Influence of landscape features on the large variation of shallow groundwater salinity in southwestern Bangladesh

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.