Arsenic in Alluvial Aquifers in the Meghna Basin, Southeastern Bangladesh : Hydrogeological and Geochemical Characterisation

Abstract:  Elevated levels of arsenic (As) in Bangladesh groundwater has emerged as a massive calamity exposing a large population to the risk of As toxicity from drinking water sources and agricultural products. Holocene alluvial aquifers in the delta- and flood-plains of the Ganges-Brahmaputra- Meghna (GBM) river systems are severely affected by high levels of As in groundwater. Groundwaters abstracted from Holocene alluvial aquifers of shallow depth (<150 m) contain As at concentrations mostly above WHO provisional drinking water guideline value of 10 μg/l whereas groundwater from the Holocene deeper aquifers (usually >150 m) and the Plio- Pleistocene aquifers contain low-As (<10 μg/l) water.The study reveals that the local and regional scale variations in groundwater composition, levels of As concentrations and the redox conditions are governed by the geological attributes of the aquifers. Groundwater in the grey to dark grey argillaceous sediments where organic matter and micas are abundant contain high concentration of dissolved As. Concentrations of As is generally low in the groundwater abstracted from the light grey to yellowish brown arenaceous sediments. A major proportion of As in the dark grey sediments is bound to poorly crystalline and amorphous metal-oxyhydroxides, particularly Fe-oxyhydroxides, that are readily mobile. On the other hand, As concentrations in the light grey to yellowish brown sediments are low and predominantly bound to less mobile stable crystalline phases. Redox reactions linked to the degradation of organic matter are the potential mechanism of As mobilisation through reductive dissolution of Fe-oxyhydroxides in grey to dark grey sediments in the Holocene shallow aquifers. This is reflected in groundwater compostion that is characterised by high concentrations of As, HCO3 -, Fe and dissolved organic carbon (DOC). However, concentration of dissolved Fe is probably controlled by the precipitation of secondary Fe-minerals like siderite (FeCO3), vivianite [Fe3(PO4)2 8H2O] and pyrite (FeS2). Weathering of biotite [K (Fe, Mg)3 AlSi3O10 (F, OH)2] is one of the major sources of Fe-oxyhydroxides in the sediment and thus plays a significant role in the processes of As mobilisation in groundwater.High concentrations of As and salinity are the major constraints for groundwater development in the Holocene alluvial aquifers of the Meghna basin. The Holocene shallow aquifers (<150 m) are high in dissolved As and salinity, while the Holocene deeper aquifers (>150 m) are low in As but contains pockets of saline groundwater. Molar ratios of Cl-/HCO3 - and Na+/Cl- indicate mixing of relict seawater with the freshly recharged water in these aquifers. Groundwater abstracted from the Pliocene Dupi Tila aquifer located at relatively higher elevations along the eastern part of the Meghna basin is not affected by As and salinity. Stable hydrogen (δ2H) and oxygen (δ18O) isotopes indicate relatively fast groundwater recharge rate with insignificant evaporation effect in the Meghna basin. The groundwater samples from shallow aquifers show relatively wider variations in isotopic composition than the deeper ones indicating multiple recharge regimes. Abstraction of groundwater from the Holocene deeper low-As aquifers for drinking purposes should thus be be properly guided to minimise the risk of cross-contamination and installation of high-capacity irrigation wells in the deeper aquifers must be avoided for sustainable drinking water supplies.