Assessment of the distribution and ecological risks of heavy metals in coastal sediments of Vietnam’s Mong Cai area

Abstract

The Mong Cai area is located on the border between Vietnam and China, with the Ka Long River forming a natural border. Coastal sediments were collected and analyzed for grain size, heavy metal content, total organic carbon, and isotopes (210Pb, 226Ra, δ15N, δ13C) to assess sediment quality.Fine sand was common in surface sediment, with abundant very fine sand in core C1; in core C2, very coarse silt and coarse silt were dominant. The sedimentation rate at C1 ranged between 0.08 and 1.62 cm/year while that at C2 ranged between 0.07 and 0.51 cm/year. The total organic carbon was highest in C2 next to the surface and lowest in C1, with content levels of 1.81%, 0.40% and 0.31%, respectively. δ13C in surface sediments ranged from − 27.43 to -17.02‰ and averaged − 24.13‰; δ15N ranged from 3.12 to 6.05‰, with an average of 4.96‰; and δ13C and δ15N in the sediment reflected the source of the organic matter mix from the marine and terrigenous sediments.The sediments’ heavy metal contents were lower than ISQGs, with the exception that As in C1 and C2 was higher than the ISQG. The heavy metal distribution was highly concentrated in the Ka Long estuary with low concentration in Ha Coi Bay. The concentration in C1 showed a decline over time, while the Cu, Pb, Zn, Cr, Co, Ni, and Mn contents in C2 increased over time between 1996 and 2019. In terms of heavy metal pollution indexes, Igeo showed C1 and C2 to be unpolluted to moderately polluted with As, with Li and Pb in C2; EF was moderately enriched with As; CF was moderately contaminated (Pb, Cd, Fe, Mo, and Li) in C2 and in C1 (Cd, As, Li) and considerably contaminated (As) in C2. ER of As showed a moderate potential ecological risk in C2. CD ranged from moderate degree contamination to considerable degree contamination (C1, C2). RI had low ecological risks.