In some high As groundwater systems, correlations are observed between dissolved organic matter (DOM) and As concentrations, but in other systems such relationships are not observed. The role of labile DOM as the main driver of microbial reductive dissolution does not explain the variation in these relationships. Other processes that may also influence arsenic mobility include complexation of As by dissolved humic substances, and competitive sorption and electron shuttling reactions mediated by humics. To evaluate such humic interactions, we characterized the optical properties of whole waters sampled from groundwater, spanning an age gradient in Araihazar, Bangladesh. Further, we analyzed fulvic acids (FA) isolated from large volume samples for optical properties, C and N content and 13C-NMR spectroscopic distribution. Old groundwater (> 30 years old) contained primarily sediment-derived DOM and had significantly higher (p < 0.001) dissolved arsenic concentration than groundwater that was < 5 years old. Younger groundwater had DOM spectroscopic signatures similar to surface water DOM and characteristic of a sewage pollution influence. Associations between dissolved arsenic, iron, and FA concentration, and fluorescence properties of isolated FA suggest that aromatic, terrestrially-derived FAs promote arsenic-iron-FA complexation reactions that enhance arsenic mobility.
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