GEOCHEMISTRY OF ARSENIC AND ITS DISTRIBUTION IN SEDIMENT AND GROUNDWATER OF SOUTH-WESTERN BANGLADESHAQUIFER

Sediments and groundwaters (GW) collected from Samta, Jessore, South-Western Bangladesh were studied with the aim of explaining the their geochemistry to elucidate the distribution of Arsenic. Results showed that trapped As on sediment matrix may act as a function of the sediment diameter, its surface area and relative radio-carbon age. Presence of As was also observed to be greatly dependent on the availability of its carrier minerals particularly Fe and Al along the aquifer depth. These observations were corroborated with laboratory leaching and adsorption tests. The partition coefficient, K_d was found to vary ranging between 1 and 40 L/kg. The relatively low K_d computed for the upper shallow aquifer may essentially reflect the occurrence of leaching process that may be followed by the reductive dissolution of the As-source minerals. Spectroscopic study done with XRD, XRF and SEM suggest that Fe and Al minerals might be the dominant secondary sources for releasing As. C-14 analysis of the 50 m grey sand sediment showed that the sediment should have deposited 7400 years ago, where as the upper shallower sediment (depth 10 m, brown clay) showed that relatively younger aged such as 1000 yrs particle could have formed the upper layer that accounted for much of Arsenic. 7 days leaching as well as adsorption tests were also conducted to get a clear view of As releasing under various leaching reagents and varying pH. It was found that PO₄among of all the considered anions could have played a significant role in releasing As with a moderate pH (7.5 to 8). Again, As⁵⁺ was observed as to be adsorbed not only in a quicker time but also in a greater quantity in compared to As³⁺. Kd of As was found to be dependent on the pH, PO₄, Fe, Al and organic carbon contents of the respective sediments. Effects of PO4 was much significant that those of SO₄ and HCO₃. In groundwater, strong correlations of As observed with Fe, Al, DOC, TOC, HCO₃ suggest that the dissolution of Fe-Al minerals. In the upper layer (brown clay) due to the smaller diameter of the particle sizes and the higher content of the Fe, Mn and Al minerals, much of the Himalayan bound As may get trapped in it, where as due to the higher particle diameter and relatively lower Fe, Mn and Al content low As might have accumulated in that sediment.