Carbon and methane cycling in arsenic-contaminated aquifers

Geogenic arsenic (As) contamination of groundwater is a health threat to millions of people worldwide, particularly in alluvial regions of South and Southeast Asia. Mitigation measures are often hindered by high heterogeneities in As concentrations, the cause(s) of which are elusive. Here we used a comprehensive suite of stable isotope analyses and hydrogeochemical parameters to shed light on the mechanisms in a typical high-As Holocene aquifer near Hanoi where groundwater is advected to a low-As Pleistocene aquifer. Carbon isotope signatures (delta C-13-CH4, delta C-13-DOC, delta C-13-DIC) provided evidence that fermentation, methanogenesis and methanotrophy are actively contributing to the As heterogeneity. Methanogenesis occurred concurrently where As levels are high (>200 mu g/L) and DOC-enriched aquitard pore water infiltrates into the aquifer. Along the flowpath to the Holocene/Pleistocene aquifer transition, methane oxidation causes a strong shift in delta C-13-CH4 from -87 parts per thousand to +47 parts per thousand, indicating high reactivity. These findings demonstrate a previously overlooked role of methane cycling and DOC infiltration in high-As aquifers. (C) 2021 The Author(s). Published by Elsevier Ltd.

Automated summary

In a high-arsenic aquifer near Hanoi, stable isotope analysis and hydrogeochemical parameters revealed the role of methane cycling and DOC infiltration in arsenic heterogeneity. Methanogenesis and methane oxidation were found to actively contribute to the distribution of arsenic in groundwater, highlighting the importance of these processes in high-arsenic aquifers.