LINKING BACTERIAL ACTIVITY AND DIVERSITY IN THE SHALLOW AQUIFERS OF BANGLADESH

Holocene and Pleistocene aquifers of Bangladesh were characterized to understand the potential relationship between the release of arsenic to groundwater and the presence of electron donors, electron acceptors, and bacteria. Two vertical profiles in the shallow Holocene aquifer (Sites A and F) and three samples from the deeper Pleistocene Dupa Tilla aquifer (Sites A, C, and F) were examined. In the Holocene aquifer, Site A has a shallow arsenic peak of 6.9 µM at 12.7 m depth and Site F has a deeper arsenic peak of 2.7 µM at 25.35 depth whereas arsenic in the Pleistocene aquifer (>30 m-bgs) was below 0.04 µM. Analysis of freshly-collected and preserved sediment Site by HCl extractions, XRD, SEM-EDS, and EXAFS indicated that the major Fe bearing minerals were hornblende, biotite, and chlorite. Potential electron acceptors include Fe(III) bearing clays and Fe(III) (hydr)oxides, with the proportion of Fe(III) (hydr)oxides increasing in the deeper Pleistocene aquifers. Electron donors that were quantified in the shallow and deep aquifers included acetate (non-detect to 33.2 µM), formate (4.4-12.7 µM), and H2 (0.4-19.1 nM). Geochemical calculations indicated that Fe(III) reduction and methanogenesis were exergonic when coupled with the available electron donors. Analysis of the 16s rRNA by terminal restriction fragment length polymorphism (TRFLP) showed no correlation to depth, arsenic, or iron concentrations, however, was correlated with the relative concentration of species from the family Geobacteraceae. Analysis of 16s rRNA clone libraries indicated a wide diversity of bacterial species and that 72% of the clones belonged to known divisions including Planctomycetes, Bacteroidetes, Actinobacteria, and Proteobacteria whereas 28% were unclassified and 55% of the unclassified clones were deeply branching and potentially form their own candidate division most similar to the candidate division OP3. Utilization of molecular methods has helped us to better understand the geochemical cycling occurring within the aquifer. Our results indicated that Fe(III) reduction may be an important process throughout the aquifer system and that these aquifers harbor a novel previously unclassified bacterial community.