3.1 Soil properties and contents of PTE in soil
As showed in Table 1, the pH of soils in Dongdagou stream valley and around Baiyin city were ranged from 6.47 to 7.91 and 6.45 to 9.4, respectively. The mean pH in soils of Dongdagou stream valley was obviously lower than the background value, which indicated that soils in Dongdagou stream valley was acidified due to mining activities (Li et al. 2019; Ma et al. 2021). Soil in study area was seriously salinized, which could be found from the EC value of soils. CaCO3 content in soils of Dongdagou stream valley was significantly lower than the background value for sierozem (118 g kg− 1) (CNEMC 1990), and the loss of CaCO3 was probably caused by soil acidification in the study area.
Table 1
Grassland soil properties and PTE contents in Dongdagou stream valley and around Baiyin city
Variable | Unit | Dongdagou (n = 17) | Around Baiyin city (n = 21) | BVc | RSVd | RIVe |
Min | Max | Mean | SD | CV | Min | Max | Mean | SDa | CVb |
pH | - | 6.47 | 7.91 | 7.20 | 0.45 | 6% | 6.45 | 9.40 | 7.58 | 0.70 | 9% | 8.5 | - | - |
EC | µs cm− 1 | 177 | 3640 | 1525 | 918 | 60% | 275 | 7837 | 2232 | 2007 | 90% | - | - | - |
CaCO3 | g kg− 1 | 41.71 | 143.15 | 96.72 | 33.68 | 35% | 35.18 | 185.02 | 112.03 | 35.34 | 32% | - | - | - |
OM | g kg− 1 | 3.22 | 31.287 | 12.158 | 8.832 | 73% | 4.259 | 26.712 | 11.242 | 6.598 | 59% | 26.5 | - | - |
Hg | mg kg− 1 | 0.14 | 18.39 | 4.45 | 6.78 | 152% | 0.10 | 2.22 | 0.35 | 0.45 | 129% | 0.02 | 1.8 | 2.5 |
As | mg kg− 1 | 15.52 | 222.69 | 116.42 | 65.14 | 56% | 5.67 | 52.98 | 19.42 | 14.25 | 73% | 12.60 | 40 | 150 |
Cu | mg kg− 1 | 26.68 | 1390.06 | 352.97 | 454.08 | 129% | 24.58 | 321.78 | 120.44 | 97.11 | 81% | 24.10 | 50 | - |
Zn | mg kg− 1 | 403.88 | 7347.50 | 2718.84 | 1834.68 | 67% | 318.78 | 4142.50 | 1103.17 | 964.67 | 87% | 68.50 | 200 | - |
Cd | mg kg− 1 | 0.12 | 148.13 | 31.84 | 46.04 | 145% | 0.14 | 44.25 | 5.61 | 10.07 | 179% | 0.12 | 0.3 | 2.0 |
Pb | mg kg− 1 | 40.26 | 5612.30 | 1122.53 | 1836.22 | 164% | 22.14 | 530.95 | 127.16 | 133.94 | 105% | 18.8 | 90 | 500 |
a Standard deviation |
b Coefficient of variation |
c Soil background value of Gansu Province (CNEMC, 1990) (in Chinese) |
d Risk screening value for soil contamination of agricultural land, pH > 5.5 (MEEPRC, 2018) (in Chinese) |
e Risk intervention values for soil contamination of agricultural land, pH > 5.5 (MEEPRC, 2018) (in Chinese) |
The concentrations of PTE in almost all soil samples of the study area exceeded the background values. It was worth noting that PTEs in 13 soil samples of Dongdagou stream valley were even exceeded the risk intervention values. The maximum concentrations of As, Cd and Pb in Dongdagou stream valley were reached 222.69 mg kg− 1, 148.13 mg kg− 1 and 5612.30 mg kg− 1, respectively, which indicated that PTEs in soils may pose risks to human health (MEEPRC 2018). Compared with the grassland around Baiyin city, the pollution of PTEs in soil of Dongdagou stream valley was more serious.
The CV of PTEs in soil from Dongdagou valley decreased in the order Pb (164%) > Hg (152%) > Cd (145%) > Cu (129%) > Zn (67%) > As (56%). The large CV for Pb, Hg, Cd and Cu indicated that PTE contents varied greatly at different sites (Xiao et al. 2015). The biggest CV of PTEs in soils around Baiyin city was Cd (179%). The contents of soil PTE in the east of Baiyin city were much higher than that in other directions of Baiyin city, because the eastern region of Baiyin was more easily affected by mining activities.
3.2 Correlation analysis
As showed in Table 2, the correlation among the total Hg, Cu, Zn, Cd and Pb in soils of Dongdagou were significant at p < 0.01, but in the soils around Baiyin city, this correlation was not so obvious. PTEs except As exhibited significant negative correlation with pH in soils of Dongdagou. These results implied that there was a high degree of homology among Hg, Cu, Zn, Cd and Pb in soils of Dongdagou stream valley (Manta et al. 2002), and Hg, Cu, Zn, Cd and Pb concentrations decreased with increasing pH in Dongdagou. However, this result was not found in the grassland soil around Baiyin city, which may be because soil pH around Baiyin city was not significantly affected by mining activities, making this correlation weaker than that of Dongdagou stream valley.
Table 2
Pearson correlations between PTE concentrations and physiochemical properties of grassland soils
| pH | EC | CaCO3 | OM | Hg | As | Cu | Zn | Cd | Pb |
pH | 1 | -0.229 | -0.563** | -0.386 | 0.052 | -0.273 | -0.336 | -0.105 | -0.187 | -0.085 |
EC | 0.312 | 1 | 0.368 | 0.519* | 0.085 | 0.522* | 0.502* | 0.048 | -0.056 | 0.485* |
CaCO3 | 0.252 | 0.222 | 1 | 0.635** | 0.226 | 0.276 | 0.348 | 0.195 | 0.186 | 0.420 |
OM | -0.343 | -0.212 | 0.174 | 1 | -0.025 | 0.698** | 0.504* | 0.015 | -0.062 | 0.331 |
Hg | -0.531* | -0.223 | -0.081 | -0.097 | 1 | -0.052 | 0.461* | 0.481* | 0.421 | 0.781** |
As | -0.220 | -0.213 | -0.607** | -0.464 | 0.148 | 1 | 0.705** | 0.159 | 0.114 | 0.423 |
Cu | -0.701** | -0.049 | -0.312 | 0.250 | 0.698** | 0.207 | 1 | 0.478* | 0.361 | 0.718** |
Zn | -0.531* | -0.267 | -0.420 | -0.049 | 0.865** | 0.204 | 0.698** | 1 | 0.890** | 0.431 |
Cd | -0.630** | -0.329 | 0.050 | 0.278 | 0.879** | -0.127 | 0.655** | 0.757** | 1 | 0.343 |
Pb | -0.674** | -0.154 | -0.153 | 0.035 | 0.924** | 0.142 | 0.880** | 0.857** | 0.843** | 1 |
Bold represents the correlation coefficients around Baiyin City, and the other stands for Dongdagou stream valley |
*p < 0.05 (2 tailed) |
**p < 0.01 (2 tailed) |
Although the As concentration in soils of Dongdagou stream valley was very high, there was no good correlation between As concentration and physicochemical properties (except CaCO3 contents) and other PTEs. Different from other metal ions, oxidation conditions and alkaline conditions promote the mobility of As (Williams et al. 2005; Biswas et al. 2014; Wu et al. 2016; Torres et al. 2017), which may be the reason for the poor correlation between As and other metals.
3.3 Migration of PTEs in soil-plant systems
The TF and BCF in plants from Dongdagou stream valley and around Baiyin city were presented in Table 3. The mean TF and BCF from Dongdagou valley decreased in the order Pb (1.12) = Zn (1.12) > Hg (0.8) > Cd (0.62) > Cu (0.36) > As (0.31) and Cu (0.46) > Hg (0.32) = Pb (0.32) > Cd (0.29) > Zn (0.21) > As (0.12), while the mean TF and BCF around Baiyin city decreased in the order Hg (2.34) > Pb (1.59) > As (1.09) > Zn (0.96) > Cu (0.8) > Cd (0.41) and Cu (0.67) > Pb (0.47) > Hg (0.39) > Zn (0.34) > As (0.28) > Cd (0.13). Both TF and BCF of Hg, As, Cu and Pb in Dongdagou stream valley were lower than that around Baiyin city, which may be because the absorption level of PTEs by plants in different regions had changed under metal stress (Bhat et al. 2019; Li et al. 2021). Dian and Giok (2017) reported that the efficiency of metal translocation may be affected by the systems responsible for capillary action in plants. Due to the adaptive responses of plant to pollutants, many plant species had become metal tolerant, as these plants were growing in the contaminated area from a long period (Sainger et al. 2011). The results also demonstrated that efficient of Pb and Zn transported from roots to aboveground parts in Dongdagou stream valley was higher than that of other PTEs which was consistent with Singh et al. (2010) and Loris et al. (2022). Migration of Cu, Hg and Pb from soil to plant was higher than that of As, Cd and Zn in this study, which indicated that Cu, Hg and Pb in plants of study area should be noted attention more than other metals. In other studies, the transfer coefficient value for Cd was higher than other metals (Bergmann 1992; Coumar et al. 2016). This may be because plants in the study area have only so much ability to accumulate heavy metals, supported by the findings of Chang et al. (1987) and Barbarick et al. (1995).
Table 3
TF and BCF of plants in Dongdagou stream valley and around Baiyin city
PTEs | TF | | BCF |
Dongdagou | Around Baiyin city | | Dongdagou | Around Baiyin city |
| Min | Max | Mean | SD | Min | Max | Mean | SD | | Min | Max | Mean | SD | Min | Max | Mean | SD |
Hg | 0.22 | 2.34 | 0.80 | 0.61 | 0.25 | 6.80 | 2.34 | 1.94 | | 0.04 | 0.59 | 0.32 | 0.16 | 0.06 | 0.89 | 0.39 | 0.20 |
As | 0.02 | 2.15 | 0.31 | 0.50 | 0.07 | 4.82 | 1.09 | 1.11 | | 0.01 | 0.43 | 0.12 | 0.11 | 0.04 | 0.76 | 0.28 | 0.22 |
Cu | 0.11 | 0.79 | 0.36 | 0.23 | 0.15 | 1.93 | 0.80 | 0.51 | | 0.10 | 1.10 | 0.46 | 0.29 | 0.14 | 1.42 | 0.67 | 0.37 |
Zn | 0.38 | 2.64 | 1.12 | 0.67 | 0.45 | 1.75 | 0.96 | 0.38 | | 0.08 | 0.68 | 0.21 | 0.15 | 0.07 | 1.00 | 0.34 | 0.24 |
Cd | 0.08 | 1.96 | 0.62 | 0.46 | 0.14 | 0.74 | 0.41 | 0.20 | | 0.02 | 0.83 | 0.29 | 0.24 | 0.04 | 0.30 | 0.13 | 0.07 |
Pb | 0.31 | 3.39 | 1.12 | 0.74 | 0.40 | 5.83 | 1.59 | 1.53 | | 0.09 | 0.78 | 0.32 | 0.19 | 0.04 | 0.89 | 0.47 | 0.25 |
As shown in Fig. 2, a good correlation was exhibited between PTEs in root/aboveground parts of plants and DTPA extractable PTEs in Dongdagou stream valley (except Hg and Zn) and around Baiyin city (except Hg and As). The results indicated that DTPA extractable PTEs could well reflect the absorption efficiency of plants for metals (Soriano-Disla et al. 2010; Wan et al. 2020; Xing et al. 2020). The DTPA extraction of Cu, Zn and Pb had been satisfactorily predicted in some studies (Hooda et al. 1997; Brun et al. 2001; Meers et al. 2007; Soriano-Disla et al. 2010). But extraction with DTPA was not a good method to assess phytoavailability of Hg, which was consistent with Luis Rodríguez et al. (2017). Higueras et al. (2003) reported that most Hg was in cinnabar form or bound to organic matter near mining area, and both forms seemed not to be phytoavailable.
It could be seen that there was a significant positive correlation between contents of PTEs in the aboveground parts and roots in Dongdagou stream valley, but this result wasn’t found in Hg and As around Baiyin city. It showed that in the study area, PTEs could still be transported from the root to the aboveground part even if plants were stressed by complex PTEs. At the same time, it showed that the absorption capacity of roots to PTEs was one of the main factors affecting the migration of PTEs from soil to aboveground parts of plants. In fact, migration of PTEs from soil to plant depended on the uptake and transport capacity of plant roots and cells for PTEs (Clemens et al. 2002; Yang et al. 2005).
3.4 Distribution of physicochemical properties and PTEs in Dongdagou stream valley soil
The bioavailability of PTEs was correlated with soil pH, organic matter content and texture (Pietrzykowski et al. 2014). Therefore, it was necessary to understand the distribution of soil physiochemical properties in the study area. As shown in Fig. 3, the soil pH was weakly acidic in the upper reaches of Dongdagou, but with the increase of distance, the soil pH increased rapidly and become alkaline. This was because the acid waste water or wastes produced by mining activities had dramatically reduced the soil pH, while the soil in the river valley had a strong buffer effect on the pH, making the soil pH return to normal soon. EC in soils first decreased and then increased, which was mainly affected by the agricultural activities in the middle reaches of Dongdagou. Salinization was one of the main impacts of agriculture on soil quality (Zalidis et al. 2002), and human activities may lead to soil salinity accumulation in multiple direct and indirect ways (Misopolinos 1990). However, the content of OM in Dongdagou stream valley always shown a significant downward trend.
The migration of PTEs in topsoil along Dongdagou valley was shown in Fig. 4. The contents of Hg, Zn, Cd and Pb in soils increased first and then decreased. It was worth noting that the content of Cd and Pb decreased to a very low level at the end of Dongdagou stream valley. The concentration of Cu in soils was almost always decreasing, but the concentration of As had no obvious trend. These results indicated that Hg, Zn, Cd and Pb had a similar migration ability, while As had a stronger migration ability than other PTEs in study area.
The mobility and bioavailability of Hg in soils was low (Santos-Francés et al. 2011), because Hg0, which had a low reactivity, accounted for a large proportion in soils near the smelting plant and mining area (Kocman et al. 2004). Hg0 was important in terms of environmental risk because it allowed Hg to accumulate near pollution sources without moving further with soil or water. Some previous studies had shown that the retention and mobility of Cd were dependent on such factors as pH, temperature, CEC and ionic strength (Gray et al. 1999; Lair 2006). Galunin et al. (2014) found that the acidity of environmental samples resulted in more intense competition between Cd2+ and protons, resulting in weakened cadmium adsorption, which was consistent with the change of Cd in Dongdagou stream valley soil. Mobility of PTEs was also affected by the high affinity in the TOM in topsoil. PTE concentrations in soil profiles decreased with depth, confirming the high accumulation of these elements in organic and organo-mineral topsoil (Godbold and Hüttermann 1985). This was due to the high adsorption and accumulation capacity of PTEs by organic matter and mineral-humus soil complexes (Kabata-Pendias and Pendias 1992). There was also easier migration of PTEs into the soil in sandy soils (Pietrzykowski et al. 2014). The migration of As was possible under extremely high pH conditions (pH > 8) because high pH promoted the dissolution of As forms under reducing conditions, and other anions promoted the desorption of As (V) from the soil solid phase, especially under alkaline conditions (Krysiak and Karczewska 2007). The release of As under alkaline conditions could be explained by its anionic character and the balance between adsorption and desorption processes (Bowell 1994; Liu et al. 2001). Biswas et al. (2014) and Torres et al. (2017) found that a high As (V) adsorption was existed at a lower pH, whereas As (V) tended to be less adsorbed on hydrous oxides at elevated pH levels. Krysiak and Karczewska (2017) also reported that As bound to soil humus and released as the humus decomposes under alkaline conditions.
3.5 Risk assessment of PTEs in study area
3.5.1 Ecological risk assessment of PTEs in plants
Cattle and sheep were the main herbivorous livestock in the study area. Therefore, the effects of plant PTEs on cattle and sheep were analyzed (Table 4). The maximum tolerable toxic metal level, defined as the dietary level that didn’t impair accepted indicators of animal health or performance when fed for a defined period of time. It could be seen from Table 4 that the average values of PTE in Dongdagou stream valley plants far exceeded the maximum levels tolerated by cattle and sheep. The mean values of metals in aboveground parts of plants from Dongdagou stream valley was much higher than that around the city. The exceedance rate of PTEs in plants of Dongdagou stream valley and around Baiyin city decreased in the following order: 82% (Hg) = 82% (As) > 65% (Pb) > 41% (Cd) and 43% (Hg) = 43% (As) > 38% (Pb) > 14% (Cd), respectively, and the average values of PTE in plants of Dongdagou stream valley and around Baiyin city exceeded the Hygienical Standard for Feeds indicating that the plants in the study area were no longer suitable for feed.
Table 4
Descriptive statistics of PTE concentrations in aboveground parts of plants and related standards
PTEs | Dongdagou | Around Baiyin city | Hygienic Standard (mg kg− 1) a | Maximum levels tolerated by livestock (mg kg− 1 dry diet) (Madejón et al., 2002) |
Min | Max | Mean | SD | Min | Max | Mean | SD | Cattle | Sheep |
Hg | 0.04 | 2.82 | 0.72 | 0.78 | 0.01 | 0.31 | 0.11 | 0.07 | 0.1 | - | - |
As | 0.49 | 16.22 | 9.64 | 5.24 | 0.68 | 10.95 | 4.10 | 3.03 | 4 | 50 | 50 |
Cu | 11.93 | 605.80 | 102.30 | 138.14 | 19.10 | 254.52 | 62.54 | 54.99 | - | 100 | 25 |
Zn | 33.91 | 1015.81 | 503.88 | 270.34 | 99.66 | 976.28 | 281.81 | 224.93 | - | 500 | 300 |
Cd | 0.03 | 23.89 | 6.77 | 9.27 | 0.02 | 2.69 | 0.38 | 0.63 | 1 | 0.5 | 0.5 |
Pb | 10.38 | 541.29 | 173.72 | 193.16 | 7.77 | 237.05 | 49.85 | 61.87 | 30 | 30 | 30 |
a Hygienical Standard for Feeds (CNSBQTS, 2017) (in Chinese) |
It was well established that toxic metals had potential adverse effects in livestock, and naturally occurring episodes of acute and toxic intoxication had been much described in the literature (López-Alonso 2012). Large amounts of PTEs may be transferred from contaminated soils to plants, leading to accumulation of these PTEs in cattle and sheep (López-Alonso et al. 2003; Miranda et al. 2005). Accumulation of PTEs could cause toxic effects in herbivores, also in humans who consume meat and milk contaminated with toxic metals (González-Weller et al. 2006; Vromman et al. 2008; Cai et al. 2009). Therefore, the study area should continue to be PTEs pollution of plants safety as a target.
3.5.2 Non-carcinogenic risk assessment of PTEs in soil
The spatial distribution maps of HQ and THI in soils from study area were presented in Fig. 5 and Fig. 6. The HQ or THI values were indicated by different colors in the map, and red indicated non-carcinogenic risk beyond the acceptable risk level. The distribution of HQ and THI in study area showed that the non-carcinogenic risk of Hg, Cu, Zn and Cd in soils of the study area to human body was at an acceptable level, and the high-risk area was distributed in the Dongdagou stream valley, especially near the mining area. The HQ values of Pb in soils near the mining area, which was the upper of Dongdagou stream, exceeded acceptable level, while the risk level of As in Dongdagou stream valley from upstream to downstream was beyond the acceptable level. These results were consistent with the migration of PTEs along Dongdagou stream valley. In terms of the total non-carcinogenic risk of PTEs in soils, Dongdagou stream valley was the risk area, while the health risk of grassland soils around Baiyin city where far away from mining area was at an acceptable level. This result revealed that Dongdagou stream was the most important way of pollutant diffusion in the study area.
Comparing HQ of As and THI distribution maps, it could be found that As contributed the most to health risk in study area. Cao et al. (2014) found that the health risks associated with arsenic exposure in people should be taken seriously, with ingestion appearing to be the main route of exposure to arsenic and other PTEs, followed by inhalation exposure. Zang et al. (2017) revealed that that Cu was slight polluted, Zn was moderate polluted, and Pb was practically unpolluted in the middle and end of Dongdagou stream valley. The non-carcinogenic risk assessment results in this study were consistent with the research results of Zang et al. (2017), and this study also found the non-negligible risk of Pb was mainly located in the upper stream. In addition to the strong mobility of As in the study area, the application of various fertilizers and pesticides in agricultural activities in Dongdagou area may also be another contributor to the serious pollution of As in soil (Luo et al. 2009; Zhou et al. 2018).