Hydrogenotrophic Methanogenesis and Aerobic Methane Oxidation in Arsenic-Prone Groundwaters in Northern Bangladesh: Stable Isotopic and Geochemical Evidence

The reductive dissolution of arsenic (As)-bearing Fe(III) (hydr)oxides is widely regarded as the primary mechanism driving As mobilization into groundwater in aquifers of Bangladesh. Recently, methane (CH4) has been proposed as a potentially important electron donor in this process, with CH4 oxidation by methanotrophic bacteria facilitating Fe(III) reduction and subsequent As release. However, the role of CH4 in As mobilization remains poorly understood. Here, we present field-based evidence for As mobilization linked to CH4 cycling, based on a comprehensive, depth-resolved data set from four groundwater wells in northern Bangladesh. This includes stable isotope measurements of CH4 and CO2 as well as other hydrogeochemical parameters. Two of the wells exhibited high dissolved As concentrations with depth-specific profiles showing significant correlations between As, Fe(II), and CH4. In particular, isotopic signatures of CH4 and CO2 revealed that CH4 formation and oxidation were closely associated with zones of elevated As. In contrast, the other two wells showed relatively low As and Fe(II) concentrations and no clear evidence of CH4-driven biogeochemical activity. Our findings suggest that, depending on site-specific hydro-biogeochemical conditions, Fe(III) (hydr)oxide reduction coupled to CH4 oxidation may substantially contribute to As mobilization in Bangladeshi aquifers and potentially other As-impacted regions worldwide.