Analysis of the Quality of Typical Acidic Groundwater of the Guangwang Mining Area and Its Associated Human Health Risks

Abstract

This study determined the hydro-chemical properties of groundwater in a typical mining area and its associated human health risks, focusing on the Guangwang mining area. Groundwater samples were analyzed for toxic metals, after which analysis of principal components, the entropy-weighted water quality index, and Spearman analysis of correlation were applied to the collected data. The Environmental Protection Agency of the United States’s health hazard appraisal was utilized to assess the hazards of toxic metals in the local water supply to the health of both grownups and juveniles. HCO₃-Na and SO₄⋅Cl-Ca⋅Mg were found to be the predominant groundwater hydro-chemical types. The eastern section of the area of study showed the greatest average total dissolved solids (16,347.00 mg/L) and SO₄²⁻ (8980.00 mg/L) levels. It was determined that the groundwater hydro-chemical type was Ca-HCO₃ and that limestone leeching and the evaporative level in the coal seam aquifer were the predominant factors regulating groundwater hydrochemistry. Six of the ten assessed metals exceeded the World Health Organization’s safe water for drinking standards, with particularly high Al (66.97 mg/L) and Cd (194.53 μg/L). Spearman correlation analysis showed significant correlations between Mn, Al, Cu, and Zn, which could be attributed to bauxite minerals associated with the coal mine. Release of metal ions was attributed to the oxidation of metal sulfide minerals, which is driven by mining-induced water–rock interaction. The intake of water for drinking was shown to be the predominant route of hazard to human health. The hazard index decreased from east to west due to the level of abandoned coal mines in the eastern region, along with well-developed fissures. The total carcinogenic hazard for grownups exceeded that of juveniles due to the greater quantity of water for drinking consumed and higher surface area of skin amongst grownups. The results can guide groundwater pollution regulation activities in mining areas to minimize potential hazards of groundwater quality to the health of humans.

1. Introduction

Water resources, predominantly surface and groundwater, are essential requirements for human survival [1]. Groundwater is an crucial source of water in many regions and sustains human socio-economic development and agriculture [2,3,4,5,6]. The advantages of groundwater as a water resource include its widespread and stable flow and water quality. Groundwater meets 70% of the water needs of urban and rural residents in China, and over 95% of the rural population in China utilize groundwater as a source of water for drinking [7]. Since groundwater quality can pose hazards to human and environmental health [3,8], there is a need for monitoring of the hydro-chemical properties of groundwater and the appraisal of its associated hazards [4].

China contains extensive coal resources. However, mining of coal alters the characteristics of groundwater flow and the water balance, which in turn break down the structure of the rock surrounding the coal seam and influence groundwater circulation and the relationships between recharge, runoff, and discharge. Complex water–rock interactions can develop due to changes in the reduction environment of sulfide deposits, including oxidation and dissolution [9,10]. These processes can lead to the acidification of groundwater and formation of acid mine drainage (AMD). AMD is characterized by extreme acidity and elevated levels of various elements and compounds, including Fe, SO₄²⁻, Pb, Cr, and As [11], thereby posing considerable hazards to human and environmental health.

There has been recent increasing attention of the effects of the mining of coal on groundwater flow and water chemistry. Elijah et al. [12] determined the hydrogeochemical characteristics and change in groundwater in a coal mining area in northern Malawi; Jiang et al. [13] and Luo et al. [14] revealed the impacts of mining activities on the spread, movement, and transformation of toxic metals in regional groundwater; Zhang et al. [1] described the hydro-chemical characteristics of nitrate-rich groundwater in the Nanchong area and the associated to human health risks. Mohammadpour et al. [15] found that chromium and arsenic were the main causes of cancer risk in a chromite mining area in Iran, and the health risk index for children was 4.48. Olivares et al. [16] studied a mining area in Latin America and found that Fe, Co and As contributed the most to the total risk and cancer risk. However, there remain few related studies on the Guangwang mining areas in Guangyuan, Wangcang, and Nanjiang in northern Sichuan [17].

The current study aimed to determine the hydro-chemical characteristics of groundwater in a typical mining area and its associated human health risks, focusing on the Guangwang mining area. The objectives of the current study were to: (1) analyze the hydrogeochemical characteristics of the Guangwang mining area and identify the associated regulating factors; (2) evaluate and the predominant sources of toxic metals in AMD in the area of study; and (3) assess the hazards of groundwater in the area of study to the health of humans. The results of the current study can guide future management of groundwater resources in the Guangwang region and in similar mining of coal areas.

2. Methods and Materials

2.1. Area of Study and Sample Positions

Most of the Guangwang mining area is in Guangyuan City, northern Sichuan Province, with only the Nanjiang Coal Mine in Nanjiang County, Bazhong City. The mining area has an elongated spread with a length of 200 km from west to east, covering an area of 230 km² and spanning the districts of Lizhou and Yuanba and Wangcang and Nanjiang in Guangyuan City and Bazhong City, respectively.

Guangyuan (31°31′–32°56′ N, 104°36′–106°45′ E) is in northern Sichuan Province and represents a region of transition between the Northwest Plateau and the Sichuan Basin, with a seasonal climate typical of an inland basin. Yearly mean temperature is 17 °C, with the maximum temperature of 40 °C occurring between July and September. Yearly mean precipitation is 973 mm, with a peak rainfall of 1518 mm in 1961. The area of study experiences seasonal rainfall, with 75% of annual rainfall occurring between June and September. Rivers flowing through the area of study form part of the Yangtze River. The Jialing River constitutes the main river, into which 75 tributary rivers flow, including the Qingshui, Bailong, Mumen, and Donghe rivers from east to west.

The elevation of the area of study decreases from north (1200 m) to south (600 m). The river valley in the area of study is deep and V-shaped, with a relative height difference of 200–500 m. The area of study has a well-developed topography. Coal strata in the area of study originate from the Permian, Jurassic, and Triassic periods. The study area is primarily dominated by the dark gray of Wujiaping group (P₂w). Four categories of clastic-rock-fissure-pore water and carbonate-rock-karst-fissure water exist in the area of study. Groundwater in the area of study is relatively abundant and is recharged by infiltration of water at the surface and atmospheric precipitation. The predominant outflows of groundwater are karst springs and underground rivers, followed by evaporation and artificial mining and drainage. Figure 1 shows a map of the area of study, including the positions of the points of sampling.

2.2. Collection, Preparation, and Analysis of Samples

The current study collected groundwater samples from 34 sites in the area, including from 10, 3, 4, 10, and 2 sample positions in Wangcang, Chaotian, Lizhou, Jiange, and Qingchuan, respectively. Field sampling was performed using a HACH portable water quality analyzer at each sample position. During the collection of water samples, in-situ pH, temperature, reduction–oxidation (REDOX) oxidation–reduction potential (ORP), and electrical conductivity (EC) were measured using field meters. Collected water samples were filtered through a 0.45-μm filter membrane and kept in a polyethylene plastic bottle under a low temperature until analysis. Samples for analysis of cations were acidified to a pH < 2 using nitric acid and kept in organic sampling bottles for transportation to the laboratory. Once reaching the laboratory, the samples bottles were kept in a refrigerator until analysis. Water samples for the determination of metal elements (Fe, Cu, Mn, Al, Zn, Cd, Pb, Cr, Hg, and As) were acidified with HNO₃ (1:1) to a HNO₃ level of 1%. Mass levels of cations and anions in water samples were calculated using the IRIS Intrepid II XSP (Thermo Electron, Airport City, IL, USA) instrument and ion chromatography (ICS-1100, Dionex, Sunnyvale, CA, USA), respectively. Inductively coupled plasma mass spectrometry (iCAP Q, Thermo Fisher, Waltham, MA, USA) was utilized to measure the metal levels in the water samples. Quality control of testing showed a variation in test results of <5%.

2.3. Data Analysis

Inverse distance weighting interpolation in ArcGIS 10.2 was utilized to produce a spread of general and metal elements in shallow groundwater in the area of study. Pearson analysis of correlation was utilized to examine the relationships between the various metals identified in the water samples. Analysis of principal components (PCA) was utilized to identify the sources of toxic metals in groundwater in the area of study.

2.4. Entropy-Weighted Water Quality Index (EWQI)

The EWQI represents a single combined water quality indicator. The predominant benefit of the EWQI approach relates to the removal of human subjectivity by including information entropy in quantitative evaluation of water quality indices. The current study followed the steps outlined below to calculate the EWQI [18,19,20]:

(1) The eigenvalue matrix “X” was estimated using Equation (1), incorporating all hydro-chemical parameters as follows:

$$\mathrm{X}=\left[\begin{array}{cccc}{x}_{11}& x_{12}& \cdots & x_{1n}\\ x_{21}& x_{22}& \cdots & x_{2n}\\ ⋮& ⋮& \ddots & ⋮\\ x_{m1}& x_{m2}& \cdots & x_{mn}\end{array}\right],$$

(1)

where m and n represent the number of groundwater samples and hydro-chemical parameters of each sample, respectively.

(2) The standard matrix Y was calculated according to Equations (2) and (3). Given the differences in scales among the different parameter units, standardization of the data before performing EWQI calculations was required.

$$y_{ij}={\displaystyle \frac{x_{ij}-{\left(x_{ij}\right)}_{min}}{{\left(x_{ij}\right)}_{max}-{\left(x_{ij}\right)}_{min}}},$$

(2)

$$\mathrm{Y}=\left[\begin{array}{cccc}{y}_{11}& y_{12}& \cdots & y_{1n}\\ y_{21}& y_{22}& \cdots & y_{2n}\\ ⋮& ⋮& \ddots & ⋮\\ y_{m1}& y_{m2}& \cdots & y_{mn}\end{array}\right],$$

(3)

where x_ij is the initial matrix and (x_ij) min and (x_ij) max represent the minimum and maximum values of the hydro-chemical parameters of the samples, respectively.

(3) Entropy e_j and entropy weight w_j were calculated according to Equations (6)–(8). The index j value for sample i is shown as P_ij.

$$e_j=-{\displaystyle \frac{1}{lnm}}{\sum }_{i=1}^m(P_{ij}\times ln{P}_{ij}),$$

(4)

$$P_{ij}={\displaystyle \frac{y_{ij}}{{\sum }_i^m{y}_{ij}}},$$

(5)

$$w_j={\displaystyle \frac{{1-e}_j}{{\sum }_{i=1}^n\left({1-e}_j\right)}},$$

(6)

(4) The quality rating scale q_j of the jth parameter was calculated as follows:

$$q_j={\displaystyle \frac{C_j}{S_j}}\times 100,$$

(7)

where q_j is the quantitative scale of categorization for hydro-chemical indices, as calculated by the hydro-chemical levels (C_j) and the World Health Organization’s (WHO) drinking standards (S_j).

(5) The EWQI was calculated as follows:

$$\mathrm{EWQI}={\sum }_{j=1}^m(w_j\times q_j),$$

(8)

Table 1. Water quality classification standard based on the index of water quality weighted by entropy (EWQI).

Rank	EWQI	Water Quality
I	<50	Excellent
II	50–100	Good
III	100–150	Medium
IV	150–200	Poor
V	>200	Extremely poor

summarizes the classification of the EWQI and shows that first- and second-grade groundwater adheres to the water for drinking standard.

2.5. Appraisal of Human Exposure and the Hazards to Health

The appraisal of human health examines the hazards posed by environmental contamination to health of humans [21]. Humans are exposed to metal elements in water predominantly through dermal contact or ingestion of water for drinking [22,23]. The current study applied the health hazard appraisal model of the Environmental Protection Agency of the United States (USEPA) [24,25]. Hazards to the health of both grownups and juveniles were assessed.

2.5.1. Model of Exposure

Chronic daily intake (CDI; mg·kg⁻¹·d⁻¹) is utilized as a measure of exposure to toxic metals and is calculated as follows:

$${CDI}_{oral}={\displaystyle \frac{CW\times IR\times EF\times ED}{BW\times AT}},$$

(9)

$${CDI}_{der}={\displaystyle \frac{CW\times SA\times PC\times ET\times EF\times ED\times IR}{BW\times AT}},$$

(10)

where CDI_oral and CDI_der are the average daily doses through ingestion of water for drinking and dermal contact, respectively; CW represents the average groundwater pollutants level; IR is average daily groundwater intake; EF represents the frequency of exposure; ED is the duration of exposure; BW represents body weight; AT denotes average time; SA is the total surface area of skin contact to groundwater; PC represents the skin permeability constant for groundwater heavy metal elements; ET is the exposure duration; and CF is a factor of conversion.

Table 2. Value of parameters of the health hazard appraisal applied in the current study.

Parameter	Abbreviation	Values		Distribution	Units
		Adult	Children
Element Concentration	C	Measured	Measured	Log-normal	mg/L
Daily Ingestion Rate	IR	2.2	1.32	Normal	L/d
Exposure Skin Area	SA	17,000	13,300	Fixed value	cm2
Conversion Factor	CF	0.001	0.001	Fixed value	L/cm3
Skin Permeability Coefficient	PC	0.0001 (Mn and Pb) 0.001 (Fe, As, Cu and Cd) 0.002 (Cr) 0.0006 (Zn)		Fixed value	cm/h
Exposure Frequency	EF	350	350	Triangular	d/a
Exposure Duration	ED	70 (Cd, Cr and As) 35 (Fe, Mn, Cu, Zn and Pb)		Fixed value	a
Exposure Time	ET	0.33	0.18	Fixed value	h/d
Body Weight	BW	60.6	42.6	Normal	kg
Average Time	AT	25,500 (Cd, Cr and As) 12,775 (Fe, Mn, Cu, Zn and Pb)		Fixed value	d
Average Life	L	70	70	Fixed value	a

provides a summary of the exposure parameter values applied in the current study.

2.5.2. Characterization of the Hazards

Chemical substances can be classed as either chemical carcinogens (e.g., Cd, Cr, and As) or chemical non-carcinogens (e.g., Fe, Mn, Cu, Zn, Hg, and Pb). Indicators of carcinomic hazards include the index of lifetime carcinogenic hazard (ILCR), whereas those of non-carcinogenic hazards include the non-carcinogenic hazard entropy (HQ) and non-carcinogenic hazard index (HI).

(1)

Carcinogenic hazards

The carcinogenic hazard of a chemical can be calculated as follows:

$$\mathrm{L}\mathrm{C}\mathrm{R}={\displaystyle \frac{CDI\times SF}{L}}(R\le 0.01),$$

(11)

$$\mathrm{I}\mathrm{L}\mathrm{C}\mathrm{R}={\displaystyle \frac{1-\exp \left(-CDI\times SF\right)}{L}}(R>0.01),$$

(12)

where ILCR represents the average carcinogenic health hazard to an individual posed by a given metal under two exposure pathways; SF denotes the metal slope factor; and L is average lifespan. The USEPA thresholds for maximum acceptable hazard and ICLR for chemical carcinogens are 1 × 10⁻⁴ and >1 × 10⁻⁴, respectively. Therefore, metals in water are considered a potential carcinogenic hazard, with an ICLR < 1 × 10⁻⁴ considered to pose negligible hazard. No interactions between chemicals were assumed during the evaluation of the carcinogenic hazards posed by various chemicals. Under this assumption, the total carcinogenic hazard can be calculated by simply adding up the individual carcinogenic hazard for each chemical.

(2)

Non-carcinogenic hazards

The non-carcinogenic hazard to the health of humans for a given chemical was calculated as follows:

$$\mathrm{H}\mathrm{Q}={\displaystyle \frac{CDI}{RfD}},$$

(13)

$$\mathrm{H}\mathrm{I}=\sum HQ={HQ}_{oral}+{HQ}_{der}$$

(14)

where HQ represents non-carcinogenic hazards posed to the health of humans; RfD denotes the daily reference doses of non-carcinogenic substances under different exposure routes; and HI is harmful intake of contaminated water by a specific individual under two pathways of exposure. The USEPA classification indicates HI < 1 and HI > 1 to be representative of acceptable and unacceptable non-carcinogenic hazards of exposure to individuals, respectively [25].

Table 3. The slope factor (SF) and reference dose (RfD) of carcinogenic metals in the groundwater of the study site.

Non-Carcinogen	RfD (mg kg−1 d−1)		Carcinogen	SF (mg kg−1 d−1)
	Ingestion	Dermal		Ingestion	Dermal
Mn	1.4 × 10−1	8.0 × 10−4	Cd	6.1	0.38
Zn	3.0 × 10−1	1.0 × 10−1	Cr	41	0.5
Pb	1.4 × 10−3	4.2 × 10−4	As	15	3.66
Cu	5.0 × 10−3	1.2 × 10−2
Fe Hg	3.0 × 10−1 1.0 × 10−4	4.5 × 10−2 3.0 × 10−4

shows the RfD values of the health hazard appraisal model parameters of toxic metals.

3. Conclusions and Discussion

3.1. Hydro-Geochemical Properties of Groundwater

The current study applied a Piper tri-linear diagram to identify the hydro-geochemical facies of the groundwater [26]. As depicted in Figure 2, water hydrochemistry can be categorized into four hydro-chemical facies [27]: (1) SO₄⋅Cl–Ca⋅Mg; (2) HCO₃–Na; (3) HCO₃–Ca⋅Mg; and (4) SO₄⋅Cl–Na. Projection of the samples of groundwater from the area of study into the Piper diagram showed groundwater to be predominantly of the SO₄⋅Cl–Ca⋅Mg (51.8%) and HCO₃–Na (48.2%) types.

Statistical analysis is commonly utilized to analyze the hydro-geochemistry of water [28,29].

Table 4. Statistical summary of conventional groundwater quality indicators in the area of study.

Parameters	Max	Min	Median	Mean	SD	CV (%)	WHO [30]	% of SEL
pH	8.06	2.49	6.28	5.48	2.12	0.39	6.5~8.5	52.17
TDS	16,347.00	299.00	985.00	3204.48	4485.41	1.40	1000.00	47.83
TH	2175.00	214.00	679.00	909.13	623.28	0.69	450.00	69.57
SO42−	8980.00	34.20	484.00	1655.15	2315.81	1.40	250.00	82.61
Cl−	61.80	1.70	2.36	5.02	12.39	2.47	250.00	0.00
F−	24.20	0.11	0.53	2.72	5.59	2.06	1.50	26.09
NO3−	7.99	0.01	1.33	1.84	2.09	1.13	45.00	0.00
K+	20.20	0.64	3.76	6.27	5.95	0.95	-	-
Na+	165.00	2.41	9.75	19.31	33.55	1.74	200.00	0.00
Ca2+	475.00	64.00	181.00	246.43	141.38	0.57	200.00	47.83
Mg2+	243.00	7.00	30.00	67.09	69.83	1.04	150.00	13.04

SD, standard deviation; CV (%), coefficient of variation; and % of SEL, % of samples exceeding acceptable limit.

provides a summary of variation in hydro-chemical parameters of the groundwater samples. Groundwater samples were acidic to neutral, with a pH of 2.49–8.06. TDS of the groundwater samples ranged between 299 mg/L and 16,347 mg/L, with 16 of 34 samples surpassing the water for drinking limit of 1000 mg/L [30]. The highest TDS levels in the groundwater were found in the eastern regions, mainly related to anthropogenic sources (drainage waste, septic tank leaks, irrigation-rerun-flows, etc. [31,32]). The hardness of the groundwater samples ranged between moderate and extreme, with total hardness (TH) ranging between 214 mg/L and 2175 mg/L (Figure 3).

As shown in Figure 4, K⁺ and Cl⁻ were predominantly concentrated in Wangcang and Jiange, with a spread consistent with that of NO₃⁻. This result indicated a common anthropogenic source amongst these ions. NO₃⁻ ranged between 0.01 mg/L and 7.99 mg/L, far below the standard for water for drinking of 45 mg/L [30]. It mainly comes from agricultural nitrogen-containing wastewater, landfill leachate, and compost [33,34,35]. Levels of Na⁺ in the groundwater ranged between 2.41 and 165.00 mg/L, with the highest levels found in Wangcang. The eastern region of the area of study showed enrichment in Ca²⁺ (64–475 mg/L) and Mg²⁺ (7–243 mg/L). F⁻ ranged between 0.11 and 24.20 mg/L. Weathering and dissolution of fluoride-containing minerals (apatite, biotite, and hornblende) and phosphate fertilization are the main source of F⁻ content in groundwater under alkaline conditions [36,37]. SO₄²⁻ levels ranged between 34.20 and 8980.00 mg/L, with the highest levels in Wangcang, surpassing the standard for water for drinking by a factor of 35.96. Analysis of the data and a field survey indicated the presence of 118 coal mines in Wangcang. Consequently, groundwater quality in this region was poor because of the seepage of mine water into the groundwater table.

The high groundwater levels of SO₄²⁻, TDS, Mg²⁺, and Ca²⁺ in the study region could be attributed to the influences of the mines. The high levels of TDS and SO₄²⁻ were due to the presence of sulfur coal in the form of pyrite (FeS₂) and other metal sulfide minerals. Mining of coal exposes metal sulfides to oxygen. Following abandonment, water accumulates in the mine and high levels of SO₄²⁻ are released into this mine water due to water–rock reactions and oxidation. Under these processes, AMD is produced, which poses a serious hazard to local groundwater.

3.2. Factors Regulating Groundwater Chemistry in the Study Region

The Gibbs diagram can be utilized to identify the origins of different ions in groundwater. The sources of groundwater ionic components can generally be classed into the evaporation dominance, rock dominance, and precipitation dominance categories [38]. The area of study contains widely distributed limestone and abundant karst water. The aquifer group in the area of study predominantly consists of limestone. Coal strata are also widely found in the area of study. Therefore, dissolution and erosion of minerals in the area of study play a crucial role in the hydrochemistry of the groundwater. Within the Gibbs diagram, most of the sample points plotted on the area of rock weathering, followed by evaporative level, indicating that these two process classes explained the majority of groundwater Ca²⁺ and HCO₃⁻ (Figure 3). Among all samples, the Na⁺/(Na⁺ + Ca²⁺) ratio was < 0.5, with Na⁺ as the dominant cation (Figure 5b). The ratio of Cl⁻/(Cl⁻ + HCO₃⁻) was < 0.5, with HCO₃⁻ being dominant (Figure 5a). The inconsistency between groundwater anions and cations could be attributed to alternating cationic adsorption regions driven by spatially varying water–rock interactions. Consequently, some water samples plotted outside the box in the Gibbs diagram.

In Figure 6a, all of the groundwater samples were distributed below the y = x line, indicating that Na⁺ comes from silicate dissolution or ion exchange. Combined with Figure 6b,c, it is shown that the dominance of Ca²⁺ and Na⁺ in the groundwater indicated the dissolution of limestone to be a major factor regulating groundwater chemistry. In the bivariate diagram of (Ca²⁺ + Mg²⁺) and (HCO₃⁻ + SO₄²⁻), most groundwater samples were plotted along or above the y = x line, indicating that the cation exchange process has an obvious impact on the groundwater chemistry (Figure 6d). Most of samples were plotted in the transition between silicate rocks and carbonate rocks (Figure 6e,f). Moreover, the existence of cation exchange was supported by the weight ratios of (Na⁺ + K⁺ − Cl⁻) against (Ca²⁺ + Mg²⁺) − (HCO₃⁻ + SO₄²⁻) (Figure 6g). Meanwhile, NO₃⁻ concentrations were affected by agriculture activities to a certain extent, as shown in Figure 6h. Figure 6i shows that the SI values of Aragonite were higher than zero, while the SI values of anhydrite, calcite, dolomite, gypsum, Halite were lower than zero. These results could be attributed mining-driven increases in groundwater TDS and Na⁺. The results also indicated that evaporative crystallization impacts groundwater chemistry in the area of study.

3.3. Characterization of Groundwater Toxic Metals

Levels of Groundwater Toxic Metals

Table 5. Heavy metal levels in the groundwater of the area of study.

Parameters	Max	Min	Median	Mean	SD	CV (%)	WHO [30]	% of SEL
Fe	2235.00	0.01	31.40	303.24	609.12	2.01	0.30	78.26
Mn	7.80	0.01	0.61	1.72	2.52	1.47	0.50	73.91
Al	545.00	0.08	1.16	66.97	147.04	2.20	0.20	78.26
Cu	3.06	0.01	0.01	0.29	0.87	3.04	1.00	8.70
Zn	19.00	0.01	0.32	2.98	5.52	1.85	3.00	26.09
Pb	58.30	0.12	0.70	4.50	12.74	2.83	0.01	100.00
Cd (μg/L)	1378.00	0.01	2.46	194.53	447.02	2.30	3.00	43.48
Cr	1.29	0.01	0.01	0.19	0.40	2.12	0.05	26.09
As (μg/L)	56.20	0.10	0.10	3.67	11.69	3.19	10.00	4.35
Hg (μg/L)	0.36	0.01	0.11	0.12	0.10	0.81	1.00	0.00

shows the properties and spread of the ten groundwater metals examined in the current study. The order of metals in groundwater according to their levels was: Fe > Al > Pb > Zn > Mn > Cu > Cr > Cd > As > Hg. Fe, Mn, Al, Pb, Cd, and Cr exceeded their respective water for drinking guidelines [30]. The average levels of groundwater Fe, Al, and Cd all significantly exceeded their respective WHO [30] guidelines for water for drinking with values of 303.24 mg/L, 66.97 mg/L, and 194.53 g/L, respectively. It can be seen from Figure 7 that the areas with a high concentration of heavy metals are mainly concentrated in Wangcang.

3.4. 2 Relationships Between Metals in the Groundwater Samples

The current study applied Spearman correlation to analyze the relationships between groundwater metals. As shown in Figure 8, there were significant correlations between Mn, Al, Cu, and Zn. These results could be predominantly attributed to the impacts of the sedimentary environment, coal and bauxite layers, metal sulfides, and generation of AMD containing metal ions. There were also weak correlations between Pb, Cd, Cr, As, Hg, Fe, Mn, Cu, and Zn.

3.5. EWQI Assessment of Groundwater Quality

The EWQI has been utilized in previous studies to comprehensively assess the impacts of hydro-chemical factors on overall water quality [39]. An EWQI < 100 indicates an overall safety of water for drinking purposes. When calculating the EWQI, the current study considered groundwater levels of K⁺, Ca²⁺, Na⁺, Mg²⁺, Cl⁻, F⁻, NO₃⁻, SO₄²⁻, and TDS. Among the different groundwater samples, the EWQI ranged between “excellent” and “very bad”, with a mean value of 166 (Figure 9). Of the groundwater samples, 71% were classed as either “excellent” or “good”, thereby complying with the WHO [30] water for drinking guidelines. However, the EWQI of ten groundwater samples exceeded 100, indicating their unsuitability for use as water for drinking.

As shown in Figure 10, the majority of groundwater in the area of study was classed as suitable for use as water for drinking. Most samples classed as unsuitable for use as water for drinking were distributed in the east. According to previous studies and a field investigation, the area of study represents an area of transition from mountain to basin. Consequently, the area of study has a relative elevation difference over 3200 m. The area of study is also characterized by a deep valley with a small population, scarce arable land, and low socioeconomic development. Given the lack of other anthropogenic activities, it can be concluded that mining is the predominant contributor to groundwater pollution in the area of study. Therefore, there is a need for increased focus on groundwater quality conservation in the east of the area of study, which will require long-term monitoring and dynamic evaluation of the groundwater quality.

3.6. Appraisal of the Hazards Posed by the Groundwater Quality to Human Health and Uncertainty Analysis

3.6.1. Appraisal of the Health Risk

The results indicated the hazards of the groundwater in the area of study to the health of adults and juveniles due to the use of the water for drinking or ingesting and dermal contact (

Table 6. Carcinogenic and non-carcinogenic hazards posed by the groundwater quality in the area of study to the health of its residents.

Carcinogen				Non-Carcinogen
Element	Cd	Cr	As	Fe	Mn	Cu	Zn	Pb
Adults ingestion	1.36 × 10−2	8.74 × 10−2	2.06 × 10−5	1.16 × 10−5	1.41 × 10−7	6.54 × 10−7	1.13 × 10−7	3.68 × 10−5
Children ingestion	1.16 × 10−2	7.46 × 10−2	1.75 × 10−5	9.87 × 10−6	1.20 × 10−7	5.59 × 10−7	9.68 × 10−8	3.14 × 10−5
Adults dermal	2.16 × 10−6	4.46 × 10−4	5.24 × 10−8	1.97 × 10−7	6.27 × 10−8	6.95 × 10−10	5.21 × 10−10	3.13 × 10−8
Children dermal	2.11 × 10−5	2.71 × 10−4	3.18 × 10−8	1.19 × 10−7	3.81 × 10−8	4.22 × 10−10	3.16 × 10−10	1.90 × 10−8

). The order of the eight metals according to human health risks was: Cr > Cd > As > Fe > Pb > Cu > Mn > Zn. Levels of Cr and Cd exceeded the maximum tolerable carcinogenic hazard of 1 × 10⁻⁴ [25]. The total carcinogenic hazards posed by the groundwater quality to grownups and juveniles ranged from 2.06 × 10⁻⁵ to 8.79 × 10⁻² and 1.76 × 10⁻⁵ to 7.49 × 10⁻². In the groundwater of a karst lead–zinc mine, the risk values of heavy metals such as Cd, Pb, and Zn are higher [40]. The risk value of heavy metals in an alum mining area is usually Cd > As > Cu > Mn [41]. The current study also calculated the total non-carcinogenic hazard to the health of humans based on the groundwater metals (Table 6), as well as the spread (Figure 9). The total non-carcinogenic hazards posed by the groundwater quality in the area of study to grownups and juveniles ranged from 1.14 × 10⁻⁷ to 3.68 × 10⁻⁵ and 9.71 × 10⁻⁸ to 3.41 × 10⁻⁵, respectively. No groundwater samples exceeded the maximum permissible non-carcinogenic hazard value of 1 × 10⁻⁴ [25].

The spread of the hazard index (HI) (Figure 11) indicated non-carcinogenic health hazards in the east that significantly exceeded that in the west, due to the levels of groundwater metals in the east. Groundwater non-carcinogenic health hazards were predominantly in the Wangcang County, southeast Chaotian County, and northeast Zhaohua County, which could be directly attributed to high groundwater metal contents. The spread of HI was consistent with that of abandoned coal mines.

The USEPA [25] categorization indicated acceptable non-carcinogenic hazards in the study region, with an HI for both grownups and juveniles < 1. The results also showed that health hazards associated with ingestion of groundwater as water for drinking exceeded those associated with dermal contact by a factor of 2–3. The hazard posed by groundwater quality to grownup health exceeded that to juvenile health. This is because exposure of adults to groundwater through consumption of water for drinking and dermal contact exceeded that to juveniles by factors of 1.17 and 1.65, respectively.

3.6.2. Sources of Uncertainty

The confounding impacts of multiple influencing factors may result in unrealistic hazard values. The results of the current study may have been impacted by the following sources of uncertainty: (1) human and instrument error during sampling, transportation, and testing of samples and (2) the applicability of the USEPA standard and hazards parameters utilized in the exposure appraisal stage and its evaluation to the population in China. While these uncertainties may have impacted the results of the health hazard appraisal, the results remain valid and useful, and the uncertainties point to directions of future research needed.

4. Conclusions

The present study assessed the hydro-chemical properties of the groundwater in the Guangwang mining area as a typical mining area and its associated human health risks. The predominant results of the current study are listed below:

(1) Past mining activities led to the highest average levels of TDS and SO₄²⁻ being located in the east. The predominant groundwater hydro-chemical types in the area of study were SO₄⋅Cl–Ca⋅Mg and HCO₃–Na.

(2) The predominant groundwater hydro-chemical type in the area of study was Ca-HCO₃. Leaching and evaporative levels within the limestone of the coal seam aquifer were identified as the predominant factors regulating groundwater hydrochemistry. Within the groundwater, levels of six of the ten assessed metals exceeded the WHO safe water for drinking standards, with levels of Al (66.97 mg/L) and Cd (194.53 μg/L) being particularly high.

(3) There were significant correlations between groundwater Mn, Al, Cu, and Zn. This could be attributed to the bauxite minerals of abandoned coal mines and the release of a range of metal sulfides through oxidation.

(4) Consumption of the groundwater as water for drinking was the predominant route of the hazards posed by the groundwater quality to the health of humans. The hazards also decreased from east to west, which is consistent with the spreads of abandoned coal mines and fissures. The hazards posed by the groundwater quality to grownup health exceeded that to juvenile health due to higher exposure for the former.

The results of the current study can guide future groundwater quality management in the Guangwang mining area and in similar mining areas in China and abroad.