Heavy metal contamination in soils from freshwater wetlands to salt marshes in the Yellow River Estuary, China

The concentration and distribution of As, Cd, Cr, Cu, Mn, Pb, Zn and Fe were determined to estimate soil contamination in freshwater wetlands (S1 and S2), brackish wetlands (S3) and salt marshes (S4 and S5). Soil cores with depths up to 50 cm were seasonally collected with three repeats. Our study revealed higher mean concentrations of Cd in top 50 cm soils than corresponding background values in the five wetlands. However, values of As, Mn, Zn, Cu and Fe exceeded background values in surface soils (0-10 cm). Heavy metal concentrations in surface soils were generally higher compared with deeper soils, with the exception of S3. Horizontally, eight heavy metals generally showed a decreasing and then increasing trend in the 0–40 cm soils, with the lowest and highest values in S4 and S5, respectively. Three hotspots of metal accmulation in the top 20 cm soils were observed in the study area.The sediment quality guidelines indicated that occasionally adverse effects of As and Cu can be anticipated. Besides, according to the enrichment factor and geo-accumulation index, Cd exhibited no pollution to moderate pollution level in these wetlands. The potential ecological risk index indicated the moderate pollution degree in soils of five wetlands. Principle component analysis and Pearson correlation analysis showed that eight heavy metals mainly derived from natural source, and SOM and soil texture were main factors influencing the accumulation and distribution of eight heavy metals in the studied wetlands.