Comprehensive Monitoring and Ecological Risk Assessment of Heavy Metals in Soil and Surface Water of Chishui River Basin in Upper Reaches of the Yangtze River

Abstract

Chishui River is an important ecological security barrier area in the upper reaches of the Yangtze River in China. Therefore, it is of great significance to conduct research on soil and water ecological risks in the Chishui River basin. In this paper, the risk of heavy metals pollution and its control factors was evaluated systematically by using surface water and soil samples from 16 tributaries in the Yunnan section of the Chishui River basin. The method of soil environmental capacity and ecological risk index were studied. The results showed that the average concentration of heavy metals in the surface water of the main stream was in the order of Fe > Mn > Zn > Cu > Pb > Cd > Hg. Except for Hg, all the concentrations of heavy metals were far lower than the Class I water limits in the Environmental Quality Standards for Surface Water (GB3838-2002) issued by the Ministry of Ecology and Environment, PRC. The average concentration of Hg concentration was 0.056 μg·L⁻¹, which was slightly higher than the limit value of Class II. Heavy metals in the surface water were distributed in a point-like manner in the main stream of the Chishui River, which was mainly affected by mining drainage, township sewage, and human production activities. Meanwhile, the environmental capacity study showed that the heavy metals in soil were in the order of Zn > Pb > Cr > Ni > As > Hg > Cu > Cd, and the environmental capacity were significant differences among different soils: purple soil > limestone soil > loess > yellow-brown soil. Soil Cd tended to migrate out of the soil under the control of the occurrence form, vegetation coverage, and human production activities, while Cr, Cu, and Ni tended to accumulate in the soil. The average comprehensive ecological risk index (RI) of heavy metals in all tributaries ranged from 44.86 to 154.15, mainly distributed in medium and low ecological risk. Therefore, it is recommended to dynamically monitor and control these pollution points in the Chishui River basin to prevent the risk of heavy metals from escalating.

1. Introduction

The shortage of water resources and the deterioration of water quality have become increasingly serious global issues with the changes in the global natural environment and the intensification of human activities [1,2]. Heavy metals are important pollutants that affect the deterioration of water quality over a long time scale because they are not easy to degrade [3,4,5,6,7,8,9,10]. They enter the water through industrial sewage, atmospheric dust, sediment release, and other ways, and directly or indirectly affect human health along with the migration of heavy metals to the food chain [11,12,13,14,15]. The Chishui River is the only primary tributary in the upper reaches of the Yangtze River in China that has not been damped and maintains a natural flow pattern. The Yunnan section of the Chishui River basin is the cradle of the basin and an important ecological security barrier in the upper reaches of the Yangtze River. The ecological environment quality of the area directly affects the water environment quality and economic development of the lower reaches of the basin.

Previous studies have identified chemical pollutants in the water environment of the Chishui River basin and classified the water environment quality level [16,17], evaluated the accumulation degree of heavy metals in the sediment of the basin, and the ecological risk [18,19]. They classified the heavy metal pollution level of soil and tried to identify the migration risk of heavy metal Cd to crops [20]. The anomaly of the water chemical index caused by man-made pollution was identified [21,22,23], and studies on biodiversity [24,25] were carried out. In a word, previous research results mainly focused on the water environmental chemistry caused by human activities in the lower reaches of the Yangtze River [12,16,18] and less on the identification and evaluation of pollutants caused by the upper reaches and natural background. Previous studies have shown that the Yunnan section of the Chishui River basin belongs to a typically high background area of soil heavy metals in southwest China, and the heavy metals (Cd, Cr, Cu, Hg, Ni, Pb, and Zn) in the soil in the area are strongly enriched [20,26,27]. However, there are few reports on the influence of heavy metals on water environment chemistry in the soil in the area. The import of heavy metals into rivers by natural origin soil and human activities and the degree of their impact on the water environment are still unclear, and the ecological security of water and soil in this critical basin is still unclear.

Given the above-mentioned research background, this paper studied the distribution and carrying capacity of heavy metal elements in soil and water, revealed the environmental capacity of heavy metals in soil and its change characteristics, and identified the migration and control factors of heavy metals in surface water of the main stream of Chishui River basin, aiming at monitoring heavy metals in the upper stream of Chishui River basin. It provides a scientific basis for the safe utilization and protection of water resources in the Chishui River basin.

2. Materials and Methods

2.1. Study Area

The study area is located in Zhenxiong and Weixin counties of Zhaotong City in northeastern Yunnan Province, southwestern China (Figure 1). The Chishui River originates in Zhenxiong country. The Yunnan section of the Chishui River basin flows through 17 townships (villages and towns). The total length of the main stream is about 97 km. The main tributaries of the Yunnan section of the Chishui River basin include 16 tributaries, namely Chishui River, Yusa River, Yuhe River, Shabatou River, Shikan River, Zhaxi River, Kuzhu River, Daoliushui River, Yudong River, Wenbi River, Longtan River, Pingqiao River, Xiaohegou River, Tongche River, Dengjiazhai River, Lanjiahegou River. The basin belongs to the plateau monsoon three-dimensional climate coexisting with the plateau subtropical zone and the warm temperate zone. It has the characteristics of more rainy days and less sunshine, and the average annual precipitation is 892.4 mm. The highest and lowest elevation are 2000 m and 726 m, respectively. It is a typical high-middle mountain-cut landform. Land use types are diverse and scattered. Karst is widely developed in the basin, and the karst hills, depressions, and caves are densely covered. The valley is shaped like a “U” or “V”, and the soil and water loss is serious. The population settlements are distributed along the river.

The study area belongs to the passive continental margin of eastern Yunnan in the Upper Yangtze ancient landmass and is mainly composed of shallow Marine carbonate facies. The outcrop is mainly Paleozoic-Mesozoic carbonate rocks, clastic rocks, and sand (mud) rock strata. Mineral resources are mainly bituminous coal, anthracite, and pyrite deposits (points).

2.2. Sampling and Analysis

2.2.1. Sample Collection

In this study, a total of 22 surface water samples were collected from the monitoring sections of the Chishui River, including 10 water samples collected from the main stream and 12 samples collected from the other 12 tributaries (Figure 1). All these surface water samples were gathered in June. A water sample was gathered in a polyethylene bottle, which had been soaked in 1:1 nitric acid and then cleaned with deionized water. Water samples were added 1:1 nitric acid solution as a protective agent and sent to the laboratory as soon as possible [28].

A total of 1972 surface soil samples were collected, covering a sampling range of 1960 km² throughout the Chishui River basin (Figure 1). The soil samples were collected at a density of 1 sample/km² in soil media at a depth of 0–20 cm. During sampling, 3~5 sub-samples were collected near a sample site, and the same amount was combined in equal quantities into a single sample. Non-soil impurities (plant residues, stones, etc.) were removed from the samples and screened through 2 mm diameter nylon screens. A composite analysis sample was composed of samples from every 4 square kilometers for analysis. Approximately 200 g of the composite analysis sample was ground into −200 mesh particle size by a pollution-free planetary ball mill, then used for analysis [29,30].

2.2.2. Analytical Methods and Quality Control

As, Cd, Cr⁶⁺, Cu, Hg, Pb, Zn, Fe, Mn, SO₄²⁻ and total nitrogen (TN) in water samples were analyzed. Meanwhile, As, Cd, Cr, Cu, Hg, Ni, Pb, Zn, Mn, and total iron (TFe₂O₃) in soil samples were tested.

The analysis and testing of soil samples were carried out using high-pressure compression of powder samples to measure TFe₂O₃ and Mn. The remaining samples were decomposed with hydrofluoric acid, nitric acid, and perchloric acid. After decomposition with aqua regia, the samples were moved to a plastic test tube, and then the volume was set and shaken. The clear solution was separated and diluted 1000 times with a 3% nitric acid solution for testing. The anion SO₄²⁻ and TN were tested on the raw water sample using ion chromatography (ICS-900 ion chromatography) and T9S ultraviolet-visible spectrophotometer, respectively. The heavy metals in the water samples were tested by adding acid samples. Inductively coupled plasma-mass spectrometry (ICP-MS) was used to test the Cd, Cu, Cr, Ni, Pb, Zn, Fe, and Mn (water samples), and atomic fluorescence spectroscopy (AFS) was used to complete the As and Hg test. TFe₂O₃ and Mn (soil) were determined by X-ray fluorescence spectroscopy, while Cr⁶⁺ was determined by spectrophotometry.

The analytical procedures followed those in China’s National Geochemical Mapping Project guidelines. Quality assurance and quality control were used throughout the testing process. A set of certified reference materials (CRMs) developed by IGGE, equal to about 8% of the total samples, were inserted randomly into each batch of 50 samples and analyzed along with field samples [31]. The results showed that the overall qualified rate of the certified reference materials was 100%, and the analyses were acceptable.

2.3. Evaluation Method

2.3.1. Environmental Capacity Index

As an important early warning index for soil heavy metal pollution level, environmental capacity can be used to measure the maximum load that the soil can bear for heavy metal pollution [32,33]. In this study, the surface soil samples in the study area are used to calculate the soil environmental capacity [33,34,35], as shown in the calculation formula:

$$PI=1/n·{\sum }_{i=1}^n{P}_i$$

(1)

$$P_i=Q_i/Q_b$$

(2)

$$Q_i=M\left(C_i-C_p\right)\times {10}^{-6}$$

(3)

$$Q_b=M\left(C_i-C_b\right)\times {10}^{-6}$$

(4)

where PI is the comprehensive environmental capacity index of soil heavy metals; n is the number of heavy metals; P_i is the single environmental capacity index of heavy metal i, which is the ratio of existing environmental capacity (Q_i) to total environmental capacity (Q_b); C_i, C_p, and C_b are the risk screening value [36], test value, and background value of heavy metal i in soil, mg·kg⁻¹; M is the weight of the cultivated soil layer per unit area, which was calculated using the measured bulk density data of the surface soil in this study, with a value of 2.48 × 10⁶ kg·hm⁻². Ye [35] divided the comprehensive environmental capacity index (PI) into five capacity levels: high-capacity area with clean soil (1 < PI), medium-capacity area with mild soil pollution (0.7 < PI ≤ 1), low-capacity area with soil moderately polluted (0.3 < PI ≤ 0.7), warning area with soil heavily polluted (0 < PI ≤ 0.3), and overloaded area with polluted soil exceeds the risk control range (PI ≤ 0). Due to the high background of soil heavy metals in the research area, when the soil heavy metal background values are high (i.e., both C_b and C_p are higher than C_i), this heavy metal does not participate in the calculation of the environmental capacity index, and only its soil environmental capacity value is calculated. In this study, the existing environmental capacity (Q_i) was used as the static environmental capacity of soil for discussion.

When considering the impact of heavy metals in soil on their migration and transformation processes, including the input and output of pollutants, adsorption and desorption, fixation, and dissolution, accumulation, and degradation, etc., their soil environmental capacity can be characterized by a dynamic change value, namely dynamic environmental capacity [33,34]:

$$Q_n={10}^{-6}M\left(S_i-C_p{K}^n\right)\left(1-K\right)/\left[K\left(1-K^n\right)\right]$$

(5)

where Q_n is the dynamic environmental capacity of soil heavy metals; S_i is the allowable limit value of heavy metal i after a certain period of change, mg·kg⁻¹; K is the residual rate of pollutants, with a constant of 90% [34,37]; n is the control period, a; C_p is the test value of heavy metal i, mg·kg⁻¹; and M is the weight of the cultivated soil layer per unit area, with a value of 2.48 × 10⁶ kg·hm⁻².

2.3.2. Potential Ecological Risk Index

In this study, the potential ecological risk index (PERI) was used to evaluate the potential ecological risk of heavy metals in the soil. The PERI of a single heavy metal ($E_r^i$) and compound with eight heavy metals (RI) were calculated as follows [38,39]:

$$C_f^i=\frac{C_s^i}{C_n^i}$$

(6)

$$E_r^i=T_r^i\times C_f^i$$

(7)

$$RI={\sum }_1^m{E}_r^i$$

(8)

in the equation, $C_f^i$ is the pollution index of heavy metal i; $C_s^i$ is the test value of heavy metal i in the soil sample; $C_n^i$ is the background value of heavy metal i; and $T_r^i$ is the biological toxicity response factor of each heavy metal [38,40].

2.3.3. Water Chemical Pollution Index

This study used the Heavy Metal Pollution Index (I_HP) to evaluate the potential risks of heavy metals in the surface water of the basin [41,42,43], as shown in the calculation formula:

$$Q_i=C_i/C_s\times 100$$

(9)

$$W_i=k/C_s$$

(10)

$$I_{HP}={\sum }_{i=1}^n\left(Q_i{W}_i\right)/{\sum }_{i=1}^n{W}_i$$

(11)

where Q_i and $W_i$ represent the sub-indicators and weight values of heavy metal i, respectively, is the dynamic environmental capacity of soil heavy metals; $C_i$ is the test concentration of the heavy metal i, mg·L⁻¹; $C_s$ is the standard concentration of the heavy metal i, mg·L⁻¹, which is the limit value for Class III water in the Environmental Quality Standard for Surface Water (GB3838-2002) issued by the Ministry of Ecology and Environment, PRC. [44]; k is a proportional constant with a value of 1. The Heavy Metal Pollution Index (I_HP) is divided into three pollution levels: low pollution (I_HP < 15), medium pollution (15 ≤ I_HP ≤ 30), and high pollution (30 < I_HP), which helps researchers analyze the risk more intuitively.

3. Results and Discussion

3.1. Monitoring of Heavy Metals in Surface Water

The statistical results of heavy metals concentration in the surface water of the Chishui River basin are listed in

Table 1. Analytic statistics of heavy metals in surface water of Chishui River.

Monitoring Section		Cd	Hg	Cu	Pb	Zn	Mn	Fe	SO42−	TN	IHP *
		μg·L−1	μg·L−1	μg·L−1	μg·L−1	μg·L−1	μg·L−1	μg·L−1	mg·L−1	mg·L−1
main stream	SW01	—	—	0.600	2.07	2.53	21.7	758	54.3	1.79	3.79
	SW02	—	—	0.713	0.730	1.10	19.1	199	49.7	2.39	1.33
	SW03	0.087	0.050	4.26	3.07	12.2	98.3	1890	132	4.86	48.1
	SW04	0.072	—	1.40	1.34	3.67	72.7	839	56.2	10.9	1.98
	SW05	0.096	—	4.18	2.02	5.82	122	1562	61.5	18.2	2.87
	SW06	0.092	0.040	3.41	1.59	5.00	99.1	1648	58.4	6.57	38.5
	SW07	—	0.070	0.595	—	—	0.383	29.3	71.8	5.76	69.9
	SW08	—	0.080	0.879	—	—	0.571	33.4	59.0	5.13	79.9
	SW09	0.070	—	3.00	1.20	4.55	83.6	1237	68.3	4.92	1.83
	SW10	0.064	0.040	3.03	1.15	5.08	85.1	1257	60.2	5.15	38.5
	average value	0.080	0.056	2.21	1.65	4.99	60.2	945	67.1	6.56
tributary	Yuhe River	0.141	0.040	2.43	1.46	5.50	89.5	2113	74.9	6.77	38.5
	Lanjiahegou River	0.077	0.090	7.73	2.30	12.0	251	2237	33.1	5.21	86.5
	Dengjiazhai River	—	0.040	0.531	0.142	—	4.23	61.9	67.5	5.31	39.2
	Shabatou River	0.064	0.800	2.08	1.25	8.95	68.3	380	42.4	5.07	78.3
	Zhaxi River	—	0.040	0.402	—	—	2.46	9.90	78.4	1.78	40.0
	Kuzhu River	—	0.040	0.607	—	—	0.289	4.52	57.9	2.94	40.0
	Shikan River	—	0.060	1.39	—	1.26	0.309	4.93	53.5	4.76	60.0
	Daoliushui River	—	0.070	0.725	—	—	0.265	3.45	55.0	3.47	70.0
	Pingqiao River	0.052	—	0.647	0.411	6.08	14.8	135	50.5	5.00	1.04
	Yusa River	—	0.090	0.733	0.106	—	8.91	148	62.5	5.37	89.8
	Xiaohegou River	—	—	1.63	0.108	2.08	10.9	43.7	88.5	5.14	0.213
	Tongche River (middle section)	0.075	0.040	8.13	2.03	14.2	215	2514	39.9	3.39	38.5

Notes: “—”means data not detected; * Dimensionless.

. As, Cd, Cr⁶⁺, Cu, Hg, Pb, and Zn in the surface water of the main stream were less than the limit values of Class III water standards in the Environmental Quality Standards for Surface Water (GB3838-2002) issued by the Ministry of Ecology and Environment, PRC [44]. As and Cr⁶⁺ were below detectable limits in all samples. The average concentrations of heavy metals followed a descending order: Fe > Mn > Zn > Cu > Pb > Cd > Hg. The average concentration of Cd, Cu, Pb, and Zn were much lower than the limit of Class I water environmental quality standards in GB3838-2002, with concentrations of 0.080 μg·L⁻¹, 2.21 μg·L⁻¹, 1.65 μg·L⁻¹, and 4.99 μg·L⁻¹, respectively. The impact of Cd on surface water was much lower than the evaluation results of the accumulation index in sediment [18], indicating that heavy metal Cd tends to accumulate in sediment after entering the water body. The average concentration of Hg was 0.056 μg·L⁻¹, which is slightly higher than the limit of Class II water environmental quality standards in GB3838-2002. The single indicator concentration of heavy metals in various monitoring sections of the main stream ranged from Class I to Class III. The average concentrations of Fe and Mn were 945 μg·L⁻¹ and 60.2 μg·L⁻¹, respectively. The concentrations of Fe in seven monitoring sections were 2.5 to 6.3 times the standard limit value of centralized drinking water of surface water sources, which is 0.300 mg·L⁻¹ [44]. The exceeding rate of Fe was 70.0%, while only the SW05 monitoring section of Mn exceeded the limit by 1.2 times. The heavy metals in each monitoring section of the tributary were below the limit of Class I water environmental quality standards in GB3838-2002, except for Fe, Mn, and Hg. The concentrations of Fe in Tongche River, Lanjiahegou River, Yuhe River, and Shabatou River were 8.4 times, 7.5 times, 7.0 times, and 1.3 times the standard limit of centralized drinking water of surface water sources, respectively. However, Mn in the Tongche River and Hg in the whole basin were 2.2 times and 8.0 times the limit value of Class III water standards.

Heavy metals showed a significant upward trend in SW03, SW05, and SW09 in the main stream from the Chishui River, respectively (Table 1). A significant increase in various heavy metal indicators compared to SW02 (Cu (6.0 times), Pb (4.2 times), Zn (11.1 times), Fe (9.5 times), and Mn (5.1 times)) was observed in the upstream of the main stream (the monitoring section SW03). According to the on-site investigation results, there are a lot of coal mining activities near the SW03 section (Figure 1). The concentration of SO₄²⁻ in this section was 132 mg·L⁻¹, which is much higher than in other monitoring sections. Therefore, it is speculated that the mining activities are the main reason for the increase of heavy metal content in the surface water of the main stream. Compared to the upstream monitoring points, the monitoring sections of SW05 and SW06 in the middle reaches of the main stream showed an upward trend in all heavy metals except for Hg, with an increased rate ranging from 1.3 to 3.0 times. The total nitrogen (TN) concentration in SW04 and SW05 sections showed a gradually increasing trend, and the concentration reached as high as 18.2 mg·L⁻¹ in the SW05 section and then decreased to the normal range in the SW06 section. This result was much higher than the monitoring results of An et al. on the total nitrogen in the upper reaches of the Chishui River [16]. It is not difficult to determine that its source may be related to the impact of anthropogenic pollutants near Dawan Town. Due to the dilution of the main river water, various indicators of the SW06 monitoring section gradually decreased to the normal range. The monitoring section SW09 downstream of the main stream showed a cliff-like increase in all heavy metals except for Hg, with Mn (146 times), Fe (37 times), and Cu (3.4 times) showing the most significant increases.

The Chishui River basin is a typical high background area of soil heavy metals in northeastern Yunnan. Heavy metals in the soil in the area are strongly influenced by natural sources of soil-forming materials and active sources such as mining and agricultural cultivation, presenting typical high background characteristics [26,45,46,47]. The background value of Cu is calculated to be 54.9 mg·kg⁻¹, which is higher than the screening value of soil pollution risk [36]. The sites with Cu content exceeding the soil pollution screening value account for 40.0% of the basin’s area, mainly distributed in the tributary areas of Dengjiazhai River, Lanjiahegou River, Xiaohegou River, Tongche River, and Yusa River, while Fe and Mn are mainly concentrated in the Dengjiazhai River, Lanjiahegou River, and Xiaohegou River regions. The mining activities within the above-mentioned tributaries are mainly coal mines and pyrite (Figure 1), and the main source of pollution is a large amount of open-air slag accumulation and scattered mineral ash. It can also be confirmed by the results that the significant increase in SO₄²⁻ in four tributaries, including Yuhe River, Dengjiazhai River, Zhaxi River, and Xiaohegou River (Table 1). These pollutants can directly infiltrate into the soil or be carried into surface rivers by surface runoff and can also pose a huge threat to the groundwater in the basin through infiltration [48,49]. Yu et al. [18] concluded that the high content of Mn and Cu in the sediment in the upper reaches of the Chishui River was mainly affected by erosion caused by slope farmland. Some studies indicated that soil erosion and rock weathering were considered important natural sources of heavy metals in surface water, and Fe and Al were considered as important products of rock weathering and soil erosion [50,51,52]. Previous studies suggested that the enrichment of heavy metals in different monitoring sections might be related to soil erosion. In this study, varying degrees of enrichment of Mn, Fe, and Cu elements have been observed in the monitoring sections of the Lanjiahegou River and the middle section of the Tongche River in this study (Table 1). It indicates that the changes in heavy metals of the SW09 monitoring section are related to soil erosion caused by farming activities in the Tongche River and its adjacent watersheds [53].

3.2. Concentration and Environmental Capacity of Heavy Metals in Soil

3.2.1. Concentration Characteristics of Heavy Metals

Soil heavy metals in the study area show an enrichment trend (

Table 2. Descriptive statistics for heavy metal concentrations in surface soil.

Items	As	Cd	Cr	Cu	Hg	Ni	Pb	Zn	Mn	TFe2O3 *
	wt/mg·kg−1									wt/%
Arithmetic Mean	13.5	0.90	136	65.1	0.16	53.7	38.1	121	1185	8.64
Std.deviation	5.21	532	53.2	35.5	70.6	18.9	8.90	26.2	422	2.86
Coefficient of variation (%)	38.5	59.2	39.0	54.6	45.4	35.3	23.3	21.8	35.6	33.1
Maximum	29.0	2.59	290	169	0.361	110	65.3	198	2448	15.4
Minimum	2.75	0.14	59.2	13.1	0.027	17.2	15.1	57.2	211	3.39
Background value	13.1	0.71	132	54.9	0.156	52.8	36.6	120	1212	7.99
Background value of Chinese soil [54]	11.2	0.097	61	23	0.065	27	26	74	585	4.2
Enrichment coefficient (Dimensionless)	1.21	9.26	2.23	2.83	2.39	1.99	1.47	1.63	2.03	2.06

Note: * Total iron.

), compared with the background values of soil (layer A) in China [54]. The enrichment coefficients of Cd, Cr, Cu, Hg, Mn, and TFe₂O₃ are all greater than 2, with Cd enrichment multiple reaching 9.26 times. Cd is strongly enriched with a high ecological security impact on soil. The coefficients of variation of Cd, Cu, and Hg in the area are greater than 45.0%, which shows that the sources of these heavy metals are complex. It has been confirmed that the main source of soil formation is the natural source of soil parent material inheritance and the artificial source generated by mining activities [26,45]. What is more, Pb and Zn show a relatively low coefficient of variation, indicating a single source, and the distribution of Pb-Zn deposits in the area may be the main enrichment reason [45,55].

3.2.2. Existing Environmental Capacity

The existing environmental capacity value (Q_i) of heavy metals in the basin was calculated by using element analysis data from surface soil (Figure 2). The results show that the existing environmental capacity in most areas of the basin is positive, with a certain capacity space. It is mainly distributed in the northeastern region of the Chishui River basin, including the Kuzhu River, Daoliushui River, Zhaxi River, Shikan River, and the lower reaches of the Chishui River. The existing environmental capacity of heavy metals followed a descending order: Zn > Pb > Cr > Ni > As > Hg > Cu > Cd. Among them, Cd shows capacity overload throughout the basin, with the most severe capacity overload near Dawan Town (Q_Cd range from −4.130 to −5.540 kg·hm⁻²) (Figure 2b). Cr and Cu overload in the areas of Yuhe River, Wenbi River, Yudong River, Pingqiao River, Tongche River, Xiaohegou River, Dengjiazhai River, and Lanjiahegou River in the southeast of the basin, while Ni overrun in the Lanjiahegou River and Tongche River (Figure 2c,d,f). Cr, Cu, and Ni are siderophile elements, and some researches suggest that the sources of these three heavy metals are related to the widely exposed basalt and Triassic carbonate rocks in the study area [45]. Therefore, it can be speculated that the overload of the three heavy metals is controlled by basalt and carbonate rocks, and the overload of Ni is also significantly affected by the pyrite in the Lanjiahegou River area (Figure 1). Pb and Zn overload only in the upstream area of Chishuiyuan Town in the southwest of the basin, while the remaining areas have high-capacity space (Figure 2g,h). It is determined that Pb and Zn are mainly affected by the Pb-Zn mines in the upper reaches of the Chishui River, and the affected areas showed value ranges of Q_Pb and Q_Zn are −45.00 to −358.0 kg·hm⁻² and −2.000 to −146.0 kg·hm⁻², respectively. As and Hg are not overloaded in the whole basin, indicating a high degree of soil tolerance (Figure 2a,e).

Figure 3 shows the distribution of the comprehensive environmental capacity index (PI) of As, Cr, Hg, Ni, Pb, and Zn in the surface soil of the Chishui River basin in 16 tributaries. As shown in Figure 3, the proportion of high-capacity areas (PI > 1) in the basin is the highest, accounting for 47.9% of the study area. The high-capacity areas of the four tributaries of Zhaxi River, Shabatou River, Daoliushui River, and Kuzhu River are over 70.0%, indicating that the soil carrying heavy metal pollutants in these four tributaries is relatively strong. The high-capacity areas of the seven tributaries of Tongche River, Pingqiao River, Yusa River, Longtan River, Lanjiahegou River, Xiaohegou River, and Dengjiazhai River are all less than 30.0%, indicating a relatively low heavy metal bearing capacity. The overload area (PI ≤ 0) accounts for 16.0% of the basin, mainly distributed within the five tributaries of Lanjiahegou River, Xiaohegou River, Tongche River, Dengjiazhai River, and Pingqiao River. The soil heavy metals in the overload area exceed the risk control range, posing a high risk of soil pollution [35]. The Tongche River and Dengjiazhai River have a relatively high proportion of warning areas (>15.0%). If heavy metal pollutants continue to accumulate, there may be a large area of pollutant overload. Therefore, further control measures need to be taken for the soil in these two tributaries.

3.2.3. Factors Affecting Soil Environmental Capacity

The influencing factors of soil heavy metal environmental capacity mainly include soil type, soil physical and chemical properties, natural environmental conditions, and biological conditions, of which soil type has the most significant impact [33]. The main soil types in the Chishui River basin include yellow soil, yellow-brown soil, limestone soil, and purple soil. In this study, the environmental capacities of soil heavy metals of different soil types in the basin are calculated and listed in

Table 3. Environmental capacity of heavy metals in surface soil of different soil types.

HMs	Existing Environmental Capacity/kg·hm−2				Dynamic Environmental Capacity/kg·hm−2·a−1
	Yellow Soil	Yellow-Brown Soil	Limestone Soil	Purple Soil	Control Period/a	Yellow Soil	Yellow-Brown Soil	Limestone Soil	Purple Soil
As	56.92	60.75	52.68	63.99	50	10.18	10.96	9.851	9.825
					100	10.15	10.92	9.821	9.788
Cd	−1.341	−1.279	−2.643	−0.343	50	0.0878	0.0819	0.0917	0.0945
					100	0.0886	0.0827	0.0932	0.0947
Cr	70.22	24.13	88.55	164.6	50	46.24	41.86	48.27	48.60
					100	46.20	41.84	48.22	48.51
Cu	−7.677	−76.94	2.255	59.81	50	17.65	14.24	18.91	18.98
					100	17.66	14.29	18.91	18.94
Hg	4.268	3.159	4.509	4.699	50	0.5184	0.4019	0.5548	0.5492
					100	0.5159	0.4001	0.5522	0.5465
Ni	67.64	21.92	84.86	112.7	50	22.50	17.57	24.77	25.04
					100	22.46	17.55	24.72	24.97
Pb	134.6	101.0	139.3	172.5	50	26.34	21.08	28.09	27.94
					100	26.26	21.02	28.02	27.84
Zn	233.6	198.7	234.0	319.3	50	60.11	55.74	62.13	62.47
					100	59.97	55.62	62.00	62.29

. The results show that the existing environmental capacity of heavy metal Cd is overloaded in all soil types; Cu is overloaded in yellow soil and yellow-brown soil, while other heavy metals are not overloaded. The allowable limits for the existing environmental capacity of heavy metal Hg are the lowest, ranging from 3.159 to 4.699 kg·hm⁻², followed by As, Cr, and Ni, with allowable limits ranging from 21.92 to 164.6 kg·hm⁻². The maximum allowable limits for existing environmental capacity are Pb and Zn, with capacity values ranging from 101.0 to 319.3 kg·hm⁻². The environmental capacities of As and Cd in different soil types follow a descending order: purple soil > yellow-brown soil ≈ yellow soil > limestone soil, while other heavy metals show in order: purple soil > limestone soil > yellow soil > yellow-brown soil.

From 50 to 100 years, the dynamic environmental capacity of soil heavy metal Cd showed an increasing trend in yellow-brown soil, yellow soil, and limestone soil (Table 3). The relative degree of change of Cd is 0.871–1.64%, while others are less than 0.5%, reflecting that Cd tends to migrate from the soil, while other heavy metals change stably in the environmental capacity of different soil types in a time scale of 50 to 100 years. Chen et al. [45,46] indicated that the source of soil Cd in Northeast Yunnan is related to human activities, and human production activities directly affect the degree of element migration and enrichment in soil. It has been reported that the proportion of agricultural land in the yellow soil distribution range of the Chishui River basin is 58.0%, much higher than 33.7% of limestone soil by the latest Natural Resources Statistical Bulletin issued by the Ministry of Natural Resources, PRC. [56]. Moreover, agricultural land, industrial land, and mining land within the basin are mainly distributed in the Wenbi River, Tongche River, and Xiaohegou River area, and these three types of land use types account for 46.8%, 45.0%, and 44.9% of the area in the three tributaries, respectively [56]. Frequent human activities have led to changes in the physical and chemical properties of the soil within the basin, leading to an increase in the activity of Cd and its migration from the soil [20,26,57,58,59], which may be the reason why the Cd environmental capacity in yellow soil is significantly higher than that in limestone soil. At the same time, with the intensification of human agricultural production, mining, and other activities, heavy metals will continue to migrate into surface water or enter agricultural products along the food chain, which will threaten human health [11,12,13] and must be given close attention.

Table 4. Dynamic environmental capacity of heavy metals in surface soils of different tributaries.

HMs	As		Cd		Cr		Cu		Hg		Ni		Pb		Zn
Control Period/a	50	100	50	100	50	100	50	100	50	100	50	100	50	100	50	100
Shabatou River	10.31	10.26	0.0866	0.0868	45.58	45.47	17.26	17.22	0.4985	0.4960	21.55	21.49	25.44	25.35	59.42	59.25
Kuzhu River	9.733	9.702	0.0926	0.0930	48.91	48.80	19.56	19.52	0.5850	0.5821	25.56	25.49	29.37	29.28	62.74	62.57
Daoliushui River	9.179	9.153	0.0993	0.1004	52.29	52.16	21.70	21.65	0.6329	0.6299	29.02	28.93	31.79	31.69	66.11	65.94
Zhaxi River	9.393	9.369	0.0932	0.0937	50.66	50.52	21.19	21.13	0.6240	0.6209	27.09	27.00	31.13	31.05	64.46	64.30
Lanjiahegou River	10.90	10.86	0.0821	0.0827	42.08	42.14	14.53	14.59	0.4185	0.4166	17.98	17.99	21.79	21.72	56.03	55.92
Shikan River	9.124	9.093	0.0985	0.0992	52.47	52.36	22.08	22.04	0.6443	0.6411	29.11	29.03	32.26	32.15	66.32	66.13
Tongche River	10.17	10.14	0.0906	0.0915	46.51	46.50	17.43	17.47	0.5206	0.5182	23.16	23.13	26.51	26.42	60.41	60.28
Xiaohegou River	11.06	11.02	0.0818	0.0827	41.28	41.33	13.72	13.78	0.3918	0.3900	17.16	17.17	20.62	20.56	55.22	55.11
Yuhe River	9.940	9.905	0.0905	0.0916	47.71	47.66	18.62	18.62	0.5455	0.5429	24.10	24.06	27.65	27.56	61.59	61.44
Longtan River	9.454	9.427	0.0993	0.1001	50.78	50.76	20.31	20.30	0.6135	0.6106	28.04	27.99	30.85	30.75	64.69	64.54
Yusa River	9.909	9.874	0.0863	0.0873	47.47	47.46	19.11	19.14	0.5245	0.5220	22.84	22.81	26.73	26.64	61.38	61.24
Dengjiazhai River	10.84	10.81	0.0816	0.0827	42.35	42.39	14.76	14.84	0.4471	0.4451	18.65	18.65	22.96	22.89	56.26	56.17
Chishui River	10.28	10.25	0.0871	0.0882	45.70	45.65	17.17	17.18	0.4922	0.4899	21.72	21.68	25.21	25.15	59.55	59.43
Wenbi River	10.33	10.30	0.0898	0.0914	45.75	45.68	16.70	16.68	0.5406	0.5381	23.26	23.20	27.19	27.12	59.59	59.46
Yudong River	10.78	10.75	0.0808	0.0827	42.72	42.71	15.13	15.16	0.4387	0.4368	18.62	18.60	22.65	22.60	56.59	56.49
Pingqiao River	9.566	9.539	0.0933	0.0954	49.84	49.81	20.15	20.14	0.6068	0.6041	26.75	26.71	30.41	30.31	63.73	63.59

Note: Unit: kg·hm⁻²·a^−1.

lists the predicted results of the dynamic environmental capacity of heavy metals in the surface soil of each tributary in the study area from 50 to 100 years. As shown in Table 4, the dynamic environmental capacity of heavy metals in the soil of each tributary of the Chishui River varies slightly from 50 to 100 years, with relative changes of less than 0.5% except for Cd. Cd shows a significant change (≥1.0%) in the Daoliushui River, Xiaohegou River, Yuhe River, Yusa River, Dengjiazhai River, Chishui River, Wenbi River, Yudong River, and Pingqiao River, indicating that soil heavy metals in this region are significantly affected by environmental changes (Figure 4). Between 50 and 100 years, As, Pb, Zn, and Hg show a decreasing trend in environmental capacity, with changes of less than 0.5%, indicating that these four heavy metals tended to remain in the soil through adsorption, fixation, and accumulation within this time scale [33]. Meanwhile, the environmental capacity of Cd continues to increase over time, indicating that it is more likely to migrate from the soil through desorption, dissolution, and degradation than other heavy metals [57].

The dynamic environmental capacity of Cd in all tributaries has changed by more than 0.5% except for the Shabatou River and Kuzhu River between 50 to 100 years (Figure 4). Previous research has shown that under the influence of the natural environment, an increase in vegetation coverage will lead to a decrease in sediment loss, thereby reducing soil heavy metal loss [60,61]. It has been pointed out that the proportion of forest (grass) land within the tributaries of the Kuzhu River and Shabatou Rivers is the highest in the basin, with 76.3% and 74.2%, respectively, in the latest Natural Resources Statistical Bulletin issued by the Ministry of Natural Resources, PRC. [56]. Some research results show that the basalt and Triassic carbonate rocks distributed in northeastern Yunnan are the main sources of the heavy metals Cr, Cu, and Ni [45], and the main components exist in the soil as residues [47], resulting in a continuous decrease in the dynamic environmental capacity of most soils in the study area over time. Therefore, it can be speculated that high vegetation coverage is the reason for the relatively stable changes in soil heavy metal environmental capacity of these two tributaries. The environmental capacities of Cr, Cu, and Ni gradually increase over time in Lanjiahegou River and Xiaohegou River, and Cu also shows an increasing trend in Tongche River, Yusa River, Dengjiazhai River, and Yudong River, while other tributaries show a decreasing trend.

3.3. Comprehensive Evaluation Results

The ecological risk index of soil heavy metals of the Chishui River basin is calculated. The potential risk index of Hg, Cd, and As are the highest, which is consistent with the analysis results of river sediment in the basin by Yu et al. [18]. The potential ecological risk index of metals follows a descending order: Hg > Cd > As > Pb > Cu > Ni > Cr > Zn. Hg shows moderate potential ecological risk in the tributaries of Pingqiao River, Dengjiazhai River, Yudong River, Wenbi River, Tongche River, Chishui River, Longtan River, and Xiaohegou River, while Cd shows moderate potential ecological risk in the tributaries of Pingqiao River, Wenbi River, and Yudong River. The remaining heavy metals show low potential ecological risk in all tributaries.

The comprehensive ecological risk index (RI) [38,39,40] of these eight heavy metals in soil was calculated (Figure 5). The comprehensive ecological risk index of soil heavy metals of the Chishui River basin is mainly distributed in low ecological risk areas (RI ≤ 150) and moderate ecological risk areas (150 ≤ RI ≤ 300), which is similar to previous research results [18]. All the tributaries are distributed in low to medium ecological risk areas. Among them, the Lanjiahegou River, Zhaxi River, Kuzhu River, and Shabatou River basins belong to low ecological risk areas. The comprehensive ecological risk value of the Pingqiao River is relatively high, and most of the samples from this tributary show moderate ecological risk.

The heavy metal pollution index (I_HP) of surface water in the basin shows that the pollution index of all five monitoring sections in the main stream is greater than 30, which belongs to the high pollution point (Table 1). However, these polluted areas are mainly distributed in a point-like manner within the main stream, indicating that they are mainly affected by local mining and other human activities. All heavy metals in the monitoring sections far from the mining area have significantly decreased with the dilution of atmospheric precipitation and the self-purification effect of rivers [62]. The pollution risk can be ignored when the pollution index reaches less than 5 [43]. Although the heavy metals index values (I_HP) of the observation sections from 12 tributaries are relatively high (with an I_HP values distribution range of 0.213~89.8) (Table 1), the impact of other tributaries on the surface water of the main stream is not significant except for Tongche River and Lanjiahegou River. It indicates that the pollution sources within the tributaries are distributed in a point-like manner, resulting in no high ecological risk under the self-purification of surface rivers under natural flow conditions.

4. Conclusions

An impact assessment of the soil and surface water system in the typical soil heavy metals high background area in southwestern China was conducted in this paper, aiming at understanding the mutual influence between soil and surface water environment. There are some new results obtained as follows:

The average mass concentration of heavy metals in the surface water of the Chishui River was in the order of Fe > Mn > Zn > Cu > Pb > Cd > Hg. In light of the Environmental Quality Standards for Surface Water (GB3838-2002) issued by the Ministry of Ecology and Environment, PRC., Hg was slightly higher than its threshold value of Class II, and other elements were far lower than their threshold values of Class I water environmental quality standards. Fe and Mn have exceeded the standard limits of surface water sources for centralized drinking water and are mainly distributed in the tributaries of Tongche River, Lanjiahegou River, Yuhe River, and Shabatou River. From the upper to the lower reaches of the main stream of the Chishui River, heavy metals in surface water increase obviously, which are parallel with mining drainage (upstream), township domestic sewage (midstream), and human agricultural production activities (downstream).

The existing environmental capacity in the surface soil of the Chishui River basin shows Zn > Pb > Cr > Ni > As > Hg > Cu > Cd. The lower reaches of the Kuzhu River, Daoliushui River, Zhaxi River, Shikan River, and the lower reaches of Chishui River are high-capacity areas, accounting for 47.9% of the basin. Overload areas (16.0%) are mainly concentrated in the seven tributaries, including Tongche River, Pingqiao River, Yusha River, Longtan River, Lanjiahegou River, Xiaohegou River, and Dengjiazhai River. The existing environmental capacity of heavy metals in the study area is mainly controlled by basalt and carbonate parent materials and mining activities. In different soil types, the soil environmental capacity is mainly ordered as purple soil > limestone soil > yellow soil > yellow-brown soil, while As and Cd are in the order of purple soil > yellow-brown soil ≈ yellow soil > limestone soil. The prediction results show that Cd tends to move out of the soil significantly under the influence of human activities in the time scale of 50 to 100 years, and its dynamic environmental capacity increases over time, while Cr, Cu, and Ni tend to accumulate in the soil. The occurrence form of soil heavy metals, vegetation coverage degree, and human production activities are the main factors controlling the dynamic change of soil heavy metal environmental capacity.

The potential risks of Hg, Cd, and As in the Chishui River basin are relatively large. The comprehensive ecological risks of heavy metals show that all tributaries are mainly distributed in the low-medium ecological risk areas, although some previous research suggests that the study area is a high heavy metal background. Five monitoring sections in the main stream of surface water are high pollution points. However, only the Tongche River and Lanjiahegou River have some influence on the main stream water in a spot-like way among the five points. The concentration of heavy metals in surface water varies with seasons and the impact of human activities, and this study only obtained monitoring results for one year. Therefore, it is recommended to dynamically monitor and control these pollution points in the Chishui River basin to prevent the risk of heavy metals from escalating.