Variable methane–arsenic interactions in groundwater of Northern Bangladesh revealed by stable isotopes and geochemical analyses

Kai Ernst, Charlotte Stirn, Martin Maier, Moritz Schroll, David Polya, Frank Keppler

The reductive dissolution of arsenic (As)-bearing Fe(III) (hydr)oxides is widely regarded as the primary mechanism driving As mobilisation into groundwater in the 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 mobilisation remains poorly constrained, particularly regarding the origin, prevalence, and vertical distribution of CH4 production and oxidation. Here, we present compelling field-based evidence for As mobilisation linked to CH4 cycling, based on a comprehensive, depth-resolved dataset from four groundwater wells in northern Bangladesh. This includes isotopic measurements of δ13CVPDB-CH4, δ2HSMOW-CH4 , δ13CVPDB-CO2 as well as of various 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 processes 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 mobilisation in Bangladeshi aquifers and potentially in other As-impacted regions worldwide.