Analysis and ecological risk assessment of heavy metal residues in wetland soils at different recovery stages

The study aims to assess the residual levels and potential ecological risks of heavy metals-Zn, Pb, As, Cr, Cu, Hg, and Cd-in the soil of returned farmland wetlands within the Naolihe national nature reserve. This assessment seeks to provide reference data for wetland restoration and protection efforts within the reserve.

A total of 57 soil samples were collected from Daxing farm, Hongqiling farm, ‘853’ farm, and ‘597’ farm, each representing different reforestation periods within the Naolihe reserve basin. The soil samples were processed using a mixed acid-electrothermal plate digestion method. The study analyzed the differences in residual heavy metals across return plots from 1-year (R1), 2-year (R2), 8-year (R8), 17-year (R17), as well as corn (CF), soybean (SF), rice (RF), and natural wetland (NW) plots. The single factor pollution index (P), geological accumulation index (I_geo), and potential ecological risk index (R_I) were employed to evaluate the potential ecological risk levels of heavy metals in the soil.

The residual concentrations of Zn, Pb, As, Cr, Cu, Hg, and Cd in the Naolihe reserve soil ranged from 0.39-0.98, 0.21-0.35, 0.63-2.85, 1.54-2.34, 1.77-3.88, 0.74-2.24, and 0.12-0.54 mg·kg^-1, respectively. The residual concentrations of Zn, Pb, As, Cr, and Cu were all below the soil background values for the Sanjiang plain, whereas Hg and Cd levels exceeded these background values. The evaluation results using the single factor pollution index indicated that Cu, Cr, Zn, As, and Pb were pollution-free across all locations. Hg pollution was moderate to severe in R1, R2, R8, and farmland plots (CF, SF, RF), while it was mild in R17 and NW plots. Cd showed mild contamination in R1 and farmland plots, with other plots remaining at clean levels. The geological accumulation index evaluation revealed that Cd, Zn, Pb, As, Cr, and Cu were pollution-free in all locations, while Hg exhibited light to moderate pollution. The potential ecological hazard index evaluation indicated that R1, R2, and farmland plots posed medium risks, while R8, R17, and NW plots posed low risks.

The residual levels of heavy metals decreased progressively with longer reforestation periods. After 17 years of reforestation, the residual levels were the lowest and comparable to those in natural wetlands. The ecological risk associated with heavy metal residues in the reclaimed wetland soils of the Naolihe reserve ranged from low to medium. Plots returned to farmland for 8 and 17 years were classified as low-risk. Heavy metals Hg and Cd are the main ecological hazard elements in the returned farmland wetland of the Naolihe reserve.