OR/14/070 Description of research activities and study areas

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

Primary Research Activity

Depth-specific groundwater monitoring in the coastal region of the Bengal Basin

The establishment of nested monitoring stations at shallow, intermediate and deep groundwater depths in collaboration with the Bangladesh Water Development Board, Dhaka University and IIT Kharagpur was designed to improve understanding of the hydraulic character of the BAS and hydraulic continuity between the shallow and deeper depths of the aquifer system. Four pairs of sites were selected in a W–E transect across the delta region (Figure 2) where hydraulic and hydrochemical studies were conducted to investigate the nature and scale of impacts of deep pumping, and vertical hydraulic continuity. Each pair of sites included an inland site (50 to 100 km from the coast) with a history of deep groundwater pumping (Kamgachhi, Khulna, Barisal) and/or a history of previous research (Khulna, Kachua), and a coastal site very proximate (<5 km) to the sea or tidal channel. The 4 pairs of sites (8 study locations in total) in Figure 2 are: Kamgachi/Digha (West Bengal); Khulna/Gabura (SW Bangladesh), Barisal/Kuakata (south-central Bangladesh), and Laksmipur/Kachua (SE Bangladesh). Through sampling and the application of automated ‘divers’ (pressure-transducer dataloggers), we have developed hydrochemical profiles of groundwater in the BAS including inorganic constituents and parameters (e.g. arsenic (As), specific conductivity (SEC)), environmental isotopes (O and H stable isotopes), and residence-time indicators (CFCs, SF₆, ¹⁴C — planned) and compiled a high-resolution (hourly) hydraulic dataset. This research is strongly complemented by a more extensive programme of manual, weekly groundwater-level monitoring in nested piezometers at 42 sites situated across coastal and southern Bangladesh led by Dr Anwar Zahid at the Bangladesh Water Development Board.

Figure 2    Map showing the location of the multi-level groundwater monitoring sites in Bangladesh and West Bengal, India. [Background image shows the SRTM digital elevation model, source: USGS].

Analysis of projected changes in rainfall intensity as a result of climate change in the IGB Delta Region

Analysis of historical climate over a 5° box centered on the IGB Delta Region of Bangladesh (Figure 3) employed gridded monthly precipitation at 0.5° resolution from the GPCC product version 5 from 1955 to 2009. Climate-change projections were obtained for the late 21st century (2070–2099) from multi-model ensembles (MMEs) compiled under the third (CMIP3) and fifth (CMIP5) Coupled Model Intercomparison Projects contributing to the 4th and 5th Assessment Reports (AR4 & AR5) of the Intergovernmental Panel on Climate Change (IPCC). In total, the MMEs contain data from 23 General Circulation Models (GCMs) for the CMIP3 dataset and 21 GCMs for the CMIP5 archive, of which 8 are newly introduced Earth System Models (ESMs). Use of data from a single greenhouse-gas emission scenario (Special Report on Emissions Scenario A1B) from the CMIP3 collection, and two emission Representative Concentration Pathway scenarios from the CMIP5 collection (RCP4.5 and RCP8.5) is also made.

Figure 3    Domain comprising a 5° box over Bangladesh for which CMIP3 (AR4) and CMIP5 (AR5) climate projections were analysed.

Allied Reseach Activity

Shallow groundwater dynamics and their relationship to arsenic concentrations in Bangladesh

The impact of very intensive use of shallow groundwater on groundwater levels (storage) across Bangladesh including the delta region has been rigorously assessed using both piezometry and Gravity Recovery And Climate Experiment (GRACE) satellite data. Localised studies of arsenic (As) in Bangladesh have, to date, generated inconsistent outcomes regarding the impact of intensive groundwater use on As concentrations in shallow (≤50 m bgl) groundwater. To evaluate associations between arsenic concentrations in shallow groundwater and intensive abstraction of shallow groundwater, we constructed generalised regression models (GRMs) to describe observed spatial variations in As concentrations both (i) nationally, and (ii) regionally within Holocene deposits where As concentrations in shallow groundwater are generally high (>10 µg L^-1). We exploited hydraulic data compiled at UCL from several sources in Bangladesh and the national- scale survey of As concentrations in shallow groundwater in 1999 conducted by the BGS and Department for Public Health Engineering (DPHE) in Dhaka.

Groundwater — surface water connections from river transects of groundwater level and salinity

Salinity is a widespread problem in shallow groundwater in the coastal region of Bangladesh, but there has been little systematic study of its origin and interaction with fresh groundwater. A BWDB programme has explored connections between shallow groundwater and surface water in cross-river transects of groundwater level and salinity in the coastal region. Here, river channels have very little fresh water flow but act as conduits for tidal flow originating in the Bay of Bengal. Very many distributaries and small tidal channels interconnect the large rivers. Five boreholes to 100 m depth were installed along each river transect to monitor of groundwater level and salinity over a 12 month period, 2013–2014, in the study of groundwater — surface water connections. Data are in the process of being compiled by BWDB. As a result, we do not at this time specifically report on findings in section 5.