Characteristics and risk assessment of arsenic contamination in a lead and zinc plant area in Yunnan Province

Taking a lead and zinc plant plot in Yunnan as the research object, the land use status and planting types of the region were investigated, the original enterprise distribution area and crop growing area of the plot were defined, and 67 surface soil samples and 14 columnar soil samples were systematically arranged in the plot for monitoring. Combined with the monitoring results and analysis, it is concluded that there is no definite basis that the high arsenic content in soil in the region is caused by the enterprise production, and the high arsenic content in the region may be caused by the background value. Based on the monitoring and investigation results, the risk assessment of the area was carried out. The results showed that the non-carcinogenic risk and carcinogenic hazard index of arsenic exceeded the prescribed acceptable level when the area was developed as the second type of construction land, and soil remediation should be carried out. At the same time, the collaborative distribution monitoring of regional crops showed that the arsenic content of crops was low, which met the requirements of food standards, possibly because the cultivated crops did not have strong adsorption and enrichment effects on the arsenic occurrence form in the regional soil.


Introduction
According to relevant investigations and research results, the soil arsenic content in some areas of my country is relatively high, and about 20 million people live in areas with high risk of soil arsenic [1] . 1 The 10th International Conference on Lead and Zinc Processing (Lead-Zinc 2023) Journal of Physics: Conference Series 2738 (2024) 012034 IOP Publishing doi:10.1088/1742-6596/2738/1/012034 2 Due to natural causes and human activities, a large amount of arsenic-containing substances released into the environment will be an important reason for soil arsenic pollution [2] .Arsenic easily enters the human body through crop absorption and contact.When the arsenic in the human body exceeds a certain limit, it will cause acute and chronic arsenic poisoning in the human body, and cause various diseases, which seriously threaten human health.Yunnan Province is rich in non-ferrous metal reserves, and the background value of arsenic in the soil is relatively high.At the same time, due to the development and smelting of non-ferrous metal minerals in the region, a large amount of tailings and metallurgical waste residues are produced.Due to historical reasons, the disposal of some arsenic-containing solid wastes was not standardized, which caused the Yangzonghai arsenic pollution accident, and also caused the excessive arsenic in the Da tun Sea in the Hong he area and the excessive arsenic in the farmland soil in the Bi jiang River Basin [3][4][5] .
According to the nature of pollutants, arsenic has both carcinogenic and non -carcinogenic hazards, and both carcinogenic and non-carcinogenic risks need to be calculated [6] .The heavy metal elements in the soil of Yunnan karst area mainly come from the regional high geological background value and pedogenesis, and the activity of heavy metals is relatively low [7,8] .Due to human production activities, the proportion of effective states of some heavy metal elements has increased relatively, and the safety of agricultural products has been seriously threatened [9] .Han et al. conducted an investigation and risk assessment of arsenic pollution in an industrial site, and concluded that the arsenic content in the surface soil of the site was high, and the regional groundwater was also polluted [10] .Guo et al. studied the characteristics of heavy metal pollution in the soil of an abandoned sulfuric acid plant, and showed that the excessive levels of heavy metals such as arsenic in different production areas were quite different, exceeding the acceptable risk level [11]   .Yao et al. conducted a survey and potential ecological risk assessment of heavy metal pollution in farmland soil in a karst area in eastern Yunnan with high cadmium and arsenic background.They believed that the high background value of heavy metals in the soil in the area seriously threatened the quality of agricultural products [12] .In recent years, there have been many literatures reporting on the risk assessment of arsenic pollution in some plots or related studies on plant uptake and restoration of arsenic-contaminated plots [13][14][15][16] .However, there are few reports on the coordinated assessment of arsenic pollution investigation, risk assessment and crop monitoring in plots.In this paper, a preliminary investigation of a plot with high arsenic content in Yunnan was taken as the research object, and the detailed monitoring and investigation, risk assessment and soil collaborative monitoring methods were used to conduct detailed and in-depth research on the plot, so as to provide a scientific basis for the subsequent development of the plot by the government.

Basic situation of the research area
The study area is a 4 km × 4 km area with high arsenic in Yunnan Province, and the latitude and longitude of the site is 23.512035° north latitude and 102.818430° east longitude, the dominant wind direction is southwest wind, the soil type is common red soil, and the land use types are dry land, forest grassland, and bare land, accounting for more than 80%, and a small part is industrial and mining enterprises land.There are no soil-sensitive targets such as hospitals, sanatoriums, and nursing homes in the area, and the soil-environmentally sensitive targets are mainly cultivated land within the survey area.The enterprises in the area are mainly distributed in the southeast of the area.The main enterprises include 10 enterprises including manganese smelting, manganese powder processing, copper ore dressing, and lead-zinc ore dressing plants.The same soil pollutants are all heavy metals, and all of them are currently in suspension.In addition, a domestic garbage sanitary landfill is distributed on the south side.The area is mainly dry land, and the main types of crops are corn, pepper, sugarcane, orange, peach, taro and so on.

Agricultural land soil and groundwater monitoring program
According to the "Technical Regulations on the Layout of Points for Detailed Investigation of Soil Pollution in Agricultural Land" (Environment Office Soil Letter [2017] No. 1021), the monitoring points are laid out in a grid with a minimum precision of 500m × 500m for various types of soil impact survey units.Combined with the fact that the arsenic content in the preliminary monitoring sampling is greater than the control value in the agricultural land risk management and control standards and the distribution of existing enterprises on the southeast side, the upwind direction control points are arranged in radial directions at different distances, and the upwind direction of the existing enterprise distribution area on the southeast side The point distribution method is densely distributed, and the two sides are densely distributed in the form of umbrella type point distribution.A total of 67 surface soil monitoring points were set up in the survey area, at 3 # , 7 # , 11 # , 14 # , 21 # , 25 # , 28 # , 36 # , 45 # , 51 # , 52 # , 55 # , 62 # , 67 # part of the point set up columnar samples, each point in the range of 20 cm -300 cm to take three samples of surface layer, middle layer and deep layer, the monitoring factor is arsenic.See Figure 1 for site layout.In addition, in order to understand the quality of groundwater in the region, three groundwater monitoring points were arranged upstream and downstream of the landfill.

Collaborative monitoring plan for crops
Among the monitoring points set within the scope of agricultural land soil quality monitoring and investigation, according to the requirements of the "Soil Environmental Quality Agricultural Land Soil Pollution Risk Management and Control Standards (Trial)" (GB15618 -2018), the soil environmental quality whose arsenic monitoring value exceeds the control value is classified as a control point, and the soil environmental quality that exceeds the screening value and is less than the control value is classified as a super-screening value point.When selecting the cooperative monitoring area for agricultural products, samples were taken from three areas beyond control, namely the southern monitoring area (for the main planted peppers, corn, and taro), and the northeastern monitoring area (for the main planted oranges, peaches, and sugarcane), and the northern monitoring area (mainly planted peppers, corn, and taro for use); The super-screening value distinguishes three areas for sampling, namely the eastern area (for the main planted corn), and the northwestern area (for the main planted peppers) Adoption), Southern District (adoption of the main planted corn).The specific collection method is: not belonging to the planting period, the corn samples that have been harvested are collected at the homes of the growers in the cultivated land in the collection area; peppers, peaches, oranges, green beans, and vegetables can be planted in season to obtain sample varieties, and refer to According to the "Technical Regulations on Preparation and Preservation of Agricultural Product Sample Collection and Transfer" (Environmental Office Soil [2017] No. 59), the collection area is divided according to 20 m × 20 m samples, and samples from 5 points are collected according to the serpentine method, and then mixed in equal amounts to form A mixed sample, the monitoring factor is arsenic.See Figure 2 for crop sampling layout.

Risk assessment methods for arsenic in soil
Arsenic is not volatile or semi volatile, and the exposure pathways for soil with arsenic monitoring values greater than the control values of construction land are considered to be oral intake of food grown in the soil, skin contact with the soil, and inhalation of soil particles.Using the health risk assessment method of the United States Environmental Protection Agency of the United States, and in combination with HJ25.3-2019Technical Guidelines for Soil Pollution Risk Assessment on Construction Land [17] , we analyzed the exposure of two types of typical land conditions and three exposure routes of the second type of land represented by industrial land in the study area, and selected carcinogenic and non-carcinogenic risk models to assess the health risks of contaminated soil on human beings, The selection of fixed parameters required for its calculation refers to the relevant provisions of HJ25.3 2019 and Regional screening levels (RSLs) general tables [18] .

Distribution characteristics of arsenic in soil
The groundwater quality of the three monitoring points in the monitoring area, 30 monitoring indicators including arsenic, can all meet the requirements of the Class III standard of the "Groundwater Quality Standard" (GB/T 14848 -2017).Compared with the "Soil Environmental Quality Agricultural Land Soil Pollution Risk Control Standard (Trial)" (GB 15618 -2018), the distribution of various value points is shown in Figure 3.According to statistics, among the 67 monitoring points, the arsenic content in the soil of 29 points exceeded the screening value, the arsenic content in the soil of 32 points exceeded the control value (up to 533 mg/kg), and only 6 points contained arsenic in the soil was lower than the screening value, and the results show that the regional soil arsenic content is high.The monitoring results show that the monitoring points for arsenic greater than the control value are mainly distributed from southwest to west to north to northeast of the region, mainly along the F1 fault zone and both sides of the region, and the monitoring points greater than the screening value are mainly concentrated in the southeast to east.The southeast side is the distribution area of the surrounding existing enterprises.
The monitoring value of arsenic in the soil around the distribution area of existing enterprises is generally higher than the monitoring value of arsenic in the soil of the distribution area of existing enterprises in the southeast.According to the monitoring results of arsenic in the columnar sample monitoring points shown in Figure 4, the surface monitoring value of the 45# columnar sample in the surrounding enterprise distribution area is greater than the monitoring value at the other two points, while the monitoring value of the 52# columnar sample at the deep layer is greater than the other two points monitoring value.Outside the distribution area of surrounding enterprises, the surface monitoring value of 51# columnar sample in the upwind direction is greater than the monitoring value at the other two points, and the deep monitoring value of 36# columnar sample in the downwind direction is greater than the monitoring value at the other two points.The monitoring data of other columnar samples in the surface layer, middle layer and deep layer also did not show a stable concentration gradient change.From the analysis of the monitoring results of the surface layer and columnar samples of the regional system, the high concentration of arsenic in the regional soil is not caused by industrial pollution, but may be caused by the high natural background value of arsenic in the regional soil layer.
The arsenic monitoring value at the surface layer of the columnar sample monitoring point 21 # at the bottom of the gully in the F1 fault zone has an obvious upward trend compared with the monitoring value of arsenic monitoring at the monitoring point 26#, 32#, 42# and 43# on the gully bank of the fault zone , indicating that the arsenic in the soil is accumulated in the surface soil of the gully area at the bottom of the terrain under the action of soil erosion such as rainfall erosion.

Crop collaborative monitoring program
The crop monitoring results are shown in Table 1.Compared with the "National Food Safety Standard Food Contaminant Limits" (GB 2762 -2017), the arsenic content in regional crops can reach the standard and can be eaten safely.The content of arsenic in the regional soil is relatively high, but the arsenic content of crops such as pumpkins, cabbage, oranges, and peppers planted is low, indicating that the arsenic absorption and enrichment effect of the crops grown in the region is weak.Note: The detection index is total arsenic (calculated as As), and the standard value of "National Food Safety Standard Limits of Contaminants in Food" (GB 2762 -2022) is 0.5 mg/kg.

Health risk assessment of arsenic in soil
According to the plan of the local government, the area will be mainly used as the second type of construction land.The average concentration of arsenic in the surface soil is calculated to be 185.8mg/kg, which is used as the concentration of arsenic in the regional soil.The calculation and evaluation results are shown in Table 2.After calculation and evaluation, under the second type of land use conditions, the total non-carcinogenic hazard index of arsenic in the regional surface soil is 2.72, which is greater than the acceptable risk level of the acceptable total hazard index 1 of a single pollutant, and the carcinogenic risk value of arsenic in regional surface soil is 1.32E -04, which is greater than the acceptable risk level of 10 -6 for a single pollutant.If the next step is to develop the area into a second-class construction land, its risk control value needs to be clarified for restoration and treatment or take measures for risk management and control.

Conclusion and suggestions
The concentration of arsenic in the regional soil is high, and most of the monitoring points exceed the control value or screening value.Sampling, monitoring and comparative analysis of the surface and deep soil are carried out through grid distribution, especially the arsenic content in the soil of the enterprise distribution area and the general crop planting area.According to the comparative analysis, the high concentration of arsenic in the regional soil has no definite basis for industrial pollution, which may be caused by the high natural background value of arsenic in the regional soil layer.Through the systematic sampling and analysis of the crops in the area, the results show that the arsenic content in the crops is low, which can meet the standard requirements, and the risk of eating the crops is low.
According to the planning of the local government, the area will be mainly used as the second type of construction land.After calculation and evaluation, under the second type of land use conditions, the non-carcinogenic risk and total carcinogenic hazard of arsenic in the regional surface soil exceed the acceptable risk level.If the area is developed into the second type of construction land in the next step, it is necessary to clarify its risk control value for restoration and treatment or take measures for risk management and control.

Table 1 .
Agricultural product monitoring results table Unit: mg/kg

Table 2 .
Table of calculation results of risk characterization under the second type of land use conditions Unit: dimensionless