Mobilisation of AS and trace metals in saline, acidic Sopolic Technosols: The role of the rhizosphere and flooding conditions

Purpose: The aim of this work was to study the risk of As and metal pollution in saline wetlands affected by slightly acidic mine wastes, in the presence or absence of a plant rhizosphere, under different flooding regimes. Some guidelines for management will be proposed in order to minimise the risk of metal leaching in metal-polluted salt marshes. Materials and methods: A microcosm experiment was conducted to assess the influence of two flooding regimes (continuous or alternating) and the presence of rhizosphere (Sarcocornia fruticosa and Phragmites australis) on the mobilisation of As, Cd, Mn, Pb, Zn and Fe in a slightly acidic, saline Spolic Technosol (pH¿6.5; total concentrations: 551 mg kg-1 As, 5,640 mg kg-1 Pb, 5,299 mg kg-1 Zn). Results and discussion: Metal and As pore water concentrations decreased with flooding time (e.g. from approximately 1,700 ¿g L-1 Cd, 190 mg L-1 Zn and 230 mg L-1 Mn to <400 ¿g L-1 Cd, <35 mg L-1 Zn, and 50 mg L-1 Mn). A sequential extraction showed that Fe and As were associated mainly with the silicate fraction of the soil, but most of the Mn, Cd, Pb and Zn belonged to the reactive fraction. The rhizosphere of Sarcocornia induced a lower pH, which facilitated higher concentrations of soluble Fe. However, in pots without plants and pots with Phragmites, the combination of Eh and pH led to conditions for formation of siderite, and hence the concentrations of soluble Fe were lower. The concentrations of Mn, Cd, Pb and Zn were lower in vegetated pots, probably due to precipitation of metal carbonates favoured by gas exchange in the rhizosphere. Conclusions: The rhizosphere modified soil conditions during the period of flooding and this affected the dynamics of metals. The high levels of soluble metals pose a risk for leaching, transfer to the food web and toxicity for living organisms. In order to reduce the risks, we propose the water saturation of the soil and the establishments of a plant cover. © 2011 Springer-Verlag.