Dissolved organic matter sources and consequences for iron and arsenic mobilization in Bangladesh aquifers.

Iron (Fe) and dissolved organic matter (DOM) cycling have been implicated in arsenic mobilization via microbially mediated Fe oxide reduction. To evaluate the sources and multiple roles of DOM in Bangladesh aquifers, we conducted spectroscopic analyses on various types of surface water and groundwater samples from a site representative of aquifer chemistry and hydrology. Surface water contained humic substances with oxidized quinone-like moieties and high concentrations of labile microbially derived DOM. In contrast, in shallow groundwater where dissolved iron and arsenic concentrations were high, the quinone-like moieties of humic substances were more reduced, with less abundant labile DOM than that of surface water. Instead, DOM at these depths was characterized by terrestrial (plant/soil) signatures. A sediment microcosm experiment demonstrated that Fe(II) and terrestrially derived DOM were released from sediment over time. The results provide new evidence to support a dual role of natural DOM in Bangladesh aquifers (1) as a labile substrate for Fe- and humic-reducing bacteria and (2) as an electron shuttle via humic substances to enhance microbial iron reduction. Fluorescence index, amino acid-like fluorescence, and redox index may serve as useful indicators of the type of DOM likely to be involved in Fe solubilization and potentially As mobilization reactions.