LINKING GEOCHEMISTRY AND DISSOLVED ORGANIC MATTER QUALITY TO MN AND AS RELEASE IN GROUNDWATER, MURSHIDABAD, WEST BENGAL, INDIA

The prevalence of manganese (Mn) and arsenic (As) and their impacts on human health have been documented extensively in the groundwater of West Bengal, India. Although the general mechanisms behind the release of these elements are understood, the link between dissolved (>0.7μm) organic matter (DOM) quality and Mn and As release is less explored. This study illustrates associations between characteristics of DOM and the spatial distribution of groundwater Mn and As. We present a dataset collected between 2011 and 2015 from six different sites within the Murshidabad district of West Bengal, India. A total of 51 shallow (<40m) groundwater samples were analyzed for cations, anions, and various DOM properties (e.g., Freshness Index (β:α), Humification Index (HIX), etc.) using 3-dimensional fluorescence excitation emission matrices followed by parallel factor (PARAFAC) modeling analyses. Mn, As, and Fe in sediments were quantified via bulk and sequential extractions, as well as µ-XRF mapping coupled with XANES and EXAFS spectroscopy to evaluate specific elemental associations in solid phases. Results indicate that two distinct biogeochemical regimes are identified. East of the river Bhagirathi, where Eh values are low and DOC is abundant (1.83 mg L^-1), dissolved Mn (0.83 mg L^-1), As (329.9 μg L^-1), and Fe (3.63 mg L^-1) are elevated. Furthermore, fluorescent signatures of DOM degradation (β:α: 0.73; HIX: 11.1) are observed, suggesting that microbial oxidation of DOM is linked to Fe and Mn-oxide reduction, releasing Mn, As, and Fe into the groundwater. West of the Bhagirathi, however, where Eh is higher and DOC values are slightly lower (1.34 mg L^-1), dissolved Mn remains elevated (1.11 mg L^-1) while Fe (0.31 mg L^-1) and As (8.98 μg L^-1) are considerably lower. Here, signatures of DOM degradation are not as pronounced (β:α: 0.93; HIX: 5.32), and thus it is hypothesized that microbially derived reductive processes are less active. As a result, Mn-oxide reduction, independent of Fe-oxide reduction and perhaps even abiotically, appears to be the dominant redox driver. Finally, associations between Mn, As, and Fe in sediments reinforce the idea that Fe and Mn-oxide reduction is most likely conducive to the release of As from aquifer sediments in Murshidabad.