OCCURRENCE AND FATE OF ARSENIC IN AN ALLUVIAL SANDY AQUIFER; KALAROA, BANGLADESH

Arsenic affected subsurface sediments collected from Kalaroa aquifer, were examined with different chemical analyses such as acid digestion, sequential extraction and anaerobic leaching in order to get an insight of the As distribution as well as its transport in the targeted sediment matrix. As contaminated groundwaters together with the sediments were also subject to batch as well as column adsorption test to determine the potentiality of the deeper (<150 m) sediments in immobilizing the elevated As concentration in groundwater. Chemical analysis revealed that due to the reductive dissolution of the As associated Fe oxihydroxides existing in the upper shallower brown clay layer coupled with its underlying reduced grey fine sands, As could have significantly released with time. A considerable amount of As (i.e. 5 μg/g) that was observed as to be remained in the labile form during the oxalic acid extraction step may also reinforce the release mechanism. Illustrating that as much as 26.2 and 18.6 μg/g of As could likely be leached out from brown clay and fine grey sand respectively during the anaerobic leaching test where bicarbonate was used as the leaching agent may further strengthen the As desorption from the sediment matrix. Besides, an evidence like availability of Fe and Al minerals on the sediment grains were also explicitly identified by the spectroscopic study like XRD, XRF, SEM and TEM. Additionally, computed low partition coefficient, Kd (i.e. 10 L/kg) for the upper reduced sandy layer resembles the high As leaching potentiality whereas high Kd (i.e. 190 L/kg) obtained for the deeper oxidized layer reflects the possibility of As being adsorbed onto the mineral surfaces. The sorption potentiality of the partly oxidized sand (fine to medium) containing relatively greater amount of Fe and Al oxyhydroxides of the deeper aquifer was found to have much better adsorption capability (92%) than that of the medium grains having relatively lower of those elements. Therefore, leaching of As triggered by the reductive dissolution in the upper reduced sand and its subsequently being adsorbed to some extent onto the Fe and Al oxide mineral surfaces at the deeper aquifer sediments are the key As transport mechanisms in that studied aquifer.