Spatial Distribution Characteristics and Evaluation of Soil Pollution in Coal Mining Area of Loess Plateau in Northern Shaanxi

doi:10.21203/rs.3.rs-1297649/v1

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Spatial Distribution Characteristics and Evaluation of Soil Pollution in Coal Mining Area of Loess Plateau in Northern Shaanxi

https://doi.org/10.21203/rs.3.rs-1297649/v1

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The ecological environment of the Loess Plateau in Northern Shaanxi is fragile, and the soil pollution caused by the development of coal resources can not be ignored. in this study, Shigetai Coal Mine in the Loess Plateau of Northern Shaanxi was selected for field investigation and sampling, the content of heavy metals in soil was analyzed, the environmental pollution status in the study area was evaluated by single factor pollution index method, comprehensive pollution index method and potential ecological risk index method, and the spatial distribution characteristics of heavy metals were discussed by geostatistics method. The results show that the average contents of heavy metals Hg, Cd, Pb and Cr are 2.03, 1.36, 1.11 and 1.23 times of the soil background value of Shaanxi Province, respectively. The average contents of other heavy metals are lower than the soil background value of Shaanxi Province. Hg and Cd show medium variation, as, Pb, Cr, Zn, Ni and Cu show strong variation. The skewness coefficient and kurtosis coefficient of soil Cd, as and Cu in the study area are high, In cumulative state. Single factor pollution index (P_i) and potential ecological risk index (E) show that heavy metal Hg is the main pollution factor, mainly concentrated in the East and north of the study area. The comprehensive index of potential ecological risk (RI) in the study area is 1336.49, showing a very high ecological risk, and its distribution characteristics are consistent with the distribution of potential ecological risk index (E) of Hg. The ecological risk early warning results show that Hg is in the light warning situation, Cd, Pb and Cr are in the early warning situation, the high value area of ecological risk early warning in the study area is mainly distributed in the East and North, and the whole study area presents an obvious zonal distribution law. In the process of coal mining, the soil is greatly disturbed. The ecological management of the mining area should be combined with the local natural conditions and regional environmental characteristics, and according to the principle of "adjusting measures to local conditions and implementing measures by classification". Adopt an environmentally sustainable approach to governance to achieve eco-environmental protection and high-quality development of coal resources.

Heavy metals have become a global environmental problem because of their high toxicity, easy accumulation and difficult degradation^{1, 2, 3} .According to statistics, the land area polluted by heavy metals in China is more than 10 million hectares, which seriously threatens China's agricultural development and ecological environment quality and safety^{4, 5}. The sources of soil heavy metal pollution are mainly divided into two categories: man-made causes and natural causes. Among them, the natural causes mainly include climate, topography, soil parent material and soil forming process, which makes a relatively small contribution to soil pollution⁶. However, factors such as mining, metal processing, chemical processing, urban transportation, oil refining, paint and pigment manufacturing, printing, agricultural fertilization and pesticides have greatly promoted soil heavy metal pollution⁷. Previous studies have shown that high concentrations of copper, zinc, arsenic, lead, mercury, cadmium and chromium in soil will cause soil dysfunction, deterioration of environmental quality and decline of crop productivity. It can not only enter the human body through the food chain, but also damage human organs through skin contact and breathing, thus endangering human health^{8, 9, 10}.For example, some respiratory diseases, fetal physical defects, cardiovascular diseases and nervous system injuries are mostly caused by the intake of heavy metal dust¹¹;Cadmium will stimulate human respiratory tract. Long term exposure to cadmium will cause loss of smell, gingival macula or gradual yellow circle. After being inhaled, cadmium will accumulate in human liver or kidney, especially in kidney¹². The "pain disease" in Japan was caused by cadmium pollution. The long-term exposure of pregnant women to the environment with high lead content may lead to the rapid transfer of lead in their bones to the fetus, thus affecting the development of the fetal nervous system¹³;In addition, lead can also damage the human cardiovascular system^{14, 15}, kidney system and liver system^{16, 17}. Although copper and zinc are essential trace elements for animals, plants and human body, long-term intake of high concentration copper and zinc can damage human organs such as pancreas, liver and kidney, and reduce the level of human HDL cholesterol¹⁸. Long term consumption of crops and vegetables grown on arsenic contaminated soil can increase the risk of skin cancer, bladder cancer and lung cancer in humans^{19, 20}.

After the founding of new China, in order to vigorously develop the economy, China adopted an extensive economic development model, that is, pollution before treatment, which led to serious environmental problems²¹. Among them, the mining of open-pit coal mine is one of the modes. No effective preventive and protective measures have been taken in the process of coal mining, resulting in serious environmental problems around the mining area. Heavy metal pollution is one of the main pollutants in the coal mining area. According to statistics, before 1996, about 3.2 million hectares of land in China were affected by mining activities, including cultivated land, forest land and pasture²². Many previous studies have shown that human activities such as coal mining and processing release heavy metals to farmland soil, surface water and plant leaves around the mining area, and then endanger human health through the food chain^{9, 23, 24}. In the process of coal mining, such as ore concentration, mineral extraction, topsoil stripping, tailings accumulation, wastewater treatment and transportation, a large number of heavy metal dust and dusty wastewater are produced in the surrounding areas^{9, 25},After mixing with soil, these refractory heavy metals migrate to the water environment through surface runoff, which amplifies their toxicity in the ecological environment²⁶. In addition, long-term over exploitation has caused land subsidence, soil erosion and deterioration of groundwater quality, which has seriously disturbed the local natural environment²⁷.

As we all know, coal gangue and fly ash contain a variety of toxic heavy metal elements, which are released to the environment through coal transportation, soot emission from coal-fired power plants, sewage discharge from mining areas and other coal industrial activities. In addition, the long-term accumulation of coal gangue and fly ash is another way to release toxic elements to the soil²⁸.Masto et al(2015)²⁹. and other studies³⁰ found that coal dust contains heavy metals such as iron, zinc, manganese, copper, lead, chromium, nickel, strontium, zirconium and arsenic, which penetrate into soil and even groundwater through surface runoff. According to statistics, the average concentration of arsenic in bituminous coal and lignite in the world is 9.0 and 7.4mg/ kg, respectively³¹. Chen et al.(2013)³² found that the contents of arsenic, selenium, mercury and antimony discharged into the atmosphere by coal-fired power plants were 236, 637, 172 and 33 tons respectively.

Northern Shaanxi coal mining area has the characteristics of large coal storage, good coal quality, high coal seam thickness and easy mining. It is known as "Kuwait of China" and an important energy base in Shaanxi Province. In recent years, the research work of domestic scholars on soil pollution in this area mostly focuses on the environmental effects of coal gangue on soil, while the research on the changes of soil heavy metal content and potential ecological environment pollution in mining areas is rare, especially for Shenmu mining area, a super large energy base located in wind and water erosion areas³³. Taking the Shenmu Shigetai Coal Mine Area as the research area, this study mainly investigated and monitored the concentration of heavy metals in the soil of the coal mine area, evaluated the pollution level of toxic substances by using the single factor pollution index, Nemero comprehensive pollution index method and potential ecological evaluation method, and discussed its spatial distribution characteristics by using geostatistical methods, In order to provide scientific support and basis for environmental management and standardization.

Analysis of heavy metal content in wasteland soil

According to the test results(Table 1), the variation ranges of Hg, Cd, As, Pb, Cr, Zn, Ni and Cu contents in the surface soil of Shigetai Coal Mine Area are 0.043 ~ 0.255, 0.44 ~ 2.23, 2.66 ~ 18.40, 11.80 ~ 42.80, 40.50 ~ 118.60, 18.90 ~ 70.10, 4.31 ~ 28.10, 4.96 ~ 46.25mg/kg respectively. The average contents of Hg, Cd, As, Pb, Cr, Zn, Ni and Cu are 0.128, 0.86, 6.55, 23.08, 76.22, 46.94, 16.11 and 12.10mg/kg respectively. The average contents of Hg, Cd, Pb and Cr in the soil of the study area are 2.03, 1.36, 1.11 and 1.23 times of the soil background value of Shaanxi Province, respectively. The average contents of As, Zn and Cu are lower than the soil background value in Shaanxi Province, but the maximum contents are 1.65, 1.01 and 2.16 times of the soil background value in Shaanxi Province, respectively. It is reported that the average concentration of Pb in agricultural soils affected by coal mines is high (433 mg/ kg)³⁴. Pb is usually related to minerals in coal and mainly exists in the form of sulfide such as PbS and PbSe³⁵, in addition, aluminosilicates and carbonates also contain Pb³⁶. Chromium is a non-volatile element related to aluminosilicate minerals³⁷, during mining, chromium may accumulate in coal, coal gangue or other tailings and enter the soil or water body through rainwater leaching³⁸.

The coefficients of variation (CV) of Hg and Cd contents in the study area were 0.050 and 0.37 respectively, showing moderate variation, indicating that the contents of these two heavy metals were less affected by the outside world. The coefficients of variation (CV) of As, Pb, Cr, Zn, Ni and Cu contents were 2.81, 7.46, 18.00, 13.51, 5.44 and 5.64 respectively, showing strong variation (CV > 0.50)³⁹ .It shows that the contents of these eight heavy metals may be affected by some local pollution sources. The value range of skewness coefficient (SK) is -3 ~ 3. The greater its absolute value, the greater the degree of skewness. When SK > 0, it is positive skewness, and when SK < 0, it is negative skewness; Kurtosis coefficient is a characteristic number that represents the peak height of probability density distribution curve at the average value⁴⁰. The skewness coefficient and kurtosis coefficient of Cd, as and Cu in the soil of the study area are high, indicating that these three elements are in a high accumulation state in the soil of the study area.

Parameters	Hg	Cd	As	Pb	Cr	Zn	Ni	Cu
Minimum(mg/kg)	0.043	0.44	2.66	11.80	40.50	18.90	4.31	4.96
Maximu(mg/kg)	0.255	2.23	18.40	42.80	118.60	70.10	28.10	46.25
Average(mg/kg)	0.128	0.86	6.55	23.80	76.22	46.94	16.11	12.10
Coefficient of variation(CV)	0.050	0.37	2.81	7.46	18.00	13.51	5.44	5.64
Skewness coefficient	0.389	0.36	0.60	0.31	0.24	0.29	0.34	0.47
Kurtosis coefficient	0.808	1.91	1.39	0.76	0.36	-0.36	-0.01	2.90
Background value of Shaanxi soil(mg/kg)	0.063	0.76	11.1	21.4	62.5	69.4	28.8	21.4

Table1 .Statistic of heavy metal concentration in wasteland soil

Heavy metal pollution characteristics in the study area

It can be seen(Table 2)that the average value of heavy metal single factor pollution index (P_i) of surface soil in the study area is Hg (2.03), Cr (1.22), Cd (1.14), Pb (1.11), Zn (0.68), As (0.59), Cu (0.57) and Ni (0.56).

The average value of each element in the study area is at or above the light pollution level, Hg element pollution is relatively serious and at the medium pollution level, Cd, Pb and Cr are at the light pollution level, and the other heavy metals are at the clean level; Among them, the single pollution index of Hg element belongs to medium pollution, accounting for 21.52% of the total sample points, and the single pollution index of Cd, Pb and Cr belongs to light pollution, accounting for 43.04%, 55.70% and 77.22% of the total sample points respectively. The NPI of 8 heavy metal elements in the surface soil of the study area is between 0.80 ~ 3.20, with an average value of 1.64, which is at the level of mild pollution. From the results, the heavy metal elements in the surface soil of the study area are affected by human activities, and Hg is the main pollution factor. This is consistent with the study of Li et al(2018)⁴¹.

Table 2

.Ecological risk of heavy metal in study area
Parameter	P_i								NPI
Parameter	Hg	Cd	As	Pb	Cr	Zn	Ni	Cu	NPI
Minimum	0.68	0.57	0.24	0.55	0.65	0.27	0.15	0.23	0.80
Maximum	4.05	2.94	1.66	2.00	1.90	1.01	0.98	2.16	3.20
Average	2.03	1.14	0.59	1.11	1.22	0.68	0.56	0.57	1.64
Pollution class	III	II	I	II	II	I	I	I	III

Spatial distribution pattern of soil heavy metal pollution

Based on GIS technology and geo-statistical analysis method, the spatial distribution pattern of Pi and NPI values of 8 kinds of heavy metals in surface soil of Shigetai Coal Mine is drawn by using ArcGIS 13.0 software (Fig. 1).

It can be seen from Figure 1 that the surface heavy metal Hg element in the study area is the element with the largest pollution degree and pollution area. In the East, South and north of the study area, the heavy metal Hg shows a moderate pollution level, and in the West and middle of the study area, it shows a slight pollution. The heavy metal Cr in the study area is the second largest element in the pollution area. Cr shows no pollution in the middle of the study area, and other areas are slightly polluted, of which the non polluted area is small. Heavy metal Pb in the study area is the third largest element in the polluted area. Pb shows no pollution in a small area in the South and middle of the study area. Other areas are slightly polluted, and the area of light pollution is wide. Heavy metal Cd in the study area is the fourth largest element in the polluted area. Cd shows no pollution in a small area in the northwest and some areas in the south of the study area, and other areas show slight pollution. Other heavy metal elements in the study area are pollution-free. Relevant studies show that heavy metal pollution in coal mine soil mainly comes from various mining activities⁹. Compared with natural soils, the elevated concentrations of heavy metals in the mining-affected soils were also reported elsewhere, e.g., Bangladesh³⁴ and India⁴².

According to the study results of Sun and Li et al. (2013)⁴³, in the process of coal mining, a large amount of coal gangue and fly ash is produced. During the process of rain leaching, many heavy metals, such as Pb, Hg, Crand Cd are released from coal gangue and fly ash. This is the most important sources of Pb, Hg, Cd, and Cr. In addition, the wear of motor vehicle brake blocks and other parts, exhaust emissions and other traffic activities are also one of the sources of heavy metal Hg pollution in soil⁴⁴. Combined with the actual situation of the study area, the study area is adjacent to the Shaanxi Inner Mongolia border in the East, so the emission of automobile exhaust may also be another cause of Hg pollution in the study area. From the spatial distribution pattern of heavy metal NPI in the study area, its distribution state is basically consistent with the P_i spatial distribution of heavy metal Hg.

Potential ecological risk assessment of soil heavy metal pollution

Taking the soil background value of Shaanxi Province as the reference ratio, the single potential ecological risk index (E) and RI of heavy metals in Shigetai Coal Mine were calculated, and the ecological risk was evaluated according to the classification standard of potential ecological risk. It can be seen from Table 3 that the average value of surface heavy metal element E in the study area is Hg (81.01), Cd (34.13), As (5.90), Pb (5.56), Cu (2.83), Ni (2.80), Cr (2.44) and Zn (0.68). In addition to Hg, the average potential ecological risk index of other heavy metals on the surface of the study area is less than 40, which is at the level of slight pollution. The potential ecological risk of Hg element varies from 27.30 to 161.90, with an average value of 81.01, which is at a high ecological risk. It can be seen that Hg is the main ecological risk factor in the study area. The range of RI in the study area is 53.44 ~ 6400.00, and the average value of comprehensive potential ecological risk index is 1336.49, which is at a very high ecological risk.

Table 3

The potential ecological risk index of heavy metal in study area.
Parameter	E								RI
Parameter	Hg	Cd	As	Pb	Cr	Zn	Ni	Cu	RI
Minimum	27.30	17.21	2.40	2.76	1.30	0.27	0.75	1.16	53.44
Maximum	161.90	88.11	16.58	10.00	3.80	1.01	4.88	10.81	6400.00
Average	81.01	34.13	5.90	5.56	2.44	0.68	2.80	2.83	1336.49
Pollution class	High	Low	Low	Low	Low	Low	Low	Low	Extremely high

Note: E stands for the individual potential ecological risk index.

Spatial distribution pattern of potential ecological risk of soil heavy metal pollution

The spatial distribution pattern of heavy metals E and RI on the surface of Shigetai Coal Mine is shown in Figure 2. It can be seen from Figure 2 that the spatial distribution of Hg element E is basically synchronized with that of P_i. The distribution area of Hg is large, and the potential ecological risk index presents medium risk, indicating that Hg is the main ecological risk factor in the mining area. The E spatial distribution of other heavy metals Cd, As,, Pb, Cr, Zn, Ni and Cu is quite different. Among them, Ni, Zn and Pb are mainly concentrated in the east and midwest of the mining area, Cr and As are mainly distributed in the east and southeast of the mining area, Cu is only concentrated in the south of the mining area, and Cd is mainly concentrated in a small area in the northwest and east of the mining area.

It can be seen from Figure 3 that from the spatial distribution pattern of RI in the study area, the ecological risk index is high in the east and north of the study area, showing extremely strong risk, slight risk in the middle and west, and medium risk in other areas. On the whole, the spatial distribution pattern of heavy metal RI value in the study area shows an obvious horizontal zonal distribution pattern, which is basically consistent with the distribution pattern of NPI.

Ecological risk early warning of soil heavy metal pollution

Ecological risk early warning evaluation originates from ecological risk evaluation, which emphasizes the possible risk warning research of ecosystem. According to the classification standard of ecological risk given by Rapant and Kordik⁴⁵, the ecological hazards caused by surface heavy metal pollution in Shigetai Coal Mine in Northern Shaanxi are early-warning evaluated. It can be seen from Table 4 that the average I_ER of Hg, Cd, As, Pb, Cr, Zn, Ni and Cu in the study area is 1.03, 0.14, -0.41, 0.11, 0.22, -0.32, -0.44 and -0.43. Among them, Hg is in light alarm, Cd, Pb and Cr are in early warning, and As, Zn, Ni and Cu are in no alarm state. The range of I_ER in the study area is -34.80 ~ 81.00, and the average value is -1.13, which belongs to no early warning.

From the spatial distribution of I_ER (Figure 4), the northern and eastern parts of the study area show heavy alarm, the transition zone from eastern to northern shows medium alarm to light alarm, the western and central regions show no alarm, and the rest parts show the state of early warning to light alarm. The high value area of ecological risk early warning is mainly distributed in the East and North, with serious pollution and obvious zonal distribution law.

Table 4

Ecological risk index of heavy metals in the study area
Parameter	I_ERi								I_ER
Parameter	Hg	Cd	As	Pb	Cr	Zn	Ni	Cu	I_ER
Minimum	-0.32	-0.43	-0.76	-0.45	-0.35	-0.73	-0.85	-0.77	-34.80
Maximum	3.05	1.94	0.66	1.00	0.90	0.01	-0.02	1.16	81.00
Average	1.03	0.14	-0.41	0.11	0.22	-0.32	-0.44	-0.43	-1.13
Warning class	Low	Early	No	Early	Early	No	No	No	No

(1) The contents of Hg, Cd, As, Pb, Cr, Zn, Ni and Cu in the soil of the study area vary from 0.043 to 0.255, 0.44 to 2.23, 2.66 to 18.40, 11.80 to 42.80, 40.50 to 118.60, 18.90 to 70.10, 4.31 to 28.10, 4.96 to 46.25mg/kg respectively. The average contents of heavy metals Hg, Cd, Pb and Cr are 2.03, 1.36, 1.11 and 1.23 times of the soil background value of Shaanxi Province, The average content of other heavy metals was lower than the background value of soil in Shaanxi Province; From the coefficient of variation, Hg and Cd showed medium variation, and As, Pb, Cr, Zn, Ni and Cu showed strong variation; The skewness coefficient and kurtosis coefficient of Cd, As and Cu in the study area are high and in a cumulative state.

(2) The single factor pollution analysis of soil heavy metals in the study area shows that the pollution of heavy metal Hg is relatively serious, which is mainly concentrated in the east and north of the study area. The south is also polluted by Hg in a small area, at a moderate pollution level, and the three elements such as Cd, Pb and Cr are at a mild pollution level; The analysis of Nemero comprehensive pollution index method shows that affected by mining, the Nemero comprehensive pollution index in the study area reaches grade III, and Hg is the main pollution factor.

(3) The average value of potential ecological risk index of soil heavy metals in the study area is Hg (81.01), Cd (34.13), as (5.90), Pb (5.56), Cu (2.83), Ni (2.80), Cr (2.44) and Zn (0.68). The range of potential ecological risk of heavy metal Hg is 27.30 ~ 161.90, with an average value of 81.01, which is at a high ecological risk level, and other heavy metals show low risk; The east and north of the study area are high-risk areas, the middle and west are low-risk areas, and the rest are medium risk areas; The high value area of ecological risk early warning in the study area is mainly distributed in the east and north, and its pollution is serious. The whole study area shows an obvious zonal distribution law.

Description of the experimental site

Shigetai Coal Mine is located in the northwest of Shenmu county, Shaanxi Province, about 55 kilometers away from the northeast of Wulun Mulan River. The administrative division is under the jurisdiction of Daliuta Town, Shenmu county. The well field is bounded by Ulan Mulun River in the west, Halagou well field in the south, batuta well field in the north, and qigaigou and Shaanxi Mongolia border in the East; The west is bounded by Ulan Mulun river. Its geographical coordinates are: 110 ° 09 ′ 41 ″ to 110 ° 18 ′ 35 ″ east longitude and 39 ° 17 ′ 02 ″ to 39 ° 35 ′ 16 ″ north latitude. Shigetai Coal Mine is about 8.4km long from north to South and 8.5km wide from east to west, covering an area of 65.25km2. The mining area is located in the north of the Loess Plateau in Northern Shaanxi and the southeast edge of the Mu Us Desert. It belongs to the north temperate semi-arid continental monsoon climate. The geomorphic units can be divided into two categories: aeolian sand is located in the north of the mine field, with continuous sand dunes, undulating waves, relatively flat terrain and undeveloped water system; The annual average temperature is 8.7 ℃, the extreme maximum temperature is 41.2 ℃, the extreme minimum temperature is - 29 ℃, and the annual average precipitation is 405.6mm.

Shigetai Coal Mine is one of the main production mines of Shendong coal group. It was officially put into operation on January 15, 2006, with 893 million tons of geological reserves and 657 million tons of recoverable reserves. The coal quality has the characteristics of low ash, low sulfur, low phosphorus and medium and high calorific value. It is a high volatile long flame coal and unbonded coal. It is a high-quality power coal, chemical and metallurgical coal.

Sample Collection

The sample collection time is June 2020, and the soil sampling depth is 0 ~ 30cm. First collect one sample of about 0.5kg at the sampling point, and select another two sampling points within 2m around the point to collect 0.5kg soil samples respectively. Mix three soil samples evenly, take 500g by quartering method to form one sample, put it into cloth bag, put the collected sample into cloth bag first, and then put it into polyethylene bag after most of the water is drained. After that, air drying was carried out. After air drying, some samples were screened for 0.149mm and then Hg, Cd, As, Pb, Cr, Zn, Ni and Cu were measured. The location of the study area and the distribution of sampling points are shown in Figure 5.

Determination method

Table 5

Execution standard for heavy metal content detection
Metal	Detection basis	Detection limit	Testing equipment
Hg	GB/T 22105.1-2008	0.002mg/kg	BAF-2000 atomic fluorescence photometer
Cd	GB/T 17141-1997	0.01 mg/kg	SOLAAR M6 atomic absorption spectrometer
As	GB/T 22105.2-2008	0.01 mg/kg	BAF-2000 atomic fluorescence photometer
Pb	GB/T 17141-1997	0.10 mg/kg	ICE 3500 atomic absorption spectrometer
Cr	HJ 350-2007	0.40 mg/kg	ICP-5000 inductively coupled plasma emission spectrometer
Zn	HJ 350-2007	0.10 mg/kg	ICP-5000 inductively coupled plasma emission spectrometer
Ni	HJ491-2019	3.0 mg/kg	SOLAAR M6 atomic fluorescence photometer
Cu	HJ491-2019	1.0 mg/kg	SOLAAR M6 atomic fluorescence photometer

Evaluation method

Nemero comprehensive pollution index method

Taking the soil background value of Shaanxi Province as a reference, the soil pollution status was evaluated by single factor pollution index (P_i) and Nemero comprehensive pollution index (NPI). The calculation formula is:

$${P_i}=\frac{{{C_i}}}{{C_{n}^{i}}}$$

$$NPI=\sqrt {\frac{{P_{{iave}}^{2}+P_{{i\hbox{max} }}^{2}}}{2}}$$

Where, C_i is the measured content of heavy metal i (mg/kg); Ci n is the background value of heavy metal i (mg/kg); P_i is the single factor pollution index of heavy metal i; NPI is the Nemero comprehensive pollution index, P_iave is the average value of each heavy metal single factor pollution index; P_imax is the maximum value of heavy metal single factor pollution index. The standard parts of pollution index classification are shown in Table 6.

Table 6

Classification of pollution degree of P_i and *NPI*
Class	P_i	Pollution level	NPI	Pollution level
1	P_i≤1	Clean	NPI≤0.7	Clean
2	1༜P_i≤2	Light	0.7༜NPI≤1	Light
3	2༜P_i≤3	Medium	1༜NPI≤2	Low
4	P_i >3	High	2༜NPI≤3	Medium
5	/	/	NPI༞3	High

Potential ecological risk index method

From the perspective of heavy metal biological toxicity, the potential ecological risk index method can not only reflect the pollution level of single heavy metal elements, but also express the comprehensive effects of all heavy metal elements. The calculation formula is:

$$R{I_j}=\sum\limits_{{i=1}}^{n} {E_{j}^{i}} =\sum\limits_{{i=1}}^{n} {{T_i} \times C_{j}^{i}} =\sum\limits_{{i=1}}^{n} {{T_i} \times \frac{{c_{j}^{i}}}{{c_{r}^{i}}}}$$

Where: RI_j is the comprehensive potential ecological risk index of various heavy metals in sample j, Ei j is the single potential ecological risk index of heavy metal i in sample j, Ci j is the pollution index of heavy metal i in sample j, ci j is the measured concentration of heavy metal i in sample j, ci r is the reference ratio of heavy metal i (the background value of soil environment in Shaanxi is taken as the reference ratio in this study), T_i is the Toxicity Coefficient of heavy metal i ^{46, 47}. ci r and T_i values are shown in Table 7.

Table 7

Environmental background values and toxic-response parameters of heavy metals in the soils
Element	Hg	Cd	As	Pb	Cr	Zn	Ni	Cu
ci r	0.063	0.76	11.1	21.4	62.5	69.4	28.8	21.4
T_i	40	30	10	5	2	1	5	5

Ecological risk early warning index method

In this study, the ecological risk early warning index (I_ER) proposed by Rapant and Kordik⁴⁵ was used to evaluate the ecological risk of heavy metal pollution in open-pit coal mine area. The calculation formula is:

$${I_{ER}}=\sum\limits_{{i=1}}^{n} {{I_{ERj}}} =\sum\limits_{{i=1}}^{n} {\invalidcharacter {P_i}-1\invalidcharacter }$$

Where: I_ER is the ecological risk early warning index, I_ERj is the ecological risk index of the i-th heavy metal, and P_i is the pollution index of heavy metal i. The pollution risk degree of RI and I_ER were shown in table 8.

Table8. Classification of risk degree of RI and I_ER

Class	RI		I_ER
I	E≤40; RI≤150	Low	I_ER≤0	No
II	40༜E≤80; 150༜RI≤300	Medium	0༜I_ER≤1	Early
III	80༜E≤160; 300༜RI≤600	High	1༜I_ER≤3	Low
IV	160༜E≤320; 600༜RI≤1200	Very high	3༜I_ER≤5	Medium
V	E༞320;RI༞1200	Extremely high	I_ER༞5	High

Statistical analyses

The data were processed and analyzed by Microsoft Excel 2013. Figure generation was performed by using ArcGIS 13.0.

Acknowledgments

This research was supported by the Scientific Research Item of Shaanxi Provincial Land Engineering Construction Group(DJNY2022-20).

Author Contributions

N.W. designed the study and wrote the paper. Y. L performed the experiments and collected the data. Z. L. contributed the statistical analyses.Y.S. reviewed and edited the manuscript. All authors read and approved the final manuscript.

Additional Information

Competing interests: The authors declare no competing interests.

Data availability

All data generated or analysed during this study are included in this published article.

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No competing interests reported.

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Spatial Distribution Characteristics and Evaluation of Soil Pollution in Coal Mining Area of Loess Plateau in Northern Shaanxi

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Abstract

Figures

Introduction

Results And Discussion

Analysis of heavy metal content in wasteland soil

Table1 .Statistic of heavy metal concentration in wasteland soil

Heavy metal pollution characteristics in the study area

Spatial distribution pattern of soil heavy metal pollution

Potential ecological risk assessment of soil heavy metal pollution

Spatial distribution pattern of potential ecological risk of soil heavy metal pollution

Ecological risk early warning of soil heavy metal pollution

Conclusion

Materials And Methods

Description of the experimental site

Sample Collection

Determination method

Evaluation method

Nemero comprehensive pollution index method

Potential ecological risk index method

Ecological risk early warning index method

Statistical analyses

Declarations

References

Additional Declarations

Status:

Version 1