Distribution fractions and potential ecological risk assessment of heavy metals in mangrove sediments of the Greater Bay Area

The restoration of mangrove in coastal wetlands of China has been started since the 1990s. However, various pollutants, especially for heavy metals (HMs), contained in wastewater might present a significant risk to mangrove forests during the restoration. In this study, sediments of five typical mangrove wetlands with varying restoration years and management measures in the Greater Bay Area were collected to evaluate the distribution fractions and potential ecological risk of HMs. Cd (0.2-1.6 mg/kg) was found in high concentrations in the exchangeable fraction (37.8-71.5%), whereas Cu (54.2-94.8 mg/kg), Zn (157.6-332.6 mg/kg), Cr (57.7-113.6 mg/kg), Pb (36.5-89.9 mg/kg), and Ni (29.7-69.5 mg/kg) primarily presented in residual fraction (30.8-91.9%). According to the geo-accumulation index (I-geo) analysis, sediment Cd presented a high level of pollution (3 <= I-geo <= 4), while Zn and Cu were associated with moderately pollution (1 <= I-geo <= 2). Besides, high ecological risk of Cd was found in sediments of five mangroves, with risk assessment code (RAC) ranging from 45.9 to 84.2. Redundancy analysis revealed that the content of NO3--N was closely related to that of HMs in sediments and, pH value and NO3--N concentration affected the distribution of HMs geochemical fractions. High concentration of HMs in QA and NS sampling sites was caused by the formerly pollutants discharge, resulting in these sediments still with a higher HM pollution level after the plant of mangrove for a long period. Fortunately, strict drainage standards for industrial activities in Shenzhen significantly availed for decreasing HMs contents in mangrove sediments. Therefore, future works on mangrove conversion and restoration should be linked to the water purification in the GBA.

Automated summary

The restoration of mangrove in coastal wetlands of China started in the 1990s, but the presence of heavy metals in wastewater poses a significant risk to the restoration process. This study evaluated the distribution and ecological risk of heavy metals in sediment samples from five mangrove wetlands in the Greater Bay Area. Cadmium (Cd) was found in high concentrations in the exchangeable fraction, while Copper (Cu), Zinc (Zn), Chromium (Cr), Lead (Pb), and Nickel (Ni) mainly presented in the residual fraction. Sediment Cd showed high pollution levels, while Zn and Cu had moderate pollution levels. The sediments also showed a high ecological risk for Cd. The content of NO3--N was closely related to the heavy metal content in sediments, and pH value and NO3--N concentration affected the distribution of heavy metal fractions. Industrial discharge in certain areas led to higher levels of heavy metal pollution, but strict drainage standards in Shenzhen helped decrease heavy metal contents in mangrove sediments. Therefore, future mangrove conversion and restoration should be connected to water purification efforts in the Greater Bay Area.