Distribution of heavy metals in mangrove soils of Red River Delta, Northern of Vietnam

Mangroves, found in intertidal zones, are vital not only for mitigating climate change and protecting against sea-related disasters but also for sustaining coastal ecosystems, preserving environmental health, and supporting local communities. Heavy metal distribution in mangrove soils is an important indicator for monitoring the coastal marine environment, assessing ecological risk, and managing mangrove ecosystems. In the present study, the eight heavy metals (HMs) including copper (Cu), lead (Pb), zinc (Zn), manganese (Mn), chromium (Cr), arsenic (As), nickel (Ni), and iron (Fe) were determined by inductively coupled plasma mass spectrometry (ICP-MS) while the relationships between different mangrove species were analyzed, while their ecological risk was assessed based on 368 soil samples in 93 sampling locations across the entire coastal region of the Red River Delta (RRD), the second largest delta in Vietnam. Mangrove species were classified based on tree count and estimated above-ground biomass. Ecological risk was assessed using Vietnam's environmental regulations, the contamination factor (CF), and the Geo-accumulation Index (I-geo). The HM amounts in the RRD mangrove soils followed the sequence Fe > Mn > Zn > Cr > Pb > Cu > Ni > As and they decreased from areas with greater species diversity to less diverse and monospecific stands. The CF and I-geo values indicated insignificant to moderate contamination levels, suggesting a low ecological risk, except for Pb, which warrants particular attention. Our results indicate that the mangrove forests play important role in HM accumulation in mangrove soils in the RRD coastal area. Most HMs were dominated in the upper soil layer (0–50 cm) and declined noticeably between 50 and 100 cm, except for Ni and Mn. Single-species stands, as well as mixed stands including Kandelia obovata and Sonneratia caseolaris, demonstrated a greater capacity for HMs accumulation in mangrove soils in RRD.