OVERVIEW OF SEDIMENTARY AND DISSOLVED ORGANIC MATTER DISTRIBUTION WITHIN ARSENIC CONTAMINATED ALLUVIAL AQUIFERS IN THE GANGA-MEGHNA-BRAHMAPUTRA BASIN

The distribution of arsenic (As) and its release mechanisms in sedimentary aquifers have been studied for several decades. Complex hydrogeology, redox biogeochemistry and presence of dissolved organic matter (DOM) in the aquifers appear to control As oxyanion release. Currently, reductive dissolution of As-bearing iron minerals by bacteria fueled by labile DOM is considered as the main mechanism of As release from sediments deposited in the Bengal Basin during Holocene and late Pleistocene. Whereas the aquifer geochemistry has been studied widely across basin, DOM characterization has only begun recently. We describe the spatial distribution of DOM across aquifers of the Bengal basin from over fourteen study sites (11 in India and 3 in Bangladesh) along an east-west transect of River Ganges (India) and a site along Meghna riverbank (Bangladesh). The dissolved As ranged between <1 to 643 µgL^-1 across these sites and correlated strongly with dissolved organic carbon (DOC), a fraction of DOM, concentrations (R = 0.94) that ranged between 0.65 to 9.6 mgL^-1. Fluorescence signatures of groundwater DOM from parallel factor (PARAFAC) models revealed a strong correlation between high As concentrations and humic-like, terrestrially-derived DOM. Further, a negative correlation between dissolved As concentration and the carbon to nitrogen (C:N) ratio was observed. Lastly, the proportion of labile and recalcitrant DOM plays an important role in As mobilization as does bulk DOC and TDN concentrations. Earlier studies indicated that humic-like DOM can enhance As mobilization via aqueous complexation, competitive sorption and electron shuttling. Within the Meghna River aquifer, the water soluble sedimentary organic matter (SOM) exhibited a mixed humic-like and protein-like character with a molecular weight of ~1,500Da, comparable to that of the adjacent groundwater DOM (~1,600Da). Whereas the bulk SOM in the As-contaminated aquifer was rich in aromatic structures with humic-like properties, the near-surface sediments exhibited protein-like signatures with more labile structures. Should the SOM contribute to the DOC load in the groundwater, the humic-like SOM may act to maintain elevated As. This study offers an overview of OM distribution across the basin and highlights processes that can help explain the heterogeneous distribution of As concentrations in groundwater.