Heavy Metal Distribution and Health Risk Assessment in Groundwater and Surface Water of Karst Lead–Zinc Mine

Abstract

Heavy metal pollution seriously threatens the drinking water safety and ecological environment in karst lead–zinc mines. Fifteen groundwater and surface water samples were collected in a karst lead–zinc mine in Daxin, Chongzuo. Ten heavy metal (Mn, Zn, As, Pb, Cr, Cd, Ni, Co, Cu, and Fe) concentrations were detected. Correlation and cluster analysis were utilized to explore the distribution characteristics and sources. The health risks were appraised using the health risk assessment model. The groundwater had more heavy metal types than the surface water, of which the concentrations and average concentrations exceeded the class III water quality standard. The mine drainage contributed most (65.10%) to the heavy metal concentrations. Pb, Zn, Cd, Mn, Co, Ni, Cu, and Fe primarily originated from the mining of the lead–zinc mine, Cr primarily came from the fuel combustion and wear of metals, and As was primarily connected with the regional geological background. The groundwater had a higher total health risk (5.12 × 10⁻⁴ a⁻¹) than the surface water (2.17 × 10⁻⁴ a⁻¹). In comparison with the non-carcinogenic risk, the carcinogenic risk increased by three to five orders of magnitude. The carcinogenic risk distribution of Cr and Cd represented the health risk pattern. The drinking pathway posed two to three orders of magnitude the amount of health risks that the dermal contact pathway posed. Children suffered greater health risks. Water security for children should be more strictly controlled. Zn, Cd, Pb, Mn, and Cr must be paid more attention in terms of water quality protection and management.

1. Introduction

Groundwater and surface water are vital water resources. Water quality is crucial to human survival and regional sustainable development [1,2]. Health risk assessment is an effective measure for water quality evaluation. Moreover, it has been widely studied by researchers [3,4,5]. Heavy metals are the usual contaminants in groundwater and surface water. Mn, Zn, As, Pb, Cr, Cd, Ni, Co, Cu, and Fe are stable and difficult to degrade. They enrich in the air, soil, and water, and they also cumulate in organisms. When the heavy metal concentration is above an appropriate level, it will threaten drinking water safety, environmental security, and human life [6,7,8,9]. Heavy metals may cause lots of diseases, including headache, stomachache, myalgia, anemia, skin disorder, nervous system disorder, cancer and so on. In terms of the health risk types, there are two health risk types that heavy metals may cause to humans. One is the carcinogenic risk and the other is the non-carcinogenic risk. Humans will suffer health risks once they intake a certain heavy metal concentration from groundwater and surface water by the drinking pathway and dermal contact pathway [10,11].

Mineral resources are abundant in China. More than 170 minerals have been discovered, including lead, zinc, manganese, copper, iron, and gold mines. Lead–zinc mines are the main minerals in the karst areas of southwest China. The exploitation of lead–zinc mineral resources has been a vital pillar of industry. However, the exploitation of lead–zinc mines brings severe pollution to water, especially heavy metal pollution. It is a severe issue because of the long-term irrational mining, and the untreated waste slag, waste water, and waste gas [12,13,14,15]. Karst areas possess a double hydrological structure, where groundwater and surface water are connected and formed by strong karstification [16]. It is difficult to recover and control the water pollution. In recent years, heavy metal pollution incidents in water affected by human activities have occurred frequently in karst lead–zinc mines, which have severely harmed drinking water safety and the ecological environment. To protect water resources in karst lead–zinc mines, comprehending the characteristics and accessing the health risks resulting from heavy metals in groundwater and surface water are extremely essential.

At present, studies on heavy metals in water from lead–zinc mines mainly center around the pollution status, pollution evaluation, and pollution treatment [17,18,19,20,21]. The health risk assessment is mainly employed to estimate the health risks caused by heavy metals in crops (vegetables, corn) and soil in lead–zinc mines [22,23,24,25,26,27,28,29]. These previous studies indicated that the Pb, Zn, Cd, and As concentrations in surface water increased, respectively, due to the production and construction of lead–zinc mines. Pb, Zn, Cd, and Mn severely polluted the groundwater. Heavy metals in crops and soil caused potential harm to humans in lead–zinc mines. However, most previous studies neglected the comparison between the groundwater and surface water. Moreover, there are few studies on assessing the health risks resulting from heavy metals in the water (both groundwater and surface water) of lead–zinc mines in karst areas. This study proposed to compare the characteristics of heavy metals between the groundwater and surface water in a karst lead–zinc mine. Furthermore, it employed the health risk assessment model. Moreover, the health risks were evaluated in the groundwater and surface water by the drinking pathway and dermal contact pathway. We believe that this study provides a theoretical foundation for karst lead–zinc mines in terms of water quality protection and management.

2. Materials and Methods

2.1. Overview of the Study Area

In this study, the Daxin lead–zinc mine (22°57′00″~22°58′15″ N, 107°16′33″~107°17′28″ E) was selected as the research area. It lies in the south margin of the Yunnan–Guizhou Plateau, southwest of Guangxi Zhuang Autonomous Region, north of Daxin County, Chongzuo City. The terrain is generally low in the south and high in the north. It is located at the intersection of the two geomorphic units, which are the low mountain and peak cluster valley. The subtropical monsoon climate is the typical local climate. There is a great deal of rain in summer. The mean annual precipitation approaches 1362 mm. The average annual temperature is 21.3 °C. The Cambrian stratum, Devonian stratum, Carboniferous stratum, and Quaternary stratum are exposed, and limestone and dolomite are the dominant lithology. The exposed area of the carbonate stratum accounts for over 90%. It is a typical karst region in China, with diverse karst features and a developed underground river system. It lies in the south of the watershed between the Heishui river and Lushui river. The southeast of the Daxin lead–zinc mine is rich in water. The atmospheric rainfall, water from the Baluashan reservoir in the west and the Naliang reservoir in the northwest are the major groundwater recharging sources. The groundwater flows into the Buli underground river through a sinkhole in the form of a spring and underground river outlet at the beginning, and then it flows to the Dongling reservoir, and finally, discharges into the Heishui river in the south of the study area. The Daxin lead–zinc mine is composed of four ore blocks. The length is 6.2 km from east to west, the width is 2.08 km from north to south, and the area is 13 km² [30,31]. The Daxin lead–zinc mine is an open-pit mine. Long-term irrational mining and low mining and extraction levels, as well as the failure to achieve timely and effective control of the waste slag, waste water, and waste gas, have brought serious pollution problems to the groundwater and surface water in the Daxin lead–zinc mine, especially the heavy metal pollution [32].

2.2. Sample Collection and Testing

Fifteen groundwater and surface water samples were collected in the Daxin lead–zinc mine in May 2024, including ten groundwater samples (G1: Mine drainage, G2: Tailings pond leachate, G3: Depression spring, G4: Groundwater from the sinkhole, G5: Ascending spring, G6: Ascending spring, G7: Groundwater from the sinkhole, G8: Mine water bursting, G9: Groundwater from the sinkhole, G10: Dongling groundwater) and five surface water samples (S1: Surface water from the Naliang reservoir, S2: Surface water from the downstream of the mine, S3: Surface water from the Shangbang reservoir, S4: Surface water from the Longyue lake, S5: Surface water from the Dongling reservoir) (Figure 1).

Water samples were collected using polyethylene bottles. The volume was 1.5 L at each sampling site. The sampling bottle was cleaned 3 times with the deionized water, and then washed 3 times using the collected water before sampling. The bottom of the sampling bottle with the cap was held in hand, and the bottle was directly placed l0~15 cm below the water surface, with the mouth of the bottle facing the water flow direction, then the cap was unscrewed off the bottle to fill the bottle with water. Next, the cap was put on the bottle, and the bottle was removed from water when the surface water samples (S1, S2, S3, S4, S5) were collected. The groundwater samples (G4, G7, G9, G10) were taken from 50 cm below the groundwater table. The groundwater samples (G3, G5, G6) were collected at the center of the outlet water flow. The groundwater samples (G1, G2, G8) were collected at the center of the sampling section from the half water depth. A 0.45 μm water microporous filter membrane was applied to percolate the collected water samples immediately, and the filtrates were loaded into polyethylene bottles, and then HNO₃ (1:1) was added to pH < 2. The polyethylene bottles were sealed with paraffin. The bottles were preserved in a freezer at 4 °C away from sunlight.

The temperature, dissolved oxygen, electrical conductivity, pH, and Eh were tested by the Portable Water Quality Multi-parameter Analyzer (Multi340i, WTW, Munich, Germany) in the field. The Ca²⁺ and HCO₃⁻ concentrations were detected using the Alkalinity Test Kit (Merck, Darmstadt, Germany). The concentrations of Cl⁻, NO₃⁻, and SO₄²⁻ were detected through the Ion Chromatograph (ICS-1100, Dionex, Sunnyvale, CA, USA). The concentrations of K⁺, Na⁺, Mg²⁺, and Fe were determined by the Full-Spectrum Direct Reading Plasma Spectrometer (IRIS Intrepid II XSP, Thermo Electron, Waltham, MA, USA) according to the GB/T 5750.6-2023 [33], and the minimum detection mass concentrations were as follows: 20, 5, 13, and 4.5 μg/L. The Mn, Zn, As, Pb, Cr, Cd, Ni, Co, and Cu concentrations were detected using the Inductively Coupled Plasma Mass Spectrometer (iCAP Q, Thermo Fisher, Waltham, MA, USA), and the minimum detection mass concentrations were as follows: 0.06, 0.8, 0.09, 0.07, 0.09, 0.06, 0.07, 0.03, and 0.09 μg/L. The tests set the blank samples as well as the parallel samples. The standard recovery method was used for quality assurance and control. The relative standard deviations (RSDs) of the detection values were less than 5.0%, and the recoveries were in the range between 80% and 120%. The detection values were all in the range of standard values.

2.3. Data Processing

Excel 2019 and Origin 2017 were utilized for the data analysis. SPSS 19 was employed for the correlation and cluster analysis. In particular, correlation analysis is a statistical method used to probe into two or more variable elements with correlation; thus, it can measure the correlation degree of two variable factors and identify whether the elements have homology [34,35]. The correlation of the heavy metal concentrations was explored through the Spearman correlation coefficient in the water in the karst lead–zinc mine. Cluster analysis is another statistical method used to obtain the potential differences and connections in data. The group linkage method was employed to carry out the cluster analysis [36].

The United States Environmental Protection Agency (USEPA) proposed the health risk assessment model (HRSM), which is a method used to probe into the health risk caused by heavy metals. It is assessed by estimating the probability of adverse effects on humans. There are two exposure pathways. One is the drinking pathway and the other is the dermal contact pathway. The carcinogenic risk and non-carcinogenic risk are the two risk types, which arise from the chemical carcinogenic heavy metals and chemical non-carcinogenic heavy metals, respectively. As, Cd, and Cr are the chemical carcinogenic heavy metals. The chemical non-carcinogenic heavy metals include Cu, Ni, Mn, Co, Pb, Zn, and Fe, according to the comprehensive analysis and assessment of the reliability of the carcinogenicity of chemicals, which were proposed by the International Agency for Research on Cancer (IARC) and World Health Organization (WHO) [37,38]. The calculation methods for the two health risk types are diverse. The parameters (exposure dose, carcinogenic intensity coefficient, and reference dose intake) are different when the same heavy metal is exposed by the drinking pathway and dermal contact pathway [39,40,41,42].

(1)

Drinking pathway

Chemical carcinogenic heavy metals:

$$R_w^c={\displaystyle \frac{\mathrm{Ad}{\mathrm{d}}_w\times \mathrm{S}{\mathrm{f}}_w}{L}}$$

(1)

When $R_w^c$ > 0.01,

$$R_w^c={\displaystyle \frac{1-\mathit{exp}(-\mathrm{Ad}{\mathrm{d}}_w\times \mathrm{S}{\mathrm{f}}_w)}{L}}$$

(2)

Chemical non-carcinogenic heavy metals:

$$R_w^n={\displaystyle \frac{\mathrm{Ad}{\mathrm{d}}_w}{Rf{D}_w\times L}}\times {10}^{-6}$$

(3)

$$\mathrm{Ad}{\mathrm{d}}_w={\displaystyle \frac{Ca\times \mathrm{Ia}\times \mathrm{E}\mathrm{f}\times \mathrm{E}\mathrm{d}}{\mathrm{Bw}\times \mathrm{Ta}}}$$

(4)

(2)

Dermal contact pathway

Chemical carcinogenic heavy metals:

$$R_d^c={\displaystyle \frac{\mathrm{Ad}{\mathrm{d}}_d\times \mathrm{S}{\mathrm{f}}_d}{L}}$$

(5)

When $R_d^c$ > 0.01,

$$R_d^c={\displaystyle \frac{1-\mathit{exp}(-\mathrm{Ad}{\mathrm{d}}_d\times \mathrm{S}{\mathrm{f}}_d)}{L}}$$

(6)

Chemical non-carcinogenic heavy metals:

$$R_d^n={\displaystyle \frac{\mathrm{A}{\mathrm{d}\mathrm{d}}_d}{Rf{D}_d\times L}}\times {10}^{-6}$$

(7)

$$\mathrm{A}{\mathrm{d}\mathrm{d}}_d={\displaystyle \frac{Ca\times \mathrm{Sa}\times \mathrm{P}\mathrm{c}\times \mathrm{E}\mathrm{t}\times \mathrm{E}\mathrm{f}\times \mathrm{E}\mathrm{d}\times \mathrm{C}\mathrm{f}}{\mathrm{Bw}\times \mathrm{Ta}}}$$

(8)

(3)

Total health risk assessment model

$$R_Z=R_w^c+R_w^n+R_d^c+R_d^n$$

(9)

$R_w^c,R_w^n$: Per capita annual health risks (PCAHRs) by the drinking pathway resulting from chemical carcinogenic heavy metals and chemical non-carcinogenic heavy metals, a⁻¹.

$R_d^c,R_d^n$: PCAHRs by the dermal contact pathway resulting from chemical carcinogenic heavy metals and chemical non-carcinogenic heavy metals, a⁻¹.

Rz: Per capita annual total health risks resulting from heavy metals.

L: Life expectancy, 70 a.

Sf_w: Carcinogenic potency factor of chemical carcinogenic heavy metals by the drinking pathway, kg·d/mg.

Sf_d: Carcinogenic potency factor of chemical carcinogenic heavy metals by the dermal contact pathway, kg·d/mg.

RfD_w, RfD_d: Reference dose for the daily intake of chemical non-carcinogenic heavy metals by the drinking pathway and dermal contact pathway, mg/(kg·d).

Add_w: Daily exposure dose per unit of body weight of heavy metal by the drinking pathway, mg/(kg·d); Ca: The average heavy metal concentration in water, mg/L; Ef: Exposure frequency, d/a (365 d/a); Ia: Average daily water intake, L/d (adults: 2.2 L/d, children: 1 L/d); Ed: Exposure duration (chemical non-carcinogenic heavy metals: 35 a, chemical carcinogenic heavy metals: 70 a); Bw: Body weight (adults: 60 kg, children: 25 kg); Ta: Average exposure time (chemical non-carcinogenic heavy metals: 12,775 d, chemical carcinogenic heavy metals: 25,550 d).

Add_d: Daily exposure dose per unit of body weight of heavy metal by the dermal contact pathway, mg/(kg·d); Pc: Skin penetration constant, cm·h⁻¹; Sa: Skin contact surface area (adults: 18,000 cm², children: 8000 cm²); Et: Exposure time, h·d⁻¹ (adults: 0.6333 h/d, children: 0.4167 h/d); Cf: Conversion factor (10⁻³ L)/(1 cm³) = 1.

Values of Pc, Sf and RfD are shown in

Table 1. Values of Pc, Sf and RfD.

Heavy Metal		Pc (cm/h)	Sf [kg·d/mg]		RfD [mg/(kg·d)]
			Sfw	Sfd	RfDw	RfDd
Carcinogenic heavy metal	As	0.0018	1.5	3.66	—	—
	Cd	0.001	6.1	6.1	—	—
	Cr	0.002	41	41	—	—
Non-carcinogenic heavy metal	Cu	0.0006	—	—	0.04	0.012
	Hg	0.0018	—	—	0.0003	0.0003
	Ni	0.0001	—	—	0.02	0.0054
	Pb	0.000004	—	—	0.0014	0.00042
	Zn	0.0006	—	—	0.3	0.01
	Fe	0.0001	—	—	0.3	0.045
	Al	0.01	—	—	0.14	0.14
	Mn	0.0001	—	—	0.046	0.0018
	Co	0.01	—	—	0.0003	0.0003

.

3. Results and Discussion

3.1. Heavy Metal Concentrations and Distribution Characteristics in Water

The average pH value of the water was 7.29. Only the groundwater sample at G1 (mine drainage) was acidic and the pH value was 4.18. The average Eh value was 240.3 mV. The average concentration of Ca²⁺ was 68.2 mg/L. The average molarity of HCO₃⁻ was 3.1 mmol/L. The dominant cation was Ca²⁺. The dominant anion was HCO₃⁻. Cao et al. [43] reported that Ca²⁺ and HCO₃⁻ are the two major ions in the water in karst areas. Therefore, these results accorded with the hydrochemical characteristics of karst areas.

The heavy metal concentrations in the groundwater are shown in

Table 2. Heavy metal concentrations in the groundwater (μg/L).

Heavy Metal	Cu	Pb	Zn	Cr	Co	Cd	Mn	As	Fe	Ni
Mean	2.95	12.00	7020.84	6.49	7.51	31.99	151.59	0.30	41.00	13.02
Maximum	12.50	103.00	48,600.00	8.70	51.00	225.00	395.00	0.44	200.00	34.00
Minimum	0.08	n.d. (2)	39.00	1.56	0.13	0.08	3.26	0.16	n.d. (2)	2.50
Standard deviation	3.78	32.26	14,888.11	2.59	15.65	69.81	153.67	0.12	81.30	10.75
Variation coefficient (%)	128.19	268.81	212.06	39.94	208.33	218.20	101.38	39.58	198.30	82.58
Standard limit (1)	1000.00	10.00	1000.00	50.00	50.00	5.00	100.00	10.00	300.00	20.00
Exceeding standard rate (%)	0.00	20.00	50.00	0.00	10.00	40.00	50.00	0.00	0.00	20.00
Pollution index (3)	0.00	1.20	7.02	0.13	0.15	6.40	1.52	0.03	0.14	0.65

Note: ⁽¹⁾ The class III water quality standard limit of the Groundwater Quality Standard (GB/T 14848-2017); ⁽²⁾ n.d.: not detected; and ⁽³⁾ pollution index: mean/standard limit.

. Compared with the class III water quality standard of the Groundwater Quality Standard (GB/T 14848-2017) [44], the average concentrations of Zn, Cd, Pb, and Mn were all above the standard, which were, respectively, 7.02, 6.40, 1.20, and 1.52 times as big as the standard. By contrast, the others all accorded with the standard. The Zn, Cd, Pb, Mn, Ni, and Co concentrations at the G1, G2, G3, G4, G5, and G10 sampling sites were above the standard, with rates of 50.00%, 40.00%, 20.00%, 50.00%, 20.00%, and 10.00%, and the maximum values were 48.60, 45.00, 10.30, 1.52, 1.70, and 1.02 times as big as the standard, respectively. The pollution index is a parameter used to reflect the heavy metal concentration level in water that approaches or exceeds the limits of the class III water quality standard. It is calculated by the mean of the heavy metal concentrations in water and the standard limit. The mean divided by the standard limit is the pollution index [45]. The order of the pollution index was Zn > Cd > Mn >Pb > Ni > Co > Fe> Cr > As > Cu. The Zn, Cd, Pb, and Mn pollution indexes were larger than the others; in addition, the average concentrations exceeded the standard. The Cu, Pb, Zn, Co, Cd, Mn, and Fe variation coefficients exceeded 100% in the groundwater, and the variation coefficients of the heavy metal determination method were all less than 10%, indicating that they varied greatly.

As shown in

Table 3. Heavy metal concentrations in the surface water (μg/L).

Heavy Metal	Cu	Pb	Zn	Cr	Co	Cd	Mn	As	Fe	Ni
Mean	1.75	5.82	585.10	4.40	1.49	2.62	24.95	0.50	11.20	6.52
Maximum	4.30	26.00	1635.00	8.20	3.70	12.00	80.30	1.30	56.00	12.20
Minimum	n.d. (2)	n.d. (2)	4.80	n.d. (2)	0.08	n.d. (2)	1.30	0.07	n.d. (2)	1.20
Standard deviation	1.69	11.34	797.58	3.58	1.83	5.25	34.37	0.49	25.04	6.09
Variation coefficient (%)	96.85	195.04	136.31	81.48	122.98	200.72	137.77	97.10	223.61	93.31
Standard limit (1)	1000.00	50.00	1000.00	50.00	1000.00	5.00	100.00	50.00	300.00	20.00
Exceeding standard rate (%)	0.00	0.00	40.00	0.00	0.00	20.00	0.00	0.00	0.00	0.00
Pollution index (3)	0.00	0.12	0.59	0.09	0.00	0.52	0.25	0.01	0.04	0.33

Note: ⁽¹⁾ The class III water quality standard limit of the Surface Water Quality Standard (GB/T 3838-2002); ⁽²⁾ n.d.: not detected; and ⁽³⁾ pollution index: mean/standard limit.

, compared with the class III water quality standard of the Surface Water Quality Standard (GB/T 3838-2002) [46], the average concentrations of each heavy metal all accorded with the standard. Moreover, the concentrations of Cu, Cr, Co, As, Fe, Pb, and Mn at each surface water sampling site accorded with the standard. However, the concentrations of Zn at S2 and S5, and Cd at S2, were above the standard, with rates of 40.00% and 20.00%, and the maximum values were 1.63 and 2.40 times as big as the standard, respectively. The pollution index was in the following order: Zn, Cd, Ni, Mn, Pb, Cr, Fe, As, Co, Cu. The Pb, Zn, Co, Cd, Mn, and Fe variation coefficients exceeded 100%, and the variation coefficients of the heavy metal determination method were all less than 10%, indicating that they varied greatly at the surface water sampling sites.

The groundwater had more heavy metal types than the surface water, of which the concentrations (Zn, Cd, Pb, Mn, Ni, and Co) and average concentrations (Zn, Cd, Pb, and Mn) exceeded the class III water quality standard. In particular, the Zn and Cd concentrations exceeded the class III water quality standard in both the groundwater and surface water. The Pb, Mn, Co, and Ni concentrations exceeded the class III water quality standard only in the groundwater. As for the average concentrations, only the groundwater appeared to exceed the standard, including Pb, Zn, Cd, and Mn. Groundwater occurs in the geological media. Physical, chemical and biogeochemical processes are intensity in groundwater. Surface water is mainly derived from atmospheric precipitation. The primary mineral composition is less in surface water due to the short contact time between surface water and rock [47,48]. Mining activities leave numerous heavy metals exposed. Therefore, physical, chemical and biogeochemical processes lead to the different hydrochemical composition between groundwater and surface water, as well as mining activities.

The total heavy metal concentrations in the groundwater and surface water at the sampling sites are illustrated in Figure 2. The total concentrations were in the range between 19.07 μg/L and 49,541.68 μg/L. Moreover, the average concentration was 5073.28 μg/L. The lowest concentration was 19.07 μg/L at S3 in the surface water. By contrast, the highest concentration was 49,541.68 μg/L at the G1 sampling site in the groundwater. The G1, G4, G3, and G2 sampling sites contributed more to the heavy metal concentrations, with contribution rates of 65.10%, 12.66%, 6.07%, and 6.04%, respectively. The concentration of Zn at the different sampling sites had the largest variation range, and the contribution rate (96.10%) was the highest. The contribution rates of Mn, Fe, Cd, Ni, and Pb were 2.16%, 0.61%, 0.43%, 0.21%, and 0.20%, respectively.

In the groundwater, the higher concentrations of Pb, Zn, Cd, Mn, Co, and Ni were mainly distributed at the G1, G2, G3, G4, G5, and G10 sampling sites, which were the sampling sites with higher heavy metal concentrations. More specifically, Zn: G1, G2, G3, G4, G10; Cd: G1, G2, G3, G4; Pb: G1 and G4; Mn: G1, G2, G3, G4, G5; Co: G1; Ni: G3 and G4. The total concentrations of Pb, Zn, Cd, Mn, Co, and Ni at the G1, G2, G3, G4, G5, and G10 sampling sites exhibited the following order: G1 > G4 > G3 > G2 > G10 >G5.

In the surface water, the higher concentrations of Zn and Cd were mainly distributed at the S2 and S5 sampling sites, which were the sampling sites with higher heavy metal concentrations. More specifically, Zn: S2 and S5; Cd: S2. The total concentrations of Zn and Cd at the S2 and S5 sampling sites exhibited the following order: S2 > S5.

G1 was the mine drainage sampling site, and it was located in the slag stacking area. The Pb, Zn, Cd, Mn, and Co concentrations all exceeded the class III water quality standard, which were 10.03, 48.60, 45.00, 3.30, and 1.02 times as big as the limits of the standard, respectively. The groundwater at G1 was acidic and the pH was 4.18. The slag stacking area where G1 was located was the main pollution source in the Daxin lead–zinc mine. The Zn and Cd concentrations decreased from G2 (Tailings pond leachate) to G3 (Depression spring). This was because Zn and Cd were easily retained in the karst pipelines by adsorption and precipitation in the process of flowing through the karst pipelines [19,49]. G3 was located downstream along the flow direction of the groundwater compared with G1 and G2. G1 and G2 had great influence on G3 in terms of the heavy metal concentrations. Therefore, in comparison with G2, the total Pb, Zn, Cd, Mn, Co, and Ni concentration at G3 was larger. The heavy metal concentrations at G4 (Groundwater from the sinkhole) were affected by G1, G2, and G3, with the result that G4 had the highest concentrations of Pb, Zn, Cd, Mn, Co, and Ni among G1, G2, G3, and G4.

The mine drainage from G1 flew to S2 (Surface water from the downstream of the mine) through surface runoff. The Pb, Zn, Cd, Mn, and Co concentrations at S2 were 0.25, 0.03, 0.05, 0.09, and 0.07 times as big as those at G1, respectively. The pH value of the water increased, but the above five heavy metal concentrations decreased from G1 to S2. This was because there was water–rock interaction that happened intensely in the runoff process.

Due to the double hydrological structure of the surface and underground formed by strong karstification [16], a close hydraulic connection and highly frequent conversion relationship exist between the groundwater and surface water. Moreover, the karst environment is fragile, and the karst hydrodynamic system is sensitive to environment changes [50]. Therefore, the influence between the groundwater and surface water is obvious. The karst characteristics make the pollutants diffuse rapidly in the water system, resulting in a larger pollution area in a short time and difficult restoration.

3.2. Multivariate Statistical Analysis of Heavy Metals in Water

3.2.1. Correlation Analysis

The correlation of the heavy metals in the groundwater and surface water within a short distance (<1 km) was determined using SPSS. The correlation coefficient was 0.925~0.957, showing that the correlation was significant between the adjacent sampling sites; moreover, the heavy metals may have homology in the groundwater and surface water. As shown in

Table 4. Correlation matrix of the heavy metals.

Heavy Metal	Cu	Pb	Zn	Cr	Co	Cd	Mn	As	Fe	Ni
Cu	1.000	0.926 **	0.902 **	0.471	0.917 **	0.926 **	0.481	−0.474	0.865 **	0.307
Pb		1.000	0.964 **	0.271	0.970 **	0966 **	0.433	−0.368	0.731 **	0.140
Zn			1.000	0.241	0.996 **	0.997 **	0.566 *	−0.327	0.799 **	0.217
Cr				1.000	0.250	0.265	0.167	−0.258	0.200	0.177
Co					1.000	0.997 **	0.575 *	−0.339	0.792 **	0.245
Cd						1.000	0.575 *	−0.333	0.811 **	0.246
Mn							1.000	−0.294	0.499	0.813 **
As								1.000	−0.386	−0.353
Fe									1.000	0.105
Ni										1.000

Note: ** The correlation is significant at 0.01. * The correlation is significant at 0.05.

, the correlation between Pb, Cd, Co, Cu, Fe and Zn was significant and positive. In particular, the correlation coefficient between Cd and Zn was the highest (0.997), indicating that the relationship between Cd and Zn in the water in the karst lead–zinc mine was the closest, and the concentration was extremely influenced by each other. Ni and Mn had a significant positive correlation, indicating that Ni and Mn had similar sources. Mn and Zn, Cd, and Co had a significant positive correlation, indicating that Mn and Zn, Cd, and Co had similar sources. Therefore, Pb, Zn, Cd, Mn, Co, Ni, Cu, and Fe had a certain homology. There was a negative correlation between As and the others, showing that the sources of the As and other heavy metals in the water in the karst lead–zinc mine were different. Cr and As had a negative correlation, while Cr and the other heavy metals had a positive correlation, but not significant, indicating that the homology between Cr and the others may not be obvious.

3.2.2. Cluster Analysis

The cluster analysis results are illustrated in Figure 3. The results gained from the cluster analysis and correlation analysis were accordant. Therefore, the heavy metals were divided into three classes. The first factor included Pb, Zn, Cd, Mn, Co, Ni, Cu, and Fe. The second factor was Cr. The third factor was As.

In the first factor, the Pb, Zn, Cd, Mn, Co, and Ni concentrations were relatively higher at G1, G2, G3, G4, G5, G10, S2, and S5. The concentrations of Cu and Fe accorded with the limits of the class III water quality standard, which varied greatly at the different sampling sites. G1, G2, G3, G4, G5, G10, S2, and S5 were primarily distributed in the tailings pond, slag stacking area and their downstream. Pb and Zn are the main heavy metals in lead–zinc mines, and Cd, Mn, Co, Ni, Cu, and Fe are the associated heavy metals in lead–zinc mines [51,52]. Pb, Zn, Cd, Mn, Co, Ni, Cu, and Fe could be disclosed because of the long-term irrational mining of the lead–zinc mine and the untreated tailings pond and slag stacking area. Pb, Zn, Cd, Mn, Co, Ni, Cu, and Fe continued entering into the groundwater and surface water after a long period of leaching, dissolution and surface runoff and underground runoff, resulting in Pb, Zn, Cd, Mn, Co, Ni, Cu, and Fe with higher concentrations in the water in the tailings pond, slag stacking area and their downstream. The eight heavy metals in the first factor were primarily connected with the long-term irrational mining of the lead–zinc mine, as well as the exposed Pb, Zn, Cd, Mn, Co, Ni, Cu, and Fe. The concentration distribution characteristics of Cr in the second factor and As in the third factor in the different sampling sites were relatively even, and there was no obvious difference compared with the other heavy metals, which accorded with the limits of the class III water quality standard. The waste gas produced by fuel combustion from vehicles transporting lead–zinc ore and slag in the mining area contained particulate Cr, and the wear of the alloy parts and electroplated metals of mining equipment and vehicles produced Cr [53,54]. Cr entered into the surface water, spring, and sinkhole after falling directly or being washed by rain through the surface runoff. Karst areas possess a double hydrological structure, where the groundwater and surface water are connected formed by strong karstification. The fissures are developed. The karst aquifer has a high degree of openness. A close hydraulic connection and highly frequent conversion relationship exist between the groundwater and surface water. The surface water leakage is serious [50,55]. Therefore, Cr entered into the groundwater easily once it entered into the surface water, which caused Cr accumulation in the groundwater. The heavy metal in the second factor mainly came from the fuel combustion and wear of metals. The Cambrian stratum, Devonian stratum, Carboniferous stratum, and Quaternary stratum are the main strata in the lead–zinc mine, and the sand and shale stone of the Cambrian stratum and Devonian stratum contain As-rich minerals [51]. As mainly came from As-rich minerals in the sand and shale stone of the Cambrian stratum and Devonian stratum in the study area. As entered into the groundwater and surface water through the combined function of groundwater hysteresis and microbial processes after a long time of water–rock interaction, leaching, and release, resulting a little As in the water [56]. The heavy metal in the third factor was primarily connected with the regional geological background.

3.3. Health Risk Assessment of Heavy Metals in Water

The per capita annual health risks (PCAHRs) of heavy metals in the groundwater and surface water by the drinking pathway and dermal contact pathway were calculated according to the HRSM (

Table 5. Per capita annual health risks by the drinking pathway (a⁻¹).

Heavy Metal	Groundwater		Surface Water
	Adults	Children	Adults	Children
As	2.37 × 10−7	2.59 × 10−7	3.94 × 10−7	4.30 × 10−7
Cd	1.02 × 10−4	1.12 × 10−4	8.37 × 10−6	9.13 × 10−6
Cr	1.41 × 10−4	1.54 × 10−4	9.44 × 10−5	1.03 × 10−4
Cu	3.86 × 10−11	4.21 × 10−11	2.29 × 10−11	2.50 × 10−11
Ni	3.41 × 10−10	3.72 × 10−10	1.71 × 10−10	1.86 × 10−10
Pb	4.49 × 10−9	4.90 × 10−9	2.18 × 10−9	2.37 × 10−9
Zn	1.23 × 10−8	1.34 × 10−8	1.02 × 10−9	1.11 × 10−9
Mn	1.73 × 10−9	1.88 × 10−9	2.94 × 10−10	3.10 × 10−10
Co	1.31 × 10−8	1.43 × 10−8	2.59 × 10−9	2.83 × 10−9
Fe	7.16 × 10−11	7.81 × 10−11	1.96 × 10−11	2.13 × 10−11
Total risk	2.34 × 10−4	2.65 × 10−4	1.03 × 10−4	1.13 × 10−4

and

Table 6. Per capita annual health risks by the dermal contact pathway (a⁻¹).

Heavy Metal	Groundwater		Surface Water
	Adults	Children	Adults	Children
As	5.40 × 10−9	3.79 × 10−9	8.98 × 10−9	6.30 × 10−9
Cd	5.30 × 10−7	3.72 × 10−7	4.33 × 10−8	3.04 × 10−8
Cr	1.46 × 10−6	1.02 × 10−6	9.78 × 10−7	6.87 × 10−7
Cu	4.00 × 10−13	2.81 × 10−13	2.37 × 10−13	1.67 × 10−13
Ni	6.54 × 10−13	4.59 × 10−13	3.28 × 10−13	2.30 × 10−13
Pb	3.10 × 10−13	2.18 × 10−13	1.50 × 10−13	1.06 × 10−13
Zn	1.14 × 10−9	8.02 × 10−10	9.53 × 10−11	6.69 × 10−11
Mn	2.29 × 10−11	1.60 × 10−11	3.76 × 10−12	2.64 × 10−12
Co	6.79 × 10−10	4.77 × 10−10	1.34 × 10−10	9.44 × 10−11
Fe	2.47 × 10−13	1.74 × 10−13	6.76 × 10−14	4.74 × 10−14
Total risk	2.00 × 10−6	1.40 × 10−6	1.03 × 10−6	7.24 × 10−7

).

The total health risks in the groundwater and surface water were both high. Moreover, the order of the total risk was groundwater (5.12 × 10⁻⁴ a⁻¹) > surface water (2.17 × 10⁻⁴ a⁻¹). This was mainly because the groundwater had a higher total heavy metal concentration than surface water. G1, G2, G3, G4, G5, G10, S2, and S5, with higher total heavy metal concentrations, contributed the most. G1, G2, G3, G4, G5, and G10 were groundwater sampling sites, and S2 and S5 were surface water sampling sites. The total health risks for adults and children in the groundwater and surface water both exceeded the maximum acceptable risk level (MARL), which is 5.0 × 10⁻⁵ a⁻¹. Moreover, children suffered greater health risks. This result was similar to the research by Real K H et al. [57]. Real K H et al. found that the total hazard index values for children exceeded one, which is the acceptable level, while those for adults were less than one, caused by heavy metals in the surface water from a coastal estuary. The result of this study indicated that children were the more sensitive risk receptors and were more seriously harmed by heavy metals than adults. Children suffered a greater total health risk by the drinking pathway, while it was the opposite by the dermal contact pathway. This result was similar to the research by Fu R J et al. [58]. Fu R J et al. reported that groundwater in the Shiqi river from the southwest sub-basin had a similar result. This was mainly connected with the selected model parameters, such as the skin contact area, per capita weight, and exposure frequency. Therefore, water security for children should be more strictly controlled.

The carcinogenic risks in the groundwater and surface water were both high. Moreover, the order of the total carcinogenic risk was groundwater > surface water. They were close to or exceeded the MARL for adults and children [59]. Children suffered a greater total carcinogenic risk. The groundwater had a higher total non-carcinogenic risk. The total non-carcinogenic risks were both less than the MARL for adults and children. It was higher for children. The per capita annual non-carcinogenic health risks were in the range between 10⁻¹⁴ a⁻¹ and 10⁻⁸ a⁻¹, and the health risks were small, which cannot cause obvious harm to the exposed humans.

In terms of the heavy metal types, the health risks by the drinking pathway were in the order: Cr, Cd, As, Co, Zn, Pb, Mn, Ni, Fe, Cu and Cr, Cd, As, Co, Pb, Zn, Mn, Ni, Cu, Fe in the groundwater and surface water. The order of health risks by the dermal contact pathway in groundwater and surface water was Cr > Cd > As > Zn > Co > Mn > Ni > Cu > Pb > Fe and Cr > Cd > As > Co > Zn > Mn > Ni > Cu > Pb > Fe, respectively, which was mainly related to the heavy metal types, concentrations, Sf, RfD, and Ed. This was because the health risk calculation methods of different heavy metal types through the same exposure pathway are different. The parameters (exposure dose, carcinogenic intensity coefficient, reference dose intake) are also different under the different exposure pathways [60,61]. In terms of the carcinogenic risk level, in comparison with the total non-carcinogenic risk, the total carcinogenic risk increased by three to five orders of magnitude. Therefore, the health risks were primarily generated by the carcinogenic heavy metals. The carcinogenic risk sources in the groundwater and surface water were both in the following order: Cr, Cd, As. The PCAHRs of Cr both exceeded the MARL in the groundwater and surface water. However, the PCAHRs of As were both less than the MARL. The PCAHR of Cd exceeded the MARL in the groundwater, but it was less than the MARL in the surface water. Cr and Cd posed greater carcinogenic risks than As, mainly due to the largest carcinogenic intensity coefficient of Cr and the highest concentration of Cd. Therefore, the carcinogenic risk distribution of Cr and Cd represented the health risk pattern. Cr and Cd must be paid more attention in the risk decision-making management. The similarity between this study and Li J et al. [62] was that the carcinogenic heavy metals posed obvious health risks. By contrast, the main heavy metals that caused carcinogenic health risks were different. Li J et al. reported that it was Cr, which was related to the different geological environment background and heavy metal sources between the Daxin lead–zinc mine and Huixian karst wetland.

In terms of the exposure pathways, in comparison with the dermal contact pathway, the total health risk and the PCAHR by the drinking pathway both increased by two to three orders of magnitude. Therefore, the health risks were primarily generated through the drinking pathway. Al-Asad H et al. [63] also showed that the drinking pathway posed greater health risks in groundwater from the southern Tangail, Bangladesh. As a consequence, the drinking pathway should be paid more attention.

4. Conclusions

This study presents novel research on the heavy metal distribution and health risk assessment in both the groundwater and surface water of a karst lead–zinc mine, and it provides a theoretical foundation for karst lead–zinc mines in terms of water quality protection and management. The heavy metal types in the groundwater, of which the concentrations and average concentrations exceeded the class III water quality standard, were more than those in the surface water. The Zn contribution rate (96.10%) was the largest. The mine drainage contributed most (65.10%) to the heavy metal concentrations. Pb, Zn, Cd, Mn, Co, Ni, Cu, and Fe primarily came from the mining of the lead–zinc mine. Cr primarily came from the fuel combustion and wear of metals. As was primarily connected with the regional geological background. The total health risk in the groundwater (5.12 × 10⁻⁴ a⁻¹) was higher than that in the surface water (2.17 × 10⁻⁴ a⁻¹). In comparison with the non-carcinogenic risk, the carcinogenic risk increased by three to five orders of magnitude. The carcinogenic risks of Cr and Cd exceeded the MCRL. The non-carcinogenic risks were three to nine orders of magnitude below 5.0 × 10⁻⁵ a⁻¹. The drinking pathway posed two to three orders of magnitude the amount of health risks that the dermal contact pathway posed. Children suffered greater health risks. Furthermore, the environmental management must be carried out from the treatment of pollution sources (abandoned mine and tailings pond). Zn, Cd, Pb, Mn, and Cr must be paid more attention in relation to water quality protection and management. Water security for children should be more strictly controlled. Water quality should be monitored regularly to obtain the current situation in time. Education and publicity concerning the treatment of the water environment should be strengthened to enhance humans’ awareness.