Changes in soluble metal concentrations induced by variable water table levels as response to liming and Phragmites australis growth in metal-polluted wetland soils: Management effectiveness

Abstract

This study aimed to assess the effectiveness of liming and Phragmites australis growth for the management of metal-polluted wetland soils under fluctuating water table levels. Soil columns (20cm in diameter and 60cm high) were constructed with two soil types (pH~6.4 and pH~3.1) and four treatments were assayed: with/without liming and with/without vegetation. The pH, Eh, EC, WSOC and soluble metals (Cd, Cu, Mn, Pb, Zn) were monitored at three depths: 5 (never under water), 30 (alternating flooding-drying conditions), and 55 (always under water) cm. In the slightly acidic soil liming decreased Cd, Mn and Zn soluble concentrations regardless of the hydric regime and the presence of Phragmites. However, it contributed to Cu and Pb mobilization under permanent flooding conditions (55cm depth). In the non-liming treatments the presence of vegetation hindered the drop of Eh in the deepest 55cm and contributed to maintain higher Cd solubility. In the strongly acidic soil liming decreased Cu, Cd, Pb and Zn soluble concentrations but not Mn. In this soil the presence of Phragmites contributed to higher soluble metal concentrations at 5cm depth due to the capillary upward movement of water with solutes (salts and metals). Hence, it is not possible to establish an unique management strategy for metal-polluted hydric soils since the effectiveness of the remediation techniques will depend on the target metal, soil type, water level regime and presence/absence of vegetation.