CHARACTERIZING SEDIMENT GEOCHEMISTRY AND MICROBIAL INTERACTIONS IN ASSESSING THE PROCESSES OF RELEASE OF ARSENIC IN THE SEDIMENTARY AQUIFERS OF MATLAB REGION, SOUTH EASTERN BANGLADESH

The incidence of high arsenic (As) in groundwaters in Matlab region of Bangladesh has been a subject of major environmental concern. The aim of our study is to examine the role of sediment geochemistry coupled with the action of microbial communities in assessing the processes of release of As from light grey, grey and dark grey sediments within the aquifers. Grey and dark grey sediments are found at shallow depths (<100 m) at the field site whereas light grey sediments are found at depths exceeding 150m. Groundwater from light grey sediment is Na-K-Cl type and contains low arsenic (<10 µg/l) and DOC (~1 mg/l) concentrations. Groundwaters from grey and dark-grey sediments, respectively contains Mg-HCO₃^- type, 150-300 µg/l As for grey and Ca-Mg-HCO₃^- type, 250-~781 µg/l of As and 26 mg/L DOC for dark grey sediments. Dissolved ionic constituents (Mn²⁺and HCO₃^-) of porewaters from dark grey-grey sediments show significant positive correlation to dissolved As while in case of light grey sediments significant positive correlation is demonstrated with only DOC variation. Sequential extractions for sediment fractionations indicate most As being bound to amorphous and poorly crystalline hydrous oxides of Fe and Al throughout three sediment types. Bulk XANES on sediments revealed occurrences of hotspots of As distributed randomly in light grey and grey sediments with As³⁺being the dominating species through most of the depths. Core sediments are also analyzed via detailed clay separations and clay mineral types are highlighted in this study that indicates specific clays in controlling distribution of As in solid phases.

Bacterial sequences obtained from sediments were classified into 101 families, including some containing species capable of iron reduction, such as Rhodocyclaceae, Enterobacteriaceae, Aeromonadaceae and Acetobacteraceae. The abundances of these groups did not vary significantly between zones defined by sediment colors, however, overall community composition did vary between sediment zones. Taken together our results highlight linkages between sediment OC, Mn, Fe, grain size fractionations that may control the release of aquifer As, but also indicates As mobilization from sediments to groundwaters are dictated by the composition/makeup of the bacterial community within those depths of sediments.