COMPARISON OF MICROBIOLOGY AND GEOCHEMISTRY FROM GRAY AND REDDISH BROWN HORIZONS OF GANGES DELTA AQUIFER, MATLAB, BANGLADESH

Arsenic is one of the greatest problems towards ensuring the safe water accessibility for local rural population in the most Bangladesh. This study focuses on the sediment geochemistry and adsorption behavior of oxidized red brown and reduced grey sediments and their respective capacities to attenuate As. Sediment cores were collected from regular intervals within a total depth of ~125m in Matlab. In this site total As in the groundwaters range in 10-600 µg/L. DOC of the groundwaters shows good correlation with As; higher DOC indicates higher As concentration in these aquifers. ORP measurements in the shallow aquifer (0-60m) show reducing conditions but with depth they turn more oxidizing. DNA recovered from Matlab core samples averaged 450ng/g from course-grained samples and 800ng/g from fine-grained samples. After sequencing the DNA, it was found that microbial community in red or light grey sediments are different than the grey-dark grey colored sediments (e.g. Burkholderiaceae and Pseudomonadacee are common in grey/ dark grey sediment but Streptomycetaceae is common in red/ light grey color ). Rhodocyclaceae, Aeromonadaceae, Shewanellaceae are the common genera present in these sediments that are capable for reducing iron in this aquifer environment. Synchrotron aided μXANES and μXRD studies conducted for solid state As and S speciation in the core samples at different depths indicate the occurrences of hotspots of As differently distributed in red-brown and grey sediments. Samples collected from North Matlab show both well-defined S^2- (2472eV) and SO₄^2-(2482eV) white lines. The sediment S^2- show correlation with the pore water SO₄^2-concentrations collected from those cores. On the other hand, samples collected from South Matlab show sharper peaks for SO₄^2- rather than S^2- . Core sediments from both North and South Matlab show a mineral assemblage as quartz, feldspars and clay minerals with various forms of mica, kaolinite and chlorite. Secondary Fe mineral assemblages are also dominant in the high As areas where the groundwater Fe is high and mostly in reduced forms. This study has broader implications towards regional scale approach for As mitigation that incorporates the enquiry of efficiency of sediment color as a simple and easy tool for identifying safe aquifers in major As prone areas like Bangladesh.