GEOGENIC MN IN DRINKING WATERS OF WEST BENGAL, INDIA

Geogenic Mn is ubiquitously present in groundwaters and soils, and can exist in multiple oxidation states in the environment. Although Mn is an essential nutrient, there is substantial work supporting its neurotoxic effects in humans at higher doses. In January of 2015, six different villages were surveyed in the Murshidabad district of West Bengal, India, in an attempt to improve the understanding of how dissolved Mn, As, and dissolved organic matter (DOM) interact with each other under varying redox potentials. Three villages east of the river Bhagirathi (Beldanga, Naoda, Hariharpara) contain groundwater with reducing oxidation-reduction potentials (ORPs) and elevated levels of dissolved Mn and As. In contrast, two villages west of the river (Nabagram, Khidirpore) contain groundwater with oxidizing ORPs and elevated levels of dissolved Mn, but low As. One village, Kandi, contains low levels of both Mn and As. Twenty-nine groundwater samples were collected and analyzed for major dissolved ions, δ¹⁸O, δ²H, and DOM characterization. Additionally, three sediment cores were drilled up to 30 m depth and analyzed for bulk total concentrations, sequential extractions, and elemental speciation of Mn and As using X-ray absorption spectroscopy (XAS). Eighty-three percent of the wells surveyed contain Mn levels that exceed the recommended WHO limit (0.4 mg/L), with Mn persisting under circumneutral pH environments and under a broad range of redox potentials. The results suggest three possible mechanisms for controlling Mn release and accumulation in the groundwater: (i) microbially mediated reduction of Mn-Fe-(oxyhydr)oxides containing sorbed As complexes (in which case Mn correlates positively with As and Fe); (ii) Mn reduction and subsequent release as a result of other species being oxidized by Mn in aquifers with positive ORP (XAS analyses of aquifer sediments in this area indicate co-existence of reduced Mn with oxidized As); and (iii) inhibition of Mn release due to the presence of protein-like DOM. Using a multivariate approach to explore the different release mechanisms of Mn in these separate geochemical regimes, our work aims to provide a better understanding of Mn behavior in the environment. Such advances will contribute to the development of more robust maximum contaminant levels for Mn in drinking waters.