Soil heavy metals in tropical coastal interface of eastern Hainan Island in China: Distribution, sources and ecological risks

Abstract

Coastal interface is the most active in the global element cycling, linking the terrestrial and ocean ecosystem. However, little attention is paid to the ecological risk of multiple heavy metals (HMs) residues in tropical coastal soil and its relationships with human activities. Here, 199 coastal topsoils were collected from the eastern Hainan Island of China in 2021. Furthermore, the pollution characteristics, spatial distribution, and ecological risk of eight HMs were evaluated. The results showed that the mean contents of eight HMs were in the order of Cr (47.96 mg·kg−1) > Zn (40.93 mg·kg−1) > Pb (21.28 mg·kg−1) > Cu (14.89 mg·kg−1) > Ni (8.67 mg·kg−1) > As (5.64 mg·kg−1) > Cd (0.06 mg·kg−1) > Hg (0.05 mg·kg−1). These HMs contents were larger than the soil background values of Hainan Province, indicating the accumulation risk. Based on the Nemerow integrated pollution index, the sample sites with moderate and severe pollution accounted for 28.5% and 28.0%, respectively. In contrast, the sum of sites with no and slight pollution was only 10%. The ecological risk assessment revealed that 25% and 5% of sites displayed moderate or extreme ecological risks with the mean RI value larger than 90, and As and Cd were the most dangerous HMs. Spatially, the pollution hotspots of Zn, Pb, Cd and Hg were distributed in the middle urban of the study area and the dense road network in the west, while that of Cr, Ni, and Cu were distributed in the woodland in the northeast and southwest. The pollution hotspots of As were distributed in northeast cropland. The PMF model further indicated that the largest contributor to Zn, Pb, and Cd were the combined sources of traffic and agricultural activities, accounting for 55.66%, 56.30%, and 55.36%, respectively. Soil parent materials contributed to the Cr, Ni, and Cu, accounting for 77.1%, 58.4%, and 38.2%, respectively. Coal combustion was the main source of Hg, accounting for 81.3%, while agricultural activities accounted for 80.8% of the As in the soils. Overall, anthropogenic activities (63.60%) were the main source for HMs residues in the studied coastal soils, but the contribution of soil parent materials (36.40%) could not be neglected.