Spatial distribution and ecological risk of heavy metals and their source apportionment in soils from a typical mining area, Inner Mongolia, China
XIE Shicheng1,2,3, LAN Tian4, XING An1,2,3, CHEN Chen1,2,3, MENG Chang1,2,3, WANG Shuiping1,2,3, XU Mingming1,2,3, HONG Mei1,2,3,*()
1Inner Mongolia Agricultural University, Hohhot 010018, China 2Inner Mongolia Key Laboratory of Soil Quality and Nutrient Resources, Hohhot 010018, China 3Key Laboratory of Agricultural Ecological Security and Green Development at Universities of Inner Mongolia Autonomous Region, Hohhot 010018, China 4School of Environment, Tsinghua University, Beijing 100084, China
Determining the distributions and sources of heavy metals in soils and assessing ecological risks are fundamental tasks in the control and management of pollution in mining areas. In this study, we selected 244 sampling sites around a typical lead (Pb) and zinc (Zn) mining area in eastern Inner Mongolia Autonomous Region of China and measured the content of six heavy metals, including cuprum (Cu), Zn, Pb, arsenic (As), cadmium (Cd), and chromium (Cr). The ecological risk of heavy metals was comprehensively evaluated using the Geo-accumulation index, Nemerow general pollution index, and potential ecological risk index. The heavy metals were traced using correlation analysis and principal component analysis. The results showed that the highest content of heavy metals was found in 0-5 cm soil layer in the study area. The average content of Zn, As, Pb, Cu, Cr, and Cd was 670, 424, 235, 162, 94, and 4 mg/kg, respectively, all exceeding the risk screening value of agricultural soil in China. The areas with high content of soil heavy metals were mainly distributed near the tailings pond. The study area was affected by a combination of multiple heavy metals, with Cd and As reaching severe pollution levels. The three pathways of exposure for carcinogenic and noncarcinogenic risks were ranked as inhalation>oral ingestion>dermal absorption. The heavy metals in the study area posed certain hazards to human health. Specifically, oral ingestion of these heavy metals carried carcinogenic risks for both children and adults, as well as noncarcinogenic risks for children. There were differences in the sources of different heavy metals. The tailings pond had a large impact on the accumulation of Cd, Zn, and Pb. The source of Cr was the soil parent material, the source of As was mainly the soil matrix, and the source of Cu was mainly the nearby Cu ore. The purpose of this study is to more accurately understand the extent, scope, and source of heavy metals pollution near a typical mining area, providing effective help to solve the problem of heavy metals pollution.
XIE Shicheng, LAN Tian, XING An, CHEN Chen, MENG Chang, WANG Shuiping, XU Mingming, HONG Mei. Spatial distribution and ecological risk of heavy metals and their source apportionment in soils from a typical mining area, Inner Mongolia, China. Journal of Arid Land, 2023, 15(10): 1196-1215.
Fig. 1Location of the study area (a) and spatial distribution of sampling sites (b)
Ei r
Potential ecological risk level
RI
Potential ecological risk level
<40
Low ecological risk
<150
Low ecological risk
40-80
Moderate ecological risk
150-300
Moderate ecological risk
80-160
Considerable ecological risk
300-600
Considerable ecological risk
160-320
Very high ecological risk
>600
Very high ecological risk
>320
Serious ecological risk
Table 1 Classification of potential ecological risk
Fig. 2Content of cuprum (Cu; a), lead (Pb; b), zinc (Zn; c), arsenic (As; d), chromium (Cr; e), cadmium (Cd; f), at different soil depths. The bar chart and the width of violin chart represent the frequency of the data. The black dots represent the medians. The bars represent the 95% confidence interval.
Fig. 3Soil pH (a) and electrical conductivity (EC; b) at different soil depths. The bar chart and the width of violin chart represent the frequency of the data. The black dots represent the medians. The bars represent the 95% confidence interval.
Fig. 4Spatial distribution of the content of Pb (a), Zn (b), Cu (c), As (d), Cr (e), and Cd (f) at 0-5 cm soil depth
Fig. 5Spatial distribution of heavy metals pollution level at 0-5, 5-10, 10-20 cm soil depths based on the Geo-accumulation index (Igeo). (a1-a3), Pb; (b1-b3), Zn; (c1-c3), Cu; (d1-d3), As; (e1-e3), Cd; (f1-f3), Cr.
Fig. 6Nemerow general pollution index (PN) of heavy metals at 0-5 (a), 5-10 (b), 10-20 (c) cm soil depths
Fig. 7Pollution distribution map based on potential ecological risk index (RI) at 0-5 (a), 5-10 (b), 10-20 (c) cm soil depths
Heavy metal
RfDing mg/(kg•d)
RfDder mg/(kg•d)
RfDinh mg/(kg•d)
HQing
HQder
HQinh
Child
Adult
Child
Adult
Child
Adult
Cu
4.0×10-2
1.2×10-2
-
1.6×10-4
9.2×10-5
3,8×10-7
3.2×10-7
3.8×10-9
8.6×10-9
Zn
3.0×10-1
6.0×10-2
-
6.4×10-4
3.8×10-4
1.6×10-6
1.3×10-6
1.5×10-8
3.5×10-8
As
3.0×10-4
1.2×10-4
3.5×10-6
3.4×10-4
2.0×10-4
8.3×10-7
7.0×10-7
8.2×10-9
1.8×10-8
Pb
3.5×10-3
5.3×10-4
8.2×10-5
1.8×10-4
1.0×10-4
4.3×10-7
3.6×10-7
4.3×10-9
9.6×10-9
Cd
1.0×10-3
1.0×10-5
2.4×10-6
2.3×10-6
1.4×10-6
5.7×10-9
4.8×10-9
5.6×10-11
1.3×10-10
HI
-
-
-
1.2
7.0×10-1
8.2×10-3
6.9×10-3
2.4×10-3
5.4×10-3
Table 2 Assessment of reference dose (RfD) values and noncarcinogenic health risks of different exposure pathways of soil heavy metals in different populations
Heavy metal
CRing
CRder
CRinh
Child
Adult
Child
Adult
Child
Adult
As
3.4×10-4
2.0×10-4
8.3×10-7
7.0×10-7
8.2×10-9
1.8×10-8
Pb
1.8×10-4
1.0×10-4
4.3×10-7
3.6×10-7
4.3×10-9
9.6×10-9
Cd
2.3×10-6
1.4×10-6
5.7×10-9
4.8×10-9
5.6×10-11
1.3×10-10
Table 3 Assessment of carcinogenic health risks of different exposure pathways of soil heavy metals in different populations
Cu
Zn
Cr
As
Cd
Pb
EC
pH
Cu
1.000
Zn
0.351**
1.000
Cr
0.103
-0.355**
1.000
As
0.196**
0.199**
-0.097
1.000
Cd
0.583**
0.411**
0.116
0.485**
1.000
Pb
0.532**
0.331**
0.214**
0.491**
0.840**
1.000
EC
0.382**
0.366**
-0.134*
0.138*
0.282**
0.229**
1.000
pH
-0.202**
-0.370**
0.226**
-0.045
-0.115
-0.059
-0.476**
1.000
Table 4 Spearman correlation analysis of heavy metals and soil physical-chemical parameters
PC
Initial eigenvalue
Sum of squared loadings
Eigenvalue
Explain variance (%)
Cumulative variance (%)
Eigenvalue
Explain variance (%)
Cumulative variance (%)
1
2.583
43.054
43.054
2.583
43.054
43.054
2
1.436
23.937
66.991
1.436
23.937
66.991
3
0.814
13.566
80.557
0.814
13.566
80.557
4
0.575
9.580
90.137
5
0.327
5.449
95.586
6
0.265
4.414
100.000
Table 5 Principal component analysis (PCA) of heavy metals in soils of the study area
Fig. 8Principal component analysis (PCA) of heavy metals in soils of the study area. PC, principal component.
Fig. 9Cluster analysis of heavy metals in soils of the study area
Fig. S1Spatial distribution of the content of lead (Pb; a), zinc (Zn; b), cuprum (Cu; c), arsenic (As; d), chromium (Cr; e), and cadmium (Cd; f)
Fig. S2Spatial distribution of the content of Pb (a), Zn (b), Cu (c), As (d), Cr (e), and Cd (f) at 10-20 cm soil depth
Fig. S3Percentage of the geo-accumulation index (Igeo) at 0-5 (a), 5-10 (b), and 10-20 (c) cm soil depths
Fig. S4Percentage of the potential ecological risk factor of a single heavy metal (Ei r) at 0-5 (a), 5-10 (b), and 10-20 (c) cm soil depths
Fig. S5Percentage of potential ecological risk index (RI) at 0-5 (a), 5-10 (b), and 10-20 (c) cm soil depths
Soil depth (cm)
Parameter
Cu (mg/kg)
Pb (mg/kg)
Zn (mg/kg)
As (mg/kg)
Cr (mg/kg)
Cd (mg/kg)
pH
EC (μS/cm)
Elevation (m)
0-5
Mean
163
235
671
425
95
4
7.7
415
747
Median
105
101
366
119
64
2
8.0
213
748
Minimum
3
0
18
6
9
0
2.5
57
703
Maximum
1025
2943
5654
8840
462
87
8.5
3098
788
SD
163
373
841
933
83
8
0.9
482
20
CV (%)
100
159
125
220
87
187
11.7
116
3
Exceeding standard rate (%)
53
34
58
78
10
73
5-10
Mean
162
200
200
354
88
3
7.7
455
747
Median
86
80
358
105
59
1
7.9
235
748
Minimum
1
1
8
2
11
0
2.4
37
703
Maximum
2776
1757
6037
6458
306
20
8.6
2841
788
SD
258
294
886
741
67
3
1.0
488
20
CV (%)
159
147
444
209
76
125
12.4
107
3
Exceeding standard rate (%)
49
29
56
73
6
66
10-20
Mean
138
168
620
257
91
4
7.6
557
747
Median
73
70
304
61
62
1
7.9
302
748
Minimum
0
1
0
2
10
0
2.7
33
703
Maximum
1924
1497
5207
5985
2015
416
8.8
2713
788
SD
198
257
828
656
142
30
0.9
546
20
CV (%)
144
153
134
256
155
722
12.3
98
3
Exceeding standard rate (%)
41
23
52
64
5
62
Table S1 Statistical analysis results of soil heavy metal content
Soil depth (cm)
Ei r
RI
Cu
Zn
Cr
As
Cd
Pb
0-5
10
2
1
160
209
7
388
5-10
9
2
1
132
123
6
273
10-20
8
2
1
95
182
4
291
Table S2 Mean value of Ei r and RI of soils at different depths
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