3.1. Hydrochemical characteristics of Groundwater
The statistical results of water quality for groundwater samples are given in
Table 3. pH is one of the most important parameters for evaluating the suitability of drinking water [
60]. The Chinese national standard proposes that the pH value of groundwater suitable for drinking is 6.5–8.5 [
59]. As
Table 3 shows, pH values of the groundwater range from 7.27 to 7.85, with a mean value of 7.59. Therefore, the groundwater in the study area is weakly alkaline water that can be used for drinking.
TH represents dissolved Ca
2+ and Mg
2+ in groundwater. High TH in groundwater may affect the taste of drinking water and reduce the efficacy of detergents [
34]. In addition, regarding human health, the long-term drinking of extremely hard water may increase the incidence of urolithiasis, anencephaly, prenatal mortality, and some cancer-related cardiovascular diseases [
68]. In this study, TH varies between 167 and 869 mg/L with a mean of 426 mg/L. According to the national Chinese drinking water standards, samples S1, S9, and S10 are extremely hard water, with TH exceeding the acceptable limit of 450 mg/L for drinking. These samples are predominantly distributed in the southern part of the study area (
Figure 2a). TH enrichment in groundwater may be due to the dissolution of soluble salts and minerals, as well as to human intervention [
2].
TDS is one of the major water quality parameters, mainly representing the various minerals present in the water [
6]. TDS varies in a wide range of 280–1312 mg/L, with a mean value of 689 mg/L (
Table 3). Based on TDS content, Liu et al. [
69] categorized waters as freshwater (TDS < 1000 mg/L) and brackish water (TDS > 1000 mg/L). Only sample S10 in Yicheng is brackish water (
Figure 2b). Generally speaking, higher TDS usually indicates stronger water-rock interaction and may also be affected by domestic wastewater, irrigation return flow, and fertilization [
1,
70]. High TDS in groundwater is generally harmless in healthy people and may cause constipation or have a laxative effect, but it may have a greater impact on people with kidney and heart disease [
6,
33,
71].
Cl
− and SO
42− in groundwater are mainly related to the regional lithological conditions and are also affected by anthropogenic sources [
68]. The concentration of Cl
− is between 7.93 and 88.1 mg/L and is lower than the Chinese national standard of 250 mg/L. The concentration of SO
42− in the study area ranged from 68 to 536 mg/L, with a mean of 182.16 mg/L. Samples S5 and S10 exceeded the acceptable limit of SO
42− for drinking. High SO
42− concentration is observed in the Yaodu and Yicheng parts of the central and south of the study area (
Figure 2c). The Ordovician karst aquifers widely distributed in the study area are affected by gypsum dissolution, and the hydrochemical type of groundwater is SO
42−·HCO
3−-Ca·Mg. In addition, the oxidation of sulfur in coal-bearing strata (S + O
2 + 2H
2O→SO
42− + 4H
+) will also cause increased sulfate concentration in groundwater [
72]. Therefore, the high mean value of SO
42− in this study is probably due to the high natural background value rather than pollution.
F
− in drinking water is essential for human health at low concentrations, such as protecting teeth from caries [
2]. However, excessive fluoride intake can cause dental fluorosis, skeletal fluorosis, and thyroid disease in adults [
17,
73]. The Chinese national standard stipulates that F
− concentration in drinking water should be less than 1.0 mg/L. In this study, F
− is in the range of 0.25–1.71 mg/L, with an average value of 0.75 mg/L. Two groundwater samples in Yaodu did not meet the requirement of the national standard (
Figure 2d). The high concentration of fluoride in groundwater may be mainly related to the lithology of the region, especially the dissolution of fluoride-bearing minerals [
16,
74].
Both cyanide and volatile phenol are toxic organics. The concentration of cyanide in all groundwater samples is less than 0.0004 mg/L. For volatile phenols, except for sample S10 in Yicheng, whose value is 0.002 mg/L, the other samples are 0.0003 mg/L. COD
Mn is an indicator that can indirectly reflect the organic pollution of groundwater [
44,
74]. The COD
Mn values for the samples are observed to be from 0.1 to 0.9 mg/L, with an average of 0.25 mg/L. Sample S10 in Yicheng has the highest volatile phenol and COD
Mn values. As shown in
Table 3, the concentrations of cyanide, volatile phenol, and COD
Mn are all within the drinking water standard limit stipulated by the national standard, indicating that the groundwater is less affected by organic pollution.
In recent years, nitrogen pollution (NO
3-N, NO
2-N, and NH
4-N) has become a hot issue for many researchers due to its adverse effects on groundwater quality and human health [
2,
12,
14,
49,
74,
75,
76]. The extensive use of nitrogenous fertilizers in agricultural activities is one of the most common sources of nitrogen pollution in groundwater [
1,
63]. Measured values of NO
3-N, NO
2-N, and NH
4-N are in the range of 0.002–11.3, 0.004–0.7 and 0.025–0.16 mg/L, respectively. Higher NO
3-N and NO
2-N concentrations are observed in the central and southwest parts of the area, while a high value of NH
4-N is mainly distributed around Yicheng (
Figure 2e–g). According to the Chinese standards, groundwater is unacceptable for drinking when the NO
3-N, NO
2-N, and NH
4-N concentration in groundwater is higher than 20, 1, and 0.5 mg/L, respectively. Therefore, the groundwater in the study area is less contaminated with nitrogen and is suitable for drinking.
PTEs content in groundwater is usually low. However, even in very low concentrations, -they can create biological toxicity and pose serious threats to aquatic ecosystems and human health [
20,
21,
41]. As shown in
Table 3, the Fe, Mn, Hg, and Cr
6+ concentrations range from 0.03 to1.41, 0.01–0.139, 0.00001–0.00006, and 0.004–0.034 mg/L, respectively. The concentrations of As, Cd, and Pb are 0.0002, 0.002, and 0.011 mg/L, respectively. The mean concentration of metals is in the following order: Fe > Mn > Pb > Cr
6+ > Cd > As >Hg. All metals, except for Fe, Mn and Pb, are within the permissible levels for drinking water. Samples with high concentrations of Fe and Mn are mainly found in the southeastern parts of the basin (
Figure 2h,i). Fe and Mn have similar geochemical behavior. Their dissolution and migration to groundwater are affected by reduction conditions, residence time, well depth, and salinity [
77]. The similarity in the spatial distribution of Cr
6+ and NO
3−-N concentrations may be related to the synergistic role of nitrogen (N)-bearing fertilizers to elevated Cr
6+ concentration in groundwater. This may be due to the production of H
+ and soil acidification during the nitrification process of NH
4+ oxidation to NO
3−, favoring the increased dissolution of Cr
3+ which is subsequently oxidized into Cr
6+ by natural and/or anthropogenic factors [
46,
47].
3.2. Groundwater Quality Assessment
In this study, pH, TDS, TH, SO
42−, Cl
−, F
−, volatile phenols, NO
3-N, NO
2-N, NH
4-N, Fe, Mn, Hg, Cd, Cr
6+ and Pb are selected as the parameters to evaluate the overall groundwater quality, using the WQI introduced previously. The values of cyanide, arsenic, and chemical oxygen demand in groundwater are very low, so they have little impact on water quality and can be ignored in water quality assessment. The weights and relative weights assigned to each parameter are shown in
Table 4.
The calculated WQI values and water types are presented in
Table 5. The results of WQI range from 23.63 to 105.96. Out of 10 groundwater samples, sample S5 is categorized as good water. Sample S10 is classified as poor water. The other 8 samples are excellent water. For the study area, the most significant parameters affecting groundwater quality are Pb, TH, F
−, SO
42−, and TDS.
From the spatial distribution of groundwater quality index results, it can be seen that poor quality water area is mainly located near Yicheng in the southeastern area of the study (
Figure 3). The main pollutants in the groundwater in this area are TH, TDS, SO
42−, Fe, Mn, and Pb, all of which exceed the upper limit for drinking purposes. The poor groundwater quality in Yicheng may be related to the buried depth of groundwater. Generally, when the groundwater is buried deeper, it takes longer for the surface pollutants to reach the aquifer. Thus, the possibility of the pollutants being adsorbed and diluted during the infiltration process becomes greater, and the degree of pollution in the groundwater system will decrease. The buried groundwater depth in the Yicheng area is shallow at 2–15 m. In addition, the lithology of the buried deep aquifer is mainly coarse sand and medium-coarse sand. The better permeability of the aquifer makes it easier for surface pollutants to seep into the groundwater, resulting in groundwater pollution. Fe and Mn in groundwater come from coal and metal deposits, especially iron ore. High TDS leads to increased ionic strength and decreased activity coefficient, which will dissolve more Fe and Mn in groundwater. In addition, the organic matter released from surface pollutants into groundwater can quickly deplete the dissolved oxygen in groundwater, resulting in a reductive hydrochemical environment more conducive to the dissolution of Fe and Mn [
77].
The assessment results indicate that the groundwater in the study area is dominated by excellent water that can be used for drinking purposes. For the Yicheng area with poor quality groundwater unsuitable for drinking, groundwater pollution remediation and safe water supply measures should be implemented as soon as possible.
3.3. Human Health Risk Assessment
The health risks of groundwater in the study area were assessed based on the model introduced previously. The calculated health risks for adults and children through drinking water and dermal contact are shown in
Table 6. For adult males, the HQ
oral values range from 0.285 to 0.827, with a mean of 0.521. The HQ
oral values for adult females and children range from 0.336 to 0.976 and 0.693–2.012, with an average of 0.615 and 1.269, respectively. The HQ
dermal values are smaller than the HQ
oral, ranging from 0.017 to 0.104 for males, 0.018 to 0.109 for females, and 0.026 to 0.156 for children, with means of 0.034, 0.036, and 0.051, respectively. This suggests that non-carcinogenic risk is mainly caused by oral exposure. The HI
total values for males and females range from 0.302 to 0.902 and 0.354–1.051, with means of 0.555 and 0.651, respectively. For children, the HI
total values are 0.719–2.100, with an average value of 1.320. For males, females, and children, HI
total values of 0%, 20%, and 80% of the samples exceed 1, indicating that males in the study area do not have associated non-carcinogenic health risks. In contrast, females and children face higher non-carcinogenic risks. Females and children have smaller body weights and therefore have higher average daily exposure dose of contaminants than males [
50,
60].
Contaminants in groundwater contribute differently to health risks. Concerning each water quality parameter, the non-carcinogenic HQ values of F
−, NO
3-N, NO
2-N, NH
4-N, Fe, Mn, Hg, Pb, Cr
6+, As, and Cd are in the ranges of 0.090–1.501, 1.809 × 10
−5–0.248, 5.789 × 10
−4–0.025, 3.730 × 10
−4–5.793 × 10
−3, 1.447 × 10
−3–0.165, 1.034 × 10
−3–3.487 × 10
−2, 5.288 × 10
−4–7.444 × 10
−3, 0.114–0.276, 0.030–0.540, 9.648 × 10
−3–0.023 and 0.033–0.076, respectively. This result suggests that besides F
−, the non-carcinogenic risks of other contaminants are acceptable to both adults and children. As shown in
Figure 4, the contribution of pollutants in groundwater to the HI
total value is observed in the following order: F
− > Pb > Cr
6+ > NO
3-N > Cd > As > Fe > Mn > NO
2-N > NH
4-N > Hg. F
− contributes the most to non-carcinogenic risk (46.86%), followed by Pb (22.78%) and Cr
6+ (11.22%). Contribution of other pollutants to the non-carcinogenic risk is less than 10%, indicating that F
−, Pb, and Cr
6+ may be drivers of adverse effects on human health.
The spatial distribution of HI
total values for males, females, and children is consistent with fluoride concentration (
Figure 5). Higher HI
total and F
− concentration mainly appear in the Yaodu, midwest of the study area. Groundwater with high F
− is found in semi-arid and arid areas of northern China, such as the middle Loess Plateau [
16], Ningxia plain [
61], Guanzhong Plain [
1,
36], Hetao Plain [
78], and Tianjin [
17]. The respective HQ mean values of F
− for males, females, and children are 0.503, 0.593, and 1.220 in the Yaodu, indicating that children are exposed to health risks from fluoride. Fluoride-bearing minerals are enriched in magmatic rocks and aluminosilicates exposed to the surface of the area, such as fluorite (CaF
2), villiaumite (NaF), and biotite [
79,
80]. There are also many active fault zones in and around Yaodu, and fluorine-containing volatile gas or hydrothermal fluid migrates upward along the faults and penetrates groundwater, increasing the fluorine content [
79]. In addition, areas with high F
− in groundwater have a higher population density, and industries such as coal mining, metallurgy, and coking are concentrated. Discharge of domestic sewage and industrial wastewater is the other reason for the increase in F
− concentration in groundwater. Although the Cr
6+ concentration of all groundwater samples is within the desirable limit for drinking, it contributes more than 10% to the health risk, similar to Pb.
The carcinogenic risks due to exposure to As, Cd, and Cr
6+ through drinking water and dermal contact are shown in
Table 6. The ranges of the CR
oral for males, females, and children are 6.169 × 10
−5–1.165 × 10
−4, 7.278 × 10
−5–1.370 × 10
−4, and 3.914 × 10
−5–7.391 × 10
−5, with means of 7.156 × 10
−5, 8.442 × 10
−5, and 4.540 × 10
−5, respectively. The results of the CR
dermal are slightly smaller than CR
oral, ranging from 1.201 × 10
−5–4.400 × 10
−5 for males, 1.263 × 10
−5–4.625 × 10
−5 for females, and 4.725 × 10
−6–1.730 × 10
−5 for children, with means of 1.777 × 10
−5, 1.868 × 10
−5, and 6.989 × 10
−6, respectively. As a result, the CR
total values for males and females are 7.371 × 10
−5–1.605 × 10
−4 and 8.541 × 10
−5–1.837 × 10
−4, with means of 8.933 × 10
−5 and 1.031 × 10
−4. Concerning children, the CR
total values range from 4.386 × 10
−5–9.121 × 10
−5 with an average value of 5.239 × 10
−5. The carcinogenic risk values of all samples exceed the acceptable limit (1 × 10
−6) recommended by the Ministry of Ecology and Environment of the P. R. China [
64] for both adults and children. Additionally, the carcinogenic risk for adults is higher than for children, especially females. Similar results have also been found by Li et al. [
60] and Zhang et al. [
50] in Weining Plain and Guanzhong Plain, respectively.
For each contaminant, only the carcinogenic risk of As to children is below the acceptable limit, with an average of 8.317 × 10
−7. As per the average values of the CR
total, Cd contributes 72.63% to the total CR, Cr
6+ and As account for 25.77% and 1.60% of the CR
total, respectively. From the spatial distribution map of carcinogenic risk, it can be seen that the south-central part of the study area has a higher CR
total value for both adults and children, especially in Xiangfen and the west Yaodu areas. The coal-bearing formations are distributed all over the Linfen Basin, except Huoshan Mountain in the east of Yaodu and Ta’ershan-Erfengshan Mountain in the south of the study area. The areas with lower CR
total values in
Figure 6 correspond to regions lacking coal-bearing formations. This result suggests that the carcinogenic risk is closely related to the regional geological environment. The natural leaching process and human mining activities will cause many hazardous substances to enter the groundwater. In agricultural activities, the application of N-bearing fertilizers and phosphorous (P)-bearing fertilizers will increase PTEs concentrations such as Cd, Cr, As, and Pb in groundwater under the appropriate favoring geochemical conditions [
46,
47]. Long-term drinking of such groundwater by residents will increase the risk of visceral cancers such as lung, liver, skin, and kidney [
18].
Residents living in the central part of the study area face high health risks (HItotal > 1 and CRtotal > 1 × 10−6) due to the groundwater being affected by the geological environment and human activities. Therefore, government officials should pay more attention to PTEs pollution in groundwater caused by mining and the production of mineral resources. Furthermore, supplying residents with high-quality drinking water with safe concentrations of F− should be the goal of sustainable groundwater management in the Yaodu area. It is urgent to take various measures to treat the polluted groundwater before direct consumption by residents to ensure water safety and people’s health.
Additionally, compared with the results of groundwater quality assessment, we found that although most of the water quality of the study area is in good condition, both adults and children face great health risks, especially carcinogenic. Therefore, the overall groundwater assessment should be accompanied by a health risk assessment to better evaluate the suitability of groundwater for drinking.