4.1. Trace Metal Concentrations
This study focuses on 18 potential trace metals, including Ag, As, Ba, Be, Cd, Co, Cr, Cu, Mo, Ni, Pb, Sb, Sn, Sr, Tl, U, V and Zn. Results are summarized in
Table 1, together with corresponding World Health Organization (WHO) (WHO, 2006) and Chinese guidelines for drinking water [
42] The median concentrations of studied metals in the raw drinking water were similar to those in the boiled water. However, trace metals in boiled water are generally higher than in raw water. The concentration of nickel is the highest both raw and boiled water among all trace metals (
Figure 2). Silver concentration is almost not detected, and the concentration of cadmium and thallium is always low.
The overall concentrations varied greatly and the maximum concentrations of most metals in the drinking water were well below the existing guideline values [
42,
43]. The exceptions were As, Pb, Zn and Tl.
As shown in
Figure 3, except for zinc, the others correspond to the right coordinate axis. The concentration of zinc is extremely high. The mean concentrations of Ag, As, Cd, Co, Sb, Th, Tl, U are relatively low. The mean concentrations of Cr, Cu, Mo, Ni, Pb, V, Zn are relatively high. The concentrations of As, Mo, Pb, V are relatively concentrated. The concentrations of Cr, Cu, Ni, Zn are relatively dispersive. The concentration of Ni present a normal dispersion, the concentration of Pb, V are left offset prior, As, Co, U are right offset prior.
Because As is a highly toxic element with many carcinogenic effects, its occurrence in drinking water deserves the most concern. The maximum tolerable concentration of 10 μg/L has been established by the national Chinese and WHO drinking water standards (WHO, 2006; MHPRC, 2006). In mainland China, it involves eight provinces, including 40 counties and 1047 villages, with a population of 2,343,238 exposed to high-arsenic water (generally between 50 and 2000 μg/L) [
44]. In this study, most of the water samples had As concentrations lower than the Chinese and WHO threshold levels. But three samples from Minhe, Qinghai province, contained significantly high As concentrations (22–100 μg/L). These samples were collected from mineral spring water sites and thus the enrichment of As might be a result of water-rock interaction processes [
45]. Indeed, Qinghai is one of the provinces that suffer contamination by high-arsenic water. A highest concentration of 318 μg/L of As in groundwater has been reported in Qinghai province [
8,
35,
42]; As levels investigated in potable water in Qinghai found the highest concentration was 100 and 23.17 μg/L, respectively. Recently, a stricter reference As guideline of 3.4 μg/L has been recommended for drinking water based on male bladder cancer with an excess risk of 10
−4 for a 75-year lifetime exposure [
46]. In comparison, 5% of the drinking water failed to meet this guideline.
Lead has numerous acute and chronic adverse effects on humans, especially children, by influencing their nervous system and reducing their intelligence. Pb is an ultratrace metal in natural water with concentrations being as low as several ng/L [
47]. But elevated Pb has usually been detected in bottled water due to the leaching of Pb from the containers [
23,
24] and in tap water due to the corrosion of water supply systems [
22,
48,
49]. In the current study, Pb occurs in drinking water at very low concentrations (51%) below the detection limit. However, several samples with Pb concentrations are higher than the WHO and Chinese guidelines of 10 μg/L. These samples were all collected from Yunnan province, which is a major lead mineralization belt in China. So, these elevated concentrations of Pb in the drinking water probably reflect the high natural background [
50].
Zinc is an essential element, but excessive intakes can cause demyelinating diseases in humans [
51]. The median concentration of Zn in this study was 8.6 μg/L, far below the Chinese guideline value of 1000 μg/L. But Zn concentrations (1060–2260 μg/L) in three samples from Hunan province exceeded the Chinese reference value. Hunan province is regarded as the heartland of Chinese non-ferrous mining and smelting, especially for Zn. It can therefore be concluded that the high concentrations of Zn in drinking water from Hunan must be a result of contamination from the Zn mining/smelting activities [
50]. This is in agreement with the findings of Cai et al. [
34], where they have reported similar concentrations of Zn (10–2090 μg/L) in contaminated groundwater from Xiangjiang watershed, Hunan province.
According to the above discussion, more attention should be paid to Zn, Pb and As content, and the distribution of these in 8 provinces are shown in
Figure 1.
No WHO limits exist for Tl in drinking water although the United States Environmental Protection Agency (USEPA) guideline value for Tl has been set to 2 μg/L [
52]. No waters in this study had Tl concentrations that exceeded the USEPA limit. However, China established a much stricter threshold level of 0.1 μg/L for Tl in drinking water considering its high toxicity. Although most waters were far below this level, two samples still surpassed this (0.11 and 0.14 μg/L). The highest value of Tl in the present study was in agreement with the maximum concentration (0.152 μg/L) found in bottled waters from British Isles [
53].
Except for the above several outliers, the overall concentrations of the trace metals would indicate a general good quality of the studied waters. In addition, our results were in good agreement with previous studies on drinking water from urban regions of Beijing [
3,
31,
32], and cities from Jiangsu province [
33], suggesting that rural drinking water quality regarding trace metals was equal to those from the urban regions. A similar result was obtained from the survey of Gao et al. [
54], where no obvious difference was found for Cu, As, Hg and Cd concentrations in drinking water between the urban and suburban areas of Beijing.
Comparing eight cities in China (
Table 2 and
Table 3), the contents of trace metals in drinking water in the Baoding, Guangdong, Shenyang and Henan are higher than in rural areas. The other three cities is lower than that in rural area. However, trace elements in Beijing were lower after 2012 than before 2011. This indicates that the quality of drinking water in the country has improved.
Comparing trace metal content in groundwater in China and abroad (
Table 4), trace metal contents in groundwater are higher than drinking water universally. Zinc, for example, is very high. The content of trace elements in groundwater in China was higher than abroad [
64,
65].
4.2. Exposure Assessment
Considering the fact that fewer rural residents drink unboiled water, we thus used the data of boiled water for the exposure assessment and the following risk assessment. The chronic daily intakes (CDI) of trace metals via drinking water ingestion were calculated based on Equation (1) listed in
Section 3. The average daily intake rate of drinking water has been set to 2.2 L. This value has been proved and widely used for the Chinese population [
31,
32]. Average body weight was taken as 65 kg according to actual measurement by previous surveys [
31,
32]. The results of CDI together with background exposure values are presented in
Table 5.
The CDI values for most trace metals were much lower than the corresponding background exposure values. Generally, the mean values of CDI accounted for less than 5% of their background values, except Ba (32%), As (15%) and Pb (7.2%). The maximum CDI value (3.37 μg/kg/d or 200 μg/d) for As was 10 times and three times higher than its background exposure value (0.3 μg/kg/d) and tolerable daily intake (TDI) value (1.0 μg/kg/d) [
69], respectively, indicating a high health risk. In the arsenic-affected areas of West Bengal, Vietnam and Thailand, daily intakes of As via drinking water ingestion were 216–266 μg/d [
70], 1.1–4.3 μg/kg/d [
71] and 1.1–4.3 μg/kg/d [
72], respectively, similar to the highest value of our study. But in the highly contaminated areas of East Punjab, Pakistan, As concentrations in groundwater could reach 2400 μg/L and the calculated CDI for As could be as high as 4800 μg/d [
73]. Although high CDI of As were found in a few sites, its median value (0.01 μg/kg/d or 0.64 μg/d) in this study was much lower than the results from Beijing (0.07 μg/kg/d) [
31], Shenzhen (0.029 μg/kg/d) [
3] and İzmir, Turkey (0.035 μg/kg/d) [
26], but similar to Maryland (0.52 μg/d) [
74]. Similarly, the median or mean CDI values for other trace metals (mainly Cd, Cr, Cu, Pb, Mo, Ni, Sb, U and Zn) were also lower than (or similar to) data from the Chinese cities of Beijing [
31], Shanghai [
32], Shenzhen [
3], and cities from Jiangsu province [
33], as well as other regions of İzmir, Turkey [
26], Kohistan, Pakistan [
52], West Bengal, India [
67], Japan [
75] and Maryland [
74]. But the mean CDI values for Cr, Pb, Ni, U and Zn in the present study were higher than the corresponding data from Japan [
75].
4.3. Risk Assessment
Lifetime carcinogenic risk in
Table 6 was calculated only for As since the slope factor values were not available for the other metals. The slope factor for As was taken as 1.5 (mg/kg/d). Reference dose values of the trace metals used for non-carcinogenic risk assessment are listed in
Table 7.
The median and mean values of lifetime carcinogenic risk (R) for As in drinking water were 1.38 × 10
−5 and 1.11 × 10
−4, respectively, which was significantly lower than those results from Beijing (median 1.05 × 10
−4) [
31] and İzmir, Turkey (5.20 × 10
−5 and 6.35 × 10
−4 for median and 90th percentile, respectively) [
26]. In this study, 98.7% of the individuals had lifetime carcinogenic risks higher than the lowest safe standard (1 in 1,000,000), while 11.3% exceeded the highest safe standards (1 in 10,000). However, people from Qinghai province had cancer risks as high as 5 in 1000. Therefore, some residents from Chinese rural areas may be exposed to drinking water with As concentrations that could lead to cancer.
For the non-carcinogenic risk, hazard quotient (HQ) values for trace metals in this study were found in the order of As >> Sr > Ba > Sb > U > Zn > Mo > Ni > Cd >> Cr ≈ Ag >> Be. Arsenic has the highest HQ values with median and 90th percentile values of 0.03 and 0.27, respectively. Furthermore, three samples had HQ values for As higher than 1 (2.7, 3.2 and 11.2 respectively), indicating that non-cancer health effects would occur if people drink these waters. Apart from these, HQ values for all the other trace metals were far below 1, indicating a negligible non-carcinogenic risk.
It is important to note that HQ was not calculated for Pb since the reference dose value was not available. In addition, the previously established provisional tolerable weekly intake (PTWI) for Pb (25 μg/kg bw) was withdrawn in 2010 by JECFA (Joint FAO/WHO Expert Committee on Food Additives), as it could no longer be considered health protective. Therefore, the health risk of Pb in drinking water has not been assessed like the other metals. In fact, CDI values for Pb were relatively high in some sites from Yunnan province and the highest value was found to exceed the background exposure value for adults. Therefore, an elevated health risk may be posed to this portion of population and this situation will be worse when aggregated exposure over all pathways/routes is considered.