Variations in hydrostratigraphy and groundwater quality between major geomorphic units of the Western Ganges Delta plain, SW Bangladesh

Relationships among geomorphology, hydrostratigraphy, and groundwater quality with special emphasis on arsenic and salinity have been analyzed in the Bangladesh part of the Western Ganges Delta (WGD). On the basis of the presence of characteristic geomorphic features, the study area is divided into two geomorphic units: fluvial deltaic plain (FDP) and fluvio-tidal deltaic plain (FTDP). Lithostratigraphic sections demonstrate that FDP is composed predominately of sandy material whereas FTDP is characterized by alternation of sand and clay/silty clay material. Hydrostratigraphically, FDP is characterized as a single aquifer system, whereas FTDP is a complex multi-aquifer system. Spatial distributions of arsenic concentrations in groundwater reveal that elevated arsenic (>0.01 mg/l) occurs mostly in the FDP. Occurrences of high arsenic in deeper part of the aquifer system (>100 m) in the FDP, particularly in the south-western part, is probably due to the absence of any prominent impermeable layer between the shallow and deeper part of the aquifer system. Distributions of chloride concentrations show an increasing trend in groundwater salinity from north to south, i.e., from FDP to FTDP.