REDOX CHARACTERISTICS OF SEDIMENTARY AQUIFERS AS A TOOL FOR TARGETTING ARSENIC-SAFE GROUNDWATER IN BANGLADESH

Natural arsenic (As) is encountered in groundwaters from the shallow aquifers of Holocene age in Bengal Delta Plain (BDP) of Bangladesh above the safe drinking water standard of WHO (10 µg/L). Groundwaters with elevated As levels are abstracted. In our ongoing studies in Matlab Upazila of SE Bangladesh, it is observed that local drillers prefer to install tube wells based on the characteristics of the aquifer sediments. The present paper attempts to link the groundwater composition to the redox characteristics of the sediments that could be used as a tool to identify and target the relatively oxidized aquifers for the installation of As-safe tube wells. The Holocene aquifers in Matlab Upazila is characterized by a sequence of sediments with considerable heterogeneity in terms of grain size, color and mineralogy. In general, a thick layer of grey (herein after referred to as black) sediments with thickness varying between 40-50 m, overlies a sequence of sediments characterized by white, off-white and red colors. The groundwater is near-neutral (pH=6-7) and predominantly of Ca-Mg-HCO3 or Na-Cl-HCO3 type. The groundwater from the was generally reducing with low concentrations of SO42- and NO3- but with high concentrations of Fetot and Mntot. The concentration of Astot range between 0-355 µg/L. The groundwater samples were classified in four groups with respect to the color of the sediments from where they were abstracted (black, white, off-white and red). Four different groups of sediments were characterized by a distinct scale of groundwater composition governed by the redox characteristics. Groundwater extracted from black sediments was most reduced, followed by white, off-white and red where the groundwater was less reduced. The reddish/yellowish color of the sediments and low concentrations of dissolved Fe in groundwater at these depths suggest a relatively oxidised condition. In these sediments, most of the Fe is likely to be present as coatings on the framework grains as oxyhydroxides, which impart reddish/yellowish colour to these sediments, thereby have the ability to adsorb As. We believe that it is possible to target safe aquifers by combining the indigenous knowledge and techniques of the local drillers along with modern geological and hydrogeochemical expertise.