Assessment of the grain size distribution and heavy metal contamination of surface and core sediments in Red River estuaries in Vietnam

In the Red River system, 45 surface and 6 core sediment samples were collected in 2024 from inside rivers, estuaries, and coastal areas to assess the distribution of heavy metals in space and time. The grain sizes were analysed via sieve and pipette methods, and the heavy metals were analysed via the ICP‒MS method. The estuaries contained fine sand, very fine sand, very coarse silt, coarse silt, and medium silt. In the surface sediments, the heavy metal concentrations of Fe, Mn, Zn, Pb, Cu, Ni, As, Co, and Cd were 21,272.91, 724.37, 71.14, 41.97, 31.06, 24.26, 13.07, 11.54, and 0.19 mg/kg, respectively. In the core sediments, the heavy metal contents of Fe, Mn, Zn, Pb, Cu, Ni, As, Co, and Cd were 27,472.60, 937.00, 96.50, 51.64, 46.73, 31.04, 19.10, 14.42, and 0.23 mg/kg, respectively. Both the surface and core sediments had concentrations of Pb, Cu, and As that were higher than the interim sediment quality guidelines (ISQG). The heavy metal pollution indices with geoaccumulation (Igeo) values for Pb, As, and Cd were moderate; the contamination factor (CF) was moderate contamination (Cu, Pb, Zn, As, Cd, and Mn), considerable contamination (Pb, As, and Cd), and high contamination (As); the enrichment factor (EF) was moderate to moderately severe enrichment (Pb, Cd, As); the potential ecological risk (ER) posed moderate to considerable potential ecological risk for As and Cd; the degree of contamination (CD) ranged from moderate to considerable; and the ecological risk (RI) ranged from low to moderate. Factor analysis and correlation analysis revealed that the heavy metals originated from mainly natural sources rather than from agricultural or local industrial sources. Overall, high concentrations of Cu, Pb, and As and moderate pollution indices in estuaries may pose risks to the environment and ecosystems.