Arsenic contamination in groundwater is a critical issue and one that causes great concern around the world with many negative health impacts on the human body. In-situ subsurface arsenic immobilization by aeration has shown to be a promising, convenient technology with high treatment efficiency. In contrast to most of other As-remediation technologies, in-situ subsurface immobilization offers the advantage of negligible waste production and hence has the potential of being a long-term, sustainable treatment option. A pilot scale plant (capacity = 2 m3 d−1) for the subsurface arsenic removal (SAR) was tested in the Mekong Delta region in South Vietnam. Within the first two weeks of operation the initial concentrations of 81 ± 14 μg As L−1 were successfully lowered to below the guideline value limit for drinking water recommended by the WHO of 10 μg L−1. Results indicated adsorption and co-precipitation with iron oxides as the principal mechanism responsible for the arsenic removal. Evaluation of the results demonstrates the feasibility of in-situ technology for arsenic mitigation. However, difficulties in manganese removal arose due to existing high ammonia concentrations and natural occurring geochemical reducing conditions.