Evaluation of Heavy Metal Pollution in Shallow Groundwater of Farmland in Huaibei Plain, China

: In order to understand the characteristics of heavy metal pollution in shallow groundwater in farmland in Huaibei Plain, 69 samples were collected from typical farmland areas in Xiaoxian County to determine the content of heavy metal elements such as Cr, Zn, Cu, Cd, and Ni. The single-factor pollution index method and Nemerow comprehensive index method are used to evaluate and analyze the pollution degree and the health risk assessment method is used to explore the health risk level. The results show that the concentrations of Cr, Zn, Cu, Cd and Ni were respectively 1.27μgꞏL -1 ~268.78μgꞏL -1 , 0.45μgꞏL -1 ~ 266.1μgꞏL -1 , 1.77μgꞏL -1 ~ 29.8μgꞏL -1 , 0.10μgꞏL -1 ~ 21.54μgꞏL -1 and 0.9μgꞏL -1 ~ 252.85μgꞏL -1 . The Nemerow comprehensive index of each heavy metal element is 0.031~5.507, and the pollution degree is Ni>Cr>Cd>Zn>Cu. The Nemerow comprehensive pollution index of each sampling point is 0.251~9.325, with an average value of 1.934. The overall value is 9.523, and the pollution level is extremely poor. The overall health risk is at a risk-free health level. There are 19 samples which the carcinogenic risk index of the Cr is greater than 10 -4 , and the number of samples with health risks accounts for 27.5%. Cr is the main source of heavy metal health risks in shallow groundwater in farmland.


Introduction
Groundwater is an important source of water for human life, as well as an important source of water for agricultural production.
With the continuous development of human society and economy, human activities have brought serious pollution to water resources [1] . Surface water is threatened by humans, and groundwater is also being polluted, especially heavy metal pollution. At the same time, due to surface water replenishment, infiltration or leaching, heavy metals in surface water will also be brought to shallow groundwater [2] . Most of the heavy metals are carcinogenic. Once too many heavy metals enter the water body, it will directly or indirectly affect the health of the human body, and even seriously affect the aquatic ecosystem [3] . When the accumulated heavy metal content in the human body exceeds the tolerable level, the normal physiological functions of the human body will be interfered, which will cause varying degrees of harm to the important functional organs of the human body, and may even threaten human life safety. Heavy metal pollution in shallow groundwater of farmland is a representative research direction of heavy metal pollution in groundwater, which mainly comes from the accumulation of chemical fertilizers, farm manure (poultry manure) and pesticides used in farming and planting in the soil [4] , which can be used in the food chain. Accumulation in farmland crops leads to excessive heavy metal content in crops and causes human health risks. Investigation and research on the status quo is of great significance for its treatment and timely restoration.
At present, many scholars and experts conduct research and analysis on the heavy metal pollution of water bodies, and put forward feasibility suggestions. Sun Jianxiong [5] and others used principal component analysis and correlation analysis to analyze the sources of heavy metals in surface waters in the coastal area of Yancheng, Jiangsu, and used the comprehensive pollution index method to evaluate heavy metal pollution; Yang Xuefu [6] and others used single-factor water quality Standard comparison method, heavy metal pollution index method and health risk assessment method are used to evaluate and analyze heavy metal pollution in water bodies and health risks. This study focuses on the heavy metal pollution of shallow groundwater in farmland in the Huaibei Plain, and evaluates the degree of heavy metal pollution and health risks, which can provide technical support for the current status of regional water environmental quality. for about 1.3% of the total area of Anhui Province. Generally speaking, the terrain is relatively flat. Most of them are plains. Due to the influence and control of the monsoon climate in Xiaoxian County, summer rainfall is relatively concentrated, and precipitation is relatively large; winter precipitation is relatively small, and the dry and wet seasons are distinct. According to statistics, the maximum annual precipitation in Xiaoxian County is 1320.3mm; the minimum annual precipitation is 562.5mm, and the annual average precipitation is 854.6mm.
The climatic conditions in Xiaoxian County determine the direction of agricultural development as dry farming, and its main crops are wheat, corn and soybeans. In 2017, Xiaoxian's annual total grain output was 771,100 tons, the sown area was about 16 hectares, and the total output value was 534 million yuan. Groundwater is the main source of irrigation water for farmland in Xiaoxian County, as well as the main source of drinking water. According to the 2017 Xiaoxian County Environmental Quality Report, the water quality of centralized drinking water sources meets the requirements of Class III water in the groundwater quality standards.

Samples collection and analysis
Select typical farmland irrigation wells from representative villages and towns in Xiaoxian County as sample collection points, store 69 water samples collected in clean bottles in 2019, and send the samples back to the school laboratory for determination after pretreatment. Strictly follow Relevant operating specifications. According to relevant national testing methods and requirements, graphite furnace atomic absorption spectrometry and flame atomic absorption spectrometry are used to determine the concentration of Cr, Cu, Zn, Cd, and Ni in water samples. Each sample needs to be measured three times. The average of the measured values is used as the final result of this research, and deionized water should be used as a blank sample for comparison during the detection process.

Single factor pollution index method
The single-factor pollution index method is an evaluation method based on the environmental quality standards used by pollutants. Its advantage is simple operation and easy implementation; but it is only used to reflect the pollution degree of each heavy metal element in the sampling point. And cannot reflect the overall pollution level caused by the combination of multiple pollution factors [7] . See formula (1) for calculation.
Pi represents the single factor index of heavy metal element i in groundwater; Ci represents the actual measured concentration of heavy metal i in groundwater, and Si refers to the evaluation standard value used for heavy metals in groundwater. The grading standard of single factor pollution index is shown in Table 1. According to the fact that shallow groundwater in Xiaoxian County is mainly used for industrial, agricultural and concentrated drinking water sources, the Class III water quality standard limit in the Groundwater Quality Standard (GB/T14848-2017) is used as the evaluation standard for the single-factor pollution index. At the same time, considering that the main crop type in Xiaoxian County is wheat, in order to investigate the use value of shallow groundwater as irrigation water, the standard limit for dry farming in the "Farmland Irrigation Water Quality Standard" (GB5084-2005) is adopted. As the "Farmland Irrigation Water Quality Standard" (GB5084-2005) lacks the standard limit of Ni in the dry farming standard, the analysis shall refer to the Ni in the category III water quality standard in the "Groundwater Quality Standard" (GB/T14848-2017) Limit.

Nemerow Comprehensive Pollution Index Method
The most commonly used method in the calculation of the comprehensive pollution index is the Nemerow comprehensive pollution index method, which has the advantages of convenient calculation, simple and clear results, and reasonable and reliable methods. More importantly, it combines the average value of the singlefactor pollution index with The maximum value is taken into consideration, and the heavy metal pollution of the sampling point is comprehensively evaluated. Compared with the single-factor pollution index method, it can reflect the comprehensive water quality more simply and is convenient for statistical analysis and utilization [8,9] , but the pollution factor is too prominent. Treat things equally. The specific expression is shown in formula (2).
P N represents the comprehensive pollution index of the shallow groundwater at the sampling point. See Table 2 for specific classification standards; (C i /S i )ave represents the average value of the single factor index of heavy metal pollutants; (C i /S i )max represents the maximum value of the single factor index of heavy metal pollutants.

Health risk assessment
Health risk assessment is based on risk as an indicator, and quantitatively describes a potential risk of environmental pollution to human health [10] . In this evaluation, HQ (hazard quotient index) and Risk (carcinogenic risk index) were used to evaluate the health risks of five heavy metal elements and two heavy metal elements, which are chemical carcinogens, Cr and Cd, to conduct carcinogenic risk evaluation. The chemical carcinogen Cr can damage the gastrointestinal tract and affect the normal circulatory metabolism in the body after being eaten into the human body. Therefore, the concentration level of Cr in the water in the study area should be paid attention to. In addition, when the chemical carcinogen Cd enters the human body with a concentration exceeding the acceptable range, it may cause chronic poisoning, cause damage to the human kidney function, and then affect the normal physiological metabolism of the human body. Therefore, although the concentration of Cd in the study area is low, it is due to the health of the human body. Harmfulness should also be paid attention to in water environmental protection.
1) The hazard quotient index HQ is calculated as formula (3).
CDI is the average chronic daily exposure, c represents the actual measured concentration of heavy metals, DI represents the daily intake dose, using 2Lꞏd -1 [11] ; BW is the average weight of Xiaoxian County adults, using 60kg; RfD is The doses of oral toxic substances and the RfD values of 5 heavy metals [12] are shown in Table 3. HQ's health risk evaluation grade [13] is shown in Table 4. 2) The carcinogenic risk index Risk is used to evaluate the health risks of carcinogens and is calculated as formula (4).
SF represents the cancer slope factor, unit: mgꞏ(kgꞏd) -1 . Among the five heavy metals, Cd and Cr are carcinogens recognized by the International Agency for Research on Cancer [14] . The cancer slope factors (SF) of heavy metal pollution factors Cd and Cr are 0.5mgꞏ(kgꞏd) -1 and 15mgꞏ(kgꞏd) -1 , respectively. USEPA believes that the risk of carcinogenic risk index is 1×10 -4 as the limit. When 1×10 -6 <Risk≤1×10 -4 , there is no risk of cancer; when Risk>1×10 -4 , it means there is a risk of cancer.

Analysis of the characteristics of heavy metal content
The content of heavy metals in shallow groundwater of farmland in Xiaoxian County is shown in Table 5. Comparing with the water quality standards of Class III water in the "Groundwater Quality Standards" (GB/T14848-2017) and comparing with the water quality standards of dry farming in the "Farmland Irrigation Water Quality Standards" (GB5084-2005), the conclusions reached are consistent, 69 Sampling points showed that the average concentration of heavy metals Cr and Ni exceeded the standard value, which was beyond the standard. The average content of the other three heavy metals did not exceed the standard value. There were 10 sampling points where the heavy metal element content exceeded the standard.

Analysis of evaluation results of single factor pollution index method
The water quality standard limits of Class III water in the "Groundwater Quality Standards" (GB/T14848-2017) and the dry farming standard limits in the "Farmland Irrigation Water Quality Standards" (GB5084-2005) are used as reference limits, using single factor The pollution index calculation formula calculates the pollution index value of heavy metals in the shallow groundwater of farmland in Xiaoxian County. The evaluation results are shown in Table 6. It can be seen that the water quality standard limit of Class III water in the "Groundwater Quality Standard" (GB/T14848-2017) is used as the reference limit for evaluation. The single factor average index of heavy metal Cr is 1.198, which belongs to light pollution; the single factor of heavy metal Zn The average factor index is 0.052<1, the pollution degree is non-polluted; the single factor average index of heavy metal Cu is 0.0087<1, and the pollution degree is non-polluted; the single factor average index of heavy metal Cd is 0.637, which is non-polluted; the single factor of heavy metal Ni The average of the factor index is 2.03, between 2-3 is considered moderate pollution.
The standard limit for dry farming in the "Farmland Irrigation Water Quality Standard" (GB5084-2005) is used as the reference limit for evaluation. The single factor average indexes of heavy metals Cr, Zn, Cu, Cd and Ni are 0.599, 0.026, 0.0087, 0.3186, and 2.03 respectively. There are 4 kinds of heavy metal pollution degrees all belong to non-polluted.
It can be seen from Table 6 that the pollution degree of various metal elements is Ni>Cr>Cd>Zn>Cu. Among them, the heavy metal Cr has the most over-standard points, and the number of samples exceeds the standard rate of 40%; the second is Ni, which The number of samples exceeded the standard rate of 35%; the number of samples of Cd accounted for 10% of the number of samples; Zn and Cu did not exceed the number of samples. Using two different standards as the reference limit, the calculated single factor pollution index of each heavy metal has the same over-standard point and the same over-standard rate.

Analysis of Nemerow Comprehensive Pollution Index Evaluation Results
Because the "Farmland Irrigation Water Quality Standard" (GB5084-2005) lacks the standard limit of the heavy metal Ni in the dry farming standard limit, and the concentration of Cr, Zn and Cr in the "Groundwater Quality Standard" (GB/T14848-2017) The limit value is relatively lower, the water quality standard requirements are higher, and the calculated single-factor pollution index value is also higher. Therefore, the more stringent "Groundwater Quality Standard" (GB /T14848-2017) The heavy metal concentration limit in the water quality standard of Class III water is used as the reference object for the shallow groundwater of farmland in Xiaoxian County, to look at the current situation of heavy metal pollution in the shallow groundwater of farmland in Xiaoxian County from a higher perspective, in order to better understand the regional water environment Provide support. See Table 7 for specific evaluation results. It can be seen from Table 7

Health Risk Assessment of Heavy Metals
The HQ values of the five heavy metal hazard quotient indexes studied in this paper are shown in  Table 9. Among the 69 sampling points, 19 of the Cr carcinogenic risk index (Risk) were greater than 10-4, accounting for 27.5%, indicating that the heavy metal Cr in the shallow groundwater of the farmland in Xiaoxian County has a greater carcinogenic risk. The carcinogenic risk index (Risk) of Cd in all sampling points is less than 10-4, indicating that there is no carcinogenic risk caused by the chemical carcinogen Cd in the shallow groundwater of farmland in Xiaoxian County. In summary, the chemical carcinogen Cr is the main source of heavy metal health risks in the shallow groundwater of Xiaoxian County farmland, followed by Cd; other heavy metal elements other than Cd and Cr chemical carcinogens have less health risks, so the chemical Carcinogens Cr and Cd should attract special attention.

Conclusions
The average value of heavy metal content in the shallow groundwater of Xiaoxian County farmland is compared with the reference standard. It can be seen that the content of heavy metals Cr and Ni exceeds the standard, and the average content of the three heavy metals of Cu, Cd and Zn does not exceed the standard value. The evaluation results of the single factor index method show that the heavy metal Cr has the largest number of overstandard samples, followed by Ni, followed by Cd; heavy metals Zn and Cu have no over-standard points. The calculation result of Nemerow comprehensive pollution index method shows that the pollution degree of 5 heavy metal elements in the shallow groundwater of farmland in Xiaoxian County is Ni>Cr>Cd>Zn>Cu. The Nemerow comprehensive pollution index of heavy metal Ni is considered to be extremely poor and should be paid more attention. The Nemerow comprehensive pollution index ranged from 0.251 to 9.325 at each sampling point, with an average value of 1.934. The number of samples belonging to good water quality is the largest; the Nemerow comprehensive pollution index in the entire study area is 9.523, and the pollution level is extremely poor.
Cr, a chemical carcinogen, is the main source of heavy metal health risks. Non-chemical carcinogens such as Cu, Zn, and Ni have relatively low health risks, and Cr carcinogen risk in 69 sampling points. The Index (Risk) has 19 samples greater than 10 -4 , and 27.5% of samples have health risks.