Impacts of periodic saltwater inundation on heavy metals in soils from the Pearl River Delta, China

The alluvial plain delta is vulnerable to complex land-sea interactions as a result of rising sea levels and increasing storm surges due to global environmental change. Topsoil samples (0-20 cm) from the Pearl River Delta (PRD) were subjected to periodic artificial saltwater inundation treatments with varying salinities (0, 35%o, 40%o, 50%o) for 50 days to explore the impacts of saltwater inundation on heavy metals (Cd, Pb, Zn) in soils. The inundation treatments reached dynamic equilibrium in approximately 20 days, and heavy metals were promoted to release into leachate. The extraction rate of heavy metals was highest with artificial saltwater at 40%o salinity, which was generally attributed to pH variation, increasing ionic strength and reductive dissolution of Fe-Mn oxyhydroxides. However, once the salinity reached 50%o, a higher SO2-4concentration could depress the release of heavy metals by providing more negative adsorption sites. Cd in soils was most likely to leach, followed by Zn, but Pb showed higher retention. After saltwater inundation, the bioavailability of heavy metals decreased in the order Cd > Zn > Pb. Redundancy analysis (RDA) results demonstrated that Cd and Zn are more susceptible to soluble salt ions in soils than Pb. The retention of Pb could be attributed to its larger ionic radius and reduced hydrated radius as well as the stable species under the pH conditions of the treatments. This study suggests that the migration of heavy metals could reduce the water quality and increase the ecological risk of the interaction zone between land and sea.