) and West Bengal (Acharyya et al 2000;Bhattacharya et al 1997;Goodbred et al 2003;Shamsudduha and Uddin 2007;Biswas et al 2014;Ghosal et al 2015) have stated the existence of multi-aquifer system in the Ganges Delta plain. Generalized classification schemes define two aquifer units: shallow aquifers (occurring within a depth of 150 m) and deep aquifers (occurring at depths below 150 m) in Bangladesh (Ahmed et al 2004).…”

“…The strategic importance of groundwater in the Bengal Basin has prompted several hydrogeolgical (e.g., Burgess et al 2002;Cuthbert et al 2002;JICA 2002;van Wonderen 2003;Nishat et al 2003;Harvey et al 2006) and hydrostratigraphic analyses (e.g., Ravenscroft et al 2005;Mukherjee et al 2007;McArthur et al 2008;Michael and Voss 2008;Burgess et al 2010;Biswas et al 2014;Ghosal et al 2015). However, comparative studies to delineate hydrostratigraphic and groundwater quality variations in different geomorphic units in the Ganges Delta are still lacking.…”

Recent Contributions in the Field of Sediment Geochemistry

“…Paleosol control of arsenic pollution in the deltaic sediments of the West Bengal was studied by McArthur et al (2011) and Ghosal et al(2015) involving detailed geochemical analyses of sedimentary cores covering a large terrane in arsenic affected area of 24 Parganas in West Bengal. McArthur et al (2012) studied Mn and other trace elements in the groundwater of parts of deltaic Bengal Basin to unravel the sedimentological control of their distribution.…”