Relationships between heavy metal concentrations in soils and reclamation history in the reclaimed coastal area of Chongming Dongtan of the Yangtze River Estuary, China

Purpose: Reclaimed coastal wetlands are an important land resource because of the shortage of cultivated land in coastal areas, but few studies have focused on contamination of heavy metals in reclaimed agricultural soils, especially in soils that have experienced long-term agricultural activities. This study was carried out in Chongming Dongtan of the Yangtze Estuary, which has undergone large-scale reclamation activities, with the aim of determining the concentrations and spatial distributions of heavy metals in different reclaimed zones.

Materials and methods: In this study, 50 soil samples were collected in the study area, which was divided into three zones based on the history of reclamation (post-1998, 1990s, and 1960s) and intertidal flats. Soil metals (Al, Fe, Mn, Cu, Cr, Ni, Zn, and Pb), soil pH, Eh, salinity, soil organic matter (SOM), and grain sizes were analyzed. Correlation analysis and factor analysis were employed to extract the main influencing factors on heavy metals in reclaimed soils. Al-normalization was used to remove the grain-size effect. The enrichment ratio (ER) was applied to reflect the chronological change of heavy metals after reclamation. Inverse distance weighted (IDW) interpolation was used to create spatial distribution maps of the measured heavy metals.

Results and discussion: There were significant positive correlations (p < 0.01) between the fine particle fractions (<20 μm) of soils and metal concentrations. Background deposition of metals in the study area increased relative to the reference values of each zone during the last 50 years. In zone post-1998, the concentrations of Cu, Zn, and Pb after reclamation were lower than those before reclamation, but not for Cr; in zone 1960s and 1990s, the concentrations of Cu, Cr, Zn, and Pb were higher than those before reclamation. In all the zones, the reclamation process had little impact on the mobilization of Ni. Our results showed that after Al-normalization, the differences of heavy metal concentrations between different zones and their spatial distribution patterns were due to the combined effects of both background deposition and anthropogenic processes, particularly in zone 1960s.

Conclusions: The current heavy metal concentrations were mainly determined by the deposition background in Chongming Dongtan, but the reclamation history had certain effects on heavy metal concentrations in the reclaimed soils. The desalination leaching effect on heavy metals was mainly found for soils with short-term reclamation (zone post-1998), but with longer reclamation history (like zone 1960s and 1990s); long-term agricultural activities had greatly influenced the concentrations of heavy metals.