OR/14/070 Motivation and research questions

Taylor, R G1, Burgess, W G1, Shamsudduha, M1, Zahid, A2, Lapworth, D J3, Ahmed, K4, Mukheriee, A5 and Nowreen, S6. 2014. Deep groundwater in the Bengal Mega-Delta: new evidence of aquifer hydraulics and the influence of intensive abstraction. British Geological Survey Internal Report, OR/14/070. 1  University College London (UCL), UK;     2  Bangladesh Water Development Board, Bangladesh;     3  British Geological Survey, UK;     4  Dhaka University, Bangladesh;     5  Indian Institute of Technology (Kharagpur), India;     6  Bangladesh University of Engineering & Technology, Dhaka, Bangladesh

The Ganges-Brahmaputra-Meghna (GBM) delta region of the Indo-Gangetic Basin is dependent upon groundwater to satisfy enormous demand for domestic and agricultural water supplies. The twin contexts of this study are: the strategic importance of deep groundwater in the Bengal Aquifer System (BAS) as a secure water resource for tens of millions of inhabitants of Bangladesh and West Bengal; and the uncertain response of the hydrological system of the GBM delta to changes in climate and large-scale anthropological interventions. In the GBM delta area, ‘deep groundwater’ (>150 m depth in the sediments of the BAS) is increasingly being developed by public authorities and private abstractors as a water source free of excessive arsenic which pervades shallow basin sediments (Figure 1). Installation of deep wells for domestic water supply has become a popular, practical and economic mitigation response to the arsenic crisis. Regionally, deep groundwater has acquired strategic importance as a secure water resource for tens of millions of inhabitants of Bangladesh and West Bengal, India. In the future there may also be pressure on deep groundwater from the enormous irrigation demand across the rural floodplain areas that is currently met by groundwater from shallow and intermediate depths (<100 m bgl). There remains, however, considerable uncertainty in the security of the BAS deep groundwater resource including: (1) the ability of recharge to balance heavy abstraction of deep groundwater and prevent excessive groundwater-level decline; (2) the vulnerability of deep groundwater throughout the floodplains to the ingress of arsenic drawn down from shallow levels and, in coastal regions, to the ingress of salinity; and (3) the impacts of climate change comprising not only sea-level rise (SLR) but also the potential intensification of rainfall.

In terms of the vulnerability of deep groundwater, the amount and timing of vertical leakage induced by deep pumping is determined by the fundamental hydraulic structure and properties of the aquifer sediments. Little is known, however, of the hydraulic character of the BAS at the depths where abstraction is rapidly accelerating, nor of the hydraulic continuity between the shallow and deeper depths of the aquifer system. Investigative modelling studies, though valuable, are constrained by a lack of data at depth in the aquifer system to enable robust model calibrations. Age-dating of deep groundwater has, to date, been piecemeal and unrelated to the municipal foci of deep pumping. The ‘Deep groundwater in the Bengal Mega-Delta: new evidence of aquifer hydraulics and the influence of intensive abstraction’ case study addresses the need to develop a greater understanding of the response of deep groundwater in the GBM delta region to increasing development and to climate change, and its future security. Our results are expected to contribute directly to development of policy for deep groundwater development by Governmental authorities. In this report, we differentiate between primary research activity funded directly through the Groundwater resilience to climate change and abstraction in the Indo-Gangetic Basin project and allied research activity, funded separately (e.g. EPSRC), that further informs our results.

Figure 1    Map showing the spatial distribution of groundwater arsenic in Bangladesh [data from the National Hydrochemical Survey in Bangladesh].