Assessment on heavy metal pollution in surface sediments off Wuchuan, Guangdong Province

To understand the pollution of heavy metals in the surface sediment in the coastal waters of Wuchuan City, Guangdong Province, 41 samples of surface sediment were collected in the end of October 2020 in nearshore waters, the content of typical heavy metals were detected, sediment particle size was analyzed, and the sources of heavy metals were discussed. The distribution of heavy metals was characterized, the pollution level of enrichment factors was evaluated, and the relationship between sediment particle size and heavy-metal concentration was explored based on correlation analysis. Results show that heavy metal content showed a high-low-high trend in west-east; high-value areas were mainly distributed in the western nearshore, northeast and southeast, low-value areas were mainly distributed in the north and southwest. The average content (in mg/kg) was in the descending order of Zn (72.62±29.94)>Cr(52.36±20.22)>Pb(32.12)>Cu(22.81±9.72)>As(8.35±2.79)>Cd(0.25±0.12), which met the national Class I of sediment quality standards. The pollution degree of the 6 heavy metal elements in the surface sediment in Wuchuan coastal area was in a descending order of Cu≈Cr>Pb>Cd>Zn>As, and the single factor pollution index was below 1. Heavy metals were mainly concentrated in strong enrichment area and the average enrichment coefficient was in a descending order of Cu>Pb>Zn>Cr>Cd>As; Cu and Pb were strongly enriched; Zn, Cr, and Cd were weakly enriched; and As was depleted. The coastal ecological environment in was polluted to some extents. The average ranking of enrichment factor was: Cd>Cu>Pb>As>Cr>Zn. Only 29.2% of the site was in slight risk, of which Cd was the biggest contributor to the overall potential ecological risk and was the dominant factor in toxicology. It is inferred that wastewater discharged from farms and factories are accountable for the high risk. In addition, heavy metal content showed a significant positive correlation with the sediment average grain size following the law of element particle size dependency. Meanwhile, correlation among heavy metals show that Cu, Pb, Zn, Cr and As were mainly derived from natural rock erosion and Cd was from human activities.