Hydrochemistry in parts of the Lower Tista Floodplain in northwest Bangladesh is dominated by alkalies and weakly acid, having highest concentration of sodium cations and bicarbonate anions respectively. Groundwater is characterized by sodium-calcium and sodium-potassium cation and bicarbonate-chloride-sulphate anion facies, and genetically ‘normal chloride’, ‘normal sulphate’, and ‘normal carbonated’ type, and soft to saline. Based on electrical conductivity values, the area is divided into northern fresh and southern saline groundwater zone, and values like salinity, Na%, SAR, and RSC and a good correlation between Na+ and Cl–, and Cl– and salinity reveals increasing salinity with depth indicating a mixing of fresh and saline bodies due to marine transgression (?) during Holocene period in the Bengal delta. The rock weathering is likely to affect the groundwater quality by dolomite dissolution and calcium precipitation, representing reverse softening process and is brine seawater. Water extracted from shallow zone (from 20 to 30 m) has suitability for drinking purpose, livestock consumption and irrigation purposes, and partially suitable for industrial use, but that from deeper zone (from 30 to 50 m) is generally poor and unsuitable especially for irrigation purposes with low alkali and moderate to high salinity hazard. So groundwater can be used for irrigation in the area of fine textured soil with proper management practices.