To study metal enrichment and identify the influencing factors, 106 surface soils were collected in the Yellow River delta, including ten vegetation types. Concentrations of 10 different metals, including As, Cd, Cr, Ni, Cu, Pb, Zn, Mn, Al, Fe, and physicochemical properties pH, salinity, particle size were detected. The pH value was 7.35~9.17, and the salinity was in the range of 0.01~2.00%. The average value of silt was 76.18%, which was the main particle size for 99% of the samples. The mean concentrations of As, Cd, Cr, and Ni were higher than the background value of Shandong Province or the background value of yellow soil in China. A higher concentration of Fe occurred in Phragmites australis (mean concentration 2.50%) and paddy field. The concentrations of Cd, Cr, Ni, Cu, Pb and Zn were lower in the Suaeda salsa soil. The Nemerow pollution index indicated that 79% of all samples showed low-level metal pollution, and 7% of all samples showed moderate-level metal pollution. In the areas with higher salinity, the concentrations of As were higher, while the concentrations of Cd and Ni were lower. The correlation analysis showed that with increasing pH, the concentrations of As and Zn decreased significantly. The metal concentrations had a significant positive correlation with clay, except for As.

Nested Delft 3D and Hydrus 1D models were applied to simulate variations in the hydrological process of tidal creeks, soil water, and salt transport in the soil profile of the reconstruction area in the Yellow River Delta under six gate dam scenarios. The results showed that the gate dam set up near the sea area was more conducive to reducing the variation range of water depth in the reconstruction area. The water depth changes in scenarios with 6 m gate valves were higher than those with 3 m sluice valves in the same gate dam location. The variations in surface water salinity, cumulative flooding time, flooding frequency, and cumulative infiltration in each scenario were similar to those for water depth. Rapid changes in soil water and salt content occurred in each scenario in periods without flooding. The fluctuation of soil salt content in different soil layers was contrary to the changes in soil water content. The overall difference in the soil salt contents and soil water content of the soil profile in scenarios with a gate dam near the sea was relatively larger than that of those with a gate dam near the shore. Obvious differences in both the soil water content and soil salt content between scenarios with 3 m and 6 m gate valves were not observed. Our results contribute to the understanding of the function of gate dams in controlling soil water and salt content in coastal wetlands.

The concentration and speciation of heavy metals in the intertidal zone play an irreplaceable role in protecting biodiversity. However, it could be changed by the constantly changing hydrological regimes and salinity. To identify the change and mechanisms of these phenomena, an incubation experiment was conducted under three hydrological regimes (no flooding, periodic flooding, and long-term flooding) and five salinities (0‰, 5‰, 10‰, 20‰, and 30‰). The concentration and speciation of Cd, Cr, Cu, Pb, and Zn in sediment cores collected at the first, third, fifth, seventh, and ninth week were detected. The results indicated that as the incubation time increased, the concentrations of Cr, Cu, and Pb decreased while the concentrations of Cd and Zn increased. The primary speciation for Cd was acid-soluble fraction, whereas the residual fraction was the dominant form for Cr, Cu, Pb, and Zn. The acid-soluble fraction of Cd was lowest in freshwater conditions. The oxidizable fraction of Cd generally increased under long-term flooding and was higher than that under no-flooding or periodic-flooding conditions. The speciation of Cr under freshwater and 5‰ salinity conditions was similar but distinctly different from that under other salinity levels. Cu was easily combined with organic matter, and the oxidizable fraction of Cu was the predominant form, aside from the residual fraction. The residual fraction of Pb observably increased in the ninth week. The general linear model revealed that hydrological regimes, salt conditions, and incubation time had an obvious influence on metal speciation. Throughout the incubation experiment, Cd posed a higher risk (ranging from 21.91% to 71.91%) and should be closely monitored. The risks associated with Cr and Zn also increased during the incubation period.