FUNCTIONS OF DISSOLVED ORGANIC MATTER IN VARIOUS GEOCHEMICAL SYSTEMS

Dissolved organic matter (DOM) which is a fraction of natural organic matter (NOM) smaller than 0.45-µm filter, and contains humic and fulvic acids, fatty acids, carbohydrates, amino acids, hydrocarbons, and humic-metal complexes. Complex nature of DOM because of its macromolecular structure, and presence of diverse functional groups such as carboxylic, phenolic, quinone, aromatic, and amine results in increased chemical reactivity and interactions with other geochemical and microbial processes in the surroundings. Role of DOM in geochemical systems has been studied only recently and it has been found that these relatively less labile DOM plays an active role in mobilization of nutrients and contaminants in the environment. One such example is involvement of humic-like DOM in mobilization of iron and arsenic in alluvial reducing aquifers of Bengal basin via microbially mediated processes. We studied the spatial distribution of DOM across the Bengal basin aquifers from fourteen study sites (eleven in India and three in Bangladesh). The dissolved As concentrations ranged between <1 to 643 µgL^-1 across these sites and correlated strongly with dissolved organic carbon (DOC), a fraction of DOM, concentrations (R² = 0.94) that ranged between 0.65 to 9.6 mgL^-1. The groundwater with high As was found to contain highly humic-like and terrestrially derived DOM based on absorbance-fluorescence spectroscopy and parallel factor (PARAFAC) model. As opposed to completely reducing shallow aquifer, in riverbank aquifers where groundwater-river water mixing occurs seasonally (e.g., Meghna riverbank in Bangladesh), we found that microbially-derived, fresh and labile DOM characterized by much lower SUVA254 (1.7 L.mg/m), humification index (3.8) and lower humic:protein ratio of 3.3, compared to 2.4 L.mg/m, 10, and 8.2 respectively in adjacent shallow aquifer. In a contrasting system in volcanic province, where volcanic (lava tube) caves host diverse microbial life in presence of meteoric water that percolates through surface sediments and fractured basalt, where lower amounts of DOM would be expected, we surprisingly find up to 12±8 mg/L of DOC in cave waters, which had strong humic-like signatures. These examples highlight the distribution, diversity, and roles of DOM in distinct geochemical systems.