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

- Liming decreased Cd, Mn and Zn solubility in slightly acidic soils but not Cu and Pb.
- Liming decreased Cu, Cd, Pb and Zn solubility in strongly acidic soils but not Mn.
- Phragmites favored metal enrichment in surface soils due to water pumping stimulation.
- Phragmites favored metal mobilization in deeper soils by hindering the drop of Eh.
- Remediation success depended on target metal, soil type, hydric regime and vegetation.

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 (20 cm in diameter and 60 cm 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 (55 cm depth). In the non-liming treatments the presence of vegetation hindered the drop of Eh in the deepest 55 cm 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 5 cm 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.