3.1 Physical and chemical properties of the soil profile and total heavy metal content in the Yellow River wetlands
The physicochemical properties and total heavy metal contents of the soils in the Yellow River wetlands are presented in Table 2. The soil pH values of the Tianhe Bay wetlands in Ningxia in the upper reaches of the Yellow River ranged from 7.63 to 8.54, with an average value of 8.12, indicating overall alkaline conditions. The total organic carbon (TOC) content of the soils varied between 0.25% and 0.57%. The soil moisture content ranged from 20.43–33.73% and decreased with increasing soil depth. The lowest TOC content and the highest moisture content were observed at sampling site NX-GD. The highest moisture content was caused by the fact that this site had been freshly tilled by human activities and exhibited evident signs of watering. The lowest TOC content was attributed to the absence of surface vegetation at the site, which led to the plant roots being the main source of the soil TOC(Guo and Gifford 2002).
Table 2
Physical and chemical properties of soil profiles in wetlands in the upper reaches of the Yellow River
| Upper reaches | Middle reaches | Lower reaches |
THW-LW | THW-CD | THW-GD | THW-YL | YJW-LS | YJW-LW | YJW-XP | MZC-LW | MZC-XP | MZC-GT | HYK | HHT | SJZ-ND | SJZ-YL | SJZ-XH |
ph | 7.63 | 8.54 | 8.27 | 8.05 | 7.81 | 7.95 | 7.90 | 7.45 | 7.57 | 7.77 | 7.89 | 7.74 | 7.95 | 8.19 | 7.98 |
TOC (%) | 0.41 | 0.35 | 0.25 | 0.57 | 0.23 | 0.12 | 0.22 | 0.63 | 0.40 | 0.15 | 0.17 | 0.20 | 0.57 | 0.90 | 0.13 |
MC (%) | 26.42 | 20.43 | 33.73 | 27.61 | 26.82 | 23.25 | 23.30 | 32.32 | 31.24 | 26.14 | 20.21 | 20.02 | 27.61 | 32.12 | 19.89 |
The soil pH values of the wetlands in Henan in the middle reaches of the Yellow River ranged from 7.44 to 7.95, with an average of 7.74, indicating mildly alkaline conditions. The soil TOC content ranged from 0.12–0.23% in the Sanmenxia reservoir wetlands (three sampling sites YJW LS–XP), ranged from 0.15–0.63% in the Luoyang reservoir wetlands (three sampling sites MZC LW–GT), and was 0.17% and 0.20% at two sampling sites (HYK and HHT) in the Zhengzhou reservoir wetlands, respectively. Among these, the MZC-GT sample site, which did not have surface vegetation, had the lowest soil TOC content among the sampling sites in the Luoyang reservoir wetlands, further confirming the positive correlation between the quantity of surface vegetation and the soil TOC content. Among the wetland sampling sites in the Henan section of the Yellow River, MZC-LW and MZC-XP had notably higher soil moisture contents than the other sites and had the highest soil TOC contents. This may have been due to the greater soil pore water at these two sites caused by the high-moisture conditions, which enhanced the transport of the dissolved organic carbon (DOC) and particulate organic carbon (POC) in the soils(A, A et al. 2018).
The soil pH values of the delta wetlands in the lower reaches of the Yellow River ranged from 7.95 to 8.19, with an average of 8.04, indicating overall alkaline conditions. The soil TOC content ranged from 0.13–0.90%, and the soil moisture content ranged from 19.89–32.12%.
3.2 Vertical distribution of heavy metals
The statistics of the contents of the various heavy metals across the different sampling sites in the Yellow River Basin are presented in Table 3. The following observations were made. (1) In the upper reaches of the Yellow River, except for Pb, the contents (in mg·kg− 1) of all of the heavy metals exceeded the environmental background values. The contents were as follows: As (16.69), Cd (0.27), Cr (81.95), Cu (26.31), Mn (838.52), Ni (33.38), Pb (19.90), Sb (1.51), and Zn (72.36). (2) In the middle reaches of the Yellow River, except for Cu, Ni, Pb, and Zn, the contents (in mg·kg− 1) of the heavy metals exceeded the environmental background values. The contents were as follows: As (11.47), Cd (0.20), Cr (69.59), Cu (19.49), Mn (609.69), Ni (27.07), Pb (18.44), Sb (1.28), and Zn (55.26). (3) In the lower reaches of the Yellow River, except for Cu, Ni, and Pb, the contents (in mg·kg− 1) of the heavy metals exceeded the environmental background values. The contents were as follows: As (12.89), Cd (0.16), Cr (71.60), Cu (20.55), Mn (702.46), Ni (28.05), Pb (19.18), Sb (1.32), and Zn (60.69). The results also revealed that the contents of the heavy metals in the alkaline wetland soils in the upper and lower reaches of the Yellow River were higher than those in the mildly alkaline wetland soils in the middle reaches, reflecting the positive correlation between the soil pH and the amount of heavy metal adsorption. This positive correlation is attributed to the fact that alkaline conditions are favorable for the precipitation and stabilization of heavy metals(Wu, Hu et al. 2015). There is no significant pattern in the spatial distributions of the heavy metals across the various locations in the Yellow River Basin.
Table 3
Heavy metal contents at various sampling sites in the Yellow River Basin
Site | As (mg·kg− 1) | Cd (mg·kg− 1) | Cr (mg·kg− 1) | Cu (mg·kg− 1) | Mn (mg·kg− 1) | Ni (mg·kg− 1) | Pb (mg·kg− 1) | Sb (mg·kg− 1) | Zn (mg·kg− 1) |
Upper reaches | THW-YL | 16.39 | 0.26 | 82.49 | 25.68 | 834.83 | 33.57 | 19.62 | 1.50 | 71.31 |
THW-LW | 18.66 | 0.32 | 84.80 | 29.14 | 899.89 | 36.96 | 22.33 | 1.61 | 79.87 |
THW-GD | 13.79 | 0.22 | 77.28 | 21.64 | 714.53 | 28.40 | 17.18 | 1.32 | 60.96 |
THW-CD | 17.91 | 0.28 | 83.23 | 28.78 | 904.81 | 34.60 | 20.45 | 1.59 | 77.28 |
Average value | 16.69 | 0.27 | 81.95 | 26.31 | 838.52 | 33.38 | 19.90 | 1.51 | 72.36 |
Background value | 12.2 | 0.07 | 62.7 | 22.10 | 497 | 21.70 | 20.06 | 1.18 | 58.8 |
Middle reaches | YJW-LS | 11.80 | 0.13 | 69.00 | 18.15 | 602.83 | 27.94 | 15.85 | 1.26 | 51.50 |
YJW-XP | 10.85 | 0.12 | 67.78 | 16.47 | 552.28 | 25.78 | 14.60 | 1.22 | 47.18 |
YJW-LW | 11.52 | 0.14 | 67.62 | 18.40 | 549.23 | 26.92 | 15.49 | 1.26 | 52.22 |
MZC-LW | 11.83 | 0.34 | 66.39 | 22.38 | 767.82 | 26.90 | 24.26 | 1.43 | 67.46 |
MZC-XP | 10.95 | 0.25 | 86.19 | 21.72 | 564.29 | 30.64 | 20.08 | 1.30 | 51.91 |
MZC-GT | 8.46 | 0.18 | 56.12 | 15.03 | 489.72 | 21.76 | 19.11 | 1.00 | 52.14 |
HYK | 11.73 | 0.20 | 69.76 | 19.32 | 610.14 | 25.62 | 17.68 | 1.26 | 53.13 |
HHT | 14.62 | 0.22 | 73.83 | 24.45 | 741.20 | 31.00 | 20.41 | 1.45 | 66.54 |
Average value | 11.47 | 0.20 | 69.59 | 19.49 | 609.69 | 27.07 | 18.44 | 1.28 | 55.26 |
Background value | 10.86 | 0.11 | 67.03 | 21.37 | 583.48 | 28.64 | 20.19 | 0.93 | 60.22 |
Lower reaches | SJZ-ND | 15.56 | 0.17 | 76.60 | 26.00 | 870.86 | 33.71 | 21.29 | 1.45 | 70.28 |
SJZ-YL | 12.85 | 0.19 | 70.64 | 22.14 | 674.21 | 29.80 | 21.47 | 1.36 | 71.05 |
SJZ-XH | 10.27 | 0.12 | 67.56 | 13.52 | 562.32 | 20.63 | 14.77 | 1.16 | 40.75 |
Average value | 12.89 | 0.16 | 71.60 | 20.55 | 702.46 | 28.05 | 19.18 | 1.32 | 60.69 |
Background value | 8.70 | 0.14 | 64.2 | 24.2 | 590 | 28.3 | 25.2 | 0.76 | 66.6 |
The vertical distribution characteristics of the heavy metals in the wetlands in the Yellow River Basin are shown in Figs. 2–4. The heavy metal contents in the soil profiles of each sampling site generally decreased with increasing soil depth. This trend is attributed to the presence of humus, which is a product of plant decomposition, accumulates in the surface layer of wetland soils, and subsequently absorbs heavy metals, forming aggregates and leading to higher contents of heavy metals in the surface soil compared to the deeper layers(Jin, Ruhai et al. 2016).
In the upper reaches of the Yellow River, at sampling site THW, the contents of the heavy metals, except for Cr, generally exhibited the following order: NX-LW > NX-CD > NX-YL > NX-GD (Fig. 2). The vertical variations in the heavy metal contents in the soil profile at NX-GD were notable, indicating that the anthropogenic tilling significantly altered the soil layer structure at this site. At sampling site NX-YL, the heavy metal contents increased at depths of 15–20 cm, and then, they decreased with increasing depth. The elevated contents at depths of 15–20 cm were attributed to the presence of a large amount of plant roots in this depth interval, which adsorbed heavy metals, thus promoting the accumulation of metals. At sampling sites NX-LW and NX-CD, the heavy metal contents linearly decreased with increasing soil depth, indicating that the natural migration and distribution processes of the heavy metals in vertical soil profiles at these two sites were not significantly disturbed by human activities.
The vertical distribution characteristics of the heavy metals in the wetland sampling sites in Henan in the middle reaches of the Yellow River are shown in Fig. 3. It ca be seen that compared to the other wetlands, the three sampling sites in sampling plot YJW in the Sanmenxia reservoir wetlands were all covered by vegetation, and the heavy metal contents did not exhibit significant vertical variations, indicating that this area was less affected by anthropogenic activities. In the Luoyang reservoir wetlands, sampling site MZC-GT (covered by bare soil) had markedly lower heavy metal contents and soil TOC contents than those of sites MZC-LW (covered by Phragmites communis) and MZC-XP (covered by Typha orientalis), confirming the strong adsorption effect of the plant roots and soil TOC on the heavy metals in the soil. In the Zhengzhou reservoir wetlands, sampling site ZZ-HYK had slightly lower heavy metal contents at depths of0–15 cm depth compared to site ZZ-HHT, and the heavy metals exhibited aggregation at depths of 5–10 cm depth. This was likely due to adsorption by plant roots. The heavy metal contents at site ZZ-HHT generally decreased with increasing soil depth and exhibited marked variations at depths of 0–15 cm, indicating a strong anthropogenic influence at this site.
For sampling plot SJZ in the lower reaches of the Yellow River, the sampling sites varied markedly in terms of the sources of the materials and their physicochemical properties, leading to both similarities and differences in the vertical distribution characteristics of the heavy metal elements. Except for Cr, the contents of the heavy metals generally exhibited the following order: SJZ-ND > SJZ-LL > SJZ-XH (Fig. 4). Site SJZ-ND was located in an area where many birds, including the red-crowned crane (Grus japonensis) and the scaly-sided merganser (Mergus squamatus), engage in feeding and resting activities, which release heavy metals that have previously accumulated at this site(A, B et al. 2017). However, a previous study has shown that the deposition of bird feces can increase the contents of heavy metals in soil(De, La et al. 2018). At site SJZ-LL, except for Cd, Mn, and Zn, the contents of the heavy metals did not change significantly with increasing soil depth, indicating minimal anthropogenic impact. At site SJZ-XH, which was in an lake area used for crab cultivation, the contents of As, Ni, Sb, and Zn peaked in the middle soil layer and were significantly different than those at the other depths. This was likely due to changes in the hydrological conditions of the wetland caused by frequent changes in the lake water level. In marked contrast to the other heavy metals, Cr exhibited peak contents at depths of 0–5 cm and 20–25 cm. The underlying cause of this pattern still needs to be identified.
3.3 Igeo analysis
The Igeo values are presented in Table 4. In the Tianhe Bay wetlands in the upper reaches of the Yellow River, the Igeo values of Cd, Mn, and Ni were 0.71, 0.17, and 0.04, respectively, indicating mild contamination, while the other heavy metals were assessed to be clean in terms of their contamination risk. In the wetlands of the middle reaches of the Yellow River, the Igeo value of Cd was 0.28, also indicating mild contamination, while the other heavy metals were assessed to be clean. In particular, the Sanmenxia reservoir wetlands were ecologically healthy, and all of the heavy metals were assessed to be healthy. In the Luoyang and Zhengzhou reservoir wetlands, the Igeo values of Cd were 0–1, indicating mild contamination. Additionally, in the Luoyang reservoir wetlands, there was mild Pb contamination at all of the sampling sites. In the Zhengzhou reservoir wetlands, mild Sb contamination only occurred at sampling site HYK. In the delta wetlands of the Yellow River, the Igeo value of Sb was 0.21, while the Igeo values of the other heavy metals were less than zero.
Table 4
Geoaccumulation index (Igeo) values at various sampling sites in the Yellow River wetlands
Site | Igeo |
As | Cd | Cr | Cu | Mn | Ni | Pb | Sb | Zn |
Upper reaches | THW | −0.13 | 0.71 | −0.20 | −0.34 | 0.17 | 0.04 | −0.64 | −0.23 | −0.29 |
Middle reaches | YJW | −0.52 | −0.36 | −0.56 | −0.86 | −0.59 | −0.68 | −0.85 | −0.16 | −0.84 |
MZC | −0.65 | 0.64 | −0.53 | −0.70 | −0.53 | −0.70 | 0.04 | −0.17 | −0.66 |
HHT | −0.47 | 0.28 | −0.53 | −0.73 | −0.52 | −0.75 | −0.78 | −0.15 | −0.77 |
HYK | −0.16 | 0.39 | −0.54 | −0.39 | −0.24 | −0.47 | −0.57 | 0.06 | −0.44 |
Average | −0.44 | 0.28 | −0.52 | −0.66 | −0.46 | −0.64 | −0.49 | −0.10 | −0.67 |
Estuary | SJZ | −0.02 | −0.37 | −0.43 | −0.82 | −0.33 | −0.60 | −0.98 | 0.21 | −0.67 |
3.4 PLI analysis
The PLI values of the heavy metals at the various sampling sites are presented in Table 5. It can be seen that except for the Sanmenxia wetlands in the middle reaches of the Yellow River, the heavy metals at the other sampling sites were in a state of mild contamination. In the upper reaches of the Yellow River, it was found that there was heavy Cd contamination, mild Pb contamination, and moderate contamination of the other heavy metals. Among the wetland sampling sites in the middle reaches of the Yellow River, Cd had the highest CF value. There was heavy Cd contamination at sampling site MZC-LW in the Luoyang wetlands and moderate Cd contamination at the other sampling sites. The CF values of all of the heavy metals were lower at site MZC-GT than at the other sampling sites, indicating that plants may have a certain enrichment effect on heavy metals. In the Zhengzhou wetlands, the CF value was lower at site HYK than at site HHT, but both values were greater than 1, indicating mild contamination. In the delta lower-reaches wetlands of the Yellow River, sampling site SJZ-XH did not have heavy metal contamination, while the other two sampling sites (SJZ-ND and SJZ-YL) were classified as mildly contaminated. Specifically, except for Pb (CF value of <!) at site SJZ-ND and for Cu and Pb (CF valeus of < 1) at site SJZ-YL, all of the heavy metals had CF values of greater than 1, indicating a high level of pollution.
Table 5
Pollution load index (PLI) values at various sampling sites in the Yellow River wetlands
Site | CF | PLI |
As | Cd | Cr | Cu | Mn | Ni | Pb | Sb | Zn |
Upper reaches | THW-LW | 1.62 | 4.63 | 1.35 | 1.32 | 1.81 | 1.70 | 1.08 | 1.36 | 1.36 | 1.63 |
THW-CD | 1.56 | 3.94 | 1.33 | 1.27 | 1.82 | 1.59 | 0.99 | 1.35 | 1.31 | 1.55 |
THW-GD | 1.20 | 3.10 | 1.23 | 0.98 | 1.44 | 1.31 | 0.83 | 1.12 | 1.04 | 1.26 |
THW-YL | 1.43 | 3.76 | 1.32 | 1.16 | 1.68 | 1.55 | 0.95 | 1.27 | 1.21 | 1.47 |
Middle reaches | YJW-LS | 1.09 | 1.23 | 1.03 | 0.85 | 1.03 | 0.98 | 0.78 | 1.36 | 0.86 | 1.01 |
YJW-XP | 1.00 | 1.08 | 1.01 | 0.77 | 0.95 | 0.90 | 0.72 | 1.32 | 0.78 | 0.93 |
YJW-LW | 1.06 | 1.20 | 1.01 | 0.86 | 1.02 | 0.94 | 0.77 | 1.35 | 0.87 | 0.99 |
MZC-LW | 1.09 | 3.06 | 0.99 | 1.05 | 1.32 | 0.94 | 1.20 | 1.54 | 1.12 | 1.27 |
MZC-XP | 1.01 | 2.28 | 1.29 | 1.02 | 0.97 | 1.07 | 0.99 | 1.39 | 0.86 | 1.16 |
MZC-GT | 0.78 | 1.65 | 0.84 | 0.70 | 0.84 | 0.76 | 0.95 | 1.08 | 0.87 | 0.91 |
HYK | 1.08 | 1.82 | 1.04 | 0.90 | 1.05 | 0.89 | 0.88 | 1.35 | 0.88 | 1.07 |
HHT | 1.35 | 1.96 | 1.10 | 1.14 | 1.27 | 1.08 | 1.01 | 1.56 | 1.10 | 1.26 |
Lower reaches | SJZ-ND | 1.75 | 1.24 | 1.19 | 1.07 | 1.48 | 1.19 | 0.84 | 1.90 | 1.09 | 1.27 |
SJZ-YL | 1.44 | 1.34 | 1.10 | 0.91 | 1.14 | 1.05 | 0.85 | 1.79 | 1.10 | 1.16 |
SJZ-XH | 1.15 | 0.89 | 1.05 | 0.56 | 0.95 | 0.73 | 0.59 | 1.53 | 0.63 | 0.85 |
3.5 EF analysis
The EF indicates the degree of enrichment of a heavy metal in a location and also reflects the main sources of the heavy metal. An EF value of 1 suggests that the heavy metal originates from crustal activities such as rock weathering. An EF value of > 1 indicates that the heavy metal came from non-crustal activities, such as pollutant emissions and biological activities. As shown in Table 6, in sampling plots THW and MZC, which included both bare soil sites and vegetated sites, the EF values of the heavy metals were lower at the bare soil sites than at the vegetated sites, indicating that the local plants had a positive effect on the accumulation of heavy metals. In the upper reaches of the Yellow River, except for Mn and Sb, the EF values of the heavy metals at the different sampling sites exhibited the following order: THW-LW > THW-CD > THW-YL > THW-GD. The EF values of Pb at all of the sampling sites were approximately 1, indicating that the Pb in the THW area primarily originated from crustal activities such as rock weathering, and anthropogenic activities exerted little influence. The EF values of the other heavy metals were all greater than 1, suggesting that the enrichment of these metals may have been influenced by human activities. Moreover, the EF values of Cd were 3–5, indicating moderate enrichment, while those of the other heavy metals were classified as mildly enriched.
Table 6
Enrichment factor (EF) values of heavy metals at various sampling sites in the Yellow River wetlands
Site | EF |
As | Cd | Cr | Cu | Mn | Ni | Pb | Sb | Zn |
Upper reaches | THW-LW | 1.43 | 4.32 | 1.26 | 1.23 | 1.69 | 1.59 | 1.01 | 1.27 | 1.27 |
THW-CD | 1.42 | 3.80 | 1.28 | 1.23 | 1.76 | 1.54 | 0.96 | 1.30 | 1.27 |
THW-GD | 1.17 | 3.21 | 1.28 | 1.01 | 1.49 | 1.35 | 0.86 | 1.16 | 1.07 |
THW-YL | 1.29 | 3.62 | 1.27 | 1.12 | 1.62 | 1.49 | 0.92 | 1.23 | 1.17 |
Middle reaches | YJW-LS | 1.23 | 1.39 | 1.17 | 0.95 | 1.17 | 1.11 | 0.89 | 1.54 | 0.97 |
YJW-LW | 1.21 | 1.37 | 1.15 | 0.91 | 1.17 | 1.08 | 0.88 | 1.55 | 0.99 |
YJW-XP | 1.18 | 1.27 | 1.19 | 0.97 | 1.12 | 1.06 | 0.85 | 1.55 | 0.92 |
MZC-LW | 1.21 | 3.41 | 1.10 | 0.99 | 1.47 | 1.05 | 1.34 | 1.71 | 1.25 |
MZC-XP | 1.15 | 2.60 | 1.46 | 1.17 | 1.10 | 1.22 | 1.13 | 1.59 | 0.98 |
MZC-GT | 0.91 | 1.92 | 0.97 | 1.16 | 0.98 | 0.88 | 1.10 | 1.26 | 1.01 |
ZZ-HYK | 1.29 | 2.18 | 1.25 | 0.82 | 1.25 | 1.07 | 1.05 | 1.62 | 1.06 |
ZZ-HHT | 1.46 | 2.12 | 1.19 | 1.08 | 1.38 | 1.17 | 1.10 | 1.69 | 1.20 |
Lower reaches | SJZ-ND | 1.86 | 1.29 | 1.23 | 1.11 | 1.53 | 1.24 | 0.88 | 1.97 | 1.13 |
SJZ-LL | 1.64 | 1.50 | 1.22 | 1.02 | 1.27 | 1.17 | 0.95 | 1.99 | 1.22 |
SJZ-XH | 1.50 | 1.13 | 1.33 | 0.71 | 1.21 | 0.92 | 0.74 | 1.94 | 0.78 |
In sampling plots YJW and MZ in the middle reaches of the Yellow River, the EF values of As, Ni, Pb, and Zn were higher at the sites covered by Phragmites communis (i.e., the sites with LW in their names) than at the sites covered by Typha orientalis (i.e., the sites with XP in their names), indicating that Typha orientalis had a mildly stronger capacity to enrich these four heavy metals compared to Phragmites communis. This is consistent with the findings of Chen et al.(Chen,Ning et al.2020). The sampling sites in plot YJW did not exhibit Cu, Ni, Pb, or Zn enrichment, and the EF values of the remaining heavy metals exhibited the following order: Sb > Cd > As > Cr > Mn. Among them, As and Sb, such as at the sampling sites in plots MZC and ZZ, exhibited more-than-mild enrichment based on the fact that their EF values ranked in the top three, indicating that the Henan section of the Yellow River Basin may be subjected to varying degrees of As and Sb pollution.
In the lower reaches of the Yellow River, sampling site in plot SJZ did not exhibit metal enrichment or it exhibited only mild enrichment, and the EF values of As, Cu, Mn, and Ni exhibited the following order: SJZ-ND > SJZ-LL > SJZ-XH. Cu, Ni, Pb, and Zn were not enriched at site SJZ-XH, while Cu, Ni, and Z exhibited mild enrichment at sites SJZ-ND and SJZ-LL. This suggests that the degree of metal enrichment in these wetland soils is related to the hydrodynamic forces in the wetlands, under which the heavy metals in the soils may migrate with the water flow toward the center of the lake.
3.6 Correlation analysis
Elements with similar chemical properties generally tend to cluster and coexist under the same or similar geological conditions(Zhang,Luo et al.2022). Therefore, correlation analysis of the heavy metals within the same research area can help to determine whether they share a common source༈Li,Zhang et al.2013). The physicochemical properties of the soils and their correlation coefficients with the heavy metals across the sampling sites in the Yellow River Basin are shown in Fig. 5, except for the sampling sites in the Zhengzhou reservoir (HYK-HHT), which were too few to conduct correlation analysis.
Except for Cr, there were positive correlations among the heavy metals across the sampling sites in plot JYW, which was in the Sanmenxia reservoir wetlands in the middle reaches of the Yellow River. There were positive correlations between the pH and heavy metals at the sampling sites in plot THW (in the upper reaches of the Yellow River) and in sampling plot MZC (in the lower reaches) where the soils were largely alkaline. In contrast, there were negative correlations between the pH and heavy metals at the sampling sites in plot JYW (in the Sanmenxia reservoir wetlands) and plot MZC (in the Luoyang reservoir wetlands), both of which were located in the middle reaches of the Yellow River and largely contained neutral soils. This was because low-pH soils contain a large amount of H+ ions, which cause heavy metals to desorbed and become more reactive, making it difficult for them to cluster(Hu,Shen et al.2020). Due to its high cation exchange capacity and the presence of numerous different functional groups, the TOC can adsorb metal elements through surface precipitation, complexation, and ion exchange, thereby promoting the enrichment of heavy metals. A higher soil moisture content also facilitates the migration of heavy metals to lower-lying areas in wetlands. The soil moisture content at the sampling sites in plot THW exhibited negative correlations with the heavy metals. This was mainly due to the application of artificial watering at sampling site THW-GD, which led to higher migration and the loss of heavy metals from the soil.
Both sampling plots THW (in the upper reaches of the Yellow River) and SZJ (in the lower reaches) had correlation coefficients of > 0.8 among the heavy metals, suggesting that the heavy metals may have originated from the same source(Bai, Xiao et al. 2011). Sampling plot YJW, located in the Henan section of the middle reaches of the Yellow River, exhibited strong correlations between As vs Cr, Ni, and Pb, a strong correlation between Cr and Pb, a moderate correlation between Cr and Mn, and strong correlations between Cu and Mn, Ni, Sb, and Zn. At sampling plot MZC, Cr exhibited weak correlations with the other heavy metals and no enrichment to mild enrichment, indicating that the primary source of the Cr in this area may be crustal activity instead of human activities. Conversely, Cd was strongly correlated with Cu, Ni, and Sb and exhibited mild to moderate enrichment, suggesting that all four of these metals may originate from human activities.