MINERALOGICAL AND GEOCHEMICAL STUDY OF AQUIFER SEDIMENTS OF THE BENGAL BASIN, BANGLADESH

Arsenic contamination in groundwater has been a grave concern over decades in many of deltaic plains around the world. The Bengal foreland basin formed due to collision of India and Asia, and drained by the Ganges, Brahmaputra, and Meghna mega river systems is also no exception. The situation is even worst in Bangladesh because of its large population density. Preliminary studies on recently drilled aquifer sediments and pore-fluid samples from eastern and west-north-western Bangladesh provide important information on distribution of arsenic in Holocene strata.

Sediments from Arsenic contaminated aquifers have higher concentration of heavy minerals relative to arsenic safe areas. Weight percentages of heavy minerals in these Holocene aquifers range from ~3% (Natore, NW Bangladesh) to ~12% (Kustia, western Bangladesh). This suggests that Holocene sediments tend to contain more heavy minerals than that of the Neogene or deeper core sediments. Arsenic polluted aquifers are found to have been composed dominantly of magnetic minerals, chlorite, biotite, apatite, kyanite, and amphibole whereas apart from some stated minerals arsenic safe aquifers are composed of staurolite, garnet, tourmaline, zircon, rutile, and Fe-bearing minerals, such as goethite. Heavy mineral concentrations are higher in drilled wells near the Ganges River compared to Brahmaputra and Meghna rivers.

Preliminary Inductively Coupled Plasma Optical Emission Spectrometry (ICP OES) data of some pore fluid samples from lower portion of the zone of aeration and upper aquifer reveal that the pore-fluids from sediments of vadose zones generally have lower concentrations of Arsenic compared to those from the aquifer sands. This observation is however not true for pore fluids from vadose zone samples in Kustia, western part of the Ganges River. Ongoing research will provide more information on mineralogical and geochemical nature of both Arsenic-contaminated and Arsenic-safe aquifer sediments and pore-water of the Bengal Basin.