Abstract
In southwestern Bangladesh, the large variation in groundwater salinity has only been elucidated in small-scale study areas and along large regional-scale gradients. We aimed to assess the regional shallow (<60 m) groundwater salinity variation with a higher resolution as a function of landscape features and associated hydrological processes. Spatial variation in groundwater salinity was assessed using 442 EC measurements from previous studies and 1998 new EC measurements. Groundwater EC values were correlated with well location data (latitude, longitude and depth of the filter) and landscape feature data (elevation, soil type, land use and surface clay thickness). Additionally, we performed a geomorphological analysis of landscape features to infer associated hydrological processes. We interpret wide fluvial zones to be remnants of sandy deposits in large paleo channels which allow freshwater recharge, resulting in groundwater that is mostly (75%) fresh. Narrow fluvial zones, tidal fluvial zones, and fluvial zones next to tidal rivers are more susceptible to lateral saline water flow or saline water recharge by occasional tidal flooding, and only contain some shallow fresh groundwater in high-lying zones. Tidal flat or tidal fringe zones hardly contain any fresh groundwater. This study is the first to demonstrate the relation between landscape features, hydrological processes and regional groundwater salinity throughout southwestern Bangladesh. The main lines of our approach may be applicable in other coastal areas with available spatial landscape feature data, enabling a first prediction of groundwater salinity variation.
Highlights
Coastal regions and deltas are among the most heavily populated areas in the world and their water resources are experiencing increasing stress (Creel, 2003; Ranjan et al, 2006; Nicholls and Cazenave, 2010)
In Satkhira district, this variation is visible as alternating areas of saline and fresh groundwater; in the Khulna district, small areas with fresher groundwater were detected between more saline areas, and in the Bagerhat district, the groundwater salinity varies the most in the eastern part
The results indicate that the groundwater Electrical Conductivity (EC) varies regionally with Shuttle Radar Topography Mission data (SRTM) elevation, and between soil classes and land use classes
Summary
Coastal regions and deltas are among the most heavily populated areas in the world and their water resources are experiencing increasing stress (Creel, 2003; Ranjan et al, 2006; Nicholls and Cazenave, 2010). One of the largest and most densely populated deltas is the Ganges–Brahmaputra–Meghna (GBM) delta Manifestations of this stress are arsenic contamination of shallow groundwater resources (Nickson et al, 1998; Harvey et al, 2002), severe pollution of surface water resources (Alam et al, 2006; Bhuiyan et al, 2011), and limited availability of the meteorological water resources, due to pronounced seasonality (Chowdhury, 2010; Sharma et al, 2010). To determine occurrence of current drinking water problems and to mitigate future risk for the drinking water supply in southwestern Bangladesh, a proper understanding of the current groundwater salinity distribution and underlying controlling processes is required Such general understanding can aid in understanding groundwater salinity variation in other coastal regions around the world
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