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 (CH 4 ) has been proposed as a potentially important electron donor in this process, with CH 4 oxidation by methanotrophic bacteria facilitating Fe(III) reduction and subsequent As release. However, the role of CH 4 in As mobilization remains poorly understood. Here, we present field-based evidence for As mobilization linked to CH 4 cycling, based on a comprehensive, depth-resolved data set from four groundwater wells in northern Bangladesh. This includes stable isotope measurements of CH 4 and CO 2 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 CH 4 . In particular, isotopic signatures of CH 4 and CO 2 revealed that CH 4 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 CH 4 -driven biogeochemical activity. Our findings suggest that, depending on site-specific hydro-biogeochemical conditions, Fe(III) (hydr)oxide reduction coupled to CH 4 oxidation may substantially contribute to As mobilization in Bangladeshi aquifers and potentially other As-impacted regions worldwide.