Analysis of Soil Heavy Metal Pollution and Investigation of Dominant Plants in Abandon Gold Ming Area

: In recent years, environmental pollution and ecological destruction in mining area has 20 seriously affected the health and survival of surrounding people. To explore the degree of heavy 21 metal pollution in soil of mining areas and phytoremediation strategies, we chose soil and dominant 22 plants in an abandoned gold mining area as the research object. In this study, Arsenic (As) content 23 in soil and plants was measured. The As pollution degree in local areas was evaluated by using geo- 24 accumulation index and potential ecological risk index. And, As content in plants was analyzed by 25 using enrichment coefficient method. The results show that 1) The pulp deposition area has the most 26 serious As pollution of soil and the largest potential ecological risk index. 2) The composition of the 27 plant community in the study area is dominated by herbaceous plants, among which gramineous, 28 composites and legumes are the dominant plant types in the vegetation community restoration in 29 the mining area. 3) The geoaccumulation index is significantly negatively correlated with species 30 diversity index, Shannon-wiener diversity index and Pielou species evenness index. 4) The As 31 content in dominant plants ranges from 0.11 to 15.92 mg/kg. The plants with strong As enrichment 32 ability are Erigeron annuus (L.) Pers., Periploca sepium Bunge, and Setaria viridis (L.) Beauv., 33 which can be considered as As-tolerant plants. This study can provide a basis and reference for plant 34 management of As pollution in local soil. 35

between the degree of soil As pollution and vegetation composition and plant diversity was 89 discussed. This study can provide a theoretical basis and suggestions for the treatment and 90 restoration of As-contaminated soil, the improvement of the local ecological environment, and the 91 construction of beautiful and livable villages. 92 93 1. Materials and methods 94 1.1 Study area 95 Shangluo City is located in the southeastern part of Shaanxi Province, China. It is between 96 108°34′20′′-111°1′25′′ E and 33°2′30′′-34°24′40′′ N. The total area is 19,851 square kilometers, 97 whose geographical distribution is in the junction area of the northern subtropical and warm 98 temperate zone. In the horizontal direction, there are transitional characteristics of two climatic 99 zones, the southern part has a northern subtropical climate and the northern part has a warm 100 temperate climate. The annual average temperature is 7.8-13.9°C, the annual average precipitation 101 is 696.8-830.1 mm, and the annual average sunshine duration is 1848.1-2055.8 hours. Shangluo's 102 terrain is complex, with large differences in elevation and vertical height, and has obvious three-103 dimensional mountain climate characteristics. There are obvious differences in light, heat, water, 104 climate resources and meteorological disasters in various places, and their distribution is extremely 105 uneven. The soil type in the south of Shangluo City is yellow cinnamon soil, and the north is mainly 106 cinnamon soil. The Gold Production Company in research area operated in 1993. In 2006, the "4.30" 107 dam failure occurred. In 2016, Shangluo City's first comprehensive treatment project for heavy 108 metal pollution in farmland soil was launched. The project adopted "microorganism + 109 phytoremediation" technology for heavy metal pollution in farmland soil, and for exposed waste 110 slag, embraced "slag removal + screening/solidification stabilization repair + safe filling Buried" 111 process. After the project was completed, the final site would be recovered and the vegetation would 112 be restored. The project was completed in 2018. 113 1.2 Acquisition and processing of soil data 114 According to the results of previous field investigations, the As content in soil in the mining 115 area is mainly affected by human activities and shows a large difference. 50 soil samples(0-20cm) 116 were collected based on the uniform grid method with a high sampling density in the mining area 117 (50 m). There were three members in the research group with unified training on the sampling 118 method. A real-time kinematic (RTK) was used to precisely locate every sampling location. Figure  119 1 shows the position of the sampling points. Stones, plant residues, and other large debris were 120 removed from each fresh sample, which was then mixed thoroughly and then stored in a labeled 121 plastic bag. Each sample weighed 500 g. All samples were air-dried at room temperature. Small 122 stones and plant residuals were manually removed and the soil samples were then run through a 123 0.15 mm sieve. The samples were divided into two parts, one was used for chemical determination 124 of pH and soil As content, the other was sealed for backup. As content in the soil was determined 125 by the atomic fluorescence spectrometry method; the pH value of the soil was measured by the glass 126 electrode method. 127 There were 27 vegetation survey samples of 16 square meters (4m*4m) set up to investigated 128 the composite of the plants. We used TRK to record the center point coordinates of the sampling 129 party, and the sampling location is shown in Figure 1 characteristics of the distribution of heavy metals, but also judge the impact of human activities on 143 the environment. Calculated as follows: where is the geological accumulation index; is the concentration of element, and is 146 the background value of element n. 147 The geological cumulative index can be divided into the following 7 levels: 00, the 148 pollution level is 0, which means unpolluted; 0＜ 01, the pollution level is 1, which means 149 unpolluted to moderately polluted; 1＜ 02 , the pollution level is 2, which means moderately 150 polluted; 2＜ 03, the pollution level is 3, which means moderately to heavily polluted; 3＜ 04, 151 the pollution level is 4, which means heavily polluted; 4＜ 05, the pollution level is 5, which 152 means heavily to extremely polluted pollution; 5＜ 06, the pollution level is 6, which means 153 extremely polluted. 154 1.3.2 Potential Ecological Risk Index 155 The potential ecological risk index (PERI) was proposed by the Swedish scientist Hakanson 156 (1980). It is mainly used to evaluate the degree of heavy metal pollution in the soil of the mining 157 area. This method links the ecological effects, environmental effects and toxicology of heavy metals 158 together. It can not only reflect the impact of various pollutants in sediments on the environment in 159 a particular environment, but also use a quantitative method to classify the potential harm of heavy calculation formula of the potential ecological risk index of a single heavy metal is: Where is the toxicity response coefficient of element i; is the measured concentration of 165 heavy metal; is the soil background value. The classification standards of PERI are classified as 166 shown in Where is the content of heavy metals in plants (mg/kg), and is the content of heavy metals 185 in the soil (mg/kg of hillside (B) and remediation field (C) adopts the risk screening value of agricultural land, which 198 is 25mg/kg. Table 3 and Table 4 present the risk screening and intervention values of soil As content 199 in development land and agricultural land. Figure 2 shows the over-standard situation of soil points 200 in the mining area. It can be seen intuitively in Figure 2 that all soil samples in area A exceeded the 201 standard, and 10 points exceeded the control value; 10 samples in area B exceeded the risk of soil 202 As contamination of agricultural land, and there were 8 points in area C that exceed the standard. 203 Table 5 provides the descriptive statistics of As contents and pH values of 50 sampling points 204 and indicates that the pH values of the soil in the whole study area are greater than 8.1, which is 205 weakly alkaline. The respective range of soil As content in the three areas was: 54.00-231.00mg/kg, 206 13.30-100.00mg/kg, and 16.10-41.5mg/kg. The average value of As in area A was 150.7mg/kg 207 belonging to serious pollution, which was 1.08 times more than the intervention value of the second 208 type land of development land. The average values of As in area B was 35.21mg/kg belonging to 209 heavy pollution, which exceeded 1.41 times the soil pollution risk screening value of agricultural 210 land. The average values of As in area C was 25.88mg/kg belonging to light pollution, which 211 exceeded 1.03 times the soil pollution risk screening value of agricultural land. The coefficient of 212 variation can be used to describe the degree of dispersion of the data values in the data set，which 213 includes the average difference coefficient, the span coefficient, and the standard deviation 214 coefficient. Due to the different pollution conditions in various regions, the average As content is not 215 equal, which makes it impossible to compare the dispersion degree of each element through an 216 absolute indicator such as standard deviation. Therefore, it is necessary to use the coefficient of 217 variation to explain the dispersion degree. The standard deviation coefficient was selected in this 218 paper. This is the ratio of the standard deviation of the group of data to the average value. The larger 219 the value is, the more uneven the spatial distributions of the elements are. The coefficients of 220 variation of soil As content in the six areas were ranked from large to small: B > A > C. Area B had 221 the largest coefficient of variation and belonged to the strong variation type. It means that the As 222 content in area B is unevenly distributed, and the source might be human activities. 223 224 the result is shown in Figure 3. Table 6 shows the accumulation index values of different areas in 241 the mining area. As shown in Table 6, the geoaccumulation index values of soil As pollution in the 242 mining area ranged from 0.34 to 3.78, and the average value was 1.57, which is moderately polluted. 243 The average value of the heavy metal pollution coefficient in each region from high to low is A>B>C.

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The Taking the toxicity response coefficient = 10 (Zheng et al 2020a), and Table 7 shows the 254 potential ecological risk index of soil As in the mining area. It can be seen from Table 7  results of the potential ecological risk index of soil As pollution in the mining area (Figure 4), it can 262 be seen that 32% of the points belonged to considerate degree hazards, in which the proportion of 263 high ecological risk reached 14% and all of them were in area A, indicating that the soil As pollution 264 in area A has been considerably serious. Generally, places far away from on-site pollution sources 265 have lower potential ecological risks. A area is the closest area to the tailings dam break area, whose 266 pollution is the most serious and the potential ecological risk index was the largest. It is also an 267 important factor for the overall potential ecological risk of the site. due to rain erosion, B area is a 268 hillside with a low soil As content. But there is a certain ecological risk because of a small amount 269 of residue. C area has been performed artificial land treatments and a restoration project with the 270 lowest soil As content, so the potential ecological risk index was the smallest. 271 Table 7 Calculation results of potential ecological risk index of soil heavy metals in mining area 272 Sampling  after the tailings dam broke. Thus, the species diversity is small and the stability of the vegetation 306 community is poor. 307 308  Figure 5 Vegetation Species diversity index in A and B area 312 2.3.2 Correlation between species diversity index and soil As pollution degree 313 The analysis of the species diversity index of the plant samples showed that the Sics abundance 314 index, Shannon-Wiener index and Pielou evences index in the severely As-polluted areas (A area) 315 was 1.018, 1.211 and 0.674 respectively, and in the non-As-polluted areas (B area) was 1.129, 1.287 316 and 0.726 respectively. It suggests that the vegetation species diversity of As-contaminated soil is 317 less than that of uncontaminated soil, which means that the stability of vegetation community in 318 polluted soil is worse than that of unpolluted soil. The Pearson correlation analysis results of the 319 plant species diversity index of contaminated soil and the corresponding accumulation index ( Table  320 9) showed that the accumulation index of As pollution had a significant negative correlation with 321 the plant species diversity at the level of 0.05, and the correlation coefficient was -0.30, -0.25, -0.23 322 respectively. 323 Table 9 Correlation between vegetation species diversity index and AS pollution index are shown in Table 10. It can be seen from The study area is located in the Qinling Mountains, which is on the diving line between North 367 and South China and the 800mm precipitation line. Qinling Mountains is rich in species resources.

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The annual average temperature is 15.6 ℃, the annual average sunshine hours is 2040.4 hours, and with good growth status, strong fecundity and large number, and can enrich arsenic in soil.

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According to the local climate environment and soil conditions, the selection of dominant plants 376 that can enrich heavy metals to control heavy metal pollution is conducive to repairing the chain 377 ecological damage caused by mining, restoring the mine ecological environment, accelerating the 378 process of vegetation restoration, and promoting the sustainable development of the ecological 379 environment in the mining area. In addition, the dominant plants screened in this experiment can 380 be used as alternative plants for remediation of As pollution in mining area soil, laying a foundation 381 for the next pollution remediation experiment of the project team. In the evaluation of soil heavy metal pollution, due to the diversity of protection objects, 390 exposure methods and standards, there are certain differences in soil environmental quality standard 391 systems in various countries. Therefore, the evaluation standards for the degree of soil heavy metal 392 pollution are also different in different regions, and there is no unified background value standard 393 at home and abroad. Using different background values for the same object, such as national soil 394 element background values, local soil element background values or national soil environmental 395 quality risk values, will lead to different results. The soil heavy metal pollution in this study area is 396 also the same. The calculation results of the national soil As element background value and the 397 national soil environmental quality As risk value are not the same, and there is currently no unified 398 regulation.

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In addition, due to the limitations of research time and cost, we only analyzed the soil As 400 pollution degree of an abandoned gold mine in Shangluo City and initially investigated the plant 401 diversity and As content of the abandoned mine area. Compared with the same type of research, the 402 plant samples are relatively single because of the small research area. In terms of heavy metal 403 analysis in plants, the difference in the enrichment of heavy metals in various parts of the plant has 404 not been analyzed. In future research, it is possible to appropriately increase the number of plots and 405 analyze heavy metal enrichment in various parts of the plant. Further work needs to be done to select 406 the dominant plant species for ecological restoration in mining areas through plant biomass and its 407 effects on As enrichment and transfer. And pot experience could be conducted to explore the 408 mechanism of As impact on plants (to quantify the impact of soil As pollution on plant physiological 409 growth), and the remediation effect of dominant plants on soil As pollution in mining areas. 410 Eventually, these research results may provide a basis for the ecological management of As 411 contaminated areas. 412 413

Conclusions 414
The heavy metal pollution of soil and crops in metal mining areas and their surrounding areas 415 has been highly concerned by countries all over the world, whi ch has become a "hot spot" issue 416 studied by domestic and foreign scholars. For China, with the implementation of "ecological 417 conservation and high-quality development of the Yellow River Basin" and "the ecological 418 restoration project of mountains, rivers, forests, farmlands, lakes and grasses", the ecological