Redox buffering and de-coupling of arsenic and iron in reducing aquifers across the Red River Delta, Vietnam, and conceptual model of de-coupling processes

Abstract

Analysis of over 500 groundwater samples from throughout the Red River Delta indicates de-coupling of dissolved arsenic (As) and dissolved iron (Fe). Sorting of all data along the redox potentials suggests re-adsorption of As released initially from Mn(IV)-oxyhydroxides and later from Fe(III)-oxyhydroxides on remaining ferric phases at moderate redox levels. A gradually decreasing specific surface area available for re-adsorption of As probably plays a role as a consequence of limited reactivity of more crystalline phases such as goethite and hematite. At low redox levels, concentrations of Fe and phosphate decrease, but As concentrations keep increasing and most As is present as As(III) with limited adsorption affinity. Based on the results of speciation modeling, the water is supersaturated with respect to siderite and vivianite. A general conceptual model of As and Fe behavior is presented, suggesting that coupled behavior is possible in two geochemical "windows", i.e., 1: between saturation of remaining adsorption sites and the onset of siderite and vivianite precipitation, and 2: after the beginning of secondary sulfide phases precipitation and during methanogenesis. The de-coupling of As from Fe is common and has been observed at many sites around the world where As is released as a consequence of redox processes, e.g., in Bangladesh, West Bengal and Assam in India, the Mekong Delta in Cambodia and Vietnam, and Taiwan. The presented general conceptual model of de-coupling processes can be applied to the interpretation of As and Fe data, and, thus, it can help in the preparation of a site conceptual model which is a necessary prerequisite for reactive transport modeling.