EVOLUTION OF COASTAL ISLANDS AT THE BENGAL BASIN: GEOCHEMISTRY, SEDIMENT COMPOSITION, SUBSIDENCE, AND SEA LEVEL CHANGES AT SANDWIP ISLAND, CHITTAGONG, BANGLADESH

The Bengal basin serves as an excellent repository of the Himalayan denudation throughout most of the Cenozoic. The large sediment accumulation has played a significant role in developing the Ganges-Brahmaputra-Meghna delta and associated coastal aquifer systems, many of which have been found to be contaminated with increased contents of arsenic and salinity. The Meghna estuary is the largest along the coast where the combined flow of several large river systems in Bangladesh is discharged into the Bay of Bengal. Among several coastal islands, Sandwip is a major one at the Meghna estuary which is morphologically active and shows accretion of ~2.0% during the last couple of decades. In this study, we evaluated regional tectonics, subsidence dynamics, sea-level changes, hydrogeology, and sediment composition to reveal the evolutionary history of the island.

Arsenic concentrations in Sandwip Island are relatively low (0.33 µg/l to 220 µg/l during dry season, and 0.64 µg/l to 6.88 µg/l during the wet season) compared to the neighboring islands such as Hatiya. Groundwater analysis from shallow (0-50m), intermediate (50-150m) and deep (>150m) aquifers of Sandwip Island reveal a salinity range from 11.95 mg/l to 5945 mg/l during the dry season, and from 398.2 mg/l to 2753.3 mg/l in the wet season. The deeper aquifers have less salinity than the shallower ones. The aquifer sediments comprise 91.73% fine-to-medium sands with the remaining 8.27% as silt and clay. Heavy mineral concentrations are 4.92%, 3.78%, and 2.25% at shallow, intermediate and deep aquifers, respectively. Varietal studies of the heavy mineral assemblages revealed the dominant presence of moderately magnetic minerals (i.e., biotite, garnet) compared to other varieties present at shallow, intermediate and deep aquifers. A low zircon-tourmaline-rutile percentage indicates compositional immaturity of heavy mineral assemblages, reflecting modest weathering from dominantly low-to-intermediate grade rocks at the source areas.

Sandwip Island hosts significant detrital sediments accumulated during the Holocene due to its proximity to the Meghna River. The bottom unit of the deeper aquifer is found at ~292m depth which implies significant subsidence of the Sandwip Island since accumulating meteoric water at the deeper aquifers.