HEAVY MINERAL DISTRIBUTION IN ARSENIC-CONTAMINATED COASTAL SEDIMENTS IN THE SOUTHEASTERN PART OF BANGLADESH

Concentrations of heavy minerals in coastal sediments provide valuable information into the hydrodynamic conditions in the nearshore water, the direction of sediment transport, and foreshore sedimentary processes, including sediment provenance. Insights into the spatial and temporal distribution of heavy minerals are important to understand hydrometeorological events (storms and strong winds), as well as a range of sedimentary conditions and morphodynamic processes. This study analyzes sediment samples collected from a depth of 5 meters from four locations along the coastal region of Hatiya Island to determine potential drainage pathways and measure the effects of chemical weathering using heavy mineral microscopy.

Mineral association in sediments offers a tool for analyzing chemical changes in groundwater composition in the aquifer through their diagenetic alteration due to water-rock interaction. The association between elevated Arsenic in sediments, groundwater, and heavy minerals assemblages in the shallow aquifer of the coastal region has been correlated. The lowest amount of heavy minerals (0.07%) has been found in the Sample 1 region which is in the northern part of Hatiya Island whereas the highest amount of heavy minerals (0.98%) has been found in the region. Sample 4 which is in the southern part. Sediments are represented by a high concentration of garnet, chlorite, amphibole, epidote, kyanite, and muscovite. Garnet, kyanite and epidote are indicative of derivation of the detritus from metamorphic rocks. Stable heavy minerals (e.g. zircon, tourmaline, rutile - ZTR) have been detected with tourmaline being comparatively higher in concentration. ZTR level is higher in the southern part compared to the northern part which indicates that mature sediments traveled further south. Some less commonly found minerals like apatite, monazite, pyrite have been identified in the sample. Detection of pyrite indicates the presence of Arsenic where precipitation of Arsenian pyrite is thought to sequester As from groundwater. The abundance of Fe-bearing minerals (e.g. magnetite, ilmenite, goethite, biotite, hematite) observed in the samples can be considered to play a crucial role as a source of Fe (III)-oxyhydroxide which acts as a sink for As causing the high concentration of Arsenic in the Hatiya Island.