Salinisation in the Bengal delta, Bangladesh
: implications for Asian delta systems

  • Sean Edward Feist

Student thesis: Doctoral Thesis

Abstract

Salinisation of water and soil resources is a global problem, with large, low-lying deltas in Asia facing significant risk associated with climate change and growing demand due to rising populations. These regions are located at the nexus of oceanic, hydrological, terrestrial (land surface), geological, atmospheric and anthropogenic domains, with the spatiotemporal distribution of salinity controlled by the complex inter-play between these systems. Increasing modification, through climate change, human activity and management practices, has led to a shift in the natural dynamics of these delta environments, resulting in changes to the saline – freshwater balance within soil, groundwater and river systems. This places the >100 million inhabitants of the Bengal delta in South Asia at increased risk from salinisation of soil and water resources, which threaten agricultural productivity and human health through the consumption of drinking water with elevated sodium content.
This study investigates the spatio-temporal distribution of salinity in Bangladesh through the application of multiple research approaches and robust non-parametric statistical methods. These were applied to extensive data-sets of tidal water level and salinity observations, which are missing for most deltas, in order to identify regional scale variation and trends. With shallow subsurface monitoring of soil and groundwater salinity, and an intensive field investigation of groundwater chemistry and its association with geological and contemporary sources of salinity in southwest Bangladesh carried out to characterise salinisation at a local scale.
Results from this study enable the identification of sub-regional patterns in delta tidal dynamics and coastal SLR along the northern Bay of Bengal. With an average rise in coastal extreme sea levels of 6.9 ± 2.1 mm yr-1 driving the 5.1 ± 4.2 mm yr-1 increase in tidal water levels within the delta. Distinct variation between the western and eastern regions is identified. Increases of 18.3 ± 4.1 mm yr-1 and 18.2 ± 6.8 mm yr-1 at high and low-tide respectively in tidally dominated rivers of southwest Bangladesh, contrast with the fluvially active central and eastern sections of the delta, where falling trends of -6.5 ± 2.3 mm yr-1 (high-tide) and -10.2 ± 3.1 mm yr-1 (low-tide) are recorded. Rapid and short term changes are observed at the site-specific level, with an acceleration of change in high-tides indicated by a rise from the long-term rate of 4.4 ± 1.5 mm yr-1 between 1950 and 2018, to 5.9 ± 3.2 mm yr-1 between 1990 and 2017, leading to increased risk for the local population.
The spatial variation of changes in tidal water levels corresponds to the intra-regional distribution of river salinity within the delta, with an east to west increasing trend in mean salinity from 21 ± 15 µS/cm year-1 to 94 ± 25 µS/cm year-1 . Monthly salinity maxima are observed to increase at faster rates than the monthly mean series, with the delta-wide increase of 100 ± 29 µS/cm year-1 corresponding to a ca. 0.5 ppt rise in salinity within a decade. The rapid shift in salinity observed in the delta between 2006 and 2007 may be linked to a period of significant cyclone activity, with a greater impact on the western delta system. The effect of sea-level rise on river water salinity is intensified by reduced freshwater discharge in the delta's west, with channel embankments increasing the inland flow of saline waters. At both local and regional scales, the inland impact of SLR within the delta is observed to be strongly modulated, leading to the observed spatial distribution of salinity within the tidal channel network.
The study also identifies significant variability within groundwater salinity in polder 31, southwest Bangladesh, with sodium concentrations between 287 to 1,766 mg/L, frequently at levels potentially harmful to human health. The concentration of salinity within the shallow aquifer is associated with the percentage of sand recorded within overlying sediments and the thickness of the silt/clay semi-confining unit. Contemporary salinisation is identified adjacent to the primary tidal channel in the east of the study area and within shallow horizons (< 30m bgl) in close proximity to artificial shrimp ponds. Results from a novel monitoring approach in shallow groundwater and soil horizons also identifies the flushing of salts into groundwater within the upper part of the saturated zone during the monsoon season.
The variation in trends identified in tidal water levels and salinity, and the association with variations in the hydro-morphology of the delta help improve the understanding of the processes controlling salinisation at both the regional and local scale within large deltas. The identification of driving mechanisms in the context of the polder system will also aid the development of appropriate mitigation strategies. Integrated management policies that can address the cascading hazards present within low-lying deltas are crucial to the regions inhabitants, where a growing population increases demand on finite water resources, and climate change threatens rising sea-levels, changing patterns of precipitation and the potential for increased cyclone activity.
Date of AwardSep 2021
Original languageEnglish
SupervisorMo Hoque (Supervisor), Mike Fowler (Supervisor) & Md. Kawser Ahmed (Supervisor)

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