Arsenic and organic carbon in shallow alluvial aquifers of two different physiographic settings in Bangladesh

Five bore holes in the Ganges flood plain and one bore hole in the Meghna flood plain were drilled and installed for collecting As-rich sediments and groundwater. Groundwater samples from two physiographic settings were collected for cations (Ca, Mg, K, Na, NH4), anions (HCO3, Cl, NO3, SO4), total organic carbon, and trace element (As, Mn, Fe, Sr, Se, Ni, Co, Cu, Mo, Sb, Pb) analyses. XRD were performed to characterize the major mineral contents of aquifer sediments. XRF were performed to analyze the major chemical composition of alluvial sediments. Results of XRF analysis clearly show that fine-grained sediments contain higher amount of trace element because of their high surface area for adsorption. Fluorescent intensities of water samples were measured by spectrofluoremeter and the results imply that groundwater in the Meghna flood plain contain more organic matter than that of the Ganges flood plain. Arsenic concentration of water ranges from 2.76-462 ppb and 10-300 ppb in the Ganges flood plain and Meghna flood plain, respectively. Arsenic level in sediments ranges from 1-11.3 mg/kg in these flood plains. TOC ranges from 0.1% to 0.3% in the Ganges flood plain and in the Meghna flood plain TOC ranges from 0.2% to 0.5%. As is positively correlated with TOC. Humic substances have been extracted from sediments in flood plains. Sediments in the Meghna flood plain contain more humic substances than that of the Ganges flood plain. The functional groups of humic substances in these plains were identified using FTIR analysis. The source of organic carbon was identified by δ13C isotope using EA-IRMS. δ13C values strongly support that the organic matter of the Meghna flood plain was primarily of terrestrial origin. In contrast, organic matter of Ganges flood plain was likely originated from both marine as well as terrestrial environment.