Hydrogeochemical Evaluation of Groundwater along An E-W Transect of the Middle Meghna Basin, Bangladesh

The Meghna basin in SE Bangladesh is characterised by low-lying (<10 m asl) flood plains and delta plains formed by the river Meghna and its tributaries. Unconsolidated Holocene alluvial sands form extensive aquifer system in the area that serves as main source of potable water for the people. Salinity in groundwater has long been a constraint for groundwater development in the area and with the occurrence of high arsenic (As) in groundwater in recent time has created a great concern for public health. Hydrogeochemical investigations along the E-W transect show that groundwater in the Holocene shallow (<150 m bgl) alluvial aquifer in western part of the area are affected by salinity (EC > 1000 µS/cm) and high As (>10 µg/L). Though the deeper part (>150 m bgl) of the Holocene aquifer is affected by salinity but low in As. On the other hand, groundwater from the older Pliocene Dupi Tila aquifer that occurs at shallow depth in the eastern part is low in EC (<1000 µS/cm) and As (<10 µg/L). Molar ratios of Cl^-/ HCO₃^- and Na⁺/Cl^- indicate ancient sea water influence rather than halite dissolution for the groundwater salinity. The Holocene shallow aquifers are high in dissolved As, HCO₃^-, Fe and dissolved organic carbon (DOC) but generally low in SO₄^2-, NO₃^-. In the contrary, the Pliocene aquifer is low in As, HCO₃^- and DOC. Molar ratio of HCO₃^- /SiO₂ suggest silicate weathering is dominant in the deeper Holocene and Pliocene aquifers. High HCO₃^-, (250.22-716.38 mg/L) in the Holocene shallow aquifers are most likely the result of the microbial degradation of organic matter in the sediment. Speciation calculations of dissolved Fe and Mn show that the groundwater is supersaturated with respect to magnetite, hematite and goethite but undersaturated to slightly oversaturated with respect to ferrihydrite siderite, pyrolusite, manganite and rhodochrosite.