ROLE OF DISSOLVED ORGANIC MATTER IN ARSENIC MOBILIZATION IN BENGAL BASIN

Elevated levels of arsenic (As) and similar oxyanions in the Bengal Basin have led to multiple studies investigating the distribution of As and its release mechanisms for several decades. The main controls of As oxyanion release appear to be complex hydrogeology of the Bengal basin, redox biogeochemistry and presence of dissolved organic matter (DOM) in the aquifers. As of now, a widely accepted mechanism of As release is the microbially mediated reductive dissolution of As-bearing iron minerals in sediments deposited in the area during Holocene and late Pleistocene. Although these mechanisms have been studied widely, the roles of DOM have only begun to be studied recently. We evaluated the spatial distribution of DOM across the Bengal basin aquifers from eleven study sites in West Bengal, India. The dissolved As concentrations ranged between <1 to 643 µgL^-1 across these sites and correlated strongly with dissolved organic carbon (DOC), which is a fraction of the total DOM, at concentrations (R = 0.94) that ranged between 0.65 to 9.6 mgL^-1. Humic:protein ratio for DOM in groundwater calculated from a parallel factor (PARAFAC) model based on fluorescence spectroscopy analysis, was found to be higher in groundwater with high As (2 – 12) while it was < 1 in groundwater with low As. In addition, an inverse correlation between dissolved As concentrations and carbon to nitrogen (C:N) molar ratio was observed. For instance, high As samples exhibit C:N molar ratio < 1 suggesting more nitrogenous DOM, while low As groundwater showed C:N molar ratio > 1 suggesting more carbonaceous DOM. Overall, high As samples were characterized by C:N molar ratio < 1 and humic:protein ratio>1. Therefore, in addition to the bulk concentrations of DOC and TDN, the proportion of humic-like or protein-like DOM also plays an important role in As mobilization. Previous studies suggested that humic-like DOM can enhance As mobilization via aqueous complexation, competitive sorption and electron shuttling. This study serves as an overview of the distribution of DOM across the basin and highlights the processes that can explain the heterogeneous distribution of As concentrations in groundwaters of the Bengal Basin.