Ecological Risk Assessment of Heavy Metals in Surface Soil of Qinghai Lake Area Based on Occurrence Forms

In this paper, the content and speciation of Cd, Cr, Pb, As, Cu, Zn and Ni in 87 groups of soil samples collected by grid method were determined by Inductively Coupled Plasma-Atomic Emission Spectrometry, to explore the speciation characteristics and ecological risk status of heavy metals in surface soil around Qinghai Lake. The results show that: Cd and Pb, compared with Cr, As, Cu, Zn and Ni, in the surface soil around Qinghai Lake are the most seriously polluted, Heavy metal speciation analysis showed that the proportion of weak acid soluble components of Cd was relatively high, with strong mobility and high environmental risk. The reducible components of Zn and Pb account for a relatively high proportion, while the oxidizable components of Cu, Cd and Pb account for a relatively high proportion. The high percentage of residual Cr, Ni and As indicates that they are relatively stable. The extractable content of Cd, Pb and Zn is relatively high, and their activities are strong, which have a great impact on the food chain. The results of Inductively Coupled Plasma-Atomic Emission Spectrometry and the ratio of secondary phase and primary phase showed that Cd and Pb had more serious ecological risk than other heavy metals. According to the results of soil speciation and ecological risk assessment of heavy metals around Chinese and foreign lakes, although the speciation characteristics of heavy metals are different, the pollution of Cd is relatively large. To sum up, heavy metal pollution in soil around Qinghai Lake area has occurred, and there is a serious ecological risk, so it is urgent to take corresponding monitoring and formulate treatment plan. Abstract The interaction between soil, organisms and surrounding environment continuously provides resources for human beings and ecology, is an important part of human environment, and is the basis of natural ecosystem and agricultural crop production [1–3]. Heavy metals are the most important persistent toxic pollutants in soil, and their effects are dicult to prevent and irreversible [4]. With the continuous development of economy, the expanding scope of people's activities and the continuous updating of technologies, pollutants

pastures, large fertile elds, rich pastures and mineral resources, which is a national key ecological protection zone. Soil nutrients and soil water resources are the focus of many geographers [27][28]. There have been some researches on the heavy metals in the soil of Qinghai Lake area, and some important results have been obtained [29][30]. For example, Wang et al. [31] analyzed the heavy metals in the northeastern surface soil of Qinghai Lake Basin, and found that copper and zinc were de cient and lead was enriched compared with the continental crust, so as to study the geochemical model, determine the possible sources and discuss the meaning of chemical weathering. Chang Huajin et al. [32] evaluated the pollution degree and source analysis of 9 heavy metals in the sediments of the lower reaches of the Shaliu River, and evaluated their risk degree. Lin et al. [33] analyzed the in uence of salinity, pH and other factors on the content of heavy metal cadmium in sediments and topsoil in Qinghai Lake area, and found that the enrichment degree of cadmium in lake sediments was signi cantly higher than that in the surface soil around the lake. But these studies mainly focus on the lake water body, and the evaluation of the river sediment of heavy metal content, and limited to a small area of Qinghai Lake or a particular landscape type, to be carried out for the entire system research of soil heavy metal pollution around Qinghai Lake region, there is almost no study of heavy metals in the region and based on the ecological risk assessment in the form of heavy metals. Recently, with the impact of climate change, tourism development and surrounding enterprises [34], the environment in the surrounding areas of Qinghai Lake has been damaged, and grassland degradation, deserti cation and other ecological problems have occurred in the land [35][36]. These uncertain factors will lead to the different distribution and proportion of heavy metals in the soil, and also have a certain impact on their migration. Therefore, the analysis of soil heavy metal speciation is an indispensable part in the study and evaluation of environmental pollution. In view of this, this paper takes the whole area around Qinghai Lake as the research object, and discusses the pollution distribution characteristics, occurrence forms and ecological risks of heavy metals in the surface soil of this region, so as to provide theoretical support and practical data reference for the protection of the ecological environment and human health in Qinghai Lake region.
1 Mathods And Materials

Overview of the study area
The study area is located in the northeast of Qinghai-tibet plateau around Qinghai Lake region, namely G109 highway, G315 road, the east and the west road within the surrounding area, surrounding monsoon region in the eastern part of our country, arid zone of northwest and southwest GaoHan District intersection zone, belongs to the typical plateau semi-arid, cold climate [37], where an average elevation of 3000 m above, and terrain tilt from west to east. It is high in the northwest and low in the southeast, and has two landforms: Qinghai-Tibet Plateau and Loess Plateau. It is snowy in winter and rainy in summer and autumn, with low temperature, big regional differences, big temperature difference between day and night, insu cient precipitation and concentrated period, distinct dry and wet season, intense sunshine and long time, solar radiation intensity increasing from southeast to northwest [38], annual average temperature ranging from − 4.6℃ to 4.0℃, annual average precipitation ranging from 291.0 to 579.0 mm, mostly concentrated in June to September [39]. Due to landform, climate, land use type and other factors, different land types in Qinghai Lake area mainly include mountain meadow soil, mountain shrub meadow soil, chernozem soil, chestnut soil, aeolian sand soil, etc. [40].

Methods for sample collection and determination
Before sampling, ArcGIS software was used to divide the remote sensing images of the area around Qinghai Lake into grids. A sampling point was laid out in each grid with a sampling density of 5 km. During the sampling in late May 2019, the sampling points were reasonably laid out by adjusting the location due to the actual factors such as terrain, buildings and Bridges, etc., through eld survey. The distribution of sampling points is shown in Fig. 1 The soil samples were pre-treated and the microwave digestion process was carried out in accordance with the "Microwave Digestion Method for Total Metal Elements in Soil and Sediments" (HJ832-2017) issued by the Ministry of Environmental Protection. After digestion, the content and speciation of heavy metals in the soil were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) after the supernatant was mixed to 25 mL at a constant volume and stood for 60 minutes.

Extraction and analysis of heavy metal speciation in samples
At present, Tessier method and BCR method are often used to classify heavy metal speciation in soil and sediments, including their improved methods. Among them, BCR three-step continuous extraction method has more practical signi cance and has been applied in the study of heavy metal speciation in soil by many scholars [41][42]. In this study, the improved BCR continuous extraction method was used to extract the heavy metal speciation from the surface soil samples around Qinghai Lake. See the references for the experimental process [43][44]. Finally, the measured values of various forms of heavy metal elements in the soil were calculated, and the recovery rate was 100% ±20%, indicating that the results of the speciation extraction in this study were reliable.

Quality control and data processing
This experiment in order to guarantee the reliability and accuracy of the experimental data, determine the time resolution of 16 samples to add two blank sample, morphological analysis process each step 2 blank samples were set up, take 10% of the sample do parallel sample analysis at the same time, the quality of the results of the analysis precision of test, the results measured the relative deviation meets the requirement of quality control [45][46]. Microsoft Excel 2016 and SPSS 20.0 software were used to make descriptive statistics and analysis of the data of soil heavy metal content, and the analysis results were plotted. The distribution map of soil sampling points was made by using   (Table 1). On the whole, only the mean contents of heavy metals Cd, Pb and As in the soil of the study area exceed the background value of heavy metals in Qinghai Lake soil [47]. From the perspective of single heavy metal elements, the content of Cd in the soil with the number of sample points accounting for about 84% exceeded the background value, accounting for the largest proportion. Among them, the content of 7 sample points even exceeded 5 mg/kg, which was about 40 times of the background value of the soil, seriously exceeding the background value. When the number of samples accounted for 59.8%, the multiple of Pb content in the soil exceeded the background value by less than 1 times. As, Cu, Zn, Cr and Ni accounted for 40, 35, 25, 12 and 8 samples that exceeded the background value, and the multiples beyond the background value were generally less than 0.5 times. In addition, the soil pH value of the sample sites in the study area ranged from 7.08 to 10.14, with an average value of 8.07. According to the soil pH classi cation standard, the surface soil in the study area was alkaline on the whole, and the pH value of all the soil sample sites was alkaline. However, the variation coe cient of the seven heavy metals in the soil was only larger, Cd, which was 2.29 (Table 1), indicating that Cd in the soil in the study area was more affected by external factors than other elements. In conclusion, the soil in the study area has been polluted by heavy metals, among which Cd and Pb are the most seriously polluted, followed by Cr, As, Cu, Zn and Ni.

Speciation analysis of heavy metals in soil
The speciation of heavy metals in soil varies according to the living environment and changes to different degrees, which is closely related to the toxicity, migration and circulation of heavy metals in nature [5,48]. In the improved BCR speciation extraction process, the heavy metals can be divided into four kinds of occurrence states: weak acid soluble state, reducible state, oxidizable state and residue state [49].
The mean analysis results of heavy metal occurrence speciation in the surface soil around Qinghai Lake are shown in Fig. 2. In terms of different heavy metals, the morphology of the 7 heavy metal elements are as follows: the weak acid soluble state and residue state of Cd are the most, followed by oxidizable state and reducible state. Cr, Pb, As, Cu, Zn and Ni have the largest proportion of residual states, and the smallest proportion of weak acid soluble states. The morphology of Cr, As, Cu and Ni is the same, and the oxidizable state is larger than the reducible state. The morphology of Pb and Zn is the same, and the oxidizable state is smaller than the reducible state.
In terms of the content ratios of different forms, the order of the proportion of the seven heavy metals in the surface soil around Qinghai Lake was Cd (34.8%) > Pb (9.8%) > Zn (

Ecological risk assessment of heavy metals in soil
The following two methods are used to evaluate the ecological risk of heavy metals in soil: risk assessment coding index and comparison value method of secondary phase and primary phase.
(1) The RAC risk assessment coding index proposed by Perin [52] in 1985 can be used to evaluate the bioavailability and availability of metals in solid media [53], which is determined by the ratio of exchangeable (i.e. weak-acid soluble) metals to the total content of a metal. According to the magnitude of the ratio, the risk was divided into ve levels [54]: RAC < 1%, no risk, 1%≤RAC ≤ 10%, low risk, 10%<RAC ≤ 30%, moderate risk, 30%<AC ≤ 50%, high risk, RAC > 50%, very high risk.
(2) The secondary phase compared to native (RSP) is a kind of value method based on soil and sediment geology to assess the size of the migration of heavy metals in sediment source and release risk method [55][56], to express the degree of heavy metal pollution said secondary phase (except the residue of other form) and heavy metal content in the primary phase (residue) the ratio of RSP < 1 for pollution-free, 1 ≤ RSP < 2 is light pollution, 2 ≤ RSP < 3 is moderate pollution, and RSP ≥ 3 is heavy pollution [57].
The results of the risk assessment coding index RAC and the comparison value of secondary phase to primary phase RSP are shown in Table 2. In terms of RAC, the mean values of the risk assessment coding index for heavy metals in 87 samples were 34.8% (Cd), 2.3% (Cr), 11.4% (Pb), 2.1% (As), 4.3% (Cu), 9.4% (Zn) and 2.5% (Ni), respectively. It can be seen that, except high risk Cd and medium risk Pb, the other ve heavy metals all have low ecological risk. According to the RSP method, the mean value of the RSP for the secondary phase and primary phase of each heavy metal in 87 samples in the study area showed that there was moderate pollution of Cd, slight pollution of Pb and Zn, and no pollution of other heavy metals. In conclusion, Cd and Pb are serious heavy metals in the surface soil around Qinghai Lake. There have been some studies on the soil heavy metal speciation around lakes or river basins at home and abroad.
Combined with the literature found, this paper summarized the study on the soil heavy metal speciation around lakes in China and compared it with the results of this study on the percentage of each heavy metal speciation in the total soil around Qinghai Lake, as shown in Table 3.
Note: the weak acid soluble state, oxidizable state, reducible state and residual state in the table are represented by a, b, c and d respectively.
Compared with the area around Qinghai Lake, the following results were obtained: (1) Compared with the area around Qinghai Lake, the forms of Cd in the soil heavy metals in Daye Lake Basin were mainly dissolved in weak acid and residual state, and the residual state accounted for the largest proportion of Pb, Cu and Zn forms. River area is relatively small, contrary to the situation in the Qinghai Lake area, and the proportion of residue state of Pb, Cu, Zn and Ni in the soil is relatively small, contrary to the situation in the Qinghai Lake area.
Based on the results in Table 3, it is further concluded that: (1)  In addition, the ecological risk assessment based on the form of heavy metals in soil around lakes at home and abroad was compared, and the results were detailed in Table 4. The data in the table show that: (1) Compared with Qinghai Lake area, the RAC index shows that the pollution of Cd is more serious, while the pollution of Pb, Cu and Zn is less. (2) Compared with Qinghai Lake area, Rac index showed that Cd was more serious than Cr, Pb, As, Cu and Zn in the Three Gorges Reservoir area. (3) RSP index showed that only As was unpolluted in Zhanjiang Bay and Qinghai Lake, and Cd was the most polluted heavy metal in both regions. (4) Compared with Qinghai Lake area, Rac index and RSP index showed that Cd was the most polluted heavy metal in Jiulong River basin, while Cr, As and Ni were less polluted in the two regions. (5) Compared with Qinghai Lake, the RAC index showed that the pollution of Cr, Cu, Zn and Ni in Sapanca Lake area was less than that in Qinghai Lake area. (6) Compared with Qinghai Lake area, the RAC index shows that the pollution of Cu, Zn and Ni is less, while the pollution of Cd is more serious in the Sinu River area.
Based on the further analysis in Table 4 and Table 5, it can be concluded that the pollution of Cd in the soil heavy metals around lakes in China is the most serious compared with other heavy metals, and the risk level is basically high, while the pollution risk in foreign regions is relatively low. Note: the reference value of heavy metals in the table is that the coe cient of variation is more than 1, The potential ecological hazard index Er > 10. Note: the reference value of heavy metals in the table is that the coe cient of variation is more than 1, The potential ecological hazard index Er > 10.
Combined with the soil heavy metal speciation distribution and ecological risk assessment in the surrounding areas of lakes at home and abroad, the comprehensive results show that the soil heavy metal pollution in the area around Qinghai Lake is not the most serious on the whole, and the indexes are not much different from those in other areas as a whole, but the indexes in some areas are relatively serious. In a word, Cd is a heavy metal that is easily affected by external factors. According to RAC and RSP indexes, Cd pollution is relatively large both at home and abroad, and the ecological risks in different regions are different to some extent, and some regions are relatively large, which should be paid attention to.

Conclusion
According to the above measurement and analysis of heavy metal content in 87 surface soil samples from the area around Qinghai Lake, the surface soil of the area around Qinghai Lake is polluted to different degrees, among which Cd, Pb and As exceed the background value of heavy metals in the soil of Qinghai Lake, while the others are not. Soil Cd pollution levels in the study area is relatively serious, has strong mobility and high environmental risk, easily in uenced by outside factors, mainly in the form of the state of weak acid soluble and Cd, Pb, and can extract the states of the zinc content is higher, activity is stronger, a greater in uence on the description of the food chain, and the ecological risk of Cd and Pb is serious. Combined with the research ndings of relevant areas at home and abroad, the law is consistent. Therefore, these two heavy metals should be the key elements to be paid attention to in the later treatment of soil heavy metal pollution in this region. It is suggested that relevant measures should be taken to reduce their biological effectiveness.

Declarations
Funding and acknowledgments: This research was nancially supported by Open fund project of Key Laboratory of environment and ecology in Qinghai Tibet Plateau No: 2018-QHS-K04. We also thank the anonymous reviewer for very helpful comments that improved the quality of the manuscript signi cantly.
Con icts of Interest: The authors declared that they have no con icts of interest to this work.
Location of surface soil sampling points around Qinghai Lake.

Figure 2
Morphological distribution of surface soil heavy metals in the area around Qinghai Lake.