Accumulation risk and sources of heavy metals in supratidal wetlands along the west coast of the Bohai Sea

Abstract

The heavy metals Al, Cr, Cu, Ni, Pb, Zn, Fe, Mn, As, and Cd in the rainfall-driven supratidal wetlands along the west coast of the Bohai Sea (the areas are named site 1, site 2, site 3, and site 4 from south to north in the gradient in this study) are tested for their accumulation risks and sources. Results show that the distribution and enrichment of the heavy metals in the supratidal wetlands are lower in site 1 than in sites 2-4. The risk indices (RIs) of all sites are less than 150, indicating low-moderate risk. However, the RI values are primarily dominated by the risk indices (E _r ⁱ ) of As and Cd. The accumulative contribution values of E _r ⁱ -As and E _r ⁱ -Cd in sites 1, 2, 3, and 4 are 79.05%, 77.80%, 80.54%, and 76.43%, respectively. Additionally, the contamination degree (C _d) and the Nemero comprehensive pollution index (PN) of the supratidal wetland in site 1 are 6.86 and 0.74 respectively, indicating a low-risk state. By contrast, the C _d and PN values of the other three supratidal wetlands are higher than those of site 1, suggesting an increasing accumulation risk for heavy metals in sites 2, 3 and 4. Our analysis indicates that the heavy metals Al, Cr, Mn, and Fe in all the supratidal wetlands mainly originate from the weathering of rocks and their parent materials. Pb is significantly correlated with anthropogenic activities, while Cu, As, and Cd are likely induced by anthropogenic activities and atmospheric deposition. The sources of Ni and Zn should be determined on the basis of the situation of the wetland and its surrounding areas. For example, Ni is mainly affected by anthropogenic activities in site 2, whereas the origins of Ni are soil parent materials or atmospheric depositions in sites 1, 3, and 4. Our results can provide data to support the utilization strategy and sustainable development plans for marine space resources on the coast of the Bohai Sea.

Fig. 1. Map of the coast of Bohai Sea and the location of our study sites. Cities are indicated by triangles, study sites are indicated by dots.

Fig. 2. Seasonal and monthly variation of mean annual precipitation along the west coast of Bohai Sea. (a) Seasonal variation of mean annual precipitation; (b) monthly variation of mean annual precipitation in sampling year (histogram indicates 2014, line indicates 2015).

Fig. 3. Spatial distribution for soil heavy metals in the four supratial wetlands along the west coast of Bohai Sea.

Fig. 4. Enrichment factors (EF) of heavy metals in the four supratidal wetlands along the west coast of Bohai Sea.

Fig. 5. Potential ecological risk index (RI) and degree of contamination (C_d) in the four supratidal wetlands along the west coast of Bohai Sea.

Fig. 6. Cluster analysis for heavy metals in the four supratidal wetlands along the west coast of Bohai Sea. (a) Site 1; (b) site 2; (c) site 3; (d) site 4.

Fig. 7. Principal component analysis (PCA) for heavy metals in the four supratidal wetlands along the west coast of Bohai Sea. (a) Site 1; (b) site 2; (c) site 3; (d) site 4.

Fig. 8. Correlation coefficients between heavy metals and soil physicochemical properties in the four supratidal wetlands along the west coast of Bohai Sea. (a) Site 1; (b) site 2; (c) site 3; (d) site 4. * correlation is significant at the 0.05 level (2-tailed), ** correlation is significant at the 0.01 level (2-tailed).