TRACE ELEMENT REACTIONS IN GROUNDWATER FLOW SYSTEM IN COASTAL AQUIFERS OF THE GANGES DELTA

The Ganges, Brahmaputra, and Meghna rivers formed one of the largest delta systems in the world called the Ganges delta. We combine groundwater geochemistry, numerical modeling techniques to study the reactive transport of metals and isotopes in the coastal aquifer of the Ganges delta.

Geochemical data show that the elevated concentrations of Fe, Mn, Sr and As can be correlated with high pH and HCO₃. These geochemical correlations suggest that elevated metal concentrations may be derived from bacterial iron and manganese reduction. Higher d¹³C values are found to be correlated with elevated As, Fe and Mn concentrations and high pH values of groundwater. This result implies carbon isotopic fractionation processes associated with bacterial Fe (III) reduction and Mn(IV) reduction. Petrographic, SEM studies of sediments show the formation of biogenic siderite and pyrite in sediments associated with Fe- rich groundwater. Filamentous and circular structures within layer suggest microbial involvement in lamina development through providing a growth medium for sulfate-reducing bacteria and potentially a template for mineral growth. Siderite and pyrite occur together as spheroids in which pyrite forms the center and siderite forms an outer rim, which is consistent with redox sequence of mineral precipitation predicted by geochemical modeling. Geochemical modeling of bacterial Fe (III) and Mn (IV) reduction shows that chemical precipitation of iron and manganese minerals could control mobility and concentrations of Fe and Mn, As in coastal aquifers. The continuous bands of siderite and pyrite observed in this study suggest that both Fe and SO₄ reductions in groundwater must have fluctuated over time. Variations of Fe and SO₄ reductions can be explained by Eh conditions where Fe-Mn oxyhydroxide dissolution and SO₄ reduction occurs.