General properties of the surface sediments
The EC, TDS, pH, and ion concentrations (K+, Na+, Ca2+, Mg2+, F-, Cl-, NO- 3, SO2- 4) in the sediment samples collected from these salt lakes are shown in Table S1. For Beida Pond, the conductivity for surface sediment is in the range of 2.45 - 29.4 mS/cm, averaging 10.66 mS/cm, the average of TDS was 7.44 g/L, with a ranged from 1.7 - 20.5 g/L, and the pH ranged from 8.49 to 8.88. For Gouchi Pond, the EC present in the surface sediment is in the ranged from 1.91 - 10.6 mS/cm, with an average of 6.89 mS/cm, ranges of TDS and pH were 1.35 - 7.36 g/L and 8.25 - 8.88, respectively. For Chagannaoer, the EC and TDS ranged from 0.45 - 17.2 mS/cm and 0.31 - 12.2 g/L, respectively. The pH ranged from 10.19 - 10.94, with an average of 10.49. For Hongjiannao, ranges of EC and TDS were 0.11 - 0.86 mS/cm and 0.07 to 0.60 g/L, respectively. The average of pH is 9.48, with a ranged from 9.05 to 9.77. In conclusion, Beida Pond and Gouchi Pond have weakly alkaline salt deposits, whereas Chagannaoer and Hongjiannao have strongly alkaline salt deposits.
The main cations in the surface sediment soluble salts of Beida Pond were Na+ and Ca2+, and the main anions were Cl- and SO24-. The K+, Na+, Ca2+, and Mg2+ concentrations ranged from 12.71–71.38, 32.16–6268.75, 55.07–672.89, and 23.43–472.22 ppm, respectively; the Cl-, SO24-, and HCO3- concentrations ranged from 5.32–5485.35, 1146.42–8165.87, and 7.32–43.93 ppm, respectively. The main cations in the surface sediment soluble salts from Gouchi Pond were Na+, Ca2+, and Mg2+, and the main anions were Cl- and SO24-. The K+, Na+, Ca2+, and Mg2+ concentrations ranged from 4.36 - 33.56, 320.11 - 1465.92, 19.34 - 663.8, and 38.13 - 417.38 ppm, respectively, and the Cl-, SO24-, and HCO3- concentrations ranged from 261.6 - 2219.07, 387.23 - 3558.07, 36.61 - 213.32 ppm, respectively. The cations in the soluble salts of Chagannaoer surface sediments were Na+, and the anions were Cl-, SO24-, and CO23-. The K+, Na+, Ca2+, and Mg2+ concentrations ranged from 33.14 - 436.44, 112.51 - 4548.57, 1.01 - 110.71, and 10.10 - 74.48 ppm, respectively, and the Cl-, SO24-, and CO23- concentrations ranged from 4.15 - 2168.54, 10.00 - 481.35, and 66.37 - 2448.42 ppm, respectively. The cation in the surface sediment soluble salts of Hongjiannao was Na+, and the anions were Cl-, SO24-, and HCO3-. The K+, Na+, Ca2+, and Mg2+ concentrations ranged from 4.58 - 11.51, 10.02 - 156.87, 5.68 - 10.92, and 2.97 - 8.93 ppm, respectively, and the Cl-, SO24-, CO23-, and HCO3- concentrations ranged from 1.89 - 94.77, 9.78 - 96.02, 5.04 - 21.60, and 35.14 - 166.92 ppm, respectively.
Concentrations of metals in the surface sediments
The concentrations of heavy metals in the sediment samples are presented in Table S1. The ranges of Cr, Ni, Cu, Zn, As, Cd, and Pb in the salt lakes of the Ordos were 50.84 - 261.73, 6.05 - 36.94, 3.59 - 24.33, 12.57 - 66.38, 0.04 - 1.24, 0.01 - 0.04, and 8.77 - 22.38 μg/g, respectively (Table.2). The mean heavy metal concentration in pond sediments was 75.49, 18.08, 9.91, 29.75, 0.37, 0.02, and 13.71, respectively. The concentration of heavy metals in Gouchi Pond followed a similar pattern to that of Gouchi Pond: Cr > Zn > Ni > Pb > Cu > As > Cd. The means of heavy metal contents in Chagannaoer sediments were 142.66 (Cr), 9.21 (Ni), 5.85 (Cu), 19.46 (Zn), 0.63 (As), 0.01 (Cd), and 15.08 (Pb), respectively, and followed a similar pattern to those of Hongjiannao sediments: Cr>Zn>Pb>Ni>Cu>As>Cd. In comparison, the Ni, Cu, and Zn concentrations in Beida Pond and Gouchi Pond were higher than those in Chagannaoer and Hongjiannao. The Cr concentration in Chagannaoer and Hongjiannao was significantly higher than that in Beida Pond and Gouchi Pond. The Pb concentration in the four salt lakes was similar, at 13.71, 15.15, 15.08, and 16.66, respectively. The concentrations of As and Cd in all the salt lake sediments were lower than the background values, within the ranges of 0.04 - 1.24 and 0.01 - 0.04, respectively.
Table.2 Heavy metal concentrations (based on dry weight basis) and background values (mg∙kg−1) in the surface sediment of the salt lake in Ordos, China
Salt lake
|
Cr
|
Ni
|
Cu
|
Zn
|
As
|
Cd
|
Pb
|
Beida pond
(mean)
|
50.84-130.89
(75.49)
|
14.75-26.73
(18.08)
|
7.72-19.51
(9.91)
|
23.94-50.15
(29.75)
|
0.07-1.08
(0.37)
|
0.02-0.03
(0.02)
|
8.77-17.10
(13.71)
|
Gouchi pond
|
65.79-89.22
(73.79)
|
17.86-36.94
(23.89)
|
10.70-24.33
(13.95)
|
30.41-66.38
(40.51)
|
0.36-0.92
(0.62)
|
0.02-0.04
(0.03)
|
14.04-22.38
(15.15)
|
Chaigannaoer
|
85.66-165.16
(142.66)
|
7.54-16.57
(9.21)
|
3.91-9.86
(5.85)
|
14.25-31.94
(19.46)
|
0.27-1.24
(0.63)
|
0.01-0.02
(0.01)
|
13.16-16.13
(15.08)
|
Hongjiannao
|
77.74-261.73
(130.12)
|
6.05-13.60
(9.20)
|
3.59-8.10
(5.33)
|
12.57-29.03
(19.00)
|
0.04-1.05
(0.83)
|
0.01-0.02
(0.02)
|
15.88-16.92
(16.66)
|
Backgrounda
|
24.8
|
11.5
|
8.8
|
23.5
|
4.3
|
0.044
|
13.8
|
a the concentration metal of soil in Ordos Plateau (Centre) 1990).
Risk assessment
Geoaccumulation index
The geoaccumulation index (Igeo) values of heavy metals detected in the sediments of the salt lake in the Ordos are shown in Fig. 2, and the details are presented in Table S1. The Igeo ranges of Cr in Beida Pond, Gouchi Pond, Chagannaoer, and Hongjiannao were 0.51 - 1.81, 0.82 - 1.26, 1.20 - 2.15, 1.06 - 2.81, respectively, suggesting moderate contamination by Cr in the four salt lakes. The Igeo of Ni in Chagannaoer and Hongjiannao were <0, and ranged from −0.22 - 0.63 (average, 0.13) and 0.05 - 1.10 (average, 0.48) in Beida Pond and Gouchi Pond, respectively, suggesting no contamination in Chagannaoer and Hongjiannao, but moderate contamination in Beida Pond and Gouchi Pond. The index values of Cu and Zn were <0 in the four salt lakes, except Gouchi Pond (average Cu, 0.12; average Zn, 0.22). The Igeo values of As, Cd, and Pb were < in the four salt lakes. Based on the Igeo values of these heavy metals, their pollution potential can be ranked as Cr>Ni>Zn>Cu>Pb>Cd>As in Beida Pond and Gouchi Pond, and Cr>Pb>Zn>Ni>Cu>Cd>As in Chagannaoer and Hongjiannao. The negative values of Ni (in Chagannaoer and Hongjiannao), Cu (in Beida Pond, Chagannaoer, and Hongjiannao), Zn (in Beida Pond, Chagannaoer, and Hongjiannao), and As, Cd, and Pb in the four lakes correspond to the uncontaminated level based on the Muller scale(Muller 1969), indicating have the potential to cause limited pollution to the salt lakes in the surroundings soil. In contrast, the Igeo values of Cr (in four lakes), Ni (in Beida Pond and Gouchi Pond), Cu (in Gouchi Pond), and Zn (in Gouchi Pond) suggested moderate contamination in the study area.
Enrichment factor
The EF is commonly used to determine the degree of anthropogenic heavy metal pollution (Li et al. 2020)(Atiemo et al., 2012). Generally, an EF of <1.5 suggests that an element is entirely controlled by natural processes, and 1.5 < EF < 3, 3 < EF < 5, and 5 < EF < 10 are interpreted as minor, moderate, severe, and very severe sediment contamination, respectively (Loska and Wiechula, 2003; Sutherland, 2000; Xu et al., 2017b). The average EF values of the heavy metals tested in this study followed the order Cr > Ni > Zn > Cu > Pb > Cd > As in Beida Pond and Gouchi Pond, and Cr > Pb > Zn > Ni > Cu > Cd > As in Chagannaoer and Hongjiannao (Fig. 3).
In Beida Pond, the average EF values of Cr (3.35), Ni (1.97), and Zn (1.55) were >1.5, and for Cu (1.47), Pb (1.06), Cd (0.64), and As (0.13) were <1.5. In Gouchi Pond, the average EF values of Cr (2.95), Ni (2.08), Zn (1.74), and Cu (1.63) were >1.5, and for Pb (1.13), Cd (0.65), and As (0.14) were <1.5. In Chagannaoer and Hongjiannao, the average EF values of all tested heavy metals were > 1.5, except for Cr, which was 5.43 and 5.52, respectively. These data indicated that the sediments in Beida Pond were moderately polluted by Ni and Zn, and severely polluted by Cr. The sediments in Gouchi Pond were moderately polluted by Cr, Ni, Cu, and Zn. The sediments in Chagannaoer and Hongjiannao were only very severely polluted by Cr. Thus, the results suggested that there was a minor anthropogenic impact from As, Cd, and Pb in the four salt lakes in the Ordos, a moderate anthropogenic impact from Ni and Zn in Beida Pond, and Cr, Ni, Cu, and Zn in Gouchi Pond, and a severe anthropogenic impact from Cr in Chagannaoer and Hongjiannao.
Potential ecological risk index
The ecological risk index (Er) was calculated to determine the potential ecological risk (RI) associated with heavy metals in sediments of the salt lakes in the Ordos. The standard level of the potential risk of heavy metals is presented in Table S1, indicating the various risk levels based on values of the index. The comprehensive RI values of individual heavy metals, individual sampling sites, and among the group sites were calculated and are presented in Table S2. The calculated mean Er for Cr, Ni, Cu, Zn, As, Cd, and Pb was 6.68 (4.10 - 21.11), 6.67 (2.63 - 16.06), 4.67 (2.04 - 13.83), 1.17 (0.53 - 2.82), 1.44 (0.10 - 2.88), 13.85 (7.24 - 30.54), and 5.60 (3.18 - 8.11), respectively. The seven heavy metals pose a low risk (Table 2). The RI values of sediments ranged from 28.50 to 79.45, and with a mean value of 41.94. The average RI for polluted habitats decreased in the following order Gouchi pond > Beidachi pond > Hongjiannao > Chagannaoer, demonstrating that the contaminated sediments in the sampling area pose a low ecological risk.
The risk assessment code (RAC)
Heavy metal analysis was performed using Tessier’s sequential extraction procedure to identify the potential bioavailability and mobility of heavy metals and their risk to the environment (Ma et al. 2016, Rosado et al. 2016). The values were categorized using the RAC classifications (Martley et al. 2004) : RAC < 1%, no risk; and metals with RAC values of 1% - 10%, 11% - 30%, 31% - 50%, and N75% were classified as low risk, medium risk, high risk, and very high risk, respectively. Cr, Ni, Cu, Zn, As, Cd, and Pb were mainly present in residual fractions, accounting for 51.14% - 68.45%, 43.35% - 54.52%, 33.02% - 66.11%, 54.65% - 65.78%, 44.32% - 61.38%, 9.52% - 45.55%, and 91.10% - 98.49% of the total, respectively (Table 3 and Table S2). This indicated that these metals were associated strongly with crystalline mineral structures, were stable under natural conditions, and had low transferability (Ma et al. 2016, Rosado et al. 2016, Xia et al. 2020). With the exception of Cu at the Ch-7 and Ch-8 sites of Chagannaoer, the proportion of these elements in fraction F1 was relatively low. Cu, Zn, and Ni were mainly present in fraction F2, accounting for 23.41% - 28.07%, and 19.35% - 31.76% of the total amount, respectively. Moreover, Cd mainly occurred in fractions F1, F2, F3, and F4, with proportions of 1.07% - 13.62%, 5.05% - 13.85%, 10.49% - 45.33%, and 19.75% - 50.15% of the total metal content, respectively.
Table 3 RACs of heavy metals from surface sediments in the salt lakes
Sites
|
Cr
|
Ni
|
Cu
|
Zn
|
As
|
Cd
|
Pb
|
BP-6
|
10.61
|
29.67
|
49.64
|
32.07
|
25.05
|
11.02
|
0.33
|
BP-7
|
9.30
|
24.75
|
42.22
|
28.74
|
13.38
|
12.61
|
0.34
|
GP-7
|
6.41
|
29.82
|
45.77
|
30.84
|
28.68
|
14.47
|
0.23
|
GP-8
|
6.18
|
28.57
|
43.68
|
26.94
|
21.99
|
15.80
|
0.69
|
Ch-7
|
10.76
|
32.72
|
48.32
|
31.23
|
14.75
|
19.32
|
3.85
|
Ch-8
|
15.39
|
36.90
|
63.55
|
38.98
|
22.94
|
18.67
|
7.67
|
HJ-5
|
7.23
|
25.80
|
29.15
|
21.14
|
10.88
|
13.11
|
1.24
|
HJ-4
|
6.36
|
25.65
|
25.56
|
22.17
|
11.04
|
8.90
|
0.95
|
Assessment of the risk posed by heavy metals in sediments at the sampling area using RAC revealed that Pb and Cr exhibited no mobility and had low potential bioavailability risk and that Zn, Ni, and As were categorized as medium risk (Fig. 3). Cu had the highest mobility, with a high risk. Based on the average RAC values, the environmental risk of the bioavailability fraction of these metals decreased in the order Cu (43.49) > Ni (29.23) > Zn (29.01) > As (18.59) > Cd (14.24) > Cr (9.03) > Pb (1.91). These RAC values indicated that Cu, Ni, and Zn posed the greatest ecological risk in this environment. Overall, heavy metals in sediments posed the greatest ecological risk at Chagannaoer, followed by Gouchi Pond, Beida Pond, and Hongjiannao.
In this study, the inconsistent results were obtained from the above evaluation methods for ecological risk of heavy metals. Igeo and EF values showed that Cr was the largest potential ecological risk heavy metal in all salt lakes. The Ni, Cu and Zn pollution in Beida Pond and Gouchi Pond was more serious than that in Chaganore and Hongjiannao. Both Er and RI show that the environmental risk of these heavy metals is low in all salt lakes, with the overall characteristic is Gouchi pond > Beida pond > Hongjiannao > Chagannaoer. However, RACs indicate that the ecological risk of Cu, Ni, and Zn was higher than Cr, with the order of Chagannaoer > Gouchi Pond > Beida Pond > Hongjiannao. These inconsistencies may be due to the different assessment objectives of the methods. Igeo, EF, Er, and RI were calculated by compare metal concentrations of pollutants with natural background levels, while RAC used metals from F1 and F2 to indicate metal mobility and bioavailability in sediments. Therefore, it is necessary to use a variety of methods and multi-purpose assessment of heavy metal levels, for the aims of understand the ecological risk of heavy metals to sediment fully and accurately.
Source identification and influence factors
The lakes in the study area were divided into two groups based on the pH of soluble salts in the sediments: the first group (group 1) was Beida Pond and Gouchi Pond (pH < 9), the second group (group 2) was Chagannaoer and Hongjiannao (pH > 9). Principal component analysis (PCA) was performed to analyze the potential sources and influence factors of heavy metals in the two groups of lake sediment. The PCA results passed the Bartlett sphericity tests (P < 0.001), indicating that the application of PCA was appropriate for the assessment of heavy metals, pH, anions, and cations in these salt lakes sediments. The principal components of the group 1 and group 2 of lakes that explained the variance accounted for 72.19% and 70.16% of the total variance, respectively (Fig. 4 and Fig. 5).
For group 1, the first principal component (PC1), which explained 49.73% of the total variance, was positively loaded with K+ (0.94), Na+ (0.85), Cl- (0.85), SO24- (0.80), Ni (0.72), Cu (0.95), Zn (0.96), Cd (0.76), and Pb (0.78). The correlation analysis coefficients showed that there were significant positive correlations between K+ and these metals, including Ni, Cu, Zn, Cd, and Pb, suggesting that these heavy metals may come from the same source (Table S4). Given that K+ and Na+ are mobile elements present in nature, it can be concluded that PC1 represents the sources associated with water - soluble elements, which are discharged by agricultural industries (Kharazi et al. 2021, Wang et al. 2021), resulting in heavy metal pollution. The second principal component (PC2) explained 22.50% of the total variance, and was positively loaded with SiO2 (0.916), Al2O3 (0.78), Cr (0.61), Pb (0.62), and As (0.43). There were positive correlations between SiO2, Al2O3 and Cr, Pb, and As, indicating that these heavy metals arose from the same source of lithospheric minerals carried by dust storms (Oliveira et al. 2011, Zhang &Wang 2020).
For group 2, PC1 was loaded by Al2O3 (0.92), Ni (0.99), Cu (0.99), Zn (0.99), Cd (0.78), Pb (0.91), and moderately by Cr (0.54), and this component accounted for 42.8% of the total variance. The correlation analysis coefficients indicated that Ni, Cu, Zn, Cd, Cr, and Pb might be derived from similar sources (Table S5). Cu, Cd, and Pb had significantly positive correlations with Al2O3, which was the essential element of the clay mineral (illite and montmorillonite) in sediment (Qiao et al. 2015, Sutherland et al. 2007); therefore, we suggested that PC1 may represent heavy metals via surface runoff and that chemicals from these sources may eventually accumulate in sediments. Moreover, As had a relatively smaller loading in PC1 and did not show a correlation with other metals, and this may also be derived from the dissolution and release of minerals. PC2, which explains 27.3% of the total variance, was mainly characterized by the positive loading of Na+ (0.92), Cl- (0.77), and SO24- (0.73), suggesting that these ions migrate in surface water and the groundwater surrounding lakes(Chen et al. 2020a, Zhang &Wang 2020).
Prior to the above, the bioavailability, toxicity and mobility of heavy metals were affected by pH, organic matter, cation substitution amount, and clay content in sediment-water. Pearson correlation analysis showed that when pH was greater than 9, metal element content was significantly negatively correlated with pH (Table S5). pH can indirectly or directly affect the solubility, adsorption, retention and movement of metals in the solid-liquid interface (Kashem &Singh 2001, Ma et al. 2016). It has been reported that increasing pH value can increase the possibility of conversion of heavy metal components into oxidizable and residual states (Zhang et al. 2014). In this study, the concentration levels of Ni, Cu, Zn and Cd in surface sediments decreased with the increase of pH, but F4 and F5 components did not show significant differences, indicating that high pH did not lead to chemical precipitation of heavy metals in the lake, but only affected the change of the total amount of metal elements in the sediments. Therefore, pH in sediment of Chagannaoer and Hongjiannao has a significant negative correlation with heavy metal concentration, which may be because high pH reduces the activity of heavy metals in water from source to sink, leading to lower heavy metal content in water and sediment of these two lakes.