Occurrence and contamination of heavy metals in urban mangroves: A case study in Shenzhen, China.

Urban mangroves are affected by surrounding anthropogenic activities. Heavy metals in urban mangroves lack systematic evaluation, and their relationships with surrounding anthropogenic activities have not been explored with quantified data. In this study, urban mangroves with different urban functional zonings were selected in Shenzhen, South China, including: Shajing mangrove (SJM) and Xixiang mangrove (XXM) featured with industry district, Futian mangrove (FTM) and Baguang mangrove (BGM) featured with central business district and ecological preserve, respectively. The distribution, ecological risk, and pollution sources of heavy metals were determined. Heavy metals' relationships with surrounding functional zonings were also explored. The levels of Cu, Zn, Pb, Ni and Cr in SJM were significantly higher than the other mangrove stands, which may be related to its proximity to point-source discharges of Maozhou River in Pearl River Estuary, China. Potential ecological risk index, pollution load index, mean PEL quotient, mean effect range median quotient, and total toxic unit showed the highest ecological risk in SJM, followed by XXM, FTM, and BGM. With the order of BGM - FTM - XXM - SJM, Zn, Ni, and Cr presented dominantly in the labile fraction, demonstrating increased anthropogenic impacts. Heavy metals in FTM and BGM were mainly from anthropogenic and lithogenic originations, with only anthropogenic origination of heavy metals to be detected in SJM and XXM. Heavy metal pollution was highest in SJM featured with industry district, and metal products industry in surrounding district should attract special attention due to its affinity for heavy metal accumulation in urban mangroves.