GOUNDWATER REDOX-FACIES AND AQUIFER SEDIMENT COLOR CHARACTERISTICS IN MATLAB UPAZILA, SE BANGLADESH

In Matlab region of SE Bangladesh, shallow groundwaters from Holocene alluvial aquifers contain as high as 1150 μg/L of As. Previous studies in Matlab suggest that groundwater composition and redox conditions are strongly correlated to the sediment colors and thus this study within the ongoing SASMIT project has been aimed to further delineate depth specific low As aquifers. Both the sediment and groundwater samples were collected from the SASMIT installed piezometer nests during March, 2009 . Field parameters were measured on-site and analyses were carried out at the laboratories of KTH and Stockholm University. Colors were assigned to the sediment samples following a process developed through assigning Munsell color codes for all collected sediment samples with the help of a Munsell soil color chart and reclassifying the colours into four colours black (B), off-white (OW), red (R), and white (W) considering local driller’s visual perceptions. The study reveals that As concentration is not only correlated with depth, but also with the sediment colors and virtually all water samples derived from black sediments exceed Bangladesh Drinking Water Standards of 50 μg/L. Concentrations of different redox-sensitive parameters in groundwater samples collected from the color-classified sediments reveals that the black and red samples are characterized by the most reducing and the least reducing end member environments with the off-white and white sediment samples in between. Three aquifers (Af1, Af2, Af3) intervened by two aquitards (At2, At3) have been identified . The shallow aquifer (Af1) extend up to a depth of 40-50 m consisting of black sediments is characterized by mainly Ca-Mg-HCO₃ water-type. The intermediate aquifer (Af2) in red/off-white/white sediments below At2 is primarily Na–Ca–Cl water-type. Groundwater from black sediments reveal highly reducing environment with high aqueous concentrations of As, Fe²⁺, NH₄⁺, HCO₃^-, PO₄^3- and DOC and low SO₄^2- and Mn²⁺ indicate that, organic matter is the principle redox driver in these sedients. Groundwaters from the red/off-white/white sediments with low organic content, show an opposite trend with low As and elevated Mn contents. Spatial distribution indicates that installation of wells at a depth of about 70 m b.g.l. anywhere in the SE part of the study area..