ARSENIC AND ORGANIC CARBON IN SHALLOW ALLUVIAL AQUIFERS OF THREE DIFFERENT PHYSIOGRAPHIC SETTINGS IN BANGLADESH

Two bore holes in the Ganges flood plain, one in the Brahmaputra flood plain and one in the Meghna flood plain were drilled and installed for collecting As-rich sediments and groundwater. Groundwater samples from these three physiographic settings were collected for cation (Ca, Mg, K, Na), anion (alkalinity, Cl, NO₃, SO₄), total organic carbon, and trace element (As, Mn, Fe, Sr, Se, Ni, Co, Cu, Mo, Sb, Pb) analyses. XRD analyses were performed to characterize the major mineral contents of aquifer sediments. XRF analyses for the major chemical compositions of alluvial sediments clearly showed that fine-grained sediments contain higher amounts of trace elements because of their high surface area for adsorption. Fluorescent intensities of water samples were measured by spectrofluorometer and the results imply that groundwater in the Meghna flood plain contains more organic matter than that of other flood plains. Arsenic concentration of groundwater ranges from 10–170 mg/L (mean value: 50 mg/L), 2.66–73.7 mg/L (mean value: 16.18 mg/L), and 10–300 mg/L (mean value: 217.85 mg/L) in the Ganges flood plain, Brahmaputra flood plain, and Meghna flood plain, respectively. Arsenic level in sediments ranges from 1–17 mg/kg in these flood plains. TOC ranges from 0.49–3.69 g/kg (mean value: 1.55 g/kg) in the Ganges flood plain, 0.51–2.07 g/kg (mean value:1.04 g/kg) in Brahmaputra flood plain, and 0.26–4.22 g/kg (mean value:1.26 g/kg) in the Meghna flood plain, respectively. As is positively correlated with TOC in sediments of Bengal basin. Humic substances have been extracted from sediments in these flood plains. FTIR analysis revealed that more humic substances were contained in the Meghna flood plain sediments than in the other two flood plains. The source of organic carbon was identified by d¹³C isotope using EA-IRMS. d¹³C values strongly supports that the organic matter of the three flood plains were primarily of terrestrial origin.