Carbon and methane cycling in arsenic-contaminated aquifers

Geogenic arsenic (As) contamination of groundwater is a health threat to millions people worldwide, particularly in alluvial regions South and Southeast Asia. Mitigation measures are often hindered by high heterogeneities As concentrations, the cause(s) which elusive. Here we used comprehensive suite stable isotope analyses hydrogeochemical parameters shed light on mechanisms typical high-As Holocene aquifer near Hanoi where advected low-As Pleistocene aquifer. Carbon signatures (?13C-CH4, ?13C-DOC, ?13C-DIC) provided evidence that fermentation, methanogenesis methanotrophy actively contributing heterogeneity. Methanogenesis occurred concurrently levels (>200 µg/L) DOC-enriched aquitard pore water infiltrates into Along flowpath Holocene/Pleistocene transition, methane oxidation causes strong shift ?13C-CH4 from -87‰ +47‰, indicating reactivity. These findings demonstrate previously overlooked role cycling DOC infiltration aquifers.