Impact of anaerobic methane oxidation coupled to iron(III) reduction on arsenic mobilization in aquifers along the Red River delta in Vietnam

Groundwater accounts for up to 30% of our drinking water resources, with over 2.5 billion people worldwide relying on its purity and availability. However, geogenic arsenic contamination in groundwater poses a serious threat to this limited resource, potentially endangering the health of over 200 million people worldwide.

Arsenic (As) contamination in aquifers is a common issue along the Red River Delta located in Vietnam. As-bearing minerals such as Fe(III) (oxyhydr)oxides are essential to immobilizing As and preventing it from contaminating groundwater. Emerging evidence in past years have linked anaerobic oxidation of methane (AOM) to the reductive dissolution of Fe(III) minerals leading to the greater release of As into groundwater. Further investigation is needed to elucidate the specific microorganisms involved and their underlying microbial mechanisms, along with the broader relevance of this process in other aquifers.

To gain deeper insight, two drilling field campaigns were performed in villages Dan Phuong and Van Phuc along the Red River. Groundwater and sediment samples were taken for geochemical and molecular biology analysis. From the geochemical analysis, we have produced chemical profiles of potential available electron donors (CH₄, NH₄⁺, H₂, CO₂, DOC) and electron acceptors (Fe(III), SO₄^2-, NO^3-, NO^2-) along sediment depth in order to confirm the relevance of CH₄ in this system. At Van Phuc and Dan Phoung CH₄ was found to be the dominant electron donor with a maximum concentration of 0.06 and 0.015 mmol kg^-1 respectively. At the Van Phuc site, novel microbial trapping devices were inserted into one of the previous drilling wells and collected 4 months later for the enrichment of organisms specialized in Fe(III) reduction and AOM. Obtained enrichment cultures are being used to measure rates of Fe(III) reduction and CH₄ oxidation with labelled ¹³C under different growth conditions. In summary, we have found CH₄ to be the primary driver of Fe(III) reduction at certain depths at both field sites. Further work will focus on the sequencing of sediment and groundwater samples to develop a profile for the present and active microbial community. With the combination of the geochemical and sequencing results we hope to confirm the relevance of Fe(III) reduction and AOM influence on As mobilization.